IP Multicast has proven to be an effective communication primitive for best effort, large-scale, multi-point audio/video conferencing applications. While the best-effort transport of real-time digital audio/video is a relatively straightforward and well understood problem, many other applications like multicast-based shared whiteboards and shared text editors are more challenging to design because their underlying media require reliable transport, i.e., a 'reliable multicast' protocol. The design of scalable end-to-end reliable multicast protocols has unfortunately proven to be an especially hard problem, exacerbated by the enormous degree of network and system heterogeneity present in the Internet. In this paper, we propose to tackle the heterogeneity problem with a hybrid model for reliable multicast that relies in part on end-to-end loss recovery mechanisms and in part on intelligent and application-aware adaptation carried out within the network. In our framework, application-aware agents --- or proxies --- use detailed knowledge of application semantics to hide the effects of heterogeneity from the rest of the system. We present a general architecture for proxy-based reliable multicast called the Reliable Multicast proXy model} and describe a prototype implementation of an RMX for a shared whiteboard application for hand-held PDAs.

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