Peer-to-peer protocols play an increasingly instrumental role in Internet content distribution. Consequently, it is important to gain a full understanding of how these protocols behave in practice and how their parameters impact overall performance. We present the first experimental investigation of the peer selection strategy of the popular BitTorrent protocol in an instrumented private torrent. By observing the decisions of more than 40 nodes, we validate three BitTorrent properties that, though widely believed to hold, have not been demonstrated experimentally. These include the clustering of similar-bandwidth peers, the effectiveness of BitTorrent’s sharing incentives, and the peers ’ high average upload utilization. In addition, our results show that BitTorrent’s new choking algorithm in seed state provides uniform service to all peers, and that an underprovisioned initial seed leads to the absence of peer clustering and less effective sharing incentives. Based on our observations, we provide guidelines for seed provisioning by content providers, and discuss a tracker protocol extension that addresses an identified limitation of the protocol. 1

Peer-to-peer (P2P) overlay networks such as BitTorrent and Avalanche are increasingly used for disseminating potentially large files from a server to many end users via the Internet. The key idea is to divide the file into many equally-sized parts and then let users download each part (or, for network coding based systems such as Avalanche, linear combinations of the parts) either from the server or from another user who has already downloaded it. However, their performance evaluation has typically been limited to comparing one system relative to another and typically been realized by means of simulation and measurements. In contrast, we provide an analytic performance analysis that is based on a new uplink-sharing version of the well-known broadcasting problem. Assuming equal upload capacities, we show that the minimal time to disseminate the file is the same as for the simultaneous send/receive version of the broadcasting problem. For general upload capacities, we provide a mixed integer linear program (MILP) solution and a complementary fluid limit solution. We thus provide a lower bound which can be used as a performance benchmark for any P2P file dissemination system. We also investigate the performance of a decentralized strategy, providing evidence that the performance of necessarily decentralized P2P file dissemination systems should be close to this bound and therefore that it is useful in practice. 1

Live peer-to-peer (P2P) streaming applications have been successfully deployed in the Internet. With relatively simple peer selection protocol design, modern live P2P streaming applications are able to provide millions of concurrent users adequately satisfying viewing experiences. That said, few existing research has provided sufficient insights on the time-varying internal characteristics of P2P topologies in live streaming. With 120 GB worth of traces in late 2006 from a commercial P2P live streaming system of UUSee Inc. in Beijing, this paper represents the first attempt in the research community to explore topological properties in practical P2P streaming, and how they behave over time. Starting from classical graph metrics, such as degree, clustering coefficient, and reciprocity, we explore and extend them in specific perspectives of streaming applications. We also compare our findings with existing insights from topological studies of P2P file sharing applications, which shed new and unique insights specific to streaming. Our characterization reveals the scalability of the commercial P2P streaming application even in case of large flash crowds, the clustering phenomenon of peers in each ISP, as well as the reciprocal behavior among peers, all of which play important roles in achieving its current success. 1

This paper develops analytic models that characterize the behavior of on-demand stored media content delivery using BitTorrent-like protocols. The models capture the effects of different piece selection policies, including Rarest-First and two variants of In-Order. Our models provide insight into transient and steady-state system behavior, and help explain the sluggishness of the system with strict In-Order streaming. We use the models to compare different retrieval policies across a wide range of system parameters, including peer arrival rate, upload/download bandwidth, and seed residence time. We also provide quantitative results on the startup delays and retrieval times for streaming media delivery. Our results provide insights into the optimal design of peer-to-peer networks for on-demand media streaming.

Abstract—BitTorrent communities, both public and private, are immensely popular in the Internet, with tens of millions of users simultaneously active at any given moment. Public and private BitTorrent communities are managed in different ways – for instance, some private communities enforce sharing ratios, have strict rules for content management, have a certain level of community oversight, and maintain a strong sense of exclusiveness. In this paper, we present the results of extensive measurements of more than half a million peers in five communities, ranging from highly popular and well-known public communities to elite private communities that can only be joined by invitation. We observe that the performance experienced by downloaders in the private communities is by far superior to the performance in the public communities, and we observe significant differences in connectability, seeder/leecher ratio, and seeding duration. Based on our results, we conjecture that when effective ratio enforcement mechanisms are in place, BitTorrent’s tit-for-tat mechanism is hardly influential anymore. Our multi-community, multi-swarm measurements are significantly broader and more extensive than any earlier measurement study on BitTorrent. I.

