Reliable Multicasting in MANETs
Multicasting is the transmission of datagramâ„¢s (packets) to a group of zero or more hosts identified by a single destination address. Maintaining group membership information and building an optimal multicast distribution structure (typically in the form of a routing tree) is challenging even in wired networks. However, nodes are increasingly mobile. One particularly challenging environment for multicast is a mobile ad-hoc network (MANET). This report provides an in-depth study of one-to-many and many-to-many communication in mobile ad-hoc networks. First we compare a range of best-effort protocols: 2 uncast routing protocols, 3 multicast routing protocols, and 2 broadcast protocols. Our results show that broadcast protocols, in particular BCAST, performs well and that this performance does not come with a high overhead. We then enhance BCAST with a NACK-based retransmission scheme to further increase the packet delivery ratio, resulting in reliable BCAST. We also explore the impact of the MAC layer on the performance of both best-effort BCAST and reliable BCAST. Varying the user traffic load and the MAC layer, the results provide a number of insights into the relationship between MAC and ROUTING layer. Overall, BCAST is a protocol that achieves high packet delivery, at the cost of an increase in packet latency. We show that the protocol performs well in a wide range of scenarios and over a number of MAC layers (all of which were variants of the 802.11 protocol family). Increasing packet delivery through a retransmission scheme is, however, only of limited value. As MAC rates increase for current and future networks, MANETs will be able to support a non-trivial amount of traffic per multicast sender. Achieving high packet delivery ratios in these networks can be achieved by adjusting the data volume through flow control to operate in the protocol sweet spot, using the best-effort protocol as basic protocol.