IT 347 Midterm 2 Review

Vocab Review

bull ATMbull CBRbull ABRbull VBRbull UBRbull MSSbull MTUbull AIMD

Transport Layer 3-3

TCP retransmission scenarios

Host A

Seq=100 20 bytes data

ACK=100

timepremature timeout

Host B

Seq=92 8 bytes data

ACK=120

Seq=92 8 bytes data

Seq=

92 ti

meo

ut

ACK=120

Host A

Seq=92 8 bytes data

ACK=100

loss

timeo

ut

lost ACK scenario

Host B

X

Seq=92 8 bytes data

ACK=100

time

Seq=

92 ti

meo

utSendBase= 100

SendBase= 120

SendBase= 120

Sendbase= 100

Transport Layer 3-4

TCP retransmission scenarios (more)Host A

Seq=92 8 bytes data

ACK=100

loss

timeo

ut

Cumulative ACK scenario

Host B

X

Seq=100 20 bytes data

ACK=120

time

SendBase= 120

Transport Layer 3-5

TCP ACK generation [RFC 1122 RFC 2581]

Event at Receiver

Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed

Arrival of in-order segment withexpected seq One other segment has ACK pending

Arrival of out-of-order segmenthigher-than-expect seq Gap detected

Arrival of segment that partially or completely fills gap

TCP Receiver action

Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK

Immediately send single cumulative ACK ACKing both in-order segments

Immediately send duplicate ACK indicating seq of next expected byte

Immediate send ACK provided thatsegment starts at lower end of gap

Transport Layer 3-6

Fast Retransmitbull time-out period often relatively long

ndash long delay before resending lost packetbull detect lost segments via duplicate ACKs

ndash sender often sends many segments back-to-backndash if segment is lost there will likely be many duplicate ACKs for that

segment

bull If sender receives 3 ACKs for same data it assumes that segment after ACKed data was lostndash fast retransmit resend

segment before timer expires

Transport Layer 3-7

Host A

timeo

ut

Host B

time

X

resend seq X2

seq x1seq x2seq x3seq x4seq x5

ACK x1

ACK x1ACK x1ACK x1

tripleduplicate

ACKs

Transport Layer 3-8

event ACK received with ACK field value of y if (y gt SendBase) SendBase = y if (there are currently not-yet-acknowledged segments) start timer else increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) resend segment with sequence number y

Fast retransmit algorithm

a duplicate ACK for already ACKed segment

fast retransmit

Transport Layer 3-9

TCP Flow Controlbull receive side of TCP

connection has a receive buffer

bull speed-matching service matching send rate to receiving applicationrsquos drain rate

app process may be slow at reading from buffer

sender wonrsquot overflowreceiverrsquos buffer bytransmitting too much too fast

flow control

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

Transport Layer 3-10

TCP Flow control how it works

(suppose TCP receiver discards out-of-order segments)

bull unused buffer space= rwnd= RcvBuffer-[LastByteRcvd -

LastByteRead]

bull receiver advertises unused buffer space by including rwnd value in segment header

bull sender limits of unACKed bytes to rwndndash guarantees receiverrsquos buffer

doesnrsquot overflow

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

rwndRcvBuffer

Transport Layer 3-11

TCP congestion control bandwidth probing

ldquoprobing for bandwidthrdquo increase transmission rate on receipt of ACK until eventually loss occurs then decrease transmission rate continue to increase on ACK decrease on loss (since available

bandwidth is changing depending on other connections in network)

ACKs being received so increase rate

X

X

XX

X loss so decrease rate

send

ing

rate

time

Q how fast to increasedecrease details to follow

TCPrsquosldquosawtoothrdquobehavior

Transport Layer 3-12

TCP Congestion Control details

bull sender limits rate by limiting number of unACKed bytes ldquoin pipelinerdquo

ndash cwnd differs from rwnd (how why)ndash sender limited by min(cwndrwnd)

bull roughly

bull cwnd is dynamic function of perceived network congestion

rate = cwnd

RTT bytessec

LastByteSent-LastByteAcked cwnd

cwndbytes

RTT

ACK(s)

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Vocab Review

bull ATMbull CBRbull ABRbull VBRbull UBRbull MSSbull MTUbull AIMD

Transport Layer 3-3

TCP retransmission scenarios

Host A

Seq=100 20 bytes data

ACK=100

timepremature timeout

Host B

Seq=92 8 bytes data

ACK=120

Seq=92 8 bytes data

Seq=

92 ti

meo

ut

ACK=120

Host A

Seq=92 8 bytes data

ACK=100

loss

timeo

ut

lost ACK scenario

Host B

X

Seq=92 8 bytes data

ACK=100

time

Seq=

92 ti

meo

utSendBase= 100

SendBase= 120

SendBase= 120

Sendbase= 100

Transport Layer 3-4

TCP retransmission scenarios (more)Host A

Seq=92 8 bytes data

ACK=100

loss

timeo

ut

Cumulative ACK scenario

Host B

X

Seq=100 20 bytes data

ACK=120

time

SendBase= 120

Transport Layer 3-5

TCP ACK generation [RFC 1122 RFC 2581]

