COMPASS – Latency Optimal Routing in Heterogeneous Chord-based P2P Systems

During the last decade, overlay networks based on distributed hash tables have become the de facto standard for data management in Peerto-Peer (P2P) systems, with Chord being its most prominent representative. Essentially, with its fully decentralized approach, Chord avoids any bottleneck and single point of failure while guaranteeing data to be retrieved in O(logN) hops in a network consisting of N nodes. By optimizing the number of hops for data access, Chord implicitly assumes that all connections between nodes have comparable bandwidth and latency characteristics. However, in heterogeneous, mobile P2P systems that consist of both mobile and fixed nodes, this is not the case. Moreover, due to the mobility of nodes, connection parameters can dynamically change. Especially in mobile P2P applications where low latency for data access is essential, such as in emergency management, routing should aim at reducing the overall latency, rather than the number of hops in the network. This technical report is based on our paper published at the MDM conference 2013 and extends it by new features and evaluations. In this report, we present COMPASS, a protocol for efficient data access in heterogeneous mobile Chord-based P2P systems. COMPASS takes into account that the network latencies of nodes in a mobile P2P network may significantly differ and thus aims at minimizing the overall latency, even if this necessitates more hops in the network. This is done by probing the network and by maintaining, in addition to Chord’s finger table, at each peer a data structure called COMPASS table. We present in detail the initialization and maintenance of the COMPASS table that dynamically adapts to changing node characteristics. Moreover in this report we first time present the new interval joining feature which reduces the COMPASS table size by joining similar intervals. Real world evaluation as well as simulation results show that COMPASS outperforms standard Chord-based routing and reduces the overall latency in heterogeneous P2P networks consisting of fixed and mobile nodes.