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Ubicast, an enhanced application-level multicast protocol, is presented in this paper. It is built upon Pastry, a robust and self-organizing peer-to-peer substrate overlay, and leverages its merits of reliable application-level routing and object location. Ubicast is mainly applied to real-time multimedia applications with QoS heterogeneity. It tries to minimize the delay radius of the multicast tree, while meeting different bandwidth requirements. It extends traditional degree-bound model and satisfies nodes' heterogeneous QoS requirements to provide customizable multicast services. Besides, Priority Joining Algorithm based on the concept of potential parents is proposed to limit the number of nodes that a participant needs to probe. The join cost is decreased substantially to guarantee the scalability for a potentially large network topology. Simulation and experimental analysis over large-scale topology confirm the performance improvement of Ubicast which is especially suited for small sized multicast groups.