1 Announcement r Homework #3 due tomorrow midnight r Project #3 is out

2 Last class r Routing in the Internet m Hierarchical routing m RIP m OSPF m BGP

3 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b Intra-AS Routing algorithm Inter-AS Routing algorithm Forwarding table 3c Hierarchical Routing: Intra- and Inter-AS Routing r Forwarding table is configured by both intra- and inter-AS routing algorithm m Intra-AS sets entries for internal dests m Inter-AS & Intra-As sets entries for external dests

4 RIP ( Routing Information Protocol) r Distance vector algorithm r Included in BSD-UNIX Distribution in 1982 r Distance metric: # of hops (max = 15 hops) m # of hops: # of subnets traversed along the shortest path from src. router to dst. subnet (e.g., src. = A) D C BA u v w x y z destination hops u 1 v 2 w 2 x 3 y 3 z 2

5 OSPF (Open Shortest Path First) r “open”: publicly available r Uses Link State algorithm m LS packet dissemination m Topology map at each node m Route computation using Dijkstra’s algorithm m Link costs configured by the network administrator r OSPF advertisement carries one entry per neighbor router r Advertisements disseminated to entire AS (via flooding) m Carried in OSPF messages directly over IP (rather than TCP or UDP

6 Hierarchical OSPF

7 Overview m BGP r Data link layer m Introduction and services m Error detection and correction m Multiple access protocols

8 Internet inter-AS routing: BGP r BGP (Border Gateway Protocol): the de facto standard r BGP provides each AS a means to: 1. Obtain subnet reachability information from neighboring ASs. 2. Propagate the reachability information to all routers internal to the AS. 3. Determine “good” routes to subnets based on reachability information and policy. r Allows a subnet to advertise its existence to rest of the Internet: “I am here”

9 BGP basics r Pairs of routers (BGP peers) exchange routing info over TCP conections: BGP sessions r Note that BGP sessions do not correspond to physical links. r When AS2 advertises a prefix to AS1, AS2 is promising it will forward any datagrams destined to that prefix towards the prefix. m AS2 can aggregate prefixes in its advertisement 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b 3c eBGP session iBGP session

10 Distributing reachability info r With eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. r 1c can then use iBGP do distribute this new prefix reach info to all routers in AS1 r 1b can then re-advertise the new reach info to AS2 over the 1b-to-2a eBGP session r When router learns about a new prefix, it creates an entry for the prefix in its forwarding table. 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b 3c eBGP session iBGP session

11 Path attributes & BGP routes r When advertising a prefix, advert includes BGP attributes. m prefix + attributes = “route” r Two important attributes: m AS-PATH: contains the ASs through which the advert for the prefix passed: AS 67 AS 17 m NEXT-HOP: Indicates the specific internal-AS router to next-hop AS. (There may be multiple links from current AS to next-hop-AS.) r When gateway router receives route advert, uses import policy to accept/decline.

12 BGP route selection r Router may learn about more than 1 route to some prefix. Router must select route. r Elimination rules: 1. Local preference value attribute: policy decision 2. Shortest AS-PATH 3. Closest NEXT-HOP router: hot potato routing 4. Additional criteria

13 BGP routing policy r A,B,C are provider networks r X,W,Y are customer (of provider networks) r X is dual-homed: attached to two networks m X does not want to route from B via X to C m.. so X will not advertise to B a route to C

14 BGP routing policy (2) r A advertises to B the path AW r B advertises to X the path BAW r Should B advertise to C the path BAW?

15 BGP routing policy (2) r A advertises to B the path AW r B advertises to X the path BAW r Should B advertise to C the path BAW? m No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers m B wants to force C to route to w via A m B wants to route only to/from its customers!

16 Why different Intra- and Inter-AS routing ? Policy: r Inter-AS: admin wants control over how its traffic routed, who routes through its net. r Intra-AS: single admin, so no policy decisions needed Scale: r hierarchical routing saves table size, reduced update traffic Performance: r Intra-AS: can focus on performance r Inter-AS: policy may dominate over performance

17 Overview m BGP r Data link layer m Introduction and services m Error detection and correction m Multiple access protocols

18 The Data Link Layer Our goals: r understand principles behind data link layer services: m error detection, correction m sharing a broadcast channel: multiple access m link layer addressing m reliable data transfer, flow control: done! r instantiation and implementation of various link layer technologies

19 Overview m BGP r Data link layer m Introduction and services m Error detection and correction m Multiple access protocols

20 Link Layer: Introduction Some terminology: r hosts and routers are nodes r communication channels that connect adjacent nodes along communication path are links m wired links m wireless links m LANs r layer-2 packet is a frame, encapsulates datagram “link” data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

21 Link layer: context r Datagram transferred by different link protocols over different links: m e.g., Ethernet on first link, frame relay on intermediate links, on last link r Each link protocol provides different services m e.g., may or may not provide rdt over link transportation analogy r trip from Princeton to Lausanne m limo: Princeton to JFK m plane: JFK to Geneva m train: Geneva to Lausanne r tourist = datagram r transport segment = communication link r transportation mode = link layer protocol r travel agent = routing algorithm

22 Link Layer Services r Framing, link access: m encapsulate datagram into frame, adding header, trailer m channel access if shared medium m “MAC” addresses used in frame headers to identify source, dest different from IP address! r Reliable delivery between adjacent nodes m we learned how to do this already (chapter 3)! m seldom used on low bit error link (fiber, some twisted pair) m wireless links: high error rates Q: why both link-level and end-end reliability?

