Such interactive, distributed multimedia applications as shared whiteboards, group editors, and simulations require reliable concurrent multicast services, i.e., the reliable dissemination of information from multiple sou~es to all the members of a group. Furthermore, it makes sense to offer that service on top of the increasingly available 1P multicast service, which offers unreliable multicasting. This paper establishes that concurrent reliable multicasting over the Intemet should be based on reliable multicast protocols based on a shared acknowledgment tree. First, we show that organizing the receivers of a reliable multicast group into an acknowledgment tree and using NAK-avoidance with periodic polling in local groups inside such a tree provides the highest maximum throughput among all classes of reliable mtdticast protocols proposed to date. Second, we introduce Lomx, which demonstrates the viability of implementing a reliable multicasting approach in the Intemet based on acknowledgment trees in a scalable mannez La-ax is the first known protocol that constructs and maintains a single acknowledgment tree for reliable concurrent multicasting, eliminates the need to maintain an acknowledgment tree for each source of a reliable multicast group, and can be used in combination with any of several tree-based reliable mtdticast protocols proposed to date.

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