chapter 6 the link layer and lans - plone site
TRANSCRIPT
Computer Networking A Top Down Approach
A note on the use of these Powerpoint slidesWersquore making these slides freely available to all (faculty students readers) Theyrsquore in PowerPoint form so you see the animations and can add modify and delete slides (including this one) and slide content to suit your needs They obviously represent a lot of work on our part In return for use we only ask the following
sect If you use these slides (eg in a class) that you mention their source (after all wersquod like people to use our book)
sect If you post any slides on a www site that you note that they are adapted from (or perhaps identical to) our slides and note our copyright of this material
Thanks and enjoy JFKKWR
All material copyright 1996-2016JF Kurose and KW Ross All Rights Reserved
7th edition Jim Kurose Keith RossPearsonAddison WesleyApril 2016
Chapter 6The Link Layer and LANs
6-1Link Layer and LANs
Chapter 6 Link layer and LANsour goalssect understand principles behind link layer
servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressingbull local area networks Ethernet VLANs
sect instantiation implementation of various link layer technologies
6-2Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-3Link Layer and LANs
Link layer introductionterminologysect hosts and routers nodessect communication channels that
connect adjacent nodes along communication path linksbull wired linksbull wireless linksbull LANs
sect layer-2 packet frameencapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
6-4Link Layer and LANs
Link layer context
sect datagram transferred by different link protocols over different linksbull eg Ethernet on first link
frame relay on intermediate links 80211 on last link
sect each link protocol provides different servicesbull eg may or may not
provide rdt over link
transportation analogysect trip from Princeton to Lausanne
bull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne
sect tourist = datagramsect transport segment =
communication linksect transportation mode = link
layer protocolsect travel agent = routing
algorithm
6-5Link Layer and LANs
Link layer servicessect framing link access
bull encapsulate datagram into frame adding header trailerbull channel access if shared mediumbull ldquoMACrdquo addresses used in frame headers to identify
source destination bull different from IP address
sect reliable delivery between adjacent nodesbull we learned how to do this already (chapter 3)bull seldom used on low bit-error link (fiber some twisted
pair)bull wireless links high error rates
bull Q why both link-level and end-end reliability
6-6Link Layer and LANs
sect flow controlbull pacing between adjacent sending and receiving nodes
sect error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors
bull signals sender for retransmission or drops frame
sect error correctionbull receiver identifies and corrects bit error(s) without resorting to
retransmission
sect half-duplex and full-duplexbull with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
6-7Link Layer and LANs
Where is the link layer implementedsect in each and every hostsect link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipbull Ethernet card 80211
card Ethernet chipsetbull implements link physical
layersect attaches into hostrsquos system
busessect combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetwork
link
linkphysical
6-8Link Layer and LANs
Adaptors communicating
sect sending sidebull encapsulates datagram in
framebull adds error checking bits
rdt flow control etc
sect receiving sidebull looks for errors rdt
flow control etcbull extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-9Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-10Link Layer and LANs
Error detectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-11Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Chapter 6 Link layer and LANsour goalssect understand principles behind link layer
servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressingbull local area networks Ethernet VLANs
sect instantiation implementation of various link layer technologies
6-2Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-3Link Layer and LANs
Link layer introductionterminologysect hosts and routers nodessect communication channels that
connect adjacent nodes along communication path linksbull wired linksbull wireless linksbull LANs
sect layer-2 packet frameencapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
6-4Link Layer and LANs
Link layer context
sect datagram transferred by different link protocols over different linksbull eg Ethernet on first link
frame relay on intermediate links 80211 on last link
sect each link protocol provides different servicesbull eg may or may not
provide rdt over link
transportation analogysect trip from Princeton to Lausanne
bull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne
sect tourist = datagramsect transport segment =
communication linksect transportation mode = link
layer protocolsect travel agent = routing
algorithm
6-5Link Layer and LANs
Link layer servicessect framing link access
bull encapsulate datagram into frame adding header trailerbull channel access if shared mediumbull ldquoMACrdquo addresses used in frame headers to identify
source destination bull different from IP address
sect reliable delivery between adjacent nodesbull we learned how to do this already (chapter 3)bull seldom used on low bit-error link (fiber some twisted
pair)bull wireless links high error rates
bull Q why both link-level and end-end reliability
6-6Link Layer and LANs
sect flow controlbull pacing between adjacent sending and receiving nodes
sect error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors
bull signals sender for retransmission or drops frame
sect error correctionbull receiver identifies and corrects bit error(s) without resorting to
retransmission
sect half-duplex and full-duplexbull with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
6-7Link Layer and LANs
Where is the link layer implementedsect in each and every hostsect link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipbull Ethernet card 80211
card Ethernet chipsetbull implements link physical
layersect attaches into hostrsquos system
busessect combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetwork
