The Link Layer

Tom Kelliher, CS 325

May 5, 2008

Administrivia

Announcements

Assignment

Read 6.1-6.3.

Projects due Wednesday. Not accepted after Friday at 2:30 pm.

From Last Time

Routing.

Outline

1. Introduction.

2. Multiple Access protocol -- CSMA/CD.

3. Link layer addressing.

4. Ethernet.

5. Link layer switches.

Coming Up

Wireless networking.

Introduction

Link layer exists between hosts on either side of a single link:

Unit of exchange is a packet/frame/datagram. Physical layer exchanges individual bits.

Example technologies: Ethernet, ATM, 802.11.

Error detection/correction codes appended frame:

Link layer services:

1. Framing -- encapsulation.

2. Link access -- protocols for shared access links.

3. Reliable delivery -- important for media with high error rates (802.11).

   Error detection/correction.

4. Flow control.

5. Duplex -- half or full?

Multiple Access Protocol -- CSMA/CD

How do you control transmission in a shared access medium?:

A number of possible ideas:

1. Take turns: fixed intervals, or a request protocol.

   Time division multiplexing.

2. Partition the bandwidth.

   Frequency division multiplexing.

3. Code division multiplexing.

4. Random access protocols:
   1. Listen. If no stations sending, begin to send.

      If a station is sending, wait a random amount of time, then listen again.

   2. Possible for two senders to begin sending. Action upon detection of collision?

      Is it always possible to detect a sending collision?

5. CSMA/CD -- Carrier Sense Multiple Access/Collision Detection:
   1. Sense media for idle state. If idle, begin transmitting.

      If media isn't idle, backoff for a randomly chosen unit of time.

   2. How can collisions occur?
      

Link Layer Addressing

1. Each network adapter has an eight byte MAC address:

   bluebird:~/Class/Cs325/Lectures
   % ifconfig eth0
   eth0      Link encap:Ethernet  HWaddr 00:1A:A0:16:65:8B  
             inet addr:10.67.1.26  Bcast:10.67.1.255  Mask:255.255.255.0
             inet6 addr: fe80::21a:a0ff:fe16:658b/64 Scope:Link
             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
             RX packets:295533 errors:0 dropped:0 overruns:0 frame:0
             TX packets:173558 errors:0 dropped:0 overruns:0 carrier:0
             collisions:0 txqueuelen:1000 
             RX bytes:139988511 (133.5 MiB)  TX bytes:31578020 (30.1 MiB)
             Interrupt:177
   

   MAC addresses are ``unique.''

2. Each host on a network has an IP address and a MAC address:
   

   Why both?

   MAC addresses are not hierarchical, beyond vendor/adapter number designation.

3. So you want to send a datagram to IP address w.x.y.z. At the link layer, you really need the MAC address. How do we get that?

   Use ARP!!

   An ARP cache:

   bluebird:~/Class/Cs325/Lectures
   % arp
   Address                  HWtype  HWaddress           Flags Mask  Iface
   10.67.1.1                ether   00:13:5F:C4:B8:0A   C           eth0
   shrike.goucher.edu       ether   00:08:74:92:71:6B   C           eth0
   goldfinch.goucher.edu    ether   00:0B:DB:A7:EC:2A   C           eth0
   

   ARP protocol screen capture. (On web site.) Ping of kingfisher from bluebird. Note multiple protocols:

   1. DNS -- A record lookup.

   2. ARP -- broadcast request, single destination response.

   3. DNS -- PTR record lookup.

   4. ICMP -- echo request/reply.

4. Trace an IP datagram from one network to the next:
   

   Example: Send an HTTP request to a server on another subnet. (Resolver target on same subnet.)

Ethernet

1. Originally, Ethernet was a shared access medium using thinwire (10B2) or thickwire (10B5) coaxial cabling:
   

2. Then, we moved on to twisted-pair and hubs in a star topology:
   

   Hubs are physical layer devices (re-generate and broadcast) -- still a shared medium.

3. Ethernet frame fields:
   
   1. Preamble (8 bytes): Used to synchronize receiver clocks with send clock.

      (Ethernet is asynchronous.)

   2. Type (2 bytes): Indicates which network layer protocol is contained in the frame's payload.

   3. Data: 46 to 1,500 bytes. Short frames are padded to 46 bytes. Padded bytes passed onto network layer.

      (IP uses datagram length to ignore padding.)

   4. CRC (4 bytes): error detection.

4. Ethernet's CSMA/CD uses exponential backoff and random choice after back-to-back collisions:
   1. After 1st collision, wait $\{0, 1\} \times 512$ bit times.

   2. After 2nd, wait $\{0, 1, 2, 3\} \times 512$ bit times.

   3. After 3rd, wait $\{0, 1, \ldots, 7\} \times 512$ bit times.

   4. $\ldots$

   5. Maximum is $\{0, 1, \ldots, 1023\} \times 512$ bit times.

5. Ethernet standards exist at both link and physical layers:
   

Link Layer Switches

1. Switches are intelligent, link layer devices.

   Store & forward.

   Eliminate collisions.

   Heterogeneous link speeds.

2. Are transparent -- hosts aren't aware they're there.

   Don't have MAC addresses.

3. Learn about the network, to eliminate broadcasting frames meant for one host.
   1. When a host first sends a frame, switch notes the source MAC address.

   2. Switch stores (MAC address, interface, timestamp) $n$-tuples in a table.

      Essentially, a small forwarding table.

      Multiple MAC addresses may be associated with the same interface. Why?

   3. On receipt of a frame, lookup the destination MAC address in the table.
      • If found, send the frame to the indicated interface.

      • Otherwise, broadcast the frame.

   4. Timestamp used to purge stale entries.

Tom Kelliher