Event at Receiver

Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed

Arrival of in-order segment withexpected seq One other segment has ACK pending

Arrival of out-of-order segmenthigher-than-expect seq Gap detected

Arrival of segment that partially or completely fills gap

TCP Receiver action

Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK

Immediately send single cumulative ACK ACKing both in-order segments

Immediately send duplicate ACK indicating seq of next expected byte

Immediate send ACK provided thatsegment starts at lower end of gap

Transport Layer 3-6

Fast Retransmitbull time-out period often relatively long

ndash long delay before resending lost packetbull detect lost segments via duplicate ACKs

ndash sender often sends many segments back-to-backndash if segment is lost there will likely be many duplicate ACKs for that

segment

bull If sender receives 3 ACKs for same data it assumes that segment after ACKed data was lostndash fast retransmit resend

segment before timer expires

Transport Layer 3-7

Host A

timeo

ut

Host B

time

X

resend seq X2

seq x1seq x2seq x3seq x4seq x5

ACK x1

ACK x1ACK x1ACK x1

tripleduplicate

ACKs

Transport Layer 3-8

event ACK received with ACK field value of y if (y gt SendBase) SendBase = y if (there are currently not-yet-acknowledged segments) start timer else increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) resend segment with sequence number y

Fast retransmit algorithm

a duplicate ACK for already ACKed segment

fast retransmit

Transport Layer 3-9

TCP Flow Controlbull receive side of TCP

connection has a receive buffer

bull speed-matching service matching send rate to receiving applicationrsquos drain rate

app process may be slow at reading from buffer

sender wonrsquot overflowreceiverrsquos buffer bytransmitting too much too fast

flow control

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

Transport Layer 3-10

TCP Flow control how it works

(suppose TCP receiver discards out-of-order segments)

bull unused buffer space= rwnd= RcvBuffer-[LastByteRcvd -

LastByteRead]

bull receiver advertises unused buffer space by including rwnd value in segment header

bull sender limits of unACKed bytes to rwndndash guarantees receiverrsquos buffer

doesnrsquot overflow

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

rwndRcvBuffer

Transport Layer 3-11

TCP congestion control bandwidth probing

ldquoprobing for bandwidthrdquo increase transmission rate on receipt of ACK until eventually loss occurs then decrease transmission rate continue to increase on ACK decrease on loss (since available

bandwidth is changing depending on other connections in network)

ACKs being received so increase rate

X

X

XX

X loss so decrease rate

send

ing

rate

time

Q how fast to increasedecrease details to follow

TCPrsquosldquosawtoothrdquobehavior

Transport Layer 3-12

TCP Congestion Control details

bull sender limits rate by limiting number of unACKed bytes ldquoin pipelinerdquo

ndash cwnd differs from rwnd (how why)ndash sender limited by min(cwndrwnd)

bull roughly

bull cwnd is dynamic function of perceived network congestion

rate = cwnd

RTT bytessec

LastByteSent-LastByteAcked cwnd

cwndbytes

RTT

ACK(s)

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-3

TCP retransmission scenarios

Host A

Seq=100 20 bytes data

ACK=100

timepremature timeout

Host B

Seq=92 8 bytes data

ACK=120

Seq=92 8 bytes data

Seq=

92 ti

meo

ut

ACK=120

Host A

Seq=92 8 bytes data

ACK=100

loss

timeo

ut

lost ACK scenario

Host B

X

Seq=92 8 bytes data

ACK=100

time

Seq=

92 ti

meo

utSendBase= 100

SendBase= 120

SendBase= 120

Sendbase= 100

Transport Layer 3-4

TCP retransmission scenarios (more)Host A

Seq=92 8 bytes data

ACK=100

loss

timeo

ut

Cumulative ACK scenario

Host B

X

Seq=100 20 bytes data

ACK=120

time

SendBase= 120

Transport Layer 3-5

TCP ACK generation [RFC 1122 RFC 2581]

Event at Receiver

Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed

Arrival of in-order segment withexpected seq One other segment has ACK pending

Arrival of out-of-order segmenthigher-than-expect seq Gap detected

Arrival of segment that partially or completely fills gap

TCP Receiver action

Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK

Immediately send single cumulative ACK ACKing both in-order segments

Immediately send duplicate ACK indicating seq of next expected byte

Immediate send ACK provided thatsegment starts at lower end of gap

Transport Layer 3-6

Fast Retransmitbull time-out period often relatively long

ndash long delay before resending lost packetbull detect lost segments via duplicate ACKs

ndash sender often sends many segments back-to-backndash if segment is lost there will likely be many duplicate ACKs for that

segment

bull If sender receives 3 ACKs for same data it assumes that segment after ACKed data was lostndash fast retransmit resend

segment before timer expires

Transport Layer 3-7

Host A

timeo

ut

Host B

time

X

resend seq X2

seq x1seq x2seq x3seq x4seq x5

ACK x1

ACK x1ACK x1ACK x1

tripleduplicate

ACKs

Transport Layer 3-8

event ACK received with ACK field value of y if (y gt SendBase) SendBase = y if (there are currently not-yet-acknowledged segments) start timer else increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) resend segment with sequence number y

Fast retransmit algorithm

a duplicate ACK for already ACKed segment

fast retransmit

Transport Layer 3-9

TCP Flow Controlbull receive side of TCP

connection has a receive buffer

bull speed-matching service matching send rate to receiving applicationrsquos drain rate

app process may be slow at reading from buffer

sender wonrsquot overflowreceiverrsquos buffer bytransmitting too much too fast

flow control

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

Transport Layer 3-10

TCP Flow control how it works

(suppose TCP receiver discards out-of-order segments)

bull unused buffer space= rwnd= RcvBuffer-[LastByteRcvd -

LastByteRead]

bull receiver advertises unused buffer space by including rwnd value in segment header

bull sender limits of unACKed bytes to rwndndash guarantees receiverrsquos buffer

doesnrsquot overflow

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

rwndRcvBuffer

Transport Layer 3-11

TCP congestion control bandwidth probing

ldquoprobing for bandwidthrdquo increase transmission rate on receipt of ACK until eventually loss occurs then decrease transmission rate continue to increase on ACK decrease on loss (since available

bandwidth is changing depending on other connections in network)

