Data Link Layer part two

1. Data Link Layer part two • How LANs work • Switching • The ARP protocol • Data link reliability • Interconnecting LANs • Hubs, bridges, routers

2. LAN technologies

3. LAN Addresses and ARP - 1 32-bit IP address: • network-layer address • used to get datagram to destination network • E.g. 138.23.169.129 LAN (or MAC or physical) address: • used to get datagram from one interface to another physically-connected interface (same network) • 48 bit MAC address (for most LANs) burned in the adapter ROM • E.g. 20:30:65:25:5a:93

4. LAN Addresses and ARP - 2 Each adapter on LAN has unique LAN address

5. LAN Address (more) • MAC address allocation administered by IEEE • manufacturer buys portion of MAC address space (to assure uniqueness) • Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address • MAC flat address => portability • can move LAN card from one LAN to another • IP hierarchical address NOT portable • depends on network to which one attaches

6. IP vs MAC address • IP address refers to communicating end points • used in network layer to find path • MAC or physical address refers to physically connected machines • Data link layer to find next hop

7. 223.1.1.1 223.1.2.1 E B A 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 223.1.3.27 223.1.1.3 223.1.3.2 223.1.3.1 Data Link and Routing Starting at A, given IP datagram addressed to B: • look up net. address of B, find B on same net. as A • link layer send datagram to B inside link-layer frame frame source, dest address datagram source, dest address A’s IP addr B’s IP addr B’s MAC addr A’s MAC addr IP payload datagram frame

8. ARP: Address Resolution Protocol Question: how to determine MAC address of B given B’s IP address? • Each IP node (Host, Router) on LAN has ARPmodule, table • ARP Table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> < ………………………….. > • TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

9. ARP protocol: in one LAN • A knows B's IP address, wants to learn physical address of B • A broadcasts ARP query pkt, containing B's IP address • all machines on LAN receive ARP query • B receives ARP packet, replies to A with its (B's) physical layer address • A caches (saves) IP-to-physical address pairs until information becomes old (times out) • soft state: information that times out (goes away) unless refreshed

10. Routing to another LAN - 1 Walkthrough: routing from A to B via R

11. Routing to another LAN - 2 • A creates IP packet with source A, destination B • Routing: A finds that R is next hop • A uses ARP to get R’s physical layer address for 111.111.111.110 • A creates Ethernet frame with • R's physical address as dest, • A, B IP datagram • A’s data link layer sends Ethernet frame

12. Routing to another LAN - 3 • R’s data link layer receives Ethernet frame • R removes IP datagram from Ethernet frame, sees its destined to B • R uses ARP to get B’s physical layer address • R creates frame containing A-to-B IP datagram sends to B

13. Data Link Reliability

14. Why retransmissions ? • Error correction although feasible, is not enough to handle all kinds of errors -- especially burst errors. • Corrupt frames cannot be deciphered and are therefore dropped. • Retransmissions needed to provide reliability. • Note this is reliability at the data link layer • Similar considerations appear at the transport layer • TCP has reliability

15. ACKs and Time-outs • When frames are sent piggyback an acknowledgement (ACK) for received packets onto sent packets. • If no ACK received up to a preset time-out, resend frame. • Called ARQ -- Automatic Repeat request. 1 Ack 1 100

16. Stop and Wait • Allow only one outstanding packet at any given time. • If ACK not received within time-out, send again.

17. How efficient is Stop and Wait? • Consider a 1.5 Mbps link with 45 ms RTT. • BW - Delay product = 67.5 Kb = 8KB. • You can fill 8 Kbytes of data prior to receiving an ACK. • However, if your frame size is 1 KB, you are using only 1/8 of the capacity. • Inefficient.

18. Sliding Window • What we really like is that the 9th frame be transmitted when ACK for the first frame arrives :). • Sliding window: • A window of packets sent • as ACKs are received window slides i.e., more packets sent. • Now what do we need in addition ? • Need to know which packets have been received and which have not. • Packets labeled using sequence numbers.

19. Some definitions • We have a window of packets sent -- Send Window Size or SWS. • Last Acknowledgement received is denoted LAR. • LFS represents the last frame sent. • NOTE: LFS - LAR <= SWS • NOTE 2: Initially we will consider “cumulative ACKs” • ACK frame seq-no=15 means I have received all packets including 15.

20. < SWS ─ ■ ■ ■ ■ ■ ■ LAR LFS Sender Functions k k+1 • When an ACK is received, the LAR moves to the right. • This allows for the transmission of an additional frames.

21. < ─ RWS ■ ■ ■ ■ ■ ■ LFR LAF Receiver functions Notation: RWS -- Receive Window Size LAF -- Largest Acceptable Frame. LFR -- Last Frame Received. • When frame with Seq_Num arrives: • If frame is outside window discard. • Seq_Num <= LFR or Seq Num > LAF • If frame within window, accept frame. • LFR < Seq_Num < LAF. • Send an ACK, if needed • Let Seq_Num_to_Ack be the largest seq-number • That I can ACK , (all frames <= Seq_Num_to_Ack have been received) • yet to be acked. • If have not sent ACK yet, send an ACK • Adjust parameters: • LFR = Seq_Num_to_Ack • Adjust LAF = LFR + RWS.

22. An Example • Let LFR =5 and RWS = 4. • This implies LAF = 9 • If packets 7 and 8 arrive (not 6), they are buffered. • Note that they are out of order. • Typically, Receiver will resend an ACK for packet 5. • When 6 arrives, it can cumulatively ACK all buffered packets i.e., it ACKs 8 and moves LFR to 8 and LAF to 12.

