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

Delivering Messages in Disconnected Mobile Ad Hoc Networks 77from node 4 and a NACK from node 2 for the same application message. In sucha situation, the HOLD message always overrides the negative acknowledgement.3.3 Message DeliveryOnce every FIND DST INTERVAL, each node tries to find a route to the destinationsof the messages it has stored in its HOLDMSG queue. If it is able to finda route to a destination then it delivers all the messages stored for the destinationin its HOLDMSG queue. When a destination node receives a message, eitherfrom the original source or from an intermediate node, it sends an acknowledgementto the sending node. For each message stored in the HOLDMSG queue ofa node, ids of the node that sent the HOLD message to the node and the nodesthat were sent an HOLD message by the current node are also kept. When anacknowledgement is received for a message in the HOLDMSG queue the nodeforwards the acknowledgement to all such nodes. These forwarded acknowledgementshelp in removing messages that have been successfully delivered to thedestination from the HOLDMSG queues of other nodes. This potentially preventsa lot of route search messages being broadcasted by all the nodes holdingthe message in the HOLDMSG queue. Some of the nodes to which the acknowledgementsare forwarded may be disconnected from the forwarder node. In thatcase the acknowledgement is either dropped or a selection process similar tothe one used for the data messages, is started depending on how important theacknowledgements are in a particular application. A message is removed froma HOLDMSG queue if an acknowledgement for the message is received or themessage has been in the HOLDMSG queue for longer than MAX HOLD TIME.3.4 Node Selection MetricWhile selecting nodes for replicating a message, a node picks the one that has thelargest number of neighbours that are not its neighbours. The intuition here is toselect a node that is connected to the most nodes outside the range of the nodecurrently involved in selection process and hence can potentially disseminate themessage farthest in the connected graph containing the source.4 Experimental Set UpWe implemented our protocol for the ns2 simulator [14] and have evaluated itsperformance for scenarios involving upto 120 nodes. The poor performance of ns2in simulating large number of nodes has prevented us from experimenting withscenarios involving a larger number of nodes. We use the modified Random WayPoint Model described in the next subsection of this paper for our experiments.The range of each wireless node is set to 100m. The traffic generator consistsof an application running at each node that tries to send a message of 64 bytesonce every five seconds to a random node (other than itself). The underlyingrouting protocol used is AODV. Table 1 shows the values of various parametersof AODV used in the simulations.