In this paper, we develop e cient admission control tests for member join leave and its associated state refresh and update procedures for receiver-initiated core-based multicast routing, e.g., Core Based Tree CBT protocol, to allow construction of QoS-capable multicast trees, while making the minimum possible impact to the existing infrastructure. Speci cally, we i derive su cient conditions for a multicast tree to maintain its QoS; ii devise e ective admission tests to verify whether or not a group member may join the multicast tree at adequate QoS, while not violating existing QoS guarantees to other on-tree members; and iii identify the minimum set of state information required for the admission tests and develop a soft state refresh and update procedure. Finally, we validate the e ectiveness of the proposed mechanism by incorporating it into the CBT protocol, and evaluate it via event-driven simulations in terms of the probability of join requests being rejected, message overhead...