23 Link Layer Services (more) r Flow Control: m pacing between adjacent sending and receiving nodes r Error Detection: m errors caused by signal attenuation, noise. m receiver detects presence of errors: signals sender for retransmission or drops frame r Error Correction: m receiver identifies and corrects bit error(s) without resorting to retransmission r Half-duplex and full-duplex m with half duplex, nodes at both ends of link can transmit, but not at same time

24 Adaptors Communicating r link layer implemented in “adaptor” (aka NIC) m Ethernet card, PCMCI card, card r sending side: m encapsulates datagram in a frame m adds error checking bits, rdt, flow control, etc. r receiving side m looks for errors, rdt, flow control, etc m extracts datagram, passes to rcving node sending node frame rcving node datagram frame adapter link layer protocol

25 Overview m BGP r Data link layer m Introduction and services m Error detection and correction m Multiple access protocols

26 Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields Error detection not 100% reliable! protocol may miss some errors, but rarely larger EDC field yields better detection and correction

27 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors 0 0

28 Checksumming: Cyclic Redundancy Check r view data bits, D, as a binary number r choose r+1 bit pattern (generator), G r goal: choose r CRC bits, R, such that m exactly divisible by G (modulo 2) m receiver knows G, divides by G. If non-zero remainder: error detected! m can detect all burst errors less than r+1 bits a burst of length greater than r+1 bits dtctd. with prob. 1-(1/2)^r r widely used in practice (ATM, HDCL)

29 CRC Example (modulo-2 arithmetic without without carries) Want: D. 2 r XOR R = nG equivalently: D. 2 r = nG XOR R equivalently: if we divide D. 2 r by G, want remainder R R = remainder[ ] D.2rGD.2rG

30 Overview m BGP r Data link layer m Introduction and services m Error detection and correction m Multiple access protocols

31 Multiple Access Links and Protocols Two types of “links”: r point-to-point m PPP for dial-up access m point-to-point link between Ethernet switch and host r broadcast (shared wire or medium) m traditional Ethernet m upstream cable m wireless LAN

32 Multiple Access protocols r single shared broadcast channel r two or more simultaneous transmissions by nodes: interference m collision if node receives two or more signals at the same time multiple access protocol r distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit r communication about channel sharing must use channel itself! m no out-of-band channel for coordination

33 Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R. 2. When M nodes want to transmit, each can send at average rate R/M 3. Fully decentralized: m no special node to coordinate transmissions m no synchronization of clocks, slots 4. Simple

34 MAC Protocols: a taxonomy Three broad classes: r Channel Partitioning m divide channel into smaller “pieces” (time slots, frequency, code) m allocate piece to node for exclusive use r Random Access m channel not divided, allow collisions m “recover” from collisions r “Taking turns” m Nodes take turns, but nodes with more to send can take longer turns

35 Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access r access to channel in "rounds" r each station gets fixed length slot (length = pkt trans time) in each round r unused slots go idle r example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle r TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load. r FDM (Frequency Division Multiplexing): frequency subdivided.

36 Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access r channel spectrum divided into frequency bands r each station assigned fixed frequency band r unused transmission time in frequency bands go idle r example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle r TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load. r FDM (Frequency Division Multiplexing): frequency subdivided. frequency bands time

37 Random Access Protocols r When node has packet to send m transmit at full channel data rate R. m no a priori coordination among nodes  two or more transmitting nodes ➜ “collision”, r random access MAC protocol specifies: m how to detect collisions m how to recover from collisions (e.g., via delayed retransmissions) r Examples of random access MAC protocols: m slotted ALOHA m ALOHA m CSMA, CSMA/CD, CSMA/CA

38 Slotted ALOHA Assumptions r all frames same size r time is divided into equal size slots, time to transmit 1 frame r nodes start to transmit frames only at beginning of slots r nodes are synchronized r if 2 or more nodes transmit in slot, all nodes detect collision Operation r when node obtains fresh frame, it transmits in next slot r no collision, node can send new frame in next slot r if collision, node retransmits frame in each subsequent slot with prob. p until success

39 Slotted ALOHA Pros r single active node can continuously transmit at full rate of channel r highly decentralized: only slots in nodes need to be in sync r simple Cons r collisions, wasting slots r idle slots r clock synchronization