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Recovery from node failure

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• Node failure
  • Effect of a node failure:

    When a node fails, the token frame sent to the node will be lost: Example:

    Losing the token is detrimental to the token bus network !!!

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• Protecting the token from being lost through node failure
  • Fact:

    Nodes must use the token passing protocol to send a token (frame) to its successor

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  • The token passing protocol of the token bus network:

    /* ============================ 1. Initial attempt ============================ */ Transmit token to successor node; Set timer; if ( transmission heard within time out ) return; // Token passed successfully /* ============================ 2. Backup attempt ============================ */ Transmit token to successor node; Set timer; if ( transmission heard within time out ) return; // Token passed successfully /* ============================================== 3. Give up: try to repair the logical ring ============================================== */ Execute the ring recovery protocol;

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• The ring recovery protocol of the token bus network
  • The ring recovery protocol:

    The current token holder will execute the ring recover protocol when: The token holder cannot pass the token to its successor due to node failure:

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  • The ring recovery protocol:

    The token holder transmits a Who_follows( my successor node ID ) frame Example: node 4 will transmit a Who_follows(1) control frame The node that has: predecessor == nodeID in "Who_follows" frame will send back: Set_successor( myID )          Example: node 0 sends back Set_successor(0) to node 4 The token holder will: Set its Successor node variable       and          Send the Token to its new successor: Example: node 4 updates succ and transmits Token to node 0: When the node receives the token (frame): frame: The node will update its predecessor variable Becomes the token holder (and transmits a frame if it has some) Example: node 0 updates its pred variable and becomes token holder And the ring is fixed: