Many real-time applications, such as video conferencing, require the transmission of flows from a sender to multiple receivers subject to Quality of Service (QoS) delivery constraints (e.g. bounded delay). Traditional multicast routing algorithms used for solving the Steiner tree problem cannot be used in this scenario, because QoS constraints on links are not considered. Moreover, with QoS metrics consideration, the problem of computing optimal constrained multicast tree is known as NP-complete. In this paper, we propose a new QoS-aware Multicast Overlay Spanning Tree (QMOST) protocol to support multiple-metric routing criteria in mobile ad hoc networks. It computes low cost trees with multiple-metric bounded paths in polynomial time using an efficient source-based optimization heuristic. Unlike common multicast protocols where data packets are transmitted in a broadcast mode, in QMOST data packets are encapsulated in unicast packets before being forwarded to the overlay tunnels. Multicasting is achieved when each node forwards multicast packets to its successors in the overlay tree. QMOST must be used in conjunction with a unicast QoS link state protocol (QOLSR) and can be integrated in a QoS model. This later offers the needed service guarantee using distributed admission control to check if the multicast QoS flows are admissible, resource reservation mechanism and congestion control. Simulation results show that our QMOST protocol achieves better performance than the QOLSR protocol.
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