lec08b - !"#$%&...

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!"#$% #$%'()*+ ,-* . /$+ 01" ), $.(2+ ,$*3-(/45 0- 66 *(.,4.%7-,4 "()8.%+9 .,-,+:)*+9 ),*$;()*+ ,-* /$+ %-,%$(,4 !"#$% $,.<2$4 *3- =-4*4 *- 4$%'($2+ ),*$(.%* 6(+>*-;(.>=)% .;($$:$,* ?,%(+>7-, !,*$;()*+ @();), .'*=$,7%.7-, @A$, '4$B ), C"04 6($.*$ D>()8.*$E ,$*3-(/4 .%(-44 *=$ >'<2)% ),*$(,$*
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Network Layer 4-99 Hierarchical Routing scale: with 200 million destinations: ! can ` t store all dest ` s in routing tables! ! routing table exchange would swamp links! administrative autonomy ! internet = network of networks ! each network admin may want to control routing in its own network Our routing study thus far - idealization ! all routers identical ! network l flat z … not true in practice
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Network Layer 4-100 Hierarchical Routing ! aggregate routers into regions, l autonomous systems z (AS) ! routers in same AS run same routing protocol " intra-AS routing protocol " routers in different AS can run different intra- AS routing protocol gateway router ! at l edge z of its own AS ! has link to router in another AS
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Network Layer 4-101 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b Intra-AS Routing algorithm Inter-AS Routing algorithm Forwarding table 3c Interconnected ASes ! forwarding table configured by both intra- and inter-AS routing algorithm " intra-AS sets entries for internal dests " inter-AS & intra-As sets entries for external dests
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Network Layer 4-102 Inter-AS tasks ! suppose router in AS1 receives datagram destined outside of AS1: " router should forward packet to gateway router, but which one? AS1 must: 1. learn which dests are reachable through AS2, which through AS3 2. propagate this reachability info to all routers in AS1 job of inter-AS routing! AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks
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Network Layer 4-103 Example: Setting forwarding table in router 1d ! suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2. " inter-AS protocol propagates reachability info to all internal routers ! router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c. " installs forwarding table entry (x,I) AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks x !
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Network Layer 4-104 Example: Choosing among multiple ASes ! now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2. ! to configure forwarding table, router 1d must determine which gateway it should forward packets towards for dest x " this is also job of inter-AS routing protocol! AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks x ! ?
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Network Layer 4-105 Learn from inter-AS protocol that subnet x is reachable via multiple gateways Use routing info from intra-AS protocol to determine costs of least-cost paths to each of the gateways Hot potato routing: Choose the gateway that has the smallest least cost Determine from forwarding table the interface I that leads to least-cost gateway. Enter (x,I) in
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lec08b - !&quot;#$%&amp;...

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