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
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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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 Spring '08
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 IP address, Subnetwork, informa

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