Managing Inter-domain Traffic in the Presence of BitTorrent File-Sharing * Srinivasan Seetharaman, Mostafa Ammar College of Computing, Georgia Institute of Technology, Atlanta, Georgia 30332 {srini,ammar}@cc.gatech.edu ABSTRACT Overlay routing operating in a selfish manner is known to cause undesired instability when it interacts with native layer routing. We observe similar selfish behavior with the BitTorrent protocol, where its performance-awareness causes it to constantly alter the routing decisions (peer and piece selection). This causes fluctuations in the load experienced by the underlying native network. By using real BitTor- rent traces and a comprehensive simulation with different network characteristics, we show that BitTorrent systems easily disrupt the load balance across inter-domain links. Further, we find that existing native layer traffic manage- ment schemes suffer from several downsides and are not con- ducive to deployment. To resolve this dilemma, we propose two BitTorrent strategies that are effective in resolving the cross-layer conflict. Categories and Subject Descriptors: C.2.2 – Computer Systems Organization [Computer-Communication Net- works]: Network Protocols; C.4 – Computer Systems Or- ganization [Performance of Systems] General Terms: Performance, Design, Experimentation Keywords: Cross-layer; Conflict; BitTorrent; Contention; Traffic management; Traffic engineering 1. INTRODUCTION BitTorrent[1] is a peer-to-peer (P2P) protocol that has achieved remarkable popularity as a well-performing file- sharing application. The main property that fueled this attractive performance is the improved tit-for-tat incentive mechanism that reduces free-riding and increases user coop- eration. However, this “performance-awareness”, which al- ters the peer selection based on measurements of the achieved download or upload rate, can have serious impact on the dy- namics of the underlying native layer. This is similar in spirit to the impact observed with ser- vice overlays in the past[2], since the process of choosing the appropriate peer to download data from is similar to a routing decision problem. For instance, a host A1 can choose to route the content through an intermediate hop A3 or A4 based on the perceived performance, with each deci- sion having different effect on the underlying native layer. Furthermore, there can be multiple “routes” using the same access link, thereby causing 1) contention for resources with each other and with other legacy traffic, 2) unfairness in load * This work was supported in part by NSF grant CNS-0721559. Copyright is held by the author/owner(s). SIGMETRICS’08, June 2–6, 2008, Annapolis, Maryland, USA. ACM 978-1-60558-005-0/08/06. distribution. This, in the presence of load-balancing efforts, can cause instability and cross-layer conflict. Our first goal is to investigate the cross-layer interac- tion between BitTorrent file-sharing and native layer traffic management operations, since BitTorrent traffic represents a large category of traffic transiting today’s Internet, with tremendous potential for future growth. Although cross- layer interaction in P2P systems has been investigated from the perspective of Internet Service Provider cost [3, 4, 5], our work is the first to investigate the impact of BitTor- rent applications on native layer traffic management. Our second goal is to propose layer-aware BitTorrent strategies (that use awareness of the dynamics at each layer) to miti- gate the detrimental effects caused by the cross-layer inter- action without compromising on BitTorrent performance. 2. MULTI-LAYER INTERACTION BitTorrent[1] is a peer-assisted file-sharing protocol that employs a tit-for-tat (TFT) incentive mechanism to reduce free-riding and increase user cooperation. It operates solely on the results of its own network monitoring process, with- out any concern for the dynamics in the native layer, to obtain as much bandwidth as possible for its own content transfer. In contrast, the objective of the native network operators is to provide as much headroom as possible on each access link at an acceptable cost, so that there are ample resources for a broader range of higher layer users. This mismatch in objective between the two layers leads to contention for limited physical resources and to cross-layer conflict. Thus, independent operation of several torrents at the higher layer can lead to disproportionate usage of differ- ent inter-domain links, thereby disrupting the load balance and resulting in high maximum utilization on some links. Furthermore, it causes undesired changes in network char- acteristics and route oscillations for the background traffic. The main reasons for the cross-layer conflict are: a) Tit- for-tat mechanism, which induces the performance-awareness and leads to overloading of certain access links, b) Geograph- ical diversity of peers and their flashcrowd arrival pattern[6], c) Seed uploading decisions, which are selfish as well, d) Un- fairness in contributions from individual peer[7, 8]. 3. IMPACT ON THE NATIVE LAYER The BitTorrent protocol is less predictable and highly complex to model accurately. Hence, we adopt a simulation- based approach, where we model the network layer spanning many ASes, and BitTorrent layer with varying torrent birth, torrent size, peer arrival and peer departure. This methodol- ogy provides insights into the fluctuation of BitTorrent traf- fic across inter-domain access links and the effect of multiple coexisting BitTorrent networks on the native layer.