While BitTorrent has been successfully used in peerto-peer content distribution, its performance is limited by the fact that typical internet users have much lower upload bandwidths than download bandwidths. This asymmetry in bandwidth results in the overall average download speed of a BitTorrent-like file download system to be bottle-necked by the much lower upload capacity. This motivates our approach in which we utilize idle users ’ spare upload capacity to significantly improve the download speed beyond what can be achieved in a conventional BitTorrent network. We show that this is possible even if these idle users (or helpers) download only a tiny fraction of the file. In fact, in terms of uplink utilization, these helpers can be almost as effective as seeders for medium to large swarms. In this work, we design such a system that is fully compatible with the existing clients who conform to the BitTorrent protocol, analyze its steady-state performance and present simulation results.

Abstract — In this paper we adopt a simulation approach to study the performance of the BitTorrent protocol in terms of the entropy that qualifies a torrent and the structure of the overlay used to distribute the content. We find that the entropy of a torrent, defined as the diversity that characterizes the distribution of pieces of the content, plays an important role for the system to achieve optimal performance. We then relate the performance of BitTorrent with the characteristics of the distribution overlay built by the peers taking part in the torrent. Our results show that the number of connections a given peer maintains with other peers and the fraction of those connections initiated by the peer itself are key factors to sustain a high entropy, hence an optimal system performance. Those results were obtained for a realistic choice of torrent sizes and system parameters, under the assumption of a flash-crowd peer arrival pattern. I.

Live peer-to-peer (P2P) streaming has recently received much research attention, with successful commercial systems showing its viability in the Internet. Nevertheless, existing analytical studies of P2P streaming systems have failed to mathematically investigate and understand their critical properties, especially with a large scale and under extreme dynamics such as a flash crowd scenario. Even more importantly, there exists no prior analytical work that focuses on an entirely new way of designing streaming protocols, with the help of network coding. In this paper, we seek to show an in-depth analytical understanding of fundamental properties of P2P streaming systems, with a particular spotlight on the benefits of network coding. We show that, if network coding is used according to certain design principles, provably good performance can be guaranteed, with respect to high playback qualities, short initial buffering delays, resilience to peer dynamics, as well as minimal bandwidth costs on dedicated streaming servers. Our results are obtained with mathematical rigor, but without sacrificing realistic assumptions of system scale, peer dynamics, and upload capacities. For further insights, streaming systems using network coding are compared with traditional pull-based streaming in large-scale simulations, with a focus on fundamentals, rather than protocol details. The scale of our simulations throughout this paper exceeds 200, 000 peers at times, which is in sharp contrast with existing empirical studies, typically with a few hundred peers involved.

In this paper, we examine the factors that contribute to the variability in download time of a self-organizing P2P file distribution application such as BitTorrent. We conducted a series of side-by-side live experiments, involving two clients running on the same machine downloading the same file at the same time. We found that the download latency varied significantly, sometimes by a factor of 2. Surprisingly, the main contributing factor isn’t the network bandwidths of the set of neighbors that a client is given. Rather, it has to do with the frequency of turn-overs in “close ” neighbors, i.e. those that are in a stable data-exchange relationship with the client. Analysis of the log data shows that a client obtains over 90 % of the file from a small set of close neighbors, and if a close neighbor leaves the network, it takes the client a long time, over half an hour, to find another one. This suggests that self-organizing P2P file distributions indeed need external help in order to provide QoS guarantees, but such guarantees are achievable with proper enhancements to the P2P network. 1.

Among all peer-to-peer (P2P) systems, BitTorrent seems to be the most prevalent one. This success has drawn a lot of research interest on the system. In particular, there have been many lines of research studying it’s scalability, performance, efficiency and fairness. However, despite the large body of work, there has been no attempt to mathematically model, in a heterogeneous (and hence realistic) environment, what is perhaps the most important performance metric from an end user’s point of view: the average file download delay. In this paper we propose a mathematical model that accurately predicts the average file download delay in a heterogeneous BitTorrent-like system. Our model is quite general, has been derived with minimal assumptions, and requires minimal system information. Then, we propose a flexible token based scheme for BitTorrent-like systems that can be used to tradeoff between overall system performance and fairness to high bandwidth users, by properly setting it’s parameters. We extend our mathematical model to predict the average file download delays in the token based system, and demonstrate how this model can be used to decide on the scheme’s parameters that achieve a target performance/fairness.