Hierarchical Structured Peer-to-Peer Networks

Structured peer-to-peer networks are scalable overlay network infrastructures that support Internet-scale network applications. A globally consistent peer-to-peer protocol maintains the structural properties of the network with peers dynamically joining, leaving and failing in the network. In this chapter, the authors discuss hierarchical distributed hash tables (DHT) as an approach to reduce the overhead of maintaining the overlay network. In a two-level hierarchical DHT, the top-level overlay consists of groups of nodes where each group is distinguished by a unique group identifi er. In each group, one or more nodes are designated as supernodes and act as gateways to nodes at the second level. Collisions of groups occur when concurrent node joins result in the creation of multiple groups with the same group identifi er. This has the adverse effects of increasing the lookup path length due to a larger top-level overlay, and the overhead of overlay network maintenance. We discuss two main approaches to address the group collision problem: collision detection-and-resolution, and collision avoidance. As an example, they describe an implementation of hierarchical DHT by extending Chord as the underlying overlay graph.