ACKs being received so increase rate

X

X

XX

X loss so decrease rate

send

ing

rate

time

Q how fast to increasedecrease details to follow

TCPrsquosldquosawtoothrdquobehavior

Transport Layer 3-12

TCP Congestion Control details

bull sender limits rate by limiting number of unACKed bytes ldquoin pipelinerdquo

ndash cwnd differs from rwnd (how why)ndash sender limited by min(cwndrwnd)

bull roughly

bull cwnd is dynamic function of perceived network congestion

rate = cwnd

RTT bytessec

LastByteSent-LastByteAcked cwnd

cwndbytes

RTT

ACK(s)

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-4

TCP retransmission scenarios (more)Host A

Seq=92 8 bytes data

ACK=100

loss

timeo

ut

Cumulative ACK scenario

Host B

X

Seq=100 20 bytes data

ACK=120

time

SendBase= 120

Transport Layer 3-5

TCP ACK generation [RFC 1122 RFC 2581]

Event at Receiver

Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed

Arrival of in-order segment withexpected seq One other segment has ACK pending

Arrival of out-of-order segmenthigher-than-expect seq Gap detected

Arrival of segment that partially or completely fills gap

TCP Receiver action

Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK

Immediately send single cumulative ACK ACKing both in-order segments

Immediately send duplicate ACK indicating seq of next expected byte

Immediate send ACK provided thatsegment starts at lower end of gap

Transport Layer 3-6

Fast Retransmitbull time-out period often relatively long

ndash long delay before resending lost packetbull detect lost segments via duplicate ACKs

ndash sender often sends many segments back-to-backndash if segment is lost there will likely be many duplicate ACKs for that

segment

bull If sender receives 3 ACKs for same data it assumes that segment after ACKed data was lostndash fast retransmit resend

segment before timer expires

Transport Layer 3-7

Host A

timeo

ut

Host B

time

X

resend seq X2

seq x1seq x2seq x3seq x4seq x5

ACK x1

ACK x1ACK x1ACK x1

tripleduplicate

ACKs

Transport Layer 3-8

event ACK received with ACK field value of y if (y gt SendBase) SendBase = y if (there are currently not-yet-acknowledged segments) start timer else increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) resend segment with sequence number y

Fast retransmit algorithm

a duplicate ACK for already ACKed segment

fast retransmit

Transport Layer 3-9

TCP Flow Controlbull receive side of TCP

connection has a receive buffer

bull speed-matching service matching send rate to receiving applicationrsquos drain rate

app process may be slow at reading from buffer

sender wonrsquot overflowreceiverrsquos buffer bytransmitting too much too fast

flow control

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

Transport Layer 3-10

TCP Flow control how it works

(suppose TCP receiver discards out-of-order segments)

bull unused buffer space= rwnd= RcvBuffer-[LastByteRcvd -

LastByteRead]

bull receiver advertises unused buffer space by including rwnd value in segment header

bull sender limits of unACKed bytes to rwndndash guarantees receiverrsquos buffer

doesnrsquot overflow

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

rwndRcvBuffer

Transport Layer 3-11

TCP congestion control bandwidth probing

ldquoprobing for bandwidthrdquo increase transmission rate on receipt of ACK until eventually loss occurs then decrease transmission rate continue to increase on ACK decrease on loss (since available

bandwidth is changing depending on other connections in network)

ACKs being received so increase rate

X

X

XX

X loss so decrease rate

send

ing

rate

time

Q how fast to increasedecrease details to follow

TCPrsquosldquosawtoothrdquobehavior

Transport Layer 3-12

TCP Congestion Control details

bull sender limits rate by limiting number of unACKed bytes ldquoin pipelinerdquo

ndash cwnd differs from rwnd (how why)ndash sender limited by min(cwndrwnd)

bull roughly

bull cwnd is dynamic function of perceived network congestion

rate = cwnd

RTT bytessec

LastByteSent-LastByteAcked cwnd

cwndbytes

RTT

ACK(s)

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-5

TCP ACK generation [RFC 1122 RFC 2581]

Event at Receiver

Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed

Arrival of in-order segment withexpected seq One other segment has ACK pending

Arrival of out-of-order segmenthigher-than-expect seq Gap detected

Arrival of segment that partially or completely fills gap

TCP Receiver action

Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK

Immediately send single cumulative ACK ACKing both in-order segments

Immediately send duplicate ACK indicating seq of next expected byte

Immediate send ACK provided thatsegment starts at lower end of gap

Transport Layer 3-6

Fast Retransmitbull time-out period often relatively long

ndash long delay before resending lost packetbull detect lost segments via duplicate ACKs

ndash sender often sends many segments back-to-backndash if segment is lost there will likely be many duplicate ACKs for that

segment

bull If sender receives 3 ACKs for same data it assumes that segment after ACKed data was lostndash fast retransmit resend

segment before timer expires

Transport Layer 3-7

Host A

timeo

ut

Host B

time

X

resend seq X2

seq x1seq x2seq x3seq x4seq x5

ACK x1

ACK x1ACK x1ACK x1

tripleduplicate

ACKs

Transport Layer 3-8

event ACK received with ACK field value of y if (y gt SendBase) SendBase = y if (there are currently not-yet-acknowledged segments) start timer else increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) resend segment with sequence number y

