Swarming peer-to-peer systems play an increasingly instrumental role in Internet content distribution. It is therefore important to better understand how these systems behave in practice. Recent research efforts have looked at various protocol parameters and have measured how they affect system performance and robustness. However, the importance of the strategy based on which peers establish connections has been largely overlooked. This work utilizes extensive simulations to examine the default overlay construction strategy in BitTorrent systems. Based on the results, we identify a critical parameter, the maximum allowable number of outgoing connections at each peer, and evaluate its impact on the robustness of the generated overlay. We find that there is no single optimal value for this parameter using the default strategy. We then propose an alternative strategy that allows certain new peer connection requests to replace existing connections. Further experiments with the new strategy demonstrate that it outperforms the default one for all considered metrics by creating an overlay more robust to churn. Additionally, our proposed strategy exhibits optimal behavior for a well-defined value of the maximum number of outgoing connections, thereby removing the need to set this parameter in an ad-hoc manner.

Although the popular BitTorrent protocol strives to limit free-riding via its tit-for-tat incentives, recent research efforts have shown that it does not strictly enforce fairness. Free-riding opportunities indeed exist, mainly via optimistic unchokes, a BitTorrent mechanism that facilitates the continuous discovery of better peers to interact with. Our results in this work also show that increasing numbers of free-riders can considerably hurt the performance of compliant peers. In an effort to address this problem, this paper proposes a new BitTorrent-like protocol that dynamically organizes peers of similar upload bandwidth in teams-groups of peers collaborating for mutual benefit. Team members mostly satisfy their data download needs inside their team and only perform optimistic unchokes when absolutely necessary. As a result, this team-based protocol improves peer performance via explicit cooperation. At the same time, it limits bandwidth spent on optimistic unchokes, thereby rendering the system more robust against free-riders. We have modified an existing BitTorrent implementation to implement the team protocol and have evaluated its impact by running real experiments on a controlled PlanetLab testbed. Our results show that the protocol enables slightly improved performance for compliant peers, while free-riders are unable to sustain high download rates, as compared to regular BitTorrent. In addition, we observe a higher degree of robustness: increasing numbers of free-riders in the system have a significantly lower negative impact on the performance of contributing peers.

Abstract This paper assesses BitTorrent's robustness against selfish peers who try to download content faster than their fair share by abusing existing protocol mechanisms. We present three exploits that can deliver potential benefits to a selfish peer and evaluate their impact on both public and private download sessions. Our results show that BitTorrent is quite robust against these exploits. Although selfish peers can sometimes attain high download throughput and compliant peers' download rates suffer slightly in consequence, we observe no significant degradation of the overall system's quality of service. We identify scenarios where a selfish peer could attain significant benefits at the expense of compliant peers, and discuss the protocol characteristics that render these scenarios unlikely and hence lead to the system's robustness. Copyright © 2007 John Wiley & Sons, Ltd.

Peer-to-peer protocols play an increasingly instrumental role in Internet content distribution. It is therefore important to gain a complete understanding of how these protocols behave in practice and how their operating parameters affect overall system performance. This paper presents the first detailed experimental investigation of the peer selection strategy in the popular BitTorrent protocol. By observing more than 40 nodes in instrumented private torrents, we validate three protocol properties that, though believed to hold, have not been previously demonstrated experimentally: the clustering of similar-bandwidth peers, the effectiveness of BitTorrent's sharing incentives, and the peers' high uplink utilization. In addition, we observe that BitTorrent's modified choking algorithmin seed state provides uniform service to all peers, and that an underprovisioned initial seed leads to absence of peer clustering and less effective sharing incentives. Based on our results, we provide guidelines for seed provisioning by content providers, and discuss a tracker protocol extension that addresses an identified limitation of the protocol.