Prithula Dhungel

Unraveling the BitTorrent Ecosystem

BitTorrent is the most successful open Internet application for content distribution. Despite its importance, both in terms of its footprint in the Internet and the influence it has on emerging P2P applications, the BitTorrent Ecosystem is only partially understood. We seek to provide a nearly complete picture of the entire public BitTorrent Ecosystem. To this end, we crawl five of the most popular torrent-discovery sites over a ine-month period, identifying all of 4.6 million and 38,996 trackers that the sites reference. We also develop a high-performance tracker crawler, and over a narrow window of 12 hours, crawl essentially all of the public Ecosystems trackers, obtaining peer lists for all referenced torrents. Complementing the torrent-discovery site and tracker crawling, we further crawl Azureus and Mainline DHTs for a random sample of torrents. Our resulting measurement data are more than an order of magnitude larger (in terms of number of torrents, trackers, or peers) than any earlier study. Using this extensive data set, we study in-depth the Ecosystems torrent-discovery, tracker, peer, user behavior, and content landscapes. For peer statistics, the analysis is based on one typical snapshot obtained over 12 hours. We further analyze the fragility of the Ecosystem upon the removal of its most important tracker service.

The pollution attack in P2P live video streaming: measurement results and defenses

P2P mesh-pull live video streaming applications ---such as Cool-Streaming, PPLive, and PPStream --- have become popular in the recent years. In this paper, we examine the stream pollution attack, for which the attacker mixes polluted chunks into the P2P distribution, degrading the quality of the rendered media at the receivers. Polluted chunks received by an unsuspecting peer not only effect that single peer, but since the peer also forwards chunks to other peers, and those peers in turn forward chunks to more peers, the polluted content can potentially spread through much of the P2P network. The contribution of this paper is twofold. First, by way of experimenting and measuring a popular P2P live video streaming system, we show that the pollution attack can be devastating. Second, we evaluate the applicability of four possible defenses to the pollution attack: blacklisting, traffic encryption, hash verification, and chunk signing. Among these, we conclude that the chunk signing solutions are most suitable.

Understanding Peer Exchange in BitTorrent Systems

Peer Exchange (PEX), in which peers directly exchange with each other lists of active peers in the torrent, has been widely implemented in modern BitTorrent clients for decentralized peer discovery. However, there is little knowledge about the behavior of PEX in operational systems. In this paper, we perform both passive measurements and Planetlab experiments to study the impact and properties of BitTorrent PEX. We first study the impact of PEX on the download efficiency of BitTorrent. We observe that PEX can significantly reduce the download time for some torrents. We then analyze the freshness, redundancy and spread speed of PEX messages. Finally, we also conduct large- scale Planetlab experiments to understand the impact of PEX on the overlay properties of BitTorrent.

Waiting for Anonymity: Understanding Delays in the Tor Overlay

Although Tor is the most widely used overlay for providing anonymity services, its users often experience very high delays. Because much of Tor usage is for Web applications, which are sensitive to latency, it is critical to reduce delays in Tor. To take an important step in this direction, we seek an in-depth understanding of delays in Tor. By taking snapshots of the entire Tor network within a short time window, we are able to study the delay distribution of the entire router population. We also monitor delays introduced by individual Tor routers over extended periods of time. Our results indicate that apart from delays introduced by routers, overlay network latency also plays a significant role in delays in Tor. We have also observed that at any time, there exist huge differences in the delays introduced by different routers. Our results reveal key performance characteristics of Tor system behavior and provide valuable insights for improving the Tor performance.

Understanding and Improving Ratio Incentives in Private Communities

Incentive mechanisms play a critical role in P2P systems. Private BitTorrent sites use a novel incentive paradigm, where the sites record upload and download amounts of users and require each user to maintain its upload-to-download ratio above a specified threshold. This paper explores in-depth incentives in private P2P file-sharing systems. Our contributions are threefold. We first conduct a measurement study on a representative private BitTorrent site, examining how incentives influence user behavior. Our measurement study shows that, as compared with public torrents, a private BitTorrent site provides more incentive for users to contribute and seed. Second, we develop a game theoretic model and analytically show that the ratio mechanism indeed provides effective incentives. But existing ratio incentives in private BitTorrent sites are vulnerable to collusions. Third, to prevent collusion, we propose an upload entropy scheme, and show through analysis and experiment that the entropy scheme successfully limits colluding, while rarely affecting normal users who do not collude.

Measurement and mitigation of BitTorrent leecher attacks

In the recent past, anti-P2P companies have successfully curtailed the distribution of targeted content over a number of P2P file-sharing systems, including Kazaa and eDonkey. More recently, anti-P2P companies have begun to attack BitTorrent. In this paper, we analyze the resilience of BitTorrent leechers to two different kinds of attacks: the connection attack and the piece attack. We present the results of both passive and active measurements. Using passive measurements, we performed a detailed analysis of a recent album that is under attack, and identified the behavior of attackers. For our active measurements, we developed a crawler that contacts all the peers in any given swarm, determines whether the swarm is under attack, and identifies the attack peers in the swarm. We used the crawler to analyze 8 top box-office movies. Based on the measurement results, we discovered that BitTorrent architecture is fundamentally resilient to Internet leecher attacks. Finally, we also propose defense mechanisms to further mitigate the attacks and evaluate their effectiveness using trace-driven simulation.

Xunlei: peer-assisted download acceleration on a massive scale

We take a close look at Xunlei, an enormously popular download acceleration application in China. Xunlei forms a complex ecosystem, with Xunlei peers extensively interacting with independent HTTP and FTP servers, cyberlockers (such as megaupload and hotfile), the BitTorrent and eDonkey file-sharing systems, as well as with other Xunlei peers. After performing a protocol analysis on Xunlei, we develop a comprehensive measurement infrastructure, enabling us to gain new insights into the scale of content, swarm sizes, and several unique characteristics of the system mechanisms in Xunlei.