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CMU 15441 Computer Networking - Lecture

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15-441 Computer NetworkingIP ForwardingGraph ModelRoutes from Node AWays to Compute Shortest PathsOutlineDistance-Vector MethodDistance-Vector UpdateAlgorithmStartIteration #1Iteration #2Distance Vector: Link Cost ChangesSlide 14Distance Vector: Split HorizonDistance Vector: Poison ReversePoison Reverse FailuresRouting Information Protocol (RIP)RIP UpdatesRIP Staleness / Oscillation ControlSlide 21Link State Protocol ConceptSending Link States by FloodingDijkstra’s AlgorithmDijkstra’s Algorithm: ConceptDijkstra’s Algorithm: InitiallySlide 27Slide 28Slide 29Slide 30Slide 31Link State CharacteristicsOSPF Routing ProtocolOSPF Reliable FloodingOSPF Flooding OperationFlooding IssuesAdoption of OSPFComparison of LS and DV AlgorithmsSlide 39Slide 40Routing HierarchiesAreasRouting HierarchyArea Hierarchy AddressingPath Sub-optimalityNext Lecture: BGP15-441 Computer NetworkingLecture 10: Intra-Domain RoutingRIP (Routing Information Protocol) & OSPF (Open Shortest Path First)9/28/2006 Lecture 10: Intra-Domain Routing 2IP Forwarding•The Story So Far… •IP addresses are structure to reflect Internet structure•IP packet headers carry these addresses•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•This/next lecture•How to generate the forwarding tableRouter9/28/2006 Lecture 10: Intra-Domain Routing 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 costsAEFCDB236411139/28/2006 Lecture 10: Intra-Domain Routing 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 7AEFCDB23641113Forwarding Table for ADest Cost Next HopA 0 AB 4 BC 6 ED 7 BE 2 EF 5 E9/28/2006 Lecture 10: Intra-Domain Routing 5Ways to Compute Shortest Paths•Centralized•Collect graph structure in one place•Use standard graph algorithm•Disseminate routing tables•Link-state•Every node collects complete graph structure•Each computes shortest paths from it•Each generates own routing table•Distance-vector•No one has copy of graph•Nodes construct their own tables iteratively•Each sends information about its table to neighbors9/28/2006 Lecture 10: Intra-Domain Routing 6Outline•Distance Vector•Link State•Routing Hierarchy9/28/2006 Lecture 10: Intra-Domain Routing 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 ADest Cost Next HopA 0 AB 4 BC–D–E 2 EF 6 F9/28/2006 Lecture 10: Intra-Domain Routing 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)9/28/2006 Lecture 10: Intra-Domain Routing 9Algorithm•Bellman-Ford algorithm•RepeatFor every node xFor every neighbor zFor every destination yd(x,y)  Update(x,y,z)•Until converge9/28/2006 Lecture 10: Intra-Domain Routing 10StartAEFCDB23641113Table for ADst Cst HopA 0 AB 4 BC–D–E 2 EF 6 FTable for BDst Cst HopA 4 AB 0 BC–D 3 DE–F 1 FTable for CDst Cst HopA–B–C 0 CD 1 DE–F 1 FTable for DDst Cst HopA–B 3 BC 1 CD 0 DE–F–Table for EDst Cst HopA 2 AB–C–D–E 0 EF 3 FTable for FDst Cst HopA 6 AB 1 BC 1 CD–E 3 EF 0 FOptimum 1-hop paths9/28/2006 Lecture 10: Intra-Domain Routing 11Iteration #1Table for ADst Cst HopA 0 AB 4 BC 7 FD 7 BE 2 EF 5 ETable for BDst Cst HopA 4 AB 0 BC 2 FD 3 DE 4 FF 1 FTable for CDst Cst HopA 7 FB 2 FC 0 CD 1 DE 4 FF 1 FTable for DDst Cst HopA 7 BB 3 BC 1 CD 0 DE–F 2 CTable for EDst Cst HopA 2 AB 4 FC 4 FD–E 0 EF 3 FTable for FDst Cst HopA 5 BB 1 BC 1 CD 2 CE 3 EF 0 FOptimum 2-hop pathsAEFCDB236411139/28/2006 Lecture 10: Intra-Domain Routing 12Iteration #2Table for ADst Cst HopA 0 AB 4 BC 6 ED 7 BE 2 EF 5 ETable for BDst Cst HopA 4 AB 0 BC 2 FD 3 DE 4 FF 1 FTable for CDst Cst HopA 6 FB 2 FC 0 CD 1 DE 4 FF 1 FTable for DDst Cst HopA 7 BB 3 BC 1 CD 0 DE 5 CF 2 CTable for EDst Cst HopA 2 AB 4 FC 4 FD 5 FE 0 EF 3 FTable for FDst Cst HopA 5 BB 1 BC 1 CD 2 CE 3 EF 0 FOptimum 3-hop pathsAEFCDB236411139/28/2006 Lecture 10: Intra-Domain Routing 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”9/28/2006 Lecture 10: Intra-Domain Routing 14Distance Vector: Link Cost ChangesLink cost changes:•Good news travels fast •Bad news travels slow - “count to infinity” problem!XZ1450Y60algorithmcontinueson!9/28/2006 Lecture 10: Intra-Domain Routing 15Distance Vector: Split HorizonIf Z routes through Y to get to X :•Z does not advertise its route to X back to YalgorithmterminatesXZ1450Y60? ? ?9/28/2006 Lecture 10: Intra-Domain Routing 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)•Eliminates some possible timeouts with split horizon•Will this completely solve count to infinity problem? XZ1450Y60algorithmterminates9/28/2006 Lecture 10: Intra-Domain Routing 17Poison Reverse Failures•Iterations don’t converge•“Count to infinity”•Solution•Make “infinity” smaller•What is upper bound on maximum path length?Table for ADst Cst HopC 7 FTable for BDst Cst HopC 8 ATable for FDst Cst HopC 1 CTable for FDst Cst HopC–Table for ADst Cst HopC–ForcedUpdateTable for BDst Cst HopC 14 AForcedUpdateF C6111BDA4Table for DDst Cst HopC 9 BForcedUpdateTable for ADst Cst HopC 13 DBetterRouteTable for DDst Cst HopC 15 BTable for ADst Cst HopC 19 DForcedUpdate•••ForcedUpdate9/28/2006 Lecture 10: Intra-Domain Routing 18Routing Information Protocol (RIP)•Earliest IP routing protocol (1982 BSD)•Current standard is version 2 (RFC 1723)•Features•Every link has cost 1•“Infinity”


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CMU 15441 Computer Networking - Lecture

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