Understanding the Properties of the BitTorrent Overlay

Anwar Al Hamra, Arnaud Legout, and Chadi Barakat

Recently, Peer-to-Peer (P2P) networks have emerged as an attractive architecture for content sharing over the Internet. By leveraging the available resources at the peers, P2P networks have the potential to scale to a large number of peers. Nowadays, P2P networks support a variety of applications, for instance, file sharing (e.g., BitTorrent, Emule), audio conferencing (e.g., Skype), or video conferencing (e.g., End System Multicast). Among all existing P2P applications, file sharing is still the most popular one. A study in 2004 by the Digital Music Weblog magazine states that P2P file sharing is responsible for 70%-80% of the overall European Internet traffic. And among the many P2P file sharing protocols, BitTorrent is the most popular one. Alone, BitTorrent generates more than half of the P2P traffic.

In this work, we conduct extensive simulations to understand the properties of the overlay generated by BitTorrent.

First, we analyze how the overlay properties impact the efficiency of BitTorrent. We focus on the average peer set size (i.e., average number of neighbors), the time for a peer to reach its maximum peer set size, and the diameter of the overlay. In particular, we show that the later a peer arrives in a torrent, the longer it takes to reach its maximum peer set size.

Second, we evaluate the impact of the maximum peer set size, the maximum number of outgoing connections per peer, and the number of NATed peers on the overlay properties. We show that BitTorrent generates a robust overlay, but that this overlay is not a random graph. In particular, the connectivity of a peer to its neighbors depends on its arriving order in the torrent. We also show that a large number of NATed peers significantly compromise the robustness of the overlay to attacks.

Finally, we evaluate the impact of peer exchange on the overlay properties, and we show that it generates a chain-like overlay with a large diameter, which will adversely impact the efficiency of large torrents.

Important Links:

The Technical Report inria-00162088

The simulator

Feel free to contact us for any further information.

Last update July 15^th, 2007 by Anwar Al Hamra