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Multimedia Multicast Transport Service for Groupware

Reliability carries different meanings for different applications. For example, in a replicated database setting, reliability means that messages are never lost, and that messages arrive in the same order at all sites. In order to guarantee this reliability property, it is acceptable to sacrifice real-time message delivery: some messages may be greatly delayed, and at certain periods message transmission may even be blocked. While this is perfectly acceptable behavior for a reliable database application, this behavior is intolerable for a reliable video server. For a continuous MPEG video player [20, 19], reliability means real-time message delivery, at a certain bandwidth; It is acceptable for some messages to be lost, as long as the available bandwidth complies with certain predetermined stochastic assumptions. Introducing database style reliability (i.e. message recovery and order constraints) may violate these assumptions, rendering the MPEG decoding algorithm incorrect. Many CSCW groupware and multimedia applications require quality of service multicast for most of their messages, and may greatly benefit from reliable multicast for a small portion of “critical” messages. Furthermore, such applications often need to be fault-tolerant, and need to support smooth reconfiguration when parties join or leave. Group communication [1] is a powerful tool for the construction of fault-tolerant applications, providing reliable multicast and membership services with strong semantics. In this paper, we incorporate Multimedia Multicast Transport Service that supports multiple quality of service options within the framework of group communications systems. This way, a single application can exploit multiple quality service options, and can also benefit from the group communication semantics.