link
linkphysical
6-8Link Layer and LANs
Adaptors communicating
sect sending sidebull encapsulates datagram in
framebull adds error checking bits
rdt flow control etc
sect receiving sidebull looks for errors rdt
flow control etcbull extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-9Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-10Link Layer and LANs
Error detectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-11Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-3Link Layer and LANs
Link layer introductionterminologysect hosts and routers nodessect communication channels that
connect adjacent nodes along communication path linksbull wired linksbull wireless linksbull LANs
sect layer-2 packet frameencapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
6-4Link Layer and LANs
Link layer context
sect datagram transferred by different link protocols over different linksbull eg Ethernet on first link
frame relay on intermediate links 80211 on last link
sect each link protocol provides different servicesbull eg may or may not
provide rdt over link
transportation analogysect trip from Princeton to Lausanne
bull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne
sect tourist = datagramsect transport segment =
communication linksect transportation mode = link
layer protocolsect travel agent = routing
algorithm
6-5Link Layer and LANs
Link layer servicessect framing link access
bull encapsulate datagram into frame adding header trailerbull channel access if shared mediumbull ldquoMACrdquo addresses used in frame headers to identify
source destination bull different from IP address
sect reliable delivery between adjacent nodesbull we learned how to do this already (chapter 3)bull seldom used on low bit-error link (fiber some twisted
pair)bull wireless links high error rates
bull Q why both link-level and end-end reliability
6-6Link Layer and LANs
sect flow controlbull pacing between adjacent sending and receiving nodes
sect error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors
bull signals sender for retransmission or drops frame
sect error correctionbull receiver identifies and corrects bit error(s) without resorting to
retransmission
sect half-duplex and full-duplexbull with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
6-7Link Layer and LANs
Where is the link layer implementedsect in each and every hostsect link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipbull Ethernet card 80211
card Ethernet chipsetbull implements link physical
layersect attaches into hostrsquos system
busessect combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetwork
link
linkphysical
6-8Link Layer and LANs
Adaptors communicating
sect sending sidebull encapsulates datagram in
framebull adds error checking bits
rdt flow control etc
sect receiving sidebull looks for errors rdt
flow control etcbull extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-9Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-10Link Layer and LANs
Error detectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-11Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Link layer introductionterminologysect hosts and routers nodessect communication channels that
connect adjacent nodes along communication path linksbull wired linksbull wireless linksbull LANs
sect layer-2 packet frameencapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
6-4Link Layer and LANs
Link layer context
sect datagram transferred by different link protocols over different linksbull eg Ethernet on first link
frame relay on intermediate links 80211 on last link
sect each link protocol provides different servicesbull eg may or may not
provide rdt over link
transportation analogysect trip from Princeton to Lausanne
bull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne
sect tourist = datagramsect transport segment =
communication linksect transportation mode = link
layer protocolsect travel agent = routing
algorithm
6-5Link Layer and LANs
Link layer servicessect framing link access
bull encapsulate datagram into frame adding header trailerbull channel access if shared mediumbull ldquoMACrdquo addresses used in frame headers to identify
source destination bull different from IP address
sect reliable delivery between adjacent nodesbull we learned how to do this already (chapter 3)bull seldom used on low bit-error link (fiber some twisted
pair)bull wireless links high error rates
bull Q why both link-level and end-end reliability
6-6Link Layer and LANs
sect flow controlbull pacing between adjacent sending and receiving nodes
sect error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors
bull signals sender for retransmission or drops frame
sect error correctionbull receiver identifies and corrects bit error(s) without resorting to
retransmission
sect half-duplex and full-duplexbull with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
6-7Link Layer and LANs
Where is the link layer implementedsect in each and every hostsect link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipbull Ethernet card 80211
card Ethernet chipsetbull implements link physical
layersect attaches into hostrsquos system
busessect combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetwork
link
linkphysical
6-8Link Layer and LANs
Adaptors communicating
sect sending sidebull encapsulates datagram in
framebull adds error checking bits
rdt flow control etc
sect receiving sidebull looks for errors rdt
flow control etcbull extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-9Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-10Link Layer and LANs
Error detectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-11Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Link layer context
sect datagram transferred by different link protocols over different linksbull eg Ethernet on first link
frame relay on intermediate links 80211 on last link
sect each link protocol provides different servicesbull eg may or may not
provide rdt over link
transportation analogysect trip from Princeton to Lausanne
bull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne
sect tourist = datagramsect transport segment =
communication linksect transportation mode = link
layer protocolsect travel agent = routing
algorithm
6-5Link Layer and LANs
Link layer servicessect framing link access
