Hierarchical Peer-to-Peer Systems

Structured peer-to-peer (P2P) lookup services—such as Chord, CAN, Pastry and Tapestry—organize peers into a flat overlay network and offer distributed hash table (DHT) functionality. In these systems, data is associated with keys and each peer is responsible for a subset of the keys. We study hierarchical DHTs, in which peers are organized into groups, and each group has its autonomous intra-group overlay network and lookup service. The groups themselves are organized in a top-level overlay network. To find a peer that is responsible for a key, the top-level overlay first determines the group responsible for the key; the responsible group then uses its intra-group overlay to determine the specific peer that is responsible for the key. After providing a general framework for hierarchical P2P lookup, we consider the specific case of a two-tier hierarchy that uses Chord for the top level. Our analysis shows that by designating the most reliable peers in the groups as superpeers, the hierarchical design can significantly reduce the expected number of hops in Chord. We also propose a scalable design for managing the groups and the superpeers.