Fast retransmit algorithm

a duplicate ACK for already ACKed segment

fast retransmit

Transport Layer 3-9

TCP Flow Controlbull receive side of TCP

connection has a receive buffer

bull speed-matching service matching send rate to receiving applicationrsquos drain rate

app process may be slow at reading from buffer

sender wonrsquot overflowreceiverrsquos buffer bytransmitting too much too fast

flow control

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

Transport Layer 3-10

TCP Flow control how it works

(suppose TCP receiver discards out-of-order segments)

bull unused buffer space= rwnd= RcvBuffer-[LastByteRcvd -

LastByteRead]

bull receiver advertises unused buffer space by including rwnd value in segment header

bull sender limits of unACKed bytes to rwndndash guarantees receiverrsquos buffer

doesnrsquot overflow

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

rwndRcvBuffer

Transport Layer 3-11

TCP congestion control bandwidth probing

ldquoprobing for bandwidthrdquo increase transmission rate on receipt of ACK until eventually loss occurs then decrease transmission rate continue to increase on ACK decrease on loss (since available

bandwidth is changing depending on other connections in network)

ACKs being received so increase rate

X

X

XX

X loss so decrease rate

send

ing

rate

time

Q how fast to increasedecrease details to follow

TCPrsquosldquosawtoothrdquobehavior

Transport Layer 3-12

TCP Congestion Control details

bull sender limits rate by limiting number of unACKed bytes ldquoin pipelinerdquo

ndash cwnd differs from rwnd (how why)ndash sender limited by min(cwndrwnd)

bull roughly

bull cwnd is dynamic function of perceived network congestion

rate = cwnd

RTT bytessec

LastByteSent-LastByteAcked cwnd

cwndbytes

RTT

ACK(s)

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-6

Fast Retransmitbull time-out period often relatively long

ndash long delay before resending lost packetbull detect lost segments via duplicate ACKs

ndash sender often sends many segments back-to-backndash if segment is lost there will likely be many duplicate ACKs for that

segment

bull If sender receives 3 ACKs for same data it assumes that segment after ACKed data was lostndash fast retransmit resend

segment before timer expires

Transport Layer 3-7

Host A

timeo

ut

Host B

time

X

resend seq X2

seq x1seq x2seq x3seq x4seq x5

ACK x1

ACK x1ACK x1ACK x1

tripleduplicate

ACKs

Transport Layer 3-8

event ACK received with ACK field value of y if (y gt SendBase) SendBase = y if (there are currently not-yet-acknowledged segments) start timer else increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) resend segment with sequence number y

Fast retransmit algorithm

a duplicate ACK for already ACKed segment

fast retransmit

Transport Layer 3-9

TCP Flow Controlbull receive side of TCP

connection has a receive buffer

bull speed-matching service matching send rate to receiving applicationrsquos drain rate

app process may be slow at reading from buffer

sender wonrsquot overflowreceiverrsquos buffer bytransmitting too much too fast

flow control

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

Transport Layer 3-10

TCP Flow control how it works

(suppose TCP receiver discards out-of-order segments)

bull unused buffer space= rwnd= RcvBuffer-[LastByteRcvd -

LastByteRead]

bull receiver advertises unused buffer space by including rwnd value in segment header

bull sender limits of unACKed bytes to rwndndash guarantees receiverrsquos buffer

doesnrsquot overflow

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

rwndRcvBuffer

Transport Layer 3-11

TCP congestion control bandwidth probing

ldquoprobing for bandwidthrdquo increase transmission rate on receipt of ACK until eventually loss occurs then decrease transmission rate continue to increase on ACK decrease on loss (since available

bandwidth is changing depending on other connections in network)

ACKs being received so increase rate

X

X

XX

X loss so decrease rate

send

ing

rate

time

Q how fast to increasedecrease details to follow

TCPrsquosldquosawtoothrdquobehavior

Transport Layer 3-12

TCP Congestion Control details

bull sender limits rate by limiting number of unACKed bytes ldquoin pipelinerdquo

ndash cwnd differs from rwnd (how why)ndash sender limited by min(cwndrwnd)

bull roughly

bull cwnd is dynamic function of perceived network congestion

rate = cwnd

RTT bytessec

LastByteSent-LastByteAcked cwnd

cwndbytes

RTT

ACK(s)

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-7

Host A

timeo

ut

Host B

time

X

resend seq X2

seq x1seq x2seq x3seq x4seq x5

ACK x1

ACK x1ACK x1ACK x1

tripleduplicate

ACKs

Transport Layer 3-8

event ACK received with ACK field value of y if (y gt SendBase) SendBase = y if (there are currently not-yet-acknowledged segments) start timer else increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) resend segment with sequence number y

Fast retransmit algorithm

a duplicate ACK for already ACKed segment

fast retransmit

Transport Layer 3-9

TCP Flow Controlbull receive side of TCP

connection has a receive buffer

bull speed-matching service matching send rate to receiving applicationrsquos drain rate

app process may be slow at reading from buffer

sender wonrsquot overflowreceiverrsquos buffer bytransmitting too much too fast

flow control

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

Transport Layer 3-10

TCP Flow control how it works

(suppose TCP receiver discards out-of-order segments)

bull unused buffer space= rwnd= RcvBuffer-[LastByteRcvd -

LastByteRead]

bull receiver advertises unused buffer space by including rwnd value in segment header

bull sender limits of unACKed bytes to rwndndash guarantees receiverrsquos buffer

doesnrsquot overflow

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

rwndRcvBuffer

Transport Layer 3-11

TCP congestion control bandwidth probing

ldquoprobing for bandwidthrdquo increase transmission rate on receipt of ACK until eventually loss occurs then decrease transmission rate continue to increase on ACK decrease on loss (since available

bandwidth is changing depending on other connections in network)

