Announcement r Homework 3 due tomorrow midnight r

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

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

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

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) u v A z C B D w x y destination hops u 1 v 2 w 2 x 3 y 3 z 2 4

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 5

Hierarchical OSPF 6

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

Internet inter-AS routing: BGP r BGP (Border Gateway Protocol): the de facto standard r BGP provides each AS a means to: 1. 2. 3. Obtain subnet reachability information from neighboring ASs. Propagate the reachability information to all routers internal to the AS. 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” 8

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 AS 2 advertises a prefix to AS 1, AS 2 is promising it will forward any datagrams destined to that prefix towards the prefix. m AS 2 can aggregate prefixes in its advertisement 3 c 3 a 3 b AS 3 1 a AS 1 2 a 1 c 1 d 1 b 2 c AS 2 2 b e. BGP session i. BGP session 9

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

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. 11

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

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 13

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? 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? 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! 15

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 16

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

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

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

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

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, 802. 11 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 21

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? 22

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 23

Adaptors Communicating datagram sending node rcving node link layer protocol frame adapter r link layer implemented in r receiving side “adaptor” (aka NIC) m looks for errors, rdt, flow control, etc m Ethernet card, PCMCI m extracts datagram, passes card, 802. 11 card to rcving node r sending side: m m encapsulates datagram in a frame adds error checking bits, rdt, flow control, etc. 24

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

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 26

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

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 m m <D, R> exactly divisible by G (modulo 2) receiver knows G, divides <D, R> by G. If non-zero remainder: error detected! 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) 28

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

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

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 802. 11 wireless LAN 31

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 32

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 m no special node to coordinate transmissions no synchronization of clocks, slots 4. Simple 33

MAC Protocols: a taxonomy Three broad classes: r Channel Partitioning m m divide channel into smaller “pieces” (time slots, frequency, code) 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 34

Channel Partitioning MAC protocols: 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 35

Channel Partitioning MAC protocols: 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 frequency bands time 36

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 r 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 CSMA, CSMA/CD, CSMA/CA 37

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 38

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 39