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Overcast is an application-level multicasting system that can be incrementally deployed using today's Internet infrastructure. These properties stem from Overcast's implementation as an overlay network. An overlay network consists of a collection of nodes placed at strategic locations in an existing network fabric. These nodes implement a network abstraction on top of the network provided by the underlying substrate network.Overcast provides scalable and reliable single-source multicast using a simple protocol for building efficient data distribution trees that adapt to changing network conditions. To support fast joins, Overcast implements a new protocol for efficiently tracking the global status of a changing distribution tree.Results based on simulations confirm that Overcast provides its added functionality while performing competitively with IP Multicast. Simulations indicate that Overcast quickly builds bandwidth-efficient distribution trees that, compared to IP Multicast, provide 70%-100% of the total bandwidth possible, at a cost of somewhat less than twice the network load. In addition, Overcast adapts quickly to changes caused by the addition of new nodes or the failure of existing nodes without causing undue load on the multicast source.