The Link Layer

1. The Link Layer

2. Announcements • No CS 415 section tomorrow, Tuesday • Project 4 is due next Monday, April 9th • Prelim II will be Thursday, April 26th, 7-9:30pm, in PH 101

3. Review: OSI Levels • Physical Layer • electrical details of bits on the wire • Data Link Layer • sending “frames” of bits and error detection • Network Layer • routing packets to the destination • Transport Layer • reliable transmission of messages, disassembly/assembly, ordering, retransmission of lost packets • Session Layer • really part of transport, typically Not implemented • Presentation Layer • data representation in the message • Application • high-level protocols (mail, ftp, etc.)

4. Review: OSI Levels Node A Application Node B Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Network

5. What is purpose of this layer? • Invoke Physical Layer • Physically encode bits on the wire • Link = pipe to send information • E.g. point to point or broadcast • Can be built out of: • Twisted pair, coaxial cable, optical fiber, radio waves, etc • Links should only be able to send data • Could corrupt, lose, reorder, duplicate, (fail in other ways)

6. Header (Dest:2) Body (Data) Message ID:3 (sender) Broadcast Networks Details • Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) • Put header on front of packet: [ Destination | Packet ] • Everyone gets packet, discards if not the target • In Ethernet, this check is done in hardware • No OS interrupt if not for particular destination • This is layering: we’re going to build complex network protocols by layering on top of the packet ID:1 (ignore) ID:4 (ignore) ID:2 (receive)

7. Switch Router Internet Point-to-point networks • Why have a shared broadcast medium? Why not simplify and only have point-to-point links + routers/switches? • Didn’t used to be cost-effective • Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. • Point-to-point network: a network in which every physical wire is connected to only two computers • Switch: a bridge that transforms a shared-bus configuration into a point-to-point network. • Router: a device that acts as a junction between two networks to transfer data packets among them.

8. Point-to-Point Networks Discussion • Advantages: • Higher link performance • Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) • Greater aggregate bandwidth than broadcast link • Can have multiple senders at once • Can add capacity incrementally • Add more links/switches to get more capacity • Better fault tolerance (as in the Internet) • Lower Latency • No arbitration to send, although need buffer in the switch • Disadvantages: • More expensive than having everyone share broadcast link • However, technology costs now much cheaper • Examples • ATM (asynchronous transfer mode) • The first commercial point-to-point LAN • Inspiration taken from telephone network • Switched Ethernet • Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet.

9. How to connect routers/machines? • WAN/Router Connections • Commercial: • T1 (1.5 Mbps), T3 (44 Mbps) • OC1 (51 Mbps), OC3 (155 Mbps) • ISDN (64 Kbps) • Frame Relay (1-100 Mbps, usually 1.5 Mbps) • ATM (some Gbps) • To your home: • DSL • Cable • Local Area: • Ethernet: IEEE 802.3 (10 Mbps, 100 Mbps, 1 Gbps) • Wireless: IEEE 802.11 b/g/a (11 Mbps, 22 Mbps, 54 Mbps)

10. Link level Issues • Encoding: map bits to analog signals • Framing: Group bits into frames (packets) • Arbitration: multiple senders, one resource • Addressing: multiple receivers, one wire

11. Encoding • Map 1s and 0s to electric signals • Simple scheme: Non-Return to Zero (NRZ) • 0 = low voltage, 1 = high voltage • Problems: • How to tell an error? When jammed? When is bus idle? • When to sample? Clock recovery is difficult. • Idea: Recover clock using encoding transitions 1 0 1 1 0

12. 0 1 1 0 Manchester Encoding • Used by Ethernet • Idea: Map 0 to low-to-high transition, 1 to high-to-low • Plusses: can detect dead-link, can recover clock • Bad: reduce bandwidth, i.e. bit rate = ½ baud rate • If wire can do X transition per second?

