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The "Counting to Infinity" problem

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• The Counting to infinite problem
  • Fact:

    Network partitioning failures can cause a very nasty problem for the Distance Vector Algorithm

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  • The problem has the interesting name of:

    The "Counting to Infinity" problem.....

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• Network partitioning failure
  • Network partitioning:

    link/node failures can partitions the network into 2 or more segments. When the network is partitioned: Nodes in one segment cannot communicate with nodes in the other segment

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• Example of network partioning and the "counting to infinity" problem
  • The distance/routing tables after the distance vector algorithm has converged:

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  • Suppose link AE fails:

    NOTE:

    The failure of link AE partitions the network into 2 disjoint portions: {A,B,C,D,F,G} {E}

  • This type of failures - as you will see - is detrimental to the distance vector routing algorithm...

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  • Result of the link failure:

    Node A detects the link failure and updates its distance table entry: Node A sends distance vector update message to all its neighbors:

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  • Let's pause and examine the situation:

    Fact: Cost (distance) to node E is infinite (because there is no route to  E !!!!) However, according to A's distance table: There is a route with distance cost 3 (from A) to E via node B !!! Here is the routing entry: This is the "route": Crux of the problem: Node A does not know that the route to E via B will use the failed link AE !!! (That's the nature of a distributed algorithm - a node does not have the information of the whole network)

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  • We continue with the example:

    Node A sends distance vector update message to all its neighbors: The neighbor nodes B, C and F processes the distance vector update message: Node B, C and F will then send another distance vector update message because: The minimum distance to E has changed from 2 to 3 Graphically: Node A will process these distance vector messages: Notice: Distance from A to E changed from 3 to 4 A send another distance vector message !!! Node A sends distance vector update message to all its neighbors:

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  • STOP !!!!

    We are back to this previous situation:

    Conlcussion:

    The distance from node A to node E increases by 1 after exchanging the distance vector messages... Therefore: The nodes will increase the distance to E by 1 again after another round of distance vector messages !!! This is the: Counting to infinity problem....

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• Proposed solutions

  • There are a number of solutions proposed to alleviate the "counting towards infinity" problem:

    Use a small integer value to represent infinity When a distance value = small integer (e.g. 20), the link cost is set to infinite Split Horizon: If the route to a destination X is via A: Do not send the distance vector update message to the node A !!!! Example: Split Horizon with poison reverse: If the route to a destination X is via A: Send the distance vector update message (Me, X, infinite) to the node A !!!! Example: