Special Issue: Recent Advances in Peer‐to‐Peer Systems and Security

Abstract

Peer-to-peer systems have revolutionized the way we store 1, 2, disseminate 3, 4 and share 5, 6 content. In recent years, researchers have examined numerous aspects of these innovative architectures and proposed numerous protocols and applications. While peer-to-peer research has impacted a number of areas, including theory, networking, databases and distributed systems, recent work has focused more on the issues of practical, deployable systems, measurements, and issues of security and privacy in peer-to-peer applications. This issue brings together recent work on several projects focused on building robust, practical peer-to-peer systems. The work described has also been presented at the 5th international workshop on peer-to-peer systems (IPTPS 2006), an annual forum for researchers and practitioners of large distributed systems, held in Santa Barbara, California, and attended by more than 80 participants. The goal of the workshop was to examine peer-to-peer technologies, applications and systems, and to identify key research issues and challenges that lie ahead. In the context of this workshop, peer-to-peer systems were characterized as being decentralized, self-organizing distributed systems, in which all or most communication is symmetric. This issue includes five papers, two of which 7, 8 present novel approaches to enhance the efficiency of current file-sharing networks, while the other three 9-11 examine security-related issues including reputations, incentives and misbehaving users. In the first paper, Epema et al. 7 propose Tribler, a new content-sharing protocol that leverages social relationships to enhance performance. While Tribler leverages BitTorrent's downloading mechanism, it is primarily a social network, where users obtain persistent, long-term identities, and self-organize according to their preferences and operational history. By leveraging shared interests, Tribler encourages users to behave altruistically, and shows that with willing helpers, downloaders can dramatically reduce their download latencies. The second paper 8 proposes the novel application of a probabilistic counting algorithm to hybrid search. To estimate whether a file is popular, the authors propose that super-peers count its occurrence by applying the Flajolet–Martin algorithm, where each member of a set flips a fair coin up to N times, and counts the number of heads seen before the first tail. The maximum value from all members is an approximate count of the set size. The remaining papers discuss issues closely related to reputations and user behavior. In the first paper, Swamynathan et al. 11 examine one of the challenges of applying reputation systems to peer-to-peer networks, that of short session histories. She proposes a proactive reputation scheme that allows peers to actively probe and test unknown peers with sample requests, all while making the probes indistinguishable from normal transactional requests and remaining anonymous. Second, Lian et al. 9 show using measurements that in the presence of colluding users, global reputation systems such as EigenTrust can produce non-ideal results. They propose instead a multi-trust reputation approach that bridges the gap between personal trust and global trust, and shows how it addresses the issues of collusion behavior found in current peer-to-peer networks. Finally, Liogkas et al. 10 explore the effectiveness of incentive and enforcement mechanisms in the popular BitTorrent network by implementing and evaluating three different exploits. They evaluate their strategies on both private and public BitTorrent networks, and present some surprising results.