1. 5: DataLink Layer5-1 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!

2. 5: DataLink Layer5-2 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

3. 5: DataLink Layer5-3 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!

4. 5: DataLink Layer5-4 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)

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

6. 5: DataLink Layer5-6 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

7. 5: DataLink Layer5-7 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

8. 5: DataLink Layer5-8 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

9. 5: DataLink Layer5-9 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

10. 5: DataLink Layer5-10 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.

11. 5: DataLink Layer5-11 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

12. 5: DataLink Layer5-12 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!

13. 5: DataLink Layer5-13 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

14. 5: DataLink Layer5-14 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

15. 5: DataLink Layer5-15 ATM architecture r adaptation layer: only at edge of ATM network m data segmentation/reassembly m roughly analogous to Internet transport layer r ATM layer: “network” layer m cell switching, routing r physical layer

16. 5: DataLink Layer5-16 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

17. 5: DataLink Layer5-17 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 ?

18. 5: DataLink Layer5-18 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 two IP routers r Switched VCs (SVC): m dynamically set up on per-call basis

19. 5: DataLink Layer5-19 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

20. 5: DataLink Layer5-20 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

21. 5: DataLink Layer5-21 Multiprotocol label switching (MPLS) r initial goal: speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding m borrowing ideas from Virtual Circuit (VC) approach m but IP datagram still keeps IP address! PPP or Ethernet header IP header remainder of link-layer frame MPLS header label Exp S TTL 20 3 1 5

22. 5: DataLink Layer5-22 MPLS capable routers r a.k.a. label-switched router r forwards packets to outgoing interface based only on label value (don’t inspect IP address) m MPLS forwarding table distinct from IP forwarding tables r signaling protocol needed to set up forwarding m RSVP-TE m forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !! m use MPLS for traffic engineering r must co-exist with IP-only routers

23. 5: DataLink Layer5-23 R1 R2 D R3 R4 R5 0 1 0 0 A R6 in out out label label dest interface 6 - A 0 in out out label label dest interface 10 6 A 1 12 9 D 0 in out out label label dest interface 10 A 0 12 D 0 1 in out out label label dest interface 8 6 A 0 0 8 A 1 MPLS forwarding tables