1. Network Layer4-1 Distance Vector: link cost changes Link cost changes: r node detects local link cost change r updates distance table (line 15) r if cost change in least cost path, notify neighbors (lines 23,24) r Notation change: D Y =D Y X Z 1 4 50 Y 1 algorithm terminates “ good news travels fast ”

2. Network Layer4-2 Distance Vector: link cost changes Link cost changes: r good news travels fast  bad news travels slow - “ count to infinity ” problem! X Z 1 4 50 Y 60 algorithm continues on!

3. Network Layer4-3 Counting to Infinity ABC 0 12 ABC 0 34 ABC 0 56 Ping-Pong to Eternity All links cost 1 11

4. Network Layer4-4 Distance Vector: poisoned reverse If 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) r will this completely solve count to infinity problem? X Z 1 4 50 Y 60 algorithm terminates

5. Network Layer4-5 Distance Vector: (Simple) Synchronized Version r Suppose in every tick each node sends its distance vector, i.e., its distance estimates to all nodes. r Assume that initial distances are link costs to neighbors, or infinity. r Just before the n th iteration, node i has as an estimate of the shortest path to node j that has <= n hops! r D n+1 (i,j) = min kεN(i) {c(i,k) + D n (k, j)} 1 3 4 6 2 5 1 32 3 5 1 3 6 2 5 1 42 4 1 6 2 1 4 1 3 4 6 2 5 1 4 1 1 4 2 3 1 1 3 4 6 2 5 1 32 3 6