Support for reliable sessions with a large number of members

The ability to localize traffic when performing distributed searches within a group of nodes that form a session is a key factor in determining how big the group can scale. In this paper we describe an algorithm using the concept of scoping that we believe significantly enhances the ability to localize traffic for the service discovery aspect of many protocols, and hence their ability to scale. The algorithm is based upon the notion of a hierarchy of administrative multicast scopes where smaller scopes nest inside larger ones. To exploit this topological structure, we describe an application-layer protocol, the Scoped Address Discovery Protocol (SADP), which provides session members with the ability to discover, for each session, which addresses to use within each of the various scopes within a hierarchy. We show via simulation that SADP affords this ability in a manner that scales, through merging the well known distribution mechanisms of announce/listen and query/response and exploiting the nested hierarchy of scopes itself.