Lecture 5

Nodes ood topology in the form of link state packets

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Unformatted text preview: 6 5 4 B +4 8 4 6 8 7 7 7 9 E +10 18 13 11 12 10 12 16 14 A’s A’s Cost Next 0 -4 B 6 B 8 B 7 B 7 B 7 B 9 B F 2 4 G 3 3 E 10 2 4 A 4 B H D 1 2 2 3 C 37 11 10/29/13 Distance Vector (5) •  Subsequent exchanges; converged To A B C D E F G H B E says says 4 7 0 3 2 1 4 2 3 0 3 2 3 6 5 4 B +4 8 4 6 8 7 7 7 9 E +10 17 13 11 12 10 12 16 14 A’s A’s Cost Next 0 -4 B 6 B 8 B 8 B 7 B 7 B 9 B F 2 4 G 3 3 E 10 2 4 A 4 B H D 1 2 2 3 C 38 Distance Vector Dynamics •  Adding routes: –  News travels one hop per exchange •  Removing routes –  When a node fails, no more exchanges, other nodes forget •  But par99ons (unreachable nodes in divided network) are a problem –  “Count to inﬁnity” scenario 39 12 10/29/13 Dynamics (2) •  Good news travels quickly, bad news slowly (inferred) X Desired convergence “Count to inﬁnity” scenario 40 Dynamics (3) •  Various heuris9cs to address –  e.g.,“Split horizon, poison reverse” (Don’t send route back to where you learned it from.) •  But none are very eﬀec9ve –  Link state now favored in prac9ce –  Except when very resource- limited 41 13 10/29/13 Introduc9on to Computer Networks Link State Rou9ng (§5.2.5) Computer Science &amp; Engineering Topic •  How to compute shortest paths in a distributed network –  The Link- State (LS) approach Flood! … then compute 52 14 10/29/13 Link- State Rou9ng •  One of two approaches to rou9ng –  Trades more computa9on than distance vector for beler dynamics •  Widely used in prac9ce –  Used in Internet/ARPANET from 1979 –  Modern networks use OSPF and IS- I...
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