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-e ort 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 o er an interesting tradeo 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 tradeo between reliability and real-time requirements. To be e ective, 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 tra c 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.