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IPTV, software replication and other large-scale distribution tasks urge the need for efficient multicast mechanisms in overlay networks. Current multicast solutions on the application layer are either efficient, structured, but inflexible, or flexible, unstructured, but of lesser efficiency. This paper introduces scalable adaptive multicast on bi-directional shared trees, a new structured but flexible approach to content distribution. BIDIRSAM is the first DHT-based overlay multicast that distributes any source multicast data according to source-specific shortest path trees. Built upon bi-directional shared prefix trees, the approach distributes packets uniquely via fully redundant paths, and allows for highly flexible network adaptivity. Guided by an overlay abstraction, it operates directly on top of a prefix routing and does not rely on any kind of rendezvous point or bootstrapping.