13. Framing • Why send packets? • Error control • How do you know when to stop reading? • Sentinel approach: send start and end sequence • For example, if sentinel is 11111 • 11111 00101001111100 11111 10101001 11111 010011 11111 • What if sentinel appears in the data? • map sentinel to something else, receiver maps it back • Bit stuffing

14. Example: HDLC • High-Level Data Link Control (HLDC) • Data link layer protocol developed by the ISO • Same sentinel for begin and end: 0111 1110 • packet format: • Bit stuffing • Sender: If 5 1s then insert a 0 • Receiver: if 5 1s followed by a 0, remove 0 • Else read next bit • Packet size now depends on the contents 0111 1110 header data CRC 0111 1110 0111 1110 0111 1101 0 0111 1101 0 0111 1110

15. Broadcast Network Arbitration • Arbitration: Act of negotiating use of shared medium • What if two senders try to broadcast at same time? • Concurrent activity but can’t use shared memory to coordinate! • Aloha network (70’s): packet radio within Hawaii • Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. • Need checksum anyway – in case airplane flies overhead • Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. • If two senders try to send at same time, both get garbled, both simply re-send later. • Problem: Stability: what if load increases? • More collisions  less gets through more resent  more load…  More collisions… • Unfortunately: some sender may have started in clear, get scrambled without finishing

16. Arbitration • One medium, multiple senders • What did we do for CPU, memory, readers/writers? • New Problem: No centralized control • Approaches • TDMA: Time Division Multiple Access • Divide time into slots, round robin among senders • If you exceed the capacity  do not admit more (busy signal) • FDMA: Frequency Division Multiple Access (AMPS) • Divide spectrum into channels, give each sender a channel • If no more channels available, give a busy signal • Good for continuous streams: fixed delay, constant data rate • Bad for bursty Internet traffic: idle slots

17. Ethernet • Developed in 1976, Metcalfe and Boggs at Xerox • Uses CSMA/CD: • Carrier Sense Multiple Access with Collision Detection • Easy way to connect LANs Metcalfe’s Ethernet sketch

18. CSMA/CD • Carrier Sense: • Listen before you speak • Multiple Access: • Multiple hosts can access the network • Collision Detection: • Can make out if someone else started speaking Older Ethernet Frame

19. CSMA Wait until carrier free

20. CSMA/CD Garbled signals If the sender detects a collision, it will stop and then retry! What is the problem?

21. No Yes attempts < 16 attempts == 16 CSMA/CD Packet? Sense Carrier Detect Collision Send Discard Packet Jam channel b=CalcBackoff(); wait(b); attempts++;

22. Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Exponential Backoff: • Goal: adapt retransmission attempts to estimated current load • heavy load: random wait will be longer • Adaptive and Random • First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. • Randomness is important to decouple colliding senders • Scheme figures out how many people are trying to send! • Example • first collision: choose K from {0,1}; delay is K x 512 bit transmission times • after second collision: choose K from {0,1,2,3}… • after ten or more collisions, choose K from {0,1,2,3,4,…,1023}

23. Packet Size • If packets are too small, the collision goes unnoticed • Limit packet size • Limit network diameter • Use CRC to check frame integrity • truncated packets are filtered out

24. Ethernet Problems • What if there is a malicious user? • Might not use exponential backoff • Might listen promiscuously to packets • Integrating Fast and Gigabit Ethernet

25. Addressing & ARP • ARP is used to discover physical addresses • ARP = Address Resolution Protocol 128.84.96.89 128.84.96.90 128.84.96.91 “What is the physical address of the host named 128.84.96.89” “I’m at 1a:34:2c:9a:de:cc”

26. Addressing & RARP • RARP is used to discover virtual addresses • RARP = Reverse Address Resolution Protocol ??? 128.84.96.90 RARP Server 128.84.96.91 “I just got here. My physical address is 1a:34:2c:9a:de:cc. What’s my name ?” “Your name is 128.84.96.89”

27. Repeaters and Bridges • Both connect LAN segments • Usually do not originate data • Repeaters (Hubs): physical layer devices • forward packets on all LAN segments • Useful for increasing range • Increases contention • Bridges: link layer devices • Forward packets only if meant on that segment • Isolates congestion • More expensive

28. Backbone Bridge

29. Summary • Data Link Layer • layer two of the seven-layer OSI model • Layer two of the five-layer TCP/IP reference model as well. • Responds to service requests from the network layer and issues service requests to the physical layer. • Broadcast vs Point-to-point • Point-to-point is often higher performance, but traditionally higher cost as well • Switched Ethernet is common now • Data Link Layer Issues • Encoding: map bits to analog signals • Manchester encoding • Framing: Group bits into frames (packets) • Bit stuffing • Arbitration: multiple senders, one resource • Ethernet uses CSMA/CD (carrier sense multiple access/collision detection) • Addressing: multiple receivers, one wire • ARP (address resolution protocol)