Optimizing Peer Relationships in a Super-Peer Network

Super-peer architectures exploit the heterogeneity of nodes in a P2P network by assigning additional responsi- bilities to higher-capacity nodes. In the design of a super- peer network for file sharing, several issues have to be ad- dressed: how client peers are related to super-peers, how super-peers locate files, how the load is balanced among the super-peers, and how the system deals with node failures. In this paper we introduce a self-organizing super-peer net- work architecture (SOSPNET) that solves these issues in a fully decentralized manner. SOSPNET maintains a super- peer network topology that reflects the semantic similarity of peers sharing content interests. Super-peers maintain se- mantic caches of pointers to files which are requested by peers with similar interests. Client peers, on the other hand, dynamically select super-peers offering the best search per- formance. We show how this simple approach can be em- ployed not only to optimize searching, but also to solve gen- erally difficult problems encountered in P2P architectures such as load balancing and fault tolerance. We evaluate SOSPNET using a model of the semantic structure derived from the 8-month traces of two large file-sharing communi- ties. The obtained results indicate that SOSPNET achieves close-to-optimal file search performance, quickly adjusts to changes in the environment (node joins and leaves), sur- vives even catastrophic node failures, and efficiently dis- tributes the system load taking into account peer capacities.