Failure Recovery for Structured P2P Networks: Protocol Design and Performance under Churn

Measurement studies indicate a high rate of node dynamics in p2p systems. In this paper, we address the question of how high a rate of node dynamics can be supported by structured p2p networks. We confine our study to the hypercube routing scheme used by several structured p2p systems. To improve system robustness and facilitate failure recovery, we introduce the property of K-consistency, K ≥ 1, which generalizes consistency defined previously. (Consistency guarantees connectivity from any node to any other node.) We design and evaluate a failure recovery protocol based upon local information for K-consistent networks. The failure recovery protocol is then integrated with a join protocol that has been proved to construct K-consistent neighbor tables for concurrent joins. The integrated protocols were evaluated by a set of simulation experiments in which nodes joined a 2000-node network and nodes (both old and new) were randomly selected to fail concurrently over 10,000 seconds of simulated time. In each such “churn ” experiment, we took a “snapshot ” of neighbor tables in the network once every 50 seconds and evaluated connectivity and consistency measures over time as a function of the churn rate, timeout value in failure recovery, and K. We found our protocols to be effective, efficient, and stable for an average node lifetime as low as 8.3 minutes (the median lifetime measured for Napster and Gnutella was 60 minutes [11]). Experiment results also show that the average routing delay of our protocols increases only slightly even when the churn rate is greatly increased. Keywords — Hypercube routing, K-consistency, failure recovery, sustainable churn rate, peer-to-peer networks