In multirate multicast different users in the same multicast group can receive services at different rates depending on their own requirements and the congestion level of the network. In this paper we present a general framework for addressing the optimal rate control problem in multirate multicast where the objective is the maximization of a social welfare function expressed by the sum of the users' utility functions. We discover properties of optimal solutions of this problem. Based on these properties we describe a market mechanism that achieves an optimal solution and satisfies the informational constraints imposed by the decentralization of information in multicast service provisioning.