GPS: a general peer-to-peer simulator and its use for modeling BitTorrent

Peer-to-Peer (P2P) systems have become popular over the past few years. However, their large scale and the open nature of the system makes studying them challenging. This paper presents an extensible framework for simulating P2P networks efficiently and accurately. Efficiency is accomplished by using message level simulation rather than packet level simulation. Moreover, accuracy is maintained by tracking the network infrastructure and using a flow model to accomplish accurate estimate of the message behavior. A second contribution of the paper is to model the BitTorrent (BT) protocol. BT is a widely-used protocol that is significantly more complex than other P2P protocols because file download occurs in chunks from many other peers concurrently. Thus, contrary to models of other P2P systems such as Gnutella or Freenet, which focus on finding the location of a file in the network, BT´s complexity occurs in downloading files (locating files in fact occurs out of band using Websites that host the Torrent files). We validate the model against a packet level simulator and also using a real, but small scale, BitTorrent experiment. The simulator is object oriented and extensible for simulating other P2P protocols and applications.