Network Layer 4-1

Distance Vector: link cost changesLink cost changes: node detects local link cost

change updates distance table (line 15) if cost change in least cost path,

notify neighbors (lines 23,24) Notation change: DY=DY

X Z14

50

Y1

algorithmterminates“good

news travelsfast”

Network Layer 4-2

Distance Vector: link cost changes

Link cost changes: good news travels fast bad news travels slow - “count to infinity” problem!

X Z14

50

Y60

algorithmcontinues

on!

Network Layer 4-3

Counting to Infinity

A B C

012

A B C

034

A B C

056

Ping-Pong to Eternity

All links cost 11 1

Network Layer 4-4

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) will this completely solve count to

infinity problem?

X Z14

50

Y60

algorithmterminates

Network Layer 4-5

Distance Vector: (Simple) Synchronized Version Suppose in every tick each node sends its distance vector, i.e., its

distance estimates to all nodes. Assume that initial distances are link costs to neighbors, or infinity. Just before the nth iteration, node i has as an estimate of the shortest

path to node j that has <= n hops! Dn+1(i,j) = minkεN(i) {c(i,k) + Dn(k, j)}

13

4

6

2

5

1

3 2

3

513

6

2

5

1

4 2

4

1

6

21

4

13

4

6

2

5

1

4

1 1

4

2

3

1

13

4

6

2

5

1

3 2

36