Applications based on multicasting such as real-time simulations or shared editors require that all data packets are delivered safely in a reasonably short time. Trying to assure such a high quality of service centrally, can easily overload both the network and the source. One technique to prevent this is clustering (organizing multicast group members into subgroups). We present an algorithm to make clustering as natural as possible by building clusters from groups of nodes that are close together in dense parts of the network. The cluster building algorithm uses only local knowledge and executes in parallel for all nodes. We have simulated our algorithm and find that it builds reasonable clusters for the topologies tested. Finally, we propose an extension of RMP, a token-ring-based, reliable multicast protocol, using our algorithm to build a ring of tree-organized clusters. This combination makes the resulting protocol scalable, which the original RMP was not.
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