ACKs being received so increase rate

X

X

XX

X loss so decrease rate

send

ing

rate

time

Q how fast to increasedecrease details to follow

TCPrsquosldquosawtoothrdquobehavior

Transport Layer 3-12

TCP Congestion Control details

bull sender limits rate by limiting number of unACKed bytes ldquoin pipelinerdquo

ndash cwnd differs from rwnd (how why)ndash sender limited by min(cwndrwnd)

bull roughly

bull cwnd is dynamic function of perceived network congestion

rate = cwnd

RTT bytessec

LastByteSent-LastByteAcked cwnd

cwndbytes

RTT

ACK(s)

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-8

event ACK received with ACK field value of y if (y gt SendBase) SendBase = y if (there are currently not-yet-acknowledged segments) start timer else increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) resend segment with sequence number y

Fast retransmit algorithm

a duplicate ACK for already ACKed segment

fast retransmit

Transport Layer 3-9

TCP Flow Controlbull receive side of TCP

connection has a receive buffer

bull speed-matching service matching send rate to receiving applicationrsquos drain rate

app process may be slow at reading from buffer

sender wonrsquot overflowreceiverrsquos buffer bytransmitting too much too fast

flow control

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

Transport Layer 3-10

TCP Flow control how it works

(suppose TCP receiver discards out-of-order segments)

bull unused buffer space= rwnd= RcvBuffer-[LastByteRcvd -

LastByteRead]

bull receiver advertises unused buffer space by including rwnd value in segment header

bull sender limits of unACKed bytes to rwndndash guarantees receiverrsquos buffer

doesnrsquot overflow

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

rwndRcvBuffer

Transport Layer 3-11

TCP congestion control bandwidth probing

ldquoprobing for bandwidthrdquo increase transmission rate on receipt of ACK until eventually loss occurs then decrease transmission rate continue to increase on ACK decrease on loss (since available

bandwidth is changing depending on other connections in network)

ACKs being received so increase rate

X

X

XX

X loss so decrease rate

send

ing

rate

time

Q how fast to increasedecrease details to follow

TCPrsquosldquosawtoothrdquobehavior

Transport Layer 3-12

TCP Congestion Control details

bull sender limits rate by limiting number of unACKed bytes ldquoin pipelinerdquo

ndash cwnd differs from rwnd (how why)ndash sender limited by min(cwndrwnd)

bull roughly

bull cwnd is dynamic function of perceived network congestion

rate = cwnd

RTT bytessec

LastByteSent-LastByteAcked cwnd

cwndbytes

RTT

ACK(s)

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-9

TCP Flow Controlbull receive side of TCP

connection has a receive buffer

bull speed-matching service matching send rate to receiving applicationrsquos drain rate

app process may be slow at reading from buffer

sender wonrsquot overflowreceiverrsquos buffer bytransmitting too much too fast

flow control

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

Transport Layer 3-10

TCP Flow control how it works

(suppose TCP receiver discards out-of-order segments)

bull unused buffer space= rwnd= RcvBuffer-[LastByteRcvd -

LastByteRead]

bull receiver advertises unused buffer space by including rwnd value in segment header

bull sender limits of unACKed bytes to rwndndash guarantees receiverrsquos buffer

doesnrsquot overflow

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

rwndRcvBuffer

Transport Layer 3-11

TCP congestion control bandwidth probing

ldquoprobing for bandwidthrdquo increase transmission rate on receipt of ACK until eventually loss occurs then decrease transmission rate continue to increase on ACK decrease on loss (since available

bandwidth is changing depending on other connections in network)

ACKs being received so increase rate

X

X

XX

X loss so decrease rate

send

ing

rate

time

Q how fast to increasedecrease details to follow

TCPrsquosldquosawtoothrdquobehavior

Transport Layer 3-12

TCP Congestion Control details

bull sender limits rate by limiting number of unACKed bytes ldquoin pipelinerdquo

ndash cwnd differs from rwnd (how why)ndash sender limited by min(cwndrwnd)

bull roughly

bull cwnd is dynamic function of perceived network congestion

rate = cwnd

RTT bytessec

LastByteSent-LastByteAcked cwnd

cwndbytes

RTT

ACK(s)

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-10

TCP Flow control how it works

(suppose TCP receiver discards out-of-order segments)

bull unused buffer space= rwnd= RcvBuffer-[LastByteRcvd -

LastByteRead]

bull receiver advertises unused buffer space by including rwnd value in segment header

bull sender limits of unACKed bytes to rwndndash guarantees receiverrsquos buffer

doesnrsquot overflow

IPdatagrams

TCP data(in buffer)

(currently)unused bufferspace

applicationprocess

rwndRcvBuffer

Transport Layer 3-11

TCP congestion control bandwidth probing

ldquoprobing for bandwidthrdquo increase transmission rate on receipt of ACK until eventually loss occurs then decrease transmission rate continue to increase on ACK decrease on loss (since available

bandwidth is changing depending on other connections in network)

ACKs being received so increase rate

X

X

XX

X loss so decrease rate

send

ing

rate

time

Q how fast to increasedecrease details to follow

TCPrsquosldquosawtoothrdquobehavior

Transport Layer 3-12

TCP Congestion Control details

bull sender limits rate by limiting number of unACKed bytes ldquoin pipelinerdquo

ndash cwnd differs from rwnd (how why)ndash sender limited by min(cwndrwnd)

bull roughly

bull cwnd is dynamic function of perceived network congestion

rate = cwnd

RTT bytessec

LastByteSent-LastByteAcked cwnd

cwndbytes

RTT

ACK(s)

