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A traffic problem created by short-circuit replies

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• A traffic problem created by the Route Request short-circuit operation
  • Consider the following scenario:

    Notice that:

    Many nodes have a source route to node G

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  • Traffic problem cause by short-circuiting a ROUTE REQUEST message:

    Suppose that node A transmits a ROUTE REQUEST message: Result: Many simultaneous broadcast transmissions due to short-circuiting the ROUTE REQUEST !!! Example:

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  • Fact about broadcast networks:

    Simultaneous transmissions will result in collision We will need to use: Exponetial backoff protocol to resolve which node gets to transmit first Better: We want to avoid simultaneous transmissions if possible !!!

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• Minimize simultaneous ROUTE REPLY transmissions
  • Engineering solution:

    Use the path length of the source route to construct a distributed scheduling mechanism !!

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  • Scheduling the transmission of a short-circuit ROUTE REPLAY message:

    Let h = length (#hops) of the source route in ROUTE REPLY message; Let r = random(0,1); // Random number between 0 and 1 Let H = hop delay (propagation delay of 1 wireless hop) wait 2×H×(h+r-1) sec; transmit the short-circuit ROUTE REPLY message;

    Example:

    In the previous example: The wait time for each node will be:

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  • The effect of the delay is:

    Reduce the number of simultaneous transmissions (of a short-circuit ROUTE REPLY operation) Allow the shortest (best) route to be transmitted first !!!

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• Further improvement: cancalling a short-circuit ROUTE REPLY
  • Monitor and cancelling a short-circuit Route Reply:

    While a node is waiting to send a short-circuit Route Reply message: the node must promiscuously receives (eavesdropping) on all packet transmissions If the node hears a transmission containing: A shorter source route from the originator (of the ROUTE REQUEST message) to the destination of the ROUTE REQUEST message then: The node will cancel the short-circuit ROUTE REPLY

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  • Example:

    Consider the previous example: If node B transmits the ROUTE REPLY first: Then node A will transmits the packet with the shortest source route: The transmission will cancel the ROUTE REPLY at nodes C, D, E and F: