Abstract: We present in this paper a simplified model of RMTP (Reliable Multicast Transport Protocol) that we have integrated in the network simulator NS. The integration of this model results in five agents that can be used with any RMTP-based multicast simulation using NS. Based on these agents we are going to analyze the performance of RMTP in heterogenous environments. For this purpose nine RMTP-based multicast scenarios with different network conditions like loss rate, delay and bandwidth will be simulated. We then present a new QoS-based mechanism for multicast group management that we call CGM (Contractual Group Membership). This mechanism is mainly based on the contract notion. A "contract" is a set of rules established between a sender and a group of receivers, which determines worst case bounds on the physical conditions that the receivers must fulfill during the whole period of a multicast session. Based on a formal definition of the contract and on the use of the "Minimum Spanning Tree-Clustering Method" a set of N receivers will be split into homogeneous subgroups. We finally propose a hybrid CGM/ Geographical subgrouping method to be used by RMTP in order to increase the performance of this protocol in heterogeneous environments.