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-11

TCP congestion control bandwidth probing

ldquoprobing for bandwidthrdquo increase transmission rate on receipt of ACK until eventually loss occurs then decrease transmission rate continue to increase on ACK decrease on loss (since available

bandwidth is changing depending on other connections in network)

ACKs being received so increase rate

X

X

XX

X loss so decrease rate

send

ing

rate

time

Q how fast to increasedecrease details to follow

TCPrsquosldquosawtoothrdquobehavior

Transport Layer 3-12

TCP Congestion Control details

bull sender limits rate by limiting number of unACKed bytes ldquoin pipelinerdquo

ndash cwnd differs from rwnd (how why)ndash sender limited by min(cwndrwnd)

bull roughly

bull cwnd is dynamic function of perceived network congestion

rate = cwnd

RTT bytessec

LastByteSent-LastByteAcked cwnd

cwndbytes

RTT

ACK(s)

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-12

TCP Congestion Control details

bull sender limits rate by limiting number of unACKed bytes ldquoin pipelinerdquo

ndash cwnd differs from rwnd (how why)ndash sender limited by min(cwndrwnd)

bull roughly

bull cwnd is dynamic function of perceived network congestion

rate = cwnd

RTT bytessec

LastByteSent-LastByteAcked cwnd

cwndbytes

RTT

ACK(s)

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-13

TCP Congestion Control more details

segment loss event reducing cwnd

bull timeout no response from receiverndash cut cwnd to 1

bull 3 duplicate ACKs at least some segments getting through (recall fast retransmit)ndash cut cwnd in half less

aggressively than on timeout

ACK received increase cwnd slowstart phase

increase exponentially fast (despite name) at connection start or following timeout

congestion avoidance increase linearly

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-14

TCP Slow Startbull when connection begins cwnd = 1

MSSndash example MSS = 500 bytes amp RTT

= 200 msecndash initial rate = 20 kbps

bull available bandwidth may be gtgt MSSRTTndash desirable to quickly ramp up to

respectable ratebull increase rate exponentially until first

loss event or when threshold reachedndash double cwnd every RTTndash done by incrementing cwnd by 1

for every ACK received

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-15

Transitioning intoout of slowstartssthresh cwnd threshold maintained by TCPbull on loss event set ssthresh to cwnd2

ndash remember (half of) TCP rate when congestion last occurred bull when cwnd gt= ssthresh transition from slowstart to congestion avoidance

phase

slow start timeout

ssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKdupACKcount++duplicate ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0 congestion

avoidance

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-16

TCP congestion avoidancebull when cwnd gt ssthresh

grow cwnd linearlyndash increase cwnd by 1 MSS

per RTT ndash approach possible

congestion slower than in slowstart

ndash implementation cwnd = cwnd + MSScwnd for each ACK received

ACKs increase cwnd by 1 MSS per RTT additive increase

loss cut cwnd in half (non-timeout-detected loss ) multiplicative decrease

AIMD

AIMD Additive IncreaseMultiplicative Decrease

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-17

TCP congestion control FSM overview

slow start

congestionavoidance

fastrecovery

cwnd gt ssthresh

losstimeout

losstimeout

new ACK loss3dupACK

loss3dupACK

losstimeout

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-18

TCP congestion control FSM details

slow start

congestionavoidance

fastrecovery

timeoutssthresh = cwnd2cwnd = 1 MSSdupACKcount = 0retransmit missing segment timeout

ssthresh = cwnd2 cwnd = 1 MSSdupACKcount = 0retransmit missing segment

Lcwnd gt ssthresh

cwnd = cwnd+MSSdupACKcount = 0transmit new segment(s)as allowed

new ACKcwnd = cwnd + MSS (MSScwnd)dupACKcount = 0transmit new segment(s)as allowed

new ACK

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3retransmit missing segment

dupACKcount == 3

dupACKcount++duplicate ACK

ssthresh= cwnd2cwnd = ssthresh + 3

retransmit missing segment

dupACKcount == 3

timeoutssthresh = cwnd2cwnd = 1 dupACKcount = 0retransmit missing segment

cwnd = cwnd + MSStransmit new segment(s) as allowed

duplicate ACK

cwnd = ssthreshdupACKcount = 0

New ACK

Lcwnd = 1 MSSssthresh = 64 KBdupACKcount = 0

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-19

Popular ldquoflavorsrdquo of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd

win

dow

size

(in

segm

ents

)

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-20

Summary TCP Congestion Control

bull when cwnd lt ssthresh sender in slow-start phase window grows exponentially

bull when cwnd gt= ssthresh sender is in congestion-avoidance phase window grows linearly

bull when triple duplicate ACK occurs ssthresh set to cwnd2 cwnd set to ~ ssthresh

bull when timeout occurs ssthresh set to cwnd2 cwnd set to 1 MSS

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-21

TCP Futures TCP over ldquolong fat pipesrdquo

bull example 1500 byte segments 100ms RTT want 10 Gbps throughput

bull requires window size W = 83333 in-flight segmentsbull throughput in terms of loss rate

bull L = 210-10 Wowbull new versions of TCP for high-speed

LRTTMSS221

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-22

fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-23

Why is TCP fairTwo competing sessionsbull Additive increase gives slope of 1 as throughout increasesbull multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

ecti o

n 2

thro

ughp

u t

congestion avoidance additive increaseloss decrease window by factor of 2

congestion avoidance additive increaseloss decrease window by factor of 2

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-24

Fairness (more)Fairness and UDPbull multimedia apps often do

not use TCPndash do not want rate throttled

by congestion controlbull instead use UDP

ndash pump audiovideo at constant rate tolerate packet loss

Fairness and parallel TCP connections

bull nothing prevents app from opening parallel connections between 2 hosts

bull web browsers do this bull example link of rate R

supporting 9 connections ndash new app asks for 1 TCP gets rate

R10ndash new app asks for 11 TCPs gets

R2

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Transport Layer 3-25

Chapter 3 Summarybull principles behind transport layer

servicesndash multiplexing demultiplexingndash reliable data transferndash flow controlndash congestion control

bull instantiation and implementation in the Internetndash UDPndash TCP

Nextbull leaving the network

ldquoedgerdquo (application transport layers)

bull into the network ldquocorerdquo

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-26

1

23

0111

value in arrivingpacketrsquos header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-27

Connection setupbull 3rd important function in some network architectures

ndash ATM frame relay X25bull before datagrams flow two end hosts and intervening routers

establish virtual connectionndash routers get involved

bull network vs transport layer connection servicendash network between two hosts (may also involve

intervening routers in case of VCs)ndash transport between two processes

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-28

Network service modelQ What service model for ldquochannelrdquo transporting datagrams from sender to receiver

example services for individual datagrams

bull guaranteed deliverybull guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams

bull in-order datagram delivery

bull guaranteed minimum bandwidth to flow

bull restrictions on changes in inter-packet spacing

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-29

Network layer service models

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-30

VC implementation

a VC consists of1 path from source to destination2 VC numbers one number for each link along path3 entries in forwarding tables in routers along path

bull packet belonging to VC carries VC number (rather than dest address)

bull VC number can be changed on each linkndash New VC number comes from forwarding table

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-31

VC Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC Outgoing interface Outgoing VC

1 12 3 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip

Forwarding table innorthwest router

Routers maintain connection state information

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-32

Virtual circuits signaling protocols

bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Initiate call 2 incoming call3 Accept call4 Call connected

5 Data flow begins 6 Receive data

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-33

Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections

ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address

ndash packets between same source-dest pair may take different paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1 Send data 2 Receive data

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-34

Datagram Forwarding table

1

23

IP destination address in arriving packetrsquos header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-35

Datagram Forwarding tableDestination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q but what happens if ranges donrsquot divide up so nicely

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-36

Longest prefix matching

Destination Address Range

11001000 00010111 00010

11001000 00010111 00011000

11001000 00010111 00011

otherwise

DA 11001000 00010111 00011000 10101010

ExamplesDA 11001000 00010111 00010110 10100001 Which interface

Which interface

when looking for forwarding table entry for given destination address use longest address prefix that matches destination address

Longest prefix matching

Link interface

0

1

2

3

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-37

Datagram or VC network whyInternet (datagram)bull data exchange among computers

ndash ldquoelasticrdquo service no strict timing req

bull ldquosmartrdquo end systems (computers)ndash can adapt perform control

error recoveryndash simple inside network

complexity at ldquoedgerdquobull many link types

ndash different characteristicsndash uniform service difficult

ATM (VC)bull evolved from telephonybull human conversation

ndash strict timing reliability requirements

ndash need for guaranteed service

bull ldquodumbrdquo end systemsndash telephonesndash complexity inside network

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-38

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =14808

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-39

SubnetsHow many 223111

223113

223114

223122223121

223126

223132223131

2231327

223112

223170

223171223180223181

223191

223192

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-40

IP addressing CIDRCIDR Classless InterDomain Routing

ndash subnet portion of address of arbitrary lengthndash address format abcdx where x is bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

2002316023

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-41

DHCP client-server scenario

223111

223112

223113

223114 223129

223122

223121

223132223131

2231327

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-42

DHCP example

bull connecting laptop needs its IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet

Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxed to IP demuxed UDP demuxed to DHCP

168111

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-43

bull DCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP server frame forwarded to client demuxing up to DHCP at client

client now knows its IP address name and IP address of DSN server IP address of its first-hop router

DHCP example

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-44

Hierarchical addressing more specific routes

ISPs-R-Us has a more specific route to Organization 1

ldquoSend me anythingwith addresses beginning 2002316020rdquo

2002316023

2002318023

2002330023

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us ldquoSend me anythingwith addresses beginning 199310016or 2002318023rdquo

2002320023Organization 2

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-45

NAT Network Address Translation

10001

10002

10003

S 10001 3345D 12811940186 80

110004

13876297

1 host 10001 sends datagram to 12811940186 80

NAT translation tableWAN side addr LAN side addr13876297 5001 10001 3345

helliphellip helliphellip

S 12811940186 80 D 10001 3345 4

S 13876297 5001D 12811940186 802

2 NAT routerchanges datagramsource addr from10001 3345 to13876297 5001updates table

S 12811940186 80 D 13876297 5001 3

3 Reply arrives dest address 13876297 5001

4 NAT routerchanges datagramdest addr from13876297 5001 to 10001 3345

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-46

Comparison of LS and DV algorithms

Message complexitybull LS with n nodes E links O(nE)

msgs sent bull DV exchange between neighbors

onlyndash convergence time varies

Speed of Convergencebull LS O(n2) algorithm requires O(nE)

msgsndash may have oscillations

bull DV convergence time variesndash may be routing loopsndash count-to-infinity problem

