7 Network Layer Network Layer BGP basics Internet inter-AS - PDF document

Network Layer Network Layer RIP ( Routing Information Protocol) RIP advertisements  distance vectors: exchanged among  distance vector algorithm neighbors every 30 sec via Response  included in BSD-UNIX Distribution in 1982 Message (also called advertisement)  distance metric: # of hops (max = 15 hops)  each advertisement: list of up to 25 From router A to subnets: destination subnets within AS destination hops u v u 1 w A B v 2 w 2 x x 3 C D z y 3 y z 2 Network Layer Network Layer 37 38 Network Layer Network Layer RIP: Example RIP: Example Dest Next hops Advertisement w - 1 D from A to D z x - 1 z C 4 w …. … ... x y z A B w x y A D B C Destination Network Next Router Num. of hops to dest. C w A 2 Destination Network Next Router Num. of hops to dest. y B 2 w A 2 z B 7 y B 2 x -- 1 z B A 7 5 …. …. .... x -- 1 …. …. Routing/Forwarding table in D .... Routing/Forwarding table in D Network Layer Network Layer 39 40 Network Layer Network Layer RIP: Link Failure and Recovery RIP Table processing  If no advertisement heard after 180 sec -->  RIP routing tables managed by application-level neighbor/link declared dead process called route-d (daemon)  routes via neighbor invalidated  advertisements sent in UDP packets, periodically repeated  new advertisements sent to neighbors  neighbors in turn send out new advertisements (if routed routed tables changed)  link failure info quickly (?) propagates to Transprt Transprt (UDP) (UDP) entire net network forwarding forwarding network  poison reverse used to prevent ping-pong loops (IP) table (IP) table (infinite distance = 16 hops) link link physical physical Network Layer Network Layer 41 42 7

Network Layer Network Layer BGP basics Internet inter-AS routing: BGP  when AS2 advertises a prefix (subnet) to AS1: AS2 promises it will forward datagrams  BGP (Border Gateway Protocol): the de towards that prefix. facto standard  BGP provides each AS a means to:  AS2 can aggregate prefixes in its advertisement 1. Obtain subnet reachability information from neighboring ASs. 2. Propagate reachability information to all AS- Reachability info internal routers. 3. Determine “good” routes to subnets based on eBGP session reachability information and policy. 3c iBGP session  allows subnet to advertise its existence to 2c 3a 3b 2a rest of Internet: “I am here” AS3 2b 1c AS2 1a 1b 1d AS1 Network Layer Network Layer 49 50 Network Layer Network Layer Distributing reachability info Path attributes & BGP routes  using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1.  advertised prefix includes BGP attributes.  1c can then use iBGP to distribute new prefix  prefix + attributes = “route” info to all routers in AS1  two important attributes:  1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session  AS-PATH: contains ASs through which prefix advertisement has passed: e.g, AS 67, AS 17  when router learns of new prefix, it creates entry for prefix in its forwarding table.  NEXT-HOP: indicates specific internal-AS router to next-hop AS. (may be multiple links from current AS to next-hop-AS) eBGP session 3c  when gateway router receives route iBGP session 2c 3a advertisement, uses import policy to 3b 2a AS3 2b accept/decline. 1c AS2 1a 1b 1d AS1 Reachability info Network Layer Network Layer 51 52 Network Layer Network Layer More than 1 route possible! BGP route selection A router may learn about more than 1 route ELIMINATION RULES to some prefix. Router must select route. are used to select a route from multiple possible routes 1. local preference value attribute: policy AS5 has two routes to AS1 reach AS1 decision 2. shortest AS-PATH AS2 AS3 AS4 3. closest NEXT-HOP router: hot potato routing 4. additional criteria AS5 Network Layer Network Layer 53 54 9

Network Layer Network Layer BGP routing policy BGP routing policy (2) legend : legend : provider provider B network B network X X W W A A ？ customer customer network: network: C C Y Y  A advertises path Aw to B  A,B,C are provider networks  B advertises path BAw to X  X,W,Y are customer (of provider networks)  Should B advertise path BAw to C?  X is dual-homed: attached to two networks  No way! B gets no “revenue” for routing CBAw since  X does not want to route from B via X to C neither W nor C are B’s customers  .. so X will not advertise to B a route to C  B wants to force C to route to w via A  B wants to route only to/from its customers! Network Layer Network Layer 55 56 Network Layer Network Layer BGP messages Why different Intra- and Inter-AS routing ? Policy:  BGP messages exchanged using TCP.  Inter-AS: admin wants control over how its traffic routed, who routes through its net.  BGP messages:  Intra-AS: single admin, so no policy decisions needed  OPEN: opens TCP connection to peer and Scale: authenticates sender  UPDATE: advertises new path (or withdraws old)  hierarchical routing saves table size, reduced update traffic  KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request Performance:  NOTIFICATION: reports errors in previous msg;  Intra-AS: can focus on performance also used to close connection  Inter-AS: policy may dominate over performance Network Layer Network Layer 57 58 Network Layer Network Layer summary Assignment #3  pp. 445  1 Introduction  5 Routing algorithms  2 Virtual circuit and  Link state datagram networks  Distance Vector  5, 8, 11, 14, 19  Hierarchical routing  3 What’s inside a  6 Routing in the router Internet  No submission required.  4 IP: Internet  RIP Protocol  OSPF  Datagram format  BGP  IPv4 addressing  ICMP  IPv6 Network Layer Network Layer 59 60 10