An ad hoc network is composed of mobile nodes without any infrastructure. Mobile nodes self organize themselves to form a network over radio links. The trend of applications of mobile ad hoc networks requires increased group oriented services. Hence multicast support is critical for ad hoc networks. As the number of participating nodes increase, scalability becomes an important issue. The on demand multicast routing protocol (ODMRP) [1] provides high packet delivery ratio in presence of high mobility. But ODMRP suffers from higher control overhead as the network size and the number of source nodes increase. In this paper we present an efficient hybrid multicast routing protocol suitable for high mobility applications which addresses the scalability issue of the ODMRP protocol. This protocol separates out the data forwarding path from the join-query forwarding path. We incorporate a low overhead local clustering technique to classify all nodes into core and normal categories. When multicast routes to destination nodes are unavailable, join-query messages are sent to all nodes in the network and data packets are forwarded by the core nodes to the destination nodes using differential destination multicast [2]. Through simulations we show that this protocol reduces control overhead and increases packet delivery ratio by 20-50% for different network scenarios.
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