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

Delivering Messages in Disconnected Mobile Ad Hoc Networks 73sites scattered in an area. The workers have mobile nodes to communicate amongthemselves. The sites may be separated by a distance that is several times theradio range of the devices. In such a case some of the sites might be disconnectedfrom each other, forming multiple partitioned mobile ad-hoc networks inthe area. While sporadic node movements between the sites may offer connectivity,it may be for brief periods of time. In such a situation it would be helpfulto have some mechanism of delivering messages between disconnected hosts.Consider a similar scenario on a long beach having several scenic-spots separatedfrom each other by a distance several times the radio range. While thereare nodes moving between the them, the scenic-spots may be disconnected forthe majority of the time. It is easy to see that the nodes moving between thesedisconnected networks could be used as carriers of messages for other nodes.Now the question is how to select the right carrier node. One option is toselect all the nodes in the connected graph containing the source as carrier nodes.This could create unnecessary replication of messages and wastage of networkbandwidth. So the goal is to find the right carrier node — the one that will comein contact with the destination within a certain period of time in the future. It isimpossible for a source to choose the right carrier node without the knowledge ofthe present and future trajectories of all nodes. So a more refined goal could be toselect those nodes as carrier nodes that have a higher probability of connectingto the destination in the future. Even this is difficult to ascertain without theknowledge of position and direction of movement of the disconnected destinationand potential carrier nodes. A further refined goal could be to select carrier nodesin every direction (as the position and direction of movement of the disconnecteddestination is not known) and to minimize redundancy in doing so. In this paperwe propose a completely decentralized protocol, Voilà, that replicates a messagedestined for a host disconnected from the source on selective nodes.2 Routing ProtocolOur protocol will work with any proactive or on-demand routing protocol includingDSDV, AODV and DSR. Besides routing messages, the only requirement thatour algorithm places on the routing protocol is that it is capable of maintaininga neighbour list at each node. We assume that the routing protocol reports withan upcall to our protocol whenever a route to a destination cannot be found oris broken.3 AlgorithmThe algorithm is described from the points of view of each of the nodes thatparticipate in it. The intuition behind this algorithm is that mobile nodes tendto exhibit correlated movement patterns. This correlated movement of nodes or”group mobility” has been studied in the past and several group based mobilitymodels [9], [11] have been proposed. Based on this we propose that nodes thatare close to each other need not store the same message, only one of them shouldbe chosen to hold a message.