ch 5. the link layer and local area networks myungchul kim [email protected]
TRANSCRIPT
2
– Frame– Error detection, retransmission, flow control, random access – Ethernet, IEEE 802.11 wireless LAN (WiFi), Token Ring, PPP, A
TM– Services offered by a link-layer protocol
Framing Link access: Medium Access Control (MAC) protocol Reliable delivery Flow control Error detection Error correction Half-duplex and full-duplex
– The services by the link layer vs the transport layer
3
4
– The link layer is implemented in a network adaptor known as a network interface card (NIC)
– LAN-on-motherboard configuration
5
Error-Detection and –Correction Techniques
6
Parirty checks
7
– Forward error correction (FEC)
Checksumming methods– The receiver checks the checksum by taking the 1s complement
of the sum of the received data (including the checksum).– Why checksumming at the Transport layer and CRC at the link l
ayer?
8
Cyclic Redundancy Check (CRC)
- generator: The sender and receiver must agree on an r+1 bit pattern- D * 2 r XOR R = nG- D * 2 r = nG XOR R- R = remainder D * 2 r / G- The CRC standards can detect burst errors of fewer than r+1 bits.
9
10
Multiple access protocols
– Point-to-point link– Broadcast link: medium access problem– Multiple access protocols– Channel partitioning protocols, random access rptocols and takin
g-turns protocols
11
Ideal Multiple Access Protocol
Broadcast channel of rate R bps
1. when one node wants to transmit, it can send at rate R.
2. when M nodes want to transmit, each can send at average rate R/M
3. fully decentralized:– no special node to coordinate transmissions– no synchronization of clocks, slots
4. simple
12
Channel partitioning protocols
– Code Division Multiple Access (CDMA)
13
– ALOHA All frames L bits Time slots of size L/R seconds The nodes are synchronized so that each node knows when the slot
s begin. If two or more frames collide in a slot, then all the nodes detect the c
ollision event before the slot ends. Max efficiency of the protocol 1/e = 0.37
14
– ALOHA Max efficiency of the protocol 1/2e
15
– Carrier Sense Multiple Access (CSMA) Listen before speaking: carrier sensing If someone else begins talking at the same time, stop talking: collisio
n detection CSMA, CSMA/CD
16
17
18
Taking-Turns protocols– Polling protocols: a master node– Token-passing protocol: no master node
Local Area Networks (LANs)– Ethernet: IEEE 802.3– Token Ring: IEEE 802.5– Fiber Distribution Data Interface (FDDI)
19
Link-layer addressing
MAC address– Link-layer address, LAN address, MAC address– 6 bytes long = 2 48 possible MAC addresses
20
– No two adaptors have the same address– A flat structure
Address Resolution Protocol (ARP)– To send a datagram, the source node must give its adapter not o
nly the IP datagram but also the MAC address for destination node.
– An ARP module in the sending node takes any IP address on the same LAN as input, and returns the corresponding MAC address.
– DNS vs ARP– ARP query and response
21
22
– Sending a datagram to a node off the subnet
23
A creates IP datagram with source A, destination B A uses ARP to get R’s MAC address for 111.111.111.110 A creates link-layer frame with R's MAC address as dest, frame
contains A-to-B IP datagram A’s NIC sends frame R’s NIC receives frame R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get B’s MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
A74-29-9C-E8-FF-55
222.222.222.221
88-B2-2F-54-1A-0F
B222.222.222.222
49-BD-D2-C7-56-2A
24
Ethernet
– Success factors First widely deployed Simple and cheap Kept up with speed race
– Hub: whenever a hub receives a bit from one of its interfaces, it sends a copy of the bit to all of its other interfaces.
25
– Ethernet frame structure Data field (46 to 1,500 bytes): MTU Destination address (6bytes) Source address (6 bytes) Type field (2 bytes): IP and other network layer protocols CRC (4 bytes) Preamble (8 bytes)
– Connectionless service– Unreliable service
26
CSMA/CD: Ethernet’s Multiple Access Protocol1. An adapter may begin to transmit at any time.
2. An adapter never transmits a frame when it senses that some other adapter is transmitting. (carrier sensing)
3. A transmitting adapter aborts its transmission as soon as it detects that another adapter is also transmitting. (collision detection)
4. Before attempting a retransmission, and adapter waits a random time that is typically samll compared with the time to transmit a frame. (exponential backoff)
27
– Ethernet efficiency = 1 / (1 + 5 d prop / d trans ) where d prop = max time to propagate between any two adapter
s and d trans = the time to transmit a maximum-size Ethernet fame.
Ethernet Technologies– Repeater– Point-to-point linkes use switches (no collision, no MAC protoco
l) while broadcast channels use hubs
28
Link-layer switches
– The role of the switch is to receive incoming link-layer frames and forward them onto outgoing links.
– Filtering– Forwarding– Switch table
29
30
– Suppose a frame arrives at the switch on interface x, the switch does If there is no entry for the destination address, the switch broadcast
s the frame. There is an entry in the table with interface x, the switch performs t
he filtering by discarding the frame. There is an entry in the table with interface y ≠ x, the switch perfor
ms its forwarding.
– As long as the switch table is complete and accurate, the switch forwards frames towards destination without any broadcasting.
31
Self-Learning– The switch table is initially empty.– For each incoming frame received on an interface, the switch st
ores in its table …– Aging time– Plug-and-play devices
32
Properties of link-layer switching– Elimination of collisions– Heterogeneous links– Management
Switches vs routers– Switch: layer 2, router: layer 3– Switch: plug-and-play, a spanning tree, hundreds hosts– Router: hierachical, thousands of hosts
33
PPP: The Point-to-Point Protocol
– Error detection– No flow control– No sequencing– Single point link
– Information: max length 1,500 bytes
34
35
Link Virtualization: A Network as a Link Layer
– ATM, MPLS: packet-switched, virtual-circuit networks
Asynchronous Transfer Mode (ATM) networks
36
– An ATM inferface will have an IP address and an ATM address.
37
Mulitprotocol Label Switching (MPLS)
– Like a switched LAN or ATM network, the MPLS-capable routers do without ever touching the IP header of a packet.
– Traffic engineering– Virtual private network
38