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

Delivering Messages in Disconnected Mobile Ad Hoc Networks 75its neighbouring nodes. However, since some nodes are known to have alreadyparticipated in the selection process for this message, node R can eliminate fromconsideration the node it received the message from (S), the original source node(X), and those nodes that are in the eliminated set sent by S.A neighbour list request message, NBREQ, consists of the following fields:A node T receiving a NBREQ message from node R replies with a neighbourlist reply message, NBREP. A NBREP control message sent by a node T consistsof the following fields:The status field in the NBREP message reports on the status of this messageat the node that sent the reply, and can be one of three values: NEW meaningthat this node knows nothing of the message, NACKED, meaning that this nodehas previously been sent a NACK for the message, or HELD, meaning that thisnode is holding the message.Nodes responding with NACKED or HELD in their status are eliminatedfrom further consideration by R. After node R receives the neighbour list fromthe nodes, it uses the same algorithm as the source node X to select othernodes to hold message M. The only difference is in the initialization of the setneigh(R). neigh(R) contains only those neighbouring nodes of R that have notbeen eliminated from consideration by R in any of the above steps. Again thenodes in the picked(R) set are sent a HOLD message containing the message Mand the set of R’s neighbours that were eliminated from further consideration.This set contains the final value of eliminated(R) together with the sending node(S) and those neighbours that were eliminated from consideration by R before.The nodes in the eliminated(R) set are sent a NACK for the message.In order to bound the size of the HOLD control message, the eliminated setsent by any node contains only those nodes that are its neighbours. Therefore,the upper bound on the size of the eliminated set is the maximum number ofneighbours a node can have. The alternative of accumulating eliminated nodes asthe HOLD message propagates would eliminate some redundancy, at the expenseof having HOLD messages that could be as large as the number of nodes in anypartition.Figure 1a shows an example network partition of 10 nodes. Suppose node 0wants to send a message to a node outside the network partition. It initiates theprocess of selecting carrier nodes by first requesting the neighbour list from itsneighbouring nodes 1, 2, 3, 4 and 5. After execution of the elimination processits picked set consists of nodes {2, 4} while its eliminated set consists of {1, 3,5} which it sends to nodes in the picked set along with the message in a HOLDcontrol message. When node 2 receives the HOLD message it starts the selectionprocess by eliminating nodes 0, 1 and 3 from consideration and requesting theneighbour list from nodes 6 and 7. After execution of the algorithm its picked