bull encapsulate datagram into frame adding header trailerbull channel access if shared mediumbull ldquoMACrdquo addresses used in frame headers to identify
source destination bull different from IP address
sect reliable delivery between adjacent nodesbull we learned how to do this already (chapter 3)bull seldom used on low bit-error link (fiber some twisted
pair)bull wireless links high error rates
bull Q why both link-level and end-end reliability
6-6Link Layer and LANs
sect flow controlbull pacing between adjacent sending and receiving nodes
sect error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors
bull signals sender for retransmission or drops frame
sect error correctionbull receiver identifies and corrects bit error(s) without resorting to
retransmission
sect half-duplex and full-duplexbull with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
6-7Link Layer and LANs
Where is the link layer implementedsect in each and every hostsect link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipbull Ethernet card 80211
card Ethernet chipsetbull implements link physical
layersect attaches into hostrsquos system
busessect combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetwork
link
linkphysical
6-8Link Layer and LANs
Adaptors communicating
sect sending sidebull encapsulates datagram in
framebull adds error checking bits
rdt flow control etc
sect receiving sidebull looks for errors rdt
flow control etcbull extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-9Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-10Link Layer and LANs
Error detectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-11Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Link layer servicessect framing link access
bull encapsulate datagram into frame adding header trailerbull channel access if shared mediumbull ldquoMACrdquo addresses used in frame headers to identify
source destination bull different from IP address
sect reliable delivery between adjacent nodesbull we learned how to do this already (chapter 3)bull seldom used on low bit-error link (fiber some twisted
pair)bull wireless links high error rates
bull Q why both link-level and end-end reliability
6-6Link Layer and LANs
sect flow controlbull pacing between adjacent sending and receiving nodes
sect error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors
bull signals sender for retransmission or drops frame
sect error correctionbull receiver identifies and corrects bit error(s) without resorting to
retransmission
sect half-duplex and full-duplexbull with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
6-7Link Layer and LANs
Where is the link layer implementedsect in each and every hostsect link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipbull Ethernet card 80211
card Ethernet chipsetbull implements link physical
layersect attaches into hostrsquos system
busessect combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetwork
link
linkphysical
6-8Link Layer and LANs
Adaptors communicating
sect sending sidebull encapsulates datagram in
framebull adds error checking bits
rdt flow control etc
sect receiving sidebull looks for errors rdt
flow control etcbull extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-9Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-10Link Layer and LANs
Error detectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-11Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
sect flow controlbull pacing between adjacent sending and receiving nodes
sect error detection bull errors caused by signal attenuation noise bull receiver detects presence of errors
bull signals sender for retransmission or drops frame
sect error correctionbull receiver identifies and corrects bit error(s) without resorting to
retransmission
sect half-duplex and full-duplexbull with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
6-7Link Layer and LANs
Where is the link layer implementedsect in each and every hostsect link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipbull Ethernet card 80211
card Ethernet chipsetbull implements link physical
layersect attaches into hostrsquos system
busessect combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetwork
link
linkphysical
6-8Link Layer and LANs
Adaptors communicating
sect sending sidebull encapsulates datagram in
framebull adds error checking bits
rdt flow control etc
sect receiving sidebull looks for errors rdt
flow control etcbull extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-9Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-10Link Layer and LANs
Error detectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-11Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Where is the link layer implementedsect in each and every hostsect link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipbull Ethernet card 80211
card Ethernet chipsetbull implements link physical
layersect attaches into hostrsquos system
busessect combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetwork
link
linkphysical
6-8Link Layer and LANs
Adaptors communicating
sect sending sidebull encapsulates datagram in
framebull adds error checking bits
rdt flow control etc
sect receiving sidebull looks for errors rdt
flow control etcbull extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-9Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-10Link Layer and LANs
Error detectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-11Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Adaptors communicating
sect sending sidebull encapsulates datagram in
framebull adds error checking bits
rdt flow control etc
sect receiving sidebull looks for errors rdt
flow control etcbull extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-9Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-10Link Layer and LANs
Error detectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-11Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-10Link Layer and LANs
Error detectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-11Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Error detectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-11Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Internet checksum (review)
sendersect treat segment contents
as sequence of 16-bit integers
