115-441 Computer NetworkingLecture 12: Intra-Domain RoutingRIP (Routing Information Protocol) & OSPF (Open Shortest Path First)Lecture 12: 2-19-2004 2Router Operation• When Packet Arrives at Router• Examine header to determine intended destination• Look up in table to determine next hop in path• Send packet out appropriate port• Today’s lecture• How to generate the routing tableRouterLecture 12: 2-19-2004 3Graph Model• Represent each router as node• Direct link between routers represented by edge• Symmetric links ⇒ undirected graph• Edge “cost” c(x,y) denotes measure of difficulty of using link• delay, $ cost, or congestion level• Task• Determine least cost path from every node to every other node• Path cost d(x,y) = sum of link costsAEFCDB23641113Lecture 12: 2-19-2004 4Routes from Node A• Properties• Some set of shortest paths forms tree• Shortest path spanning tree• Solution not unique• E.g., A-E-F-C-D also has cost 7AEFCDB23641113Table for AA0AB4BE6CB7DE2EE5FNext HopCostDest2Lecture 12: 2-19-2004 5Ways to Compute Shortest Paths• Centralized• Collect graph structure in one place• Use standard graph algorithm• Disseminate routing tables• Partially Distributed• Every node collects complete graph structure• Each computes shortest paths from it• Each generates own routing table• “Link-state” algorithm• Fully Distributed• No one has copy of graph• Nodes construct their own tables iteratively• Each sends information about its table to neighbors• “Distance-Vector” algorithmLecture 12: 2-19-2004 6Outline• Distance Vector• Link StateLecture 12: 2-19-2004 7Distance-Vector Method• Idea• At any time, have cost/next hop of best known path to destination• Use cost ∞ when no path known• Initially• Only have entries for directly connected nodesAEFCDB23641113Initial Table for AA0AB4B–∞C–∞DE2EF6FNext HopCostDestLecture 12: 2-19-2004 8Distance-Vector Update• Update(x,y,z)d ← c(x,z) + d(z,y) # Cost of path from x to y with first hop zif d < d(x,y)# Found better pathreturn d,z # Updated cost / next hopelsereturn d(x,y), nexthop(x,y) # Existing cost / next hopxzyc(x,z)d(z,y)d(x,y)3Lecture 12: 2-19-2004 9Algorithm• Bellman-Ford algorithm• RepeatFor every node xFor every neighbor zFor every destination yd(x,y) ← Update(x,y,z)• Until ConvergeLecture 12: 2-19-2004 10StartAEFCDB23641113Table for AA0AB4B–∞C–∞DE2EF6FHopCstDstTable for BA4AB0B–∞CD3D–∞EF1FHopCstDstTable for C–∞A–∞BC0CD1D–∞EF1FHopCstDstTable for D–∞AB3BC1CD0D–∞E–∞FHopCstDstTable for EA2A–∞B–∞C–∞DE0EF3FHopCstDstTable for FA6AB1BC1C–∞DE3EF0FHopCstDstOptimum 1-hop pathsLecture 12: 2-19-2004 11Iteration #1Table for AA0AB4BF7CB7DE2EE5FHopCstDstTable for BA4AB0BF2CD3DF4EF1FHopCstDstTable for CF7AF2BC0CD1DF4EF1FHopCstDstTable for DB7AB3BC1CD0D–∞EC2FHopCstDstTable for EA2AF4BF4C–∞DE0EF3FHopCstDstTable for FB5AB1BC1CC2DE3EF0FHopCstDstOptimum 2-hop pathsAEFCDB23641113Lecture 12: 2-19-2004 12Iteration #2Table for AA0AB4BE6CB7DE2EE5FHopCstDstTable for BA4AB0BF2CD3DF4EF1FHopCstDstTable for CF6AF2BC0CD1DF4EF1FHopCstDstTable for DB7AB3BC1CD0DC5EC2FHopCstDstTable for EA2AF4BF4CF5DE0EF3FHopCstDstTable for FB5AB1BC1CC2DE3EF0FHopCstDstOptimum 3-hop pathsAEFCDB236411134Lecture 12: 2-19-2004 13Distance Vector: Link Cost ChangesLink cost changes:• Node detects local link cost change • Updates distance table • If cost change in least cost path, notify neighborsXZ1450Y1algorithmterminates“goodnews travelsfast”Lecture 12: 2-19-2004 14Distance Vector: Link Cost ChangesLink cost changes:• Good news travels fast • Bad news travels slow -“count to infinity” problem!XZ1450Y60algorithmcontinueson!Lecture 12: 2-19-2004 15Distance Vector: Split HorizonIf Z routes through Y to get to X :• Z does not advertise its route to X back to YalgorithmterminatesXZ1450Y60? ? ?Lecture 12: 2-19-2004 16Distance Vector: Poison ReverseIf Z routes through Y to get to X :• Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)• Will this completely solve count to infinity problem? XZ1450Y60algorithmterminates5Lecture 12: 2-19-2004 17Poison Reverse Failures• Iterations don’t converge• “Count to infinity”• Solution• Make “infinity” smaller• What is upper bound on maximum path length?Table for AF7CHopCstDstTable for BA8CHopCstDstTable for FC1CHopCstDstTable for F–∞CHopCstDstTable for A–∞CHopCstDstForcedUpdateTable for BA14CHopCstDstForcedUpdateF C6111BDA4∞∞Table for DB9CHopCstDstForcedUpdateTable for AD13CHopCstDstBetterRouteTable for DB15CHopCstDstTable for AD19CHopCstDstForcedUpdate•••ForcedUpdateLecture 12: 2-19-2004 18Routing Information Protocol (RIP)• Earliest IP routing protocol (1982 BSD)• Current standard is version 2 (RFC 1723)• Features• Every link has cost 1• “Infinity” = 16• Limits to networks where everything reachable within 15 hops• Sending Updates• Every router listens for updates on UDP port 520• RIP message can contain entries for up to 25 table entriesLecture 12: 2-19-2004 19RIP Updates• Initial• When router first starts, asks for copy of table for every neighbor• Uses it to iteratively generate own table• Periodic• Every 30 seconds, router sends copy of its table to each neighbor• Neighbors use to iteratively update their tables• Triggered• When every entry changes, send copy of entry to neighbors• Except for one causing update (split horizon rule)• Neighbors use to update their tablesLecture 12: 2-19-2004 20RIP Staleness / Oscillation Control• Small Infinity• Count to infinity doesn’t take very long• Route Timer• Every route has timeout limit of 180 seconds• Reached when haven’t received update from next hop for 6 periods• If not updated, set to infinity• Soft-state refresh Æ important concept!!!• Behavior• When router or link fails, can take minutes to stabilize6Lecture 12: 2-19-2004 21RIP Table Processing• RIP routing tables managed by application-level process called route-d (daemon)• advertisements sent in UDP packets, periodically repeatedLecture 12: 2-19-2004 22Outline• Distance Vector• Link StateLecture 12: 2-19-2004 23Link State Protocol Concept• Every node gets complete copy of graph• Every node “floods” network with data about its outgoing links• Every node computes routes to every other node• Using single-source,
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