Page 2 Lecture Notes in Computer Science 2865 Edited by G. Goos ...

76 R. Shah and N.C. Hutchinsonset consists of node {6} and its eliminated set consists of nodes {0, 1, 3, 7}.When node 6 receives a HOLD message it starts the selection process but endsup eliminating all nodes from its neighbour list and thus the process terminatesat this node. Nodes 4 and 9 are the other nodes that hold the message.1620573498162305 4 978(a)(b)Fig. 1. (a) A network partition of 10 nodes (b) Node positions after some mobilityAfter a while if node 0 has another application message for an unreachablehost, the nodes that are selected this time may be different from the ones previouslyselected as some of the nodes have moved to different positions as shown inFigure 1b. When node 0 executes the algorithm it again ends up with the pickedset {2, 4} but this time the eliminated set does not contain node 3. This timethe eliminated set is {1, 5}. When node 2 starts its selection process it requeststhe neighbour list from nodes 3 and 6. Node 3 is also being considered by node4 at the same time. If node 3 receives a HOLD or a NACK message for thesame application message from node 4 before the NBREQ message from node 2arrives at node 3 then node 3 reports its status appropriately to node 2 eitherstating that it already holds the message (status=HELD) or it has been eliminatedfrom consideration (status=NACKED) by some node which has selectedone of its neighbours to hold the message (in the present scenario node 3 wouldreceive a HOLD message from node 4). Ideally we would like a node to receiveeither a NACK or a HOLD control message for a particular application messageand not both. This cannot be achieved because of distributed nature of the algorithmand changing neighbour lists of the nodes during the execution of thealgorithm. Thus it is possible for a node to receive both a NACK and a HOLDmessage for the same application message and in any order. Again referring toFigure 1b, if node 3 receives the NBREQ message from node 2 before it receivesthe HOLD control message from node 4, then it will receive a HOLD message