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Compung paths inialize variables m s set of

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Unformatted text preview: loops, but increases adver)sement sizes Warning •  Split horizon/split horizon with poison reverse only help between two nodes –  Can s)ll get loop with three nodes involved –  Might need to delay adver)sing routes ajer changes, but affects convergence )me Link State Rou)ng •  Strategy –  send to all nodes informa)on about directly connected neighbors •  Link State Packet (LSP) –  ID of the node that created the LSP –  Cost of link to each directly connected neighbor –  Sequence number (SEQNO) –  TTL Reliable Flooding •  Store most recent LSP from each node –  Ignore earlier versions of the same LSP •  Forward LSP to all nodes but the one that sent it •  Generate new LSP periodically –  Increment SEQNO •  Start at SEQNO=0 when reboot –  If you hear your own packet with SEQNO=n, set your next SEQNO to n+1 •  Decrement TTL of each stored LSP –  Discard when TTL=0 Calcula)ng best path •  Djikstra’s single- source shortest path algorithm –  Each node computes shortest paths from itself •  Let: –  N denote set of nodes in the graph –  l(i,j) denote the non- nega)ve link between i,j •  ∞ if there is no direct link between i and j –  C(n) denote the cost of path from s to n –  s denotes yourself (node compu)ng paths) •  Ini)alize variables –  M = {s} (set of nodes incorporated thus far) –  For each n in N- {s}, C(n) = l(s,n) –  R(n) = n if l(s,n) < ∞, – otherwise Djikstra’s Algorithm •  While N≠M –  Let w ∈(N- M) be the node with lowest C(w) –  M = M ∪ {w} –  Foreach n ∈ (N- M), if C(w) + l(w,n) < C(n) –  then C(n) = C(w) + l(w,n), R(n) = R(w) •  Example: D: (D,0,- ) (C,2,C) (B,5,C) (A,10,C) B 5 A 3 10 C 11 D 2 Distance Vector vs. Link State •  # of messages (per node) –  DV: O(d), where d is degree of node –  LS: O(nd) for n nodes in system •  Computa)on –  DV: convergence )me varies (e.g., count- to- infinity) –  LS: O(n2) with O(nd) messages •  Robustness: what happens with malfunc)oning router? –  DV: Nodes can adver)se incorrect path cost –  DV: Others can use the cost, propagates through network –  LS: Nodes can adver)se incorrect link cost Examples •  RIPv2 –  Fairly simple implementa)on of DV –  RFC 2453 (38 pages) •  OSPF (Open Shortest Path First) –  More complex link- state protocol –  Add...
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