1. 5: DataLink Layer5-1 Chapter 5 Link Layer and LANs Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright 1996-2004 J.F Kurose and K.W. Ross, All Rights Reserved

2. 5: DataLink Layer5-2 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 Link-Layer Addressing r 5.5 Ethernet r 5.6 Hubs and switches r 5.7 PPP r 5.8 Link Virtualization: ATM

3. 5: DataLink Layer5-3 Point to Point Data Link Control r one sender, one receiver, one link: easier than broadcast link: m no Media Access Control m no need for explicit MAC addressing m e.g., dialup link, ISDN line r popular point-to-point DLC protocols: m PPP (point-to-point protocol) m HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!

4. 5: DataLink Layer5-4 PPP Design Requirements [RFC 1557] r packet framing: encapsulation of network-layer datagram in data link frame m carry network layer data of any network layer protocol (not just IP) at same time m ability to demultiplex upwards r bit transparency: must carry any bit pattern in the data field r error detection (no correction) r connection liveness: detect, signal link failure to network layer r network layer address negotiation: endpoint can learn/configure each other’s network address

5. 5: DataLink Layer5-5 PPP non-requirements r no error correction/recovery r no flow control r out of order delivery OK r no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers!

6. 5: DataLink Layer5-6 PPP Data Frame r Flag: delimiter (framing) r Address: does nothing (only one option) r Control: does nothing; in the future possible multiple control fields r Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc)

7. 5: DataLink Layer5-7 PPP Data Frame r info: upper layer data being carried r check: cyclic redundancy check for error detection

8. 5: DataLink Layer5-8 Byte Stuffing r “data transparency” requirement: data field must be allowed to include flag pattern m Q: is received data or flag? r Sender: adds (“stuffs”) extra byte after each data byte r Receiver: m two 01111110 bytes in a row: discard first byte, continue data reception m single 01111110: flag byte

9. 5: DataLink Layer5-9 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

10. 5: DataLink Layer5-10 PPP Data Control Protocol Before exchanging network- layer data, data link peers must r configure PPP link (max. frame length, authentication) r learn/configure network layer information m for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

11. 5: DataLink Layer5-11 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 Link-Layer Addressing r 5.5 Ethernet r 5.6 Hubs and switches r 5.7 PPP r 5.8 Link Virtualization: ATM and MPLS

12. 5: DataLink Layer5-12 Virtualization of networks Virtualization of resources: a powerful abstraction in systems engineering: r computing examples: virtual memory, virtual devices m Virtual machines: e.g., java m IBM VM os from 1960’s/70’s r layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly

13. 5: DataLink Layer5-13 The Internet: virtualizing networks 1974: multiple unconnected nets m ARPAnet m data-over-cable networks m packet satellite network (Aloha) m packet radio network … differing in: m addressing conventions m packet formats m error recovery m routing ARPAnet satellite net "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, May, 1974, pp. 637-648.

14. 5: DataLink Layer5-14 The Internet: virtualizing networks ARPAnet satellite net gateway Internetwork layer (IP): r addressing: internetwork appears as a single, uniform entity, despite underlying local network heterogeneity r network of networks Gateway: r “embed internetwork packets in local packet format or extract them” r route (at internetwork level) to next gateway

15. 5: DataLink Layer5-15 Cerf & Kahn’s Internetwork Architecture What is virtualized? r two layers of addressing: internetwork and local network r new layer (IP) makes everything homogeneous at internetwork layer r underlying local network technology m cable m satellite m 56K telephone modem m today: ATM, MPLS … “invisible” at internetwork layer. Looks like a link layer technology to IP!

16. 5: DataLink Layer5-16 ATM and MPLS r ATM, MPLS separate networks in their own right m different service models, addressing, routing from Internet r viewed by Internet as logical link connecting IP routers m just like dialup link is really part of separate network (telephone network) r ATM, MPSL: of technical interest in their own right

17. 5: DataLink Layer5-17 Asynchronous Transfer Mode: ATM r 1990’s/00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture r Goal: integrated, end-end transport of carry voice, video, data m meeting timing/QoS requirements of voice, video (versus Internet best-effort model) m “next generation” telephony: technical roots in telephone world m packet-switching (fixed length packets, called “cells”) using virtual circuits

18. 5: DataLink Layer5-18 ATM architecture r adaptation layer: only at edge of ATM network m data segmentation/reassembly m roughly analagous to Internet transport layer r ATM layer: “network” layer m cell switching, routing r physical layer

19. 5: DataLink Layer5-19 ATM: network or link layer? Vision: end-to-end transport: “ATM from desktop to desktop” m ATM is a network technology Reality: used to connect IP backbone routers m “IP over ATM” m ATM as switched link layer, connecting IP routers ATM network IP network

20. 5: DataLink Layer5-20 ATM Adaptation Layer (AAL) r ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM layer below r AAL present only in end systems, not in switches r AAL layer segment (header/trailer fields, data) fragmented across multiple ATM cells m analogy: TCP segment in many IP packets

21. 5: DataLink Layer5-21 ATM Adaptation Layer (AAL) [more] Different versions of AAL layers, depending on ATM service class: r AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation r AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video r AAL5: for data (eg, IP datagrams) AAL PDU ATM cell User data

22. 5: DataLink Layer5-22 ATM Layer Service: transport cells across ATM network r analogous to IP network layer r very different services than IP network layer Network Architecture Internet ATM Service Model best effort CBR VBR ABR UBR Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes no Order no yes Timing no yes no Congestion feedback no (inferred via loss) no congestion no congestion yes no Guarantees ?

23. 5: DataLink Layer5-23 ATM Layer: Virtual Circuits r VC transport: cells carried on VC from source to dest m call setup, teardown for each call before data can flow m each packet carries VC identifier (not destination ID) m every switch on source-dest path maintain “state” for each passing connection m link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf. r Permanent VCs (PVCs) m long lasting connections m typically: “permanent” route between to IP routers r Switched VCs (SVC): m dynamically set up on per-call basis

24. 5: DataLink Layer5-24 ATM VCs r Advantages of ATM VC approach: m QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) r Drawbacks of ATM VC approach: m Inefficient support of datagram traffic m one PVC between each source/dest pair) does not scale (N*2 connections needed) m SVC introduces call setup latency, processing overhead for short lived connections

25. 5: DataLink Layer5-25 ATM Layer: ATM cell r 5-byte ATM cell header r 48-byte payload m Why?: small payload -> short cell-creation delay for digitized voice m halfway between 32 and 64 (compromise!) Cell header Cell format

26. 5: DataLink Layer5-26 ATM cell header r VCI: virtual channel ID m will change from link to link thru net r PT: Payload type (e.g. RM cell versus data cell) r CLP: Cell Loss Priority bit m CLP = 1 implies low priority cell, can be discarded if congestion r HEC: Header Error Checksum m cyclic redundancy check

27. 5: DataLink Layer5-27 ATM Physical Layer (more) Two pieces (sublayers) of physical layer: r Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD sublayer below r Physical Medium Dependent: depends on physical medium being used TCS Functions: m Header checksum generation: 8 bits CRC m Cell delineation m With “unstructured” PMD sublayer, transmission of idle cells when no data cells to send

28. 5: DataLink Layer5-28 ATM Physical Layer Physical Medium Dependent (PMD) sublayer r SONET/SDH: transmission frame structure (like a container carrying bits); m bit synchronization; m bandwidth partitions (TDM); m several speeds: OC3 = 155.52 Mbps; OC12 = 622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps r TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps r unstructured: just cells (busy/idle)

29. 5: DataLink Layer5-29 IP-Over-ATM Classic IP only r 3 “networks” (e.g., LAN segments) r MAC (802.3) and IP addresses IP over ATM r replace “network” (e.g., LAN segment) with ATM network r ATM addresses, IP addresses ATM network Ethernet LANs Ethernet LANs

30. 5: DataLink Layer5-30 IP-Over-ATM AAL ATM phy Eth IP ATM phy ATM phy app transport IP AAL ATM phy app transport IP Eth phy

31. 5: DataLink Layer5-31 Datagram Journey in IP-over-ATM Network r at Source Host: m IP layer maps between IP, ATM dest address (using ARP) m passes datagram to AAL5 m AAL5 encapsulates data, segments cells, passes to ATM layer r ATM network: moves cell along VC to destination r at Destination Host: m AAL5 reassembles cells into original datagram m if CRC OK, datagram is passed to IP

32. 5: DataLink Layer5-32 IP-Over-ATM Issues: r IP datagrams into ATM AAL5 PDUs r from IP addresses to ATM addresses m just like IP addresses to 802.3 MAC addresses! ATM network Ethernet LANs

33. 5: DataLink Layer5-33 Chapter 5: Summary r principles behind data link layer services: m error detection, correction m sharing a broadcast channel: multiple access m link layer addressing r instantiation and implementation of various link layer technologies m Ethernet m switched LANS m PPP m virtualized networks as a link layer: ATM, MPLS