Hierarchical architectures in structured peer-to-peer overlay networks

Distributed Hash Tables (DHT) are presently used in several large-scale systems in the Internet and envisaged as a key mechanism to provide identifier-locator separation for mobile hosts in Future Internet. Such P2P-based systems become increasingly complex serving popular social networking, resource sharing applications, and Internet-scale infrastructures. Hierarchy is a standard mechanism for coping with heterogeneity and scalability in distributed systems. To address the shortcomings of flat DHT designs, many hierarchical P2P designs have been proposed over recent years. The last generation is hierarchical DHTs (HDHTs) where nodes are organized onto layers and groups. This article discusses hierarchical architectures applied in structured P2P overlay networks, focusing on HDHT designs. We introduce a framework consisting of conceptual models of network hierarchy, multi-layer hierarchical DHT architectures, principles affecting the design choices, and cost models for system tradeoff analysis, performance evaluation, and scalability estimation. Based on the framework we provide a taxonomy and survey more than 20 hierarchical HDHT proposals.