IP multicast has been known to have deployment difficulties. Among many contributing factors, the model per se is probably one of the most critical. Overlay multicast does not solve all the problems either. We therefore proposed a new multicast architecture named MUST, inspired by previous efforts. In essence, the architecture uses an overlay network to interconnect statically configured IP multicast trees to create the data paths for improvised multicast groups. While being incrementally deployed, it can evolve towards higher efficiency. In this paper, we described the model in detail, theoretically estimated its cost, and compared the cost with that of overlay multicast.