Robustness what happens if router malfunctions

LS ndash node can advertise incorrect

link costndash each node computes only its

own table

DVndash DV node can advertise

incorrect path costndash each nodersquos table used by

others bull error propagate thru network

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-47

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

bull forwarding table configured by both intra- and inter-AS routing algorithmndash intra-AS sets entries for

internal destsndash inter-AS amp intra-As sets

entries for external dests

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-48

Inter-AS tasksbull suppose router in AS1

receives datagram destined outside of AS1ndash router should

forward packet to gateway router but which one

AS1 must1 learn which dests are

reachable through AS2 which through AS3

2 propagate this reachability info to all routers in AS1

job of inter-AS routing

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-49

Example Setting forwarding table in router 1d

bull suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2ndash inter-AS protocol propagates reachability info to all internal routers

bull router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1cndash installs forwarding table entry (xI)

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

xhellip

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-50

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine which gateway it should forward packets towards for dest x ndash this is also job of inter-AS routing protocol

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x helliphelliphellip

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-51

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routingChoose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway Enter (xI) in

forwarding table

Example Choosing among multiple ASes

bull now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2

bull to configure forwarding table router 1d must determine towards which gateway it should forward packets for dest x ndash this is also job of inter-AS routing protocol

bull hot potato routing send packet towards closest of two routers

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-52

Intra-AS Routing

bull also known as Interior Gateway Protocols (IGP)bull most common Intra-AS routing protocols

ndash RIP Routing Information Protocol (DV)

ndash OSPF Open Shortest Path First (LS)

ndash IGRP Interior Gateway Routing Protocol (Cisco proprietary)(LS)

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-53

BGP basics distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

bull using eBGP session between 3a and 1c AS3 sends prefix reachability info to AS1ndash 1c can then use iBGP do distribute new prefix info to all routers in AS1ndash 1b can then re-advertise new reachability info to AS2 over 1b-to-2a

eBGP sessionbull when router learns of new prefix it creates entry for prefix in

its forwarding table

eBGP session

iBGP session

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-54

Path attributes amp BGP routesbull advertised prefix includes BGP attributes

ndash prefix + attributes = ldquorouterdquobull two important attributes

ndash AS-PATH contains ASs through which prefix advertisement has passed eg AS 67 AS 17

ndash NEXT-HOP indicates specific internal-AS router to next-hop AS (may be multiple links from current AS to next-hop-AS)

bull gateway router receiving route advertisement uses import policy to acceptdeclinendash eg never route through AS xndash policy-based routing

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-55

BGP route selectionbull router may learn about more than 1 route

to destination AS selects route based on1 local preference value attribute policy

decision2 shortest AS-PATH 3 closest NEXT-HOP router hot potato routing4 additional criteria

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-56

BGP messages

bull BGP messages exchanged between peers over TCP connectionbull BGP messages

ndash OPEN opens TCP connection to peer and authenticates sender

ndash UPDATE advertises new path (or withdraws old)ndash KEEPALIVE keeps connection alive in absence of

UPDATES also ACKs OPEN requestndash NOTIFICATION reports errors in previous msg

also used to close connection

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-57

BGP routing policy

ABC are provider networks XWY are customer (of provider networks) X is dual-homed attached to two networks

X does not want to route from B via X to C so X will not advertise to B a route to C

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-58

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C

No way B gets no ldquorevenuerdquo for routing CBAW since neither W nor C are Brsquos customers

B wants to force C to route to w via A B wants to route only tofrom its customers

A

B

C

W X

Y

legend

customer network

provider network

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-59

Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic routed who

routes through its net bull Intra-AS single admin so no policy decisions needed

Scalebull hierarchical routing saves table size reduced update trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary

Network Layer 4-60

Chapter 4 summary4 1 Introduction42 Virtual circuit and datagram networks43 Whatrsquos inside a router44 IP Internet Protocol

ndash Datagram formatndash IPv4 addressingndash ICMPndash IPv6

45 Routing algorithmsndash Link statendash Distance Vectorndash Hierarchical routing

46 Routing in the Internetndash RIPndash OSPFndash BGP

47 Broadcast and multicast routing

• IT 347 Midterm 2 Review
• Vocab Review
• TCP retransmission scenarios
• TCP retransmission scenarios (more)
• TCP ACK generation [RFC 1122 RFC 2581]
• Fast Retransmit
• Slide 7
• Fast retransmit algorithm
• TCP Flow Control
• TCP Flow control how it works
• TCP congestion control bandwidth probing
• TCP Congestion Control details
• TCP Congestion Control more details
• TCP Slow Start
• Transitioning intoout of slowstart
• TCP congestion avoidance
• TCP congestion control FSM overview
• TCP congestion control FSM details
• Popular ldquoflavorsrdquo of TCP
• Summary TCP Congestion Control
• TCP Futures TCP over ldquolong fat pipesrdquo
• TCP Fairness
• Why is TCP fair
• Fairness (more)
• Chapter 3 Summary
• Slide 26
• Connection setup
• Network service model
• Network layer service models
• VC implementation
• VC Forwarding table
• Virtual circuits signaling protocols
• Datagram networks
• Datagram Forwarding table
• Datagram Forwarding table
• Longest prefix matching
• Datagram or VC network why
• IP Fragmentation and Reassembly
• Subnets
• IP addressing CIDR
• DHCP client-server scenario
• DHCP example
• DHCP example (2)
• Hierarchical addressing more specific routes
• NAT Network Address Translation
• Comparison of LS and DV algorithms
• Interconnected ASes
• Inter-AS tasks
• Example Setting forwarding table in router 1d
• Example Choosing among multiple ASes
• Example Choosing among multiple ASes (2)
• Intra-AS Routing
• BGP basics distributing path information
• Path attributes amp BGP routes
• BGP route selection
• BGP messages
• BGP routing policy
• BGP routing policy (2)
• Why different Intra- and Inter-AS routing
• Chapter 4 summary