23. Other possibilities • Send NAK (negative acknowledgement) for lost packets -- example for 6, when 7 is received. • Duplicate ACKs -- send an ACK for 5 again when 7 is received to trigger retransmission of 6. • Selective ACKs : Explicitly ACK frames that are received -- more complex.

24. Setting the Window Size • Sender window: SWS, set considering the BW delay product and link quality • How many packets should be “on flight”? • Receiver window: RWS, set to something appropriate -- may depend on buffering resources. • If RWS = 1 what happens to out of order frames ?

25. Sequence number wrapping • Sequence numbers are finite -- thus there is a need to reuse -- called wrap around. • What is the relationship between the SWS and MaxSeqNum ?

26. Maximum Sequence Number and SWS • Should it not be SWS <= MaxSeqNum + 1 ? • Let us consider an example: • Sender has eight Seq Nums from 0 to 7. • Assume SWS = RWS = 7. • Sender transmits frames 0-6 • Receiver gets them, ACKs but ACKs get lost. • Receiver expects 7 and next 0...5. But sender sends the previous 0..5. • When the receiver gets these, he cannot distinguish. • Thus, when RWS = SWS, SWS < (MaxSeqNum+1)/2. • Example: consider MaxSeqNum = 1 (one bit) • I can only send one packet at a time, alternating between 1 and 0

27. Sequence Numbers and SWS • For other cases i.e., when RWS is not equal to SWS, other rules may apply. • Depends on the specific case. • A different way with TCP -- we will see later. • Easy solution -- large Sequence number space.

28. Interconnecting LANs • Hubs • Bridges • Routers

29. Interconnecting LANs Q: Why not just one big LAN? • Limited amount of supportable traffic: on single LAN, all stations must share bandwidth • Limited length: 802.3 specifies maximum cable length • Large “collision domain” (can collide with many stations) • Limited number of stations: 802.5 have token passing delays at each station

30. Overview: Order of increasing “intelligence’ • Hubs: • repeat everything to all ports except the incoming • only on tree topologies (otherwise -> loops) • Bridges: even with non-tree topologies • Select a tree structure and use only that • Default: behave like a hub • Learn: if known, forward only towards destination • Routers: • Handle arbitrary topologies • Perform complex routing decisions

31. Hubs - 1 • Physical Layer devices: essentially repeaters operating at bit levels: repeat received bits on one interface to all other interfaces • Each connected LAN referred to as LAN segment • Hubs do not isolate collisiondomains: node may collide with any node residing at any segment in LAN

32. Hub Advantages • simple, inexpensive device • Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions • extends maximum distance between node pairs (100m per Hub)

33. Hub limitations • Single collision domain results in no increase in max throughput • multi-tier throughput same as single segment throughput • Individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage • Cannot connect different Ethernet types (e.g., 10BaseT and 100baseT)

34. Bridges - 1 • Link Layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination • Bridge isolates collision domains since it buffers frames • When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit

35. Bridges - 2 • Bridge advantages: • Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage • Can connect different type Ethernet since it is a store and forward device • Transparent: no need for any change to hosts LAN adapters

36. Bridges: frame filtering, forwarding • Bridges filter packets • Same-LAN -segment frames not forwarded onto other LAN segments • Forwarding: • How to know which LAN segment on which to forward frame? • Looks like a routing problem (more shortly!)

37. Backbone Bridge

38. Bridge Filtering - 1 • Bridges learn which hosts can be reached through which interfaces: maintain filtering tables • when frame received, bridge “learns” location of sender: incoming LAN segment • records sender location in filtering table • Filtering table entry: • (Node LAN Address, Bridge Interface, Time Stamp) • stale entries in Filtering Table dropped (TTL can be 60 minutes)

39. Bridge Filtering - 2 • Filtering procedure: if destination is on LAN on which frame was received then drop the frame else { lookup filtering table if entry found for destination then forward the frame on interface indicated; else flood; /* forward on all but the interface on which the frame arrived*/ }

40. Bridge Learning: example - 1 Suppose C sends frame to D and D replies back with frame to C • C sends frame, bridge has no info about D, so floods to both LANs • bridge notes that C is on port 1 • frame ignored on upper LAN • frame received by D

41. Bridge Learning: example - 2 C | 1 • D generates reply to C, sends • bridge sees frame from D • bridge notes that D is on interface 2 • bridge knows C on interface 1, so selectively forwards frame out via interface 1

42. Disabled Bridges Spanning Tree • For increased reliability, desirable to have redundant, alternate paths from source to dest • With multiple simultaneous paths, cycles result - bridges may multiply and forward frame forever • Solution: organize bridges in a spanning tree by disabling subset of interfaces

43. The Spanning Tree Algorithm • Purpose: select a subset of ports for fwd-ing • Root bridge election: lowest ID wins • Each bridge: select one port towards root • Shortest path to root (break ties) • In each LAN: select designated bridge - select port • Root is designated for all adjacent LANs • Designated bridge: fwds frames towards root • Select the bridge closest to root (IDs to break ties) • Note: a bridge can be designated for many LANs • Active ports: the selected ones, others are stand by PetDavie 3.2.2 for more details

44. WWF Bridges vs. Routers • Both store-and-forward devices • routers: network layer devices (examine network layer headers) • bridges are Link Layer devices • Routers maintain routing tables, implement routing algorithms • Bridges maintain filtering tables, implement filtering, learning and spanning tree algorithms

45. Routers vs. Bridges - 1 Bridges + and - + Bridge operation is simpler requiring less processing bandwidth - Topologies are restricted with bridges: a spanning tree must be built to avoid cycles - Bridges do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a bridge)

46. Routers vs. Bridges - 2 Routers + and - + arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols) + provide firewall protection against broadcast storms - require IP address configuration (not plug and play) - require higher processing bandwidth • Bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)