sect checksum addition (1rsquos complement sum) of segment contents
sect sender puts checksum value into UDP checksum field
receiversect compute checksum of
received segmentsect check if computed
checksum equals checksum field valuebull NO - error detectedbull YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
6-13Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Cyclic redundancy checksect more powerful error-detection codingsect view data bits D as a binary numbersect choose r+1 bit pattern (generator) Gsect goal choose r CRC bits R such that
bull ltDRgt exactly divisible by G (modulo 2) bull receiver knows G divides ltDRgt by G If non-zero remainder
error detectedbull can detect all burst errors less than r+1 bits
sect widely used in practice (Ethernet 80211 WiFi ATM)
6-14Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-15Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-16Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquosect point-to-point
bull PPP for dial-up accessbull point-to-point link between Ethernet switch host
sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-17Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Multiple access protocolssect single shared broadcast channel sect two or more simultaneous transmissions by nodes
interference bull collision if node receives two or more signals at the same
time
multiple access protocolsect distributed algorithm that determines how nodes share
channel ie determine when node can transmitsect communication about channel sharing must use channel itself
bull no out-of-band channel for coordination
6-18Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
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6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-19Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
MAC protocols taxonomy
three broad classessect channel partitioning
bull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use
sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions
sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer
turns
6-20Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accesssect access to channel in rounds sect each station gets fixed length slot (length = packet
transmission time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-21Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
FDM cable
Channel partitioning MAC protocols FDMA
6-22Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Random access protocolssect when node has packet to send
bull transmit at full channel data rate Rbull no a priori coordination among nodes
sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies
bull how to detect collisionsbull how to recover from collisions (eg via delayed
retransmissions)sect examples of random access MAC protocols
bull slotted ALOHAbull ALOHAbull CSMA CSMACD CSMACA
6-23Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Slotted ALOHA
assumptionssect all frames same sizesect time divided into equal size
slots (time to transmit 1 frame)
sect nodes start to transmit only slot beginning
sect nodes are synchronizedsect if 2 or more nodes transmit
in slot all nodes detect collision
operationsect when node obtains fresh
frame transmits in next slotbull if no collision node can send
new frame in next slotbull if collision node retransmits
frame in each subsequent slot with prob p until success
6-24Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Prossect single active node can
continuously transmit at full rate of channel
sect highly decentralized only slots in nodes need to be in sync
sect simple
Conssect collisions wasting slotssect idle slotssect nodes may be able to
detect collision in less than time to transmit packet
sect clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-25Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
sect suppose N nodes with many frames to send each transmits in slot with probability p
sect prob that given node has success in a slot = p(1-p)N-1
sect prob that any node has a success = Np(1-p)N-1
sect max efficiency find p that maximizes Np(1-p)N-1
sect for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-26Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Pure (unslotted) ALOHA
sect unslotted Aloha simpler no synchronizationsect when frame first arrives
bull transmit immediately sect collision probability increases
bull frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-27Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-28Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire framesect if channel sensed busy defer
transmission
sect human analogy donrsquot interrupt others
6-29Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
CSMA collisions
sect collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
sect collision entire packet transmission time wastedbull distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-30Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAbull collisions detected within short timebull colliding transmissions aborted reducing channel wastage
sect collision detectionbull easy in wired LANs measure signal strengths compare
transmitted received signalsbull difficult in wireless LANs received signal strength
overwhelmed by local transmission strength sect human analogy the polite conversationalist
6-31Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-32Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff bull after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
bull longer backoff interval with more collisions
6-33Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
CSMACD efficiency
sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame
sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity
sect better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-34Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node random access MAC protocols
sect efficient at low load single node can fully utilize channel
sect high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
6-35Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
pollingsect master node ldquoinvitesrdquo
slave nodes to transmit in turn
sect typically used with ldquodumbrdquo slave devices
