As the use of IP multicast sessions becomes widespread, the potential benefits derived from currently unavailable topological information on multicast distribution trees may become increasingly critical. In this paper we propose a framework for discovering the topology of shared multicast trees based on a novel fan-out decrement mechanism analogous to time-to-live (TTL) decrementing in IP. We propose an algorithm for topology discovery based on the matrix of path/fan-out distances among a set of E session members