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Many Internet multicast applications such as teleconferencing and remote diagnosis have quality-of-service (QoS) requirements. It is a challenging task to build QoS constrained multicast trees with high performance, high success ratio, low overhead, and low system requirements. This paper presents a new scalable QoS multicast routing protocol (SoMR) that has very small communication overhead and requires no state outside the multicast tree. SoMR achieves the favorable tradeoff between routing performance and overhead by carefully selecting the network sub-graph in which it conducts the search for a path that can support the QoS requirement, and by auto-tuning the selection according to the current network conditions. Its early-warning mechanism helps to detect and route around the real bottlenecks in the network, which increases the chance of finding feasible paths for additive QoS requirements. SoMR minimizes the system requirements; it relies only on the local state stored at each router. The routing operations are completely decentralized.