sect concernsbull polling overhead bull latencybull single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-36Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-37Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-38Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-39Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Summary of MAC protocols
sect channel partitioning by time frequency or codebull Time Division Frequency Division
sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring
6-40Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-41Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
MAC addresses and ARP
sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwarding
sect MAC (or LAN or physical or Ethernet) addressbull function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
bull 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
bull eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-42Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-43Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
LAN addresses (more)
sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space
(to assure uniqueness)sect analogy
bull MAC address like Social Security Numberbull IP address like postal address
sect MAC flat address portability bull can move LAN card from one LAN to another
sect IP hierarchical address not portablebull address depends on IP subnet to which node is
attached
6-44Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
bull IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-45Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
ARP protocol same LANsect A wants to send datagram
to Bbull Brsquos MAC address not in
Arsquos ARP tablesect A broadcasts ARP query
packet containing Bs IP address
bull destination MAC address = FF-FF-FF-FF-FF-FF
bull all nodes on LAN receive ARP query
sect B receives ARP packet replies to A with its (Bs) MAC address
bull frame sent to Arsquos MAC address (unicast)
sect A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
bull soft state information that times out (goes away) unless refreshed
sect ARP is ldquoplug-and-playrdquobull nodes create their ARP
tables without intervention from net administrator
6-46Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-47Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-48Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-49Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-50Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-51Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-52Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-53Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Ethernetldquodominantrdquo wired LAN technology sect single chip multiple speeds (eg Broadcom BCM5761)sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-54Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Ethernet physical topologysect bus popular through mid 90s
bull all nodes in same collision domain (can collide with each other)
sect star prevails todaybull active switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-55Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble sect 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011sect used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-56Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Ethernet frame structure (more)sect addresses 6 byte source destination MAC addresses
bull if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
bull otherwise adapter discards framesect type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)sect CRC cyclic redundancy check at receiver
bull error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-57Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs
Ethernet unreliable connectionless
sect connectionless no handshaking between sending and receiving NICs
sect unreliable receiving NIC doesnt send acks or nacks to sending NICbull data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
sect Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-58Link Layer and LANs
8023 Ethernet standards link amp physical layers
sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps
10 Gbps 40 Gbpsbull different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-59Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-60Link Layer and LANs
Ethernet switchsect link-layer device takes an active role
bull store forward Ethernet framesbull examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
sect transparentbull hosts are unaware of presence of switches
sect plug-and-play self-learningbull switches do not need to be configured
6-61Link Layer and LANs
Switch multiple simultaneous transmissions
sect hosts have dedicated direct connection to switch
sect switches buffer packetssect Ethernet protocol used on each
incoming link but no collisions full duplexbull each link is its own collision
domainsect switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-62Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learningsect switch learns which hosts
can be reached through which interfaces
bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
bull records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-64Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
6-65Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
sect frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-66Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-67Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-68Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-69Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
framedatagram
6-70Link Layer and LANs
VLANs motivationconsidersect CS user moves office to EE
but wants connect to CS switch
sect single broadcast domainbull all layer-2 broadcast
traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
bull securityprivacy efficiency issues
Computer Science Electrical
Engineering
ComputerEngineering
6-71Link Layer and LANs
VLANsport-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting VLAN capabilities can be configured to define multiple virtualLANS over single physical LAN infrastructure
Virtual Local Area Network 1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
6-72Link Layer and LANs
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
bull can also define VLAN based on MAC addresses of endpoints rather than switch port
sect dynamic membership ports can be dynamically assigned among VLANs
router
sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined
switches plus routers
6-73Link Layer and LANs
VLANS spanning multiple switches
sect trunk port carries frames between VLANS defined over multiple physical switches
bull frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
bull 8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
6-74Link Layer and LANs
type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
destaddress
sourceaddress data (payload) CRCpreamble
destaddress
sourceaddresspreamble data (payload) CRC
type
6-75Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-76Link Layer and LANs
Multiprotocol label switching (MPLS)
sect initial goal high-speed IP forwarding using fixed length label (instead of IP address)
bull fast lookup using fixed length identifier (rather than shortest prefix matching)
bull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address
PPP or Ethernet header IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
6-77Link Layer and LANs
MPLS capable routers
sect aka label-switched routersect forward packets to outgoing interface based only on
label value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables
sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to
same destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup
paths (useful for VoIP)
6-78Link Layer and LANs
R2
DR3
R5
A
R6
MPLS versus IP paths
IP routersect IP routing path to destination determined
by destination address alone
R4
6-79Link Layer and LANs
R2
DR3R4
R5
A
R6
MPLS versus IP paths
IP-onlyrouter
sect IP routing path to destination determined by destination address alone
MPLS and IP router
sect MPLS routing path to destination can be based on source and destination address
bull fast reroute precompute backup routes in case of link failure
entry router (R4) can use different MPLS routes to A based eg on source address
6-80Link Layer and LANs
MPLS signalingsect modify OSPF IS-IS link-state flooding protocols to
carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth
DR4
R5
A
R6
sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
6-81Link Layer and LANs
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
6-82Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-83Link Layer and LANs
Data center networks
sect 10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitybull e-business (eg Amazon)bull content-servers (eg YouTube Akamai Apple Microsoft)bull search engines data mining (eg Google)
sect challengessect multiple applications each
serving massive numbers of clients
sect managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container Chicago data center
6-84Link Layer and LANs
Serverracks
TORswitches
Tier-1switches
Tier-2switches
Loadbalancer
Loadbalancer
B
1 2 3 4 5 6 7 8
A C
Borderrouter
Accessrouter
Data center networks load balancer application-layer routingsect receives external client requestssect directs workload within data centersect returns results to external client (hiding data
center internals from client)
6-85Link Layer and LANs
Internet
Serverracks
TORswitches
Tier-1switches
Tier-2switches
1 2 3 4 5 6 7 8
Data center networks sect rich interconnection among switches racks
bull increased throughput between racks (multiple routing paths possible)
bull increased reliability via redundancy
6-86Link Layer and LANs
Link layer LANs outline
61 introduction services62 error detection
correction 63 multiple access
protocols64 LANs
bull addressing ARPbull Ethernetbull switchesbull VLANS
65 link virtualization MPLS
66 data center networking
67 a day in the life of a web request
6-87Link Layer and LANs
Synthesis a day in the life of a web request
sect journey down protocol stack completebull application transport network link
sect putting-it-all-together synthesisbull goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
bull scenario student attaches laptop to campus network requestsreceives wwwgooglecom
6-88Link Layer and LANs
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
web page
browser
6-89Link Layer and LANs
router(runs DHCP)
A day in the lifehellip connecting to the Internet
sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
sect DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8023Ethernet
sect Ethernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP
6-90Link Layer and LANs
router(runs DHCP)
sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
sect encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
sect DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
6-91Link Layer and LANs
router(runs DHCP)
A day in the lifehellip ARP (before DNS before HTTP)
sect before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface
sect client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
6-92Link Layer and LANs
router(runs DHCP)
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGProuting protocols) to DNS server
sect demuxed to DNS serversect DNS server replies to client
with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
6-93Link Layer and LANs
router(runs DHCP)
A day in the lifehellipTCP connection carrying HTTP
HTTPTCPIPEthPhy
HTTP
sect to send HTTP request client first opens TCP socket to web server
sect TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
sect TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIPEthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
sect web server responds with TCP SYNACK (step 2 in 3-way handshake)
6-94Link Layer and LANs
router(runs DHCP)
A day in the lifehellip HTTP requestreply HTTPTCPIPEthPhy
HTTP
sect HTTP request sent into TCP socket
sect IP datagram containing HTTP request routed to wwwgooglecom
sect IP datagram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIPEthPhy
sect web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
sect web page finally () displayed
6-95Link Layer and LANs
Chapter 6 Summarysect principles behind data link layer services
bull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing
sect instantiation and implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS
sect synthesis a day in the life of a web request
6-96Link Layer and LANs
Chapter 6 letrsquos take a breath
sect journey down protocol stack complete (except PHY)
sect solid understanding of networking principles practice
sect hellip could stop here hellip but lots of interesting topicsbull wirelessbull multimediabull security
6-97Link Layer and LANs