---

Trick: Automatic Route Shortening

---

• Shortening a route
  • Source routes can be shortened if:

    one or more intermediate hops in the route not necessary

    Example:

    Node A has an inferior source route: The source route can be shorted as follows:

  ---

  ---

  ---

  ---

• Inferior routes....
  • Consider the following scenario that we have seen before:

    Many neighbor node of A has cached routes to G: When node A transmits a ROUTE REQUEST to find a route to node G: The nodes will delay the reply: Node B will pick the smallest delay and reply:

    ---

    ---

  • Now, suppose B's reply was lost:

    Then the following can happen:

    Node C picks the second smallest delay and sends a ROUTE REPLY to node A: And node A will have an inferior route:

  ---

  ---

  ---

  ---

• Detecting an opportunity to fix inferior routes
  • Detecting that a route can be shortened:

    Notice that: When node A transmits a message to node D: Node B can overhear the transmission !!! When a node overhear a packet where: The next hop is not itself The unused portion (the prefix) of the source route contain its node ID Example: Then The node can shorten the route !!!

  ---

  ---

  ---

  ---

• The automatic route shortening mechanism

  • The automatic route shortening procedure:

    A node has detected the above scenario: Node is not the next hop Node is in the unused portion of the source route Then: The node transmits a gratuitous (unsolicited) ROUTE REPLY message to the sender of the packet.

    Example:

    Result:

    Node A gets a better route to node G