CONCURRENCY AND COMPUTATION: PRACTICE AND EXPERIENCE Concurrency Computat.: Pract. Exper. 2007; 19:1–11 Prepared using cpeauth.cls [Version: 2002/09/19 v2.02] Exploring the Robustness of Unstructured Peer-to-Peer Content Distribution Systems * Nikitas Liogkas, Robert Nelson, Eddie Kohler, and Lixia Zhang † University of California, Los Angeles - Los Angeles, CA, U.S.A. SUMMARY 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. KEY WORDS: BitTorrent exploits free-riding robustness selfish 1. Introduction The popular unstructured peer-to-peer (P2P) BitTorrent protocol, designed for scalable content distribution, strives to provide a form of fairness: clients who do not contribute data to others should not be able to attain high download throughput. However, although BitTorrent emphasizes fair interactions to increase performance and scalability [7], it does not strictly enforce fairness. In this paper, we study the behavior of selfish BitTorrent clients who abuse protocol mechanisms to maximize their download rate while minimizing their own contributions to the system. We identify three exploits that can deliver potential benefits to a selfish peer and may negatively impact the download rates of compliant peers. These exploits were derived after careful consideration of the core BitTorrent mechanisms and attack * A shorter description of this work appeared at IPTPS’06 [13] † E-mails: {nikitas, rlnelson, kohler, lixia}@cs.ucla.edu Received 30 September 2006 Copyright c  2007 John Wiley & Sons, Ltd. Revised 24 January 2007