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1、The Network Layer Part 5Contents OSPFThe Interior Gateway Routing Protocol(Section 5.6.4)BGPThe Exterior Gateway Routing Protocol(Section 5.6.5)IPv6(Section 5.6.8)3b1d3a1c2aAS3AS1AS21a2c2b1bIntra-ASRouting algorithmInter-ASRouting algorithmForwardingtable3cInterconnected ASes Forwarding table is con
2、figured by both intra-and inter-AS routing algorithm Intra-AS sets entries for internal dests Inter-AS&Intra-As sets entries for external dests From slides of Computer Networking course,by Prof.J.Kurose Routing in the Internet Interior Gateway Protocol RIP(Routing Information Protocol):distance vect
3、or protocol based on the Bellman-Ford algorithm OSPF(Open Shortest Path First):Exterior Gateway Protocol BGPOSPF Open Shortest Path First The Interior Gateway Routing Protocol(a)An autonomous system.(b)A graph representation of (a).OSPF(2)The relation between ASes,backbones,and areas in OSPF.OSPF(3)
4、The five types of OSPF messages.5-66BGP Border Gateway ProtocolThe Exterior Gateway Routing Protocol(a)A set of BGP routers.(b)Information sent to F.BGP basics Pairs of routers(BGP peers)exchange routing info over semi-permanent TCP connections:BGP sessions Note that BGP sessions do not correspond t
5、o physical links.When AS2 advertises a prefix to AS1,AS2 is promising it will forward any datagrams destined to that prefix towards the prefix.AS2 can aggregate prefixes in its advertisement3b1d3a1c2aAS3AS1AS21a2c2b1b3ceBGP sessioniBGP sessionFrom slides of Computer Networking course,by Prof.J.Kuros
6、e 3b1d3a1c2aAS3AS1AS21a2c2b1b3cInter-AS tasks Suppose router in AS1 receives datagram for which dest is outside of AS1 Router should forward packet towards one of the gateway routers,but which one?AS1 needs:1.to learn which dests are reachable through AS2 and which through AS32.to propagate this rea
7、chability info to all routers in AS1Job of inter-AS routing!From slides of Computer Networking course,by Prof.J.Kurose Example:Setting forwarding table in router 1d Suppose AS1 learns from the inter-AS protocol that subnet x is reachable from AS3(gateway 1c)but not from AS2.Inter-AS protocol propaga
8、tes 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.Puts in forwarding table entry(x,I).From slides of Computer Networking course,by Prof.J.Kurose Learn from inter-AS protocol that subnet x is reachable vi
9、a multiple gatewaysUse routing infofrom intra-AS protocol to determinecosts of least-cost paths to eachof the gatewaysHot potato routing:Choose the gatewaythat has the smallest least costDetermine fromforwarding table the interface I that leads to least-cost gateway.Enter(x,I)in forwarding tableExam
10、ple:Choosing among multiple ASes Now suppose AS1 learns from the inter-AS protocol that subnet x is reachable from AS3 and from AS2.To configure forwarding table,router 1d must determine towards which gateway it should forward packets for dest x.This is also the job on inter-AS routing protocol!Hot
11、potato routing:send packet towards closest of two routers.From slides of Computer Networking course,by Prof.J.Kurose BGP messages BGP messages exchanged using TCP.BGP messages:OPEN:opens TCP connection to peer and authenticates sender UPDATE:advertises new path(or withdraws old)KEEPALIVE keeps conne
12、ction alive in absence of UPDATES;also ACKs OPEN request NOTIFICATION:reports errors in previous msg;also used to close connectionFrom slides of Computer Networking course,by Prof.J.Kurose BGP route selectionRouter may learn about more than 1 route to some prefix.Router must select route.Elimination
13、 rules:1.Local preference value attribute:policy decision2.Shortest AS-PATH 3.Closest NEXT-HOP router:hot potato routing4.Additional criteria From slides of Computer Networking course,by Prof.J.Kurose IPv6 Major goals of IPv6:Support billions of hosts,even with inefficient address space allocation R
14、educe the size of the routing tables Simplify the protocol,to allow routers to process packets faster Provide better security(authentication and privacy)than current IP Pay more attention to type of service,particularly for real-time data Aid multicasting by allowing scopes to be specified Make it possible for a host to roam without changing its address Allow the protocol to evolve in the future.Permit the old and new protocols to coexist for years The Main IPv6 HeaderThe IPv6 fixed header(required).Summary IGP OSPF EGP BGP IPv6
计算机网络英文版课件：slide 09 The Network Layer – Part 5
