Obtaining feedback information regarding the state of receivers in a multicast session is a fundamental problem that often arises in collaborative multimedia systems.In this paper, we present a definition of the general form of the state feedback problem.Then, we present a feedback protocol for addressing some of the special cases that commonly arise.The presented feedback protocol is suitable for application in best-effort unreliable networks such as the Internet. It allows for determining the worst case state among a group of receivers, where each receiver may be in one of a set of finite states, and is applicable in receiver-driven as well as in sender-driven adaptive multimedia systems.The efficiency of the proposed protocol in eliminating the reply implosion problem, its robustness in facing network losses, as well as its responsiveness are illustrated bysimulation experiments.