Resilient multicast support for continuous-media applications

The IP multicast delivery mechanism provides a popular basis for delivery of continuous media to many participants in a conferencing application. However, the best-effort nature of multicast delivery results in poor playback quality in the presence of network congestion and packet loss. Contrary to widespread belief that the real-time nature of continuous media applications precludes the possibility of recovery of lost packets using retransmissions, we have found that these applications offer an interesting tradeoff between the desired playback quality and the desired degree of interactivity. In particular, we propose a new model of multicast delivery called resilient multicast in which each receiver in a multicast group can decide its own tradeoff between reliability and real-time requirements. To be effective, error recovery mechanisms in such a model need to be both fast (due to the real-time constraint) and have a low overhead (due to high volume of continuous media data). We have designed a resilient multicast protocol called STORM (STructure-Oriented Resilient Multicast) in which senders and receivers collaborate to recover from lost packets using two key ideas. First, group participants self-organize themselves into a distribution structure and use the structure to recover lost packets from adjacent nodes. Second, the distribution structure is dynamic and a lightweight algorithm is used to adapt the structure to changing network traffic conditions and group membership. We have implemented STORM in both VAT and a packet level simulator. Experimental results using both the MBONE and a simulation model demonstrate the effectiveness of our approach.