Peer-to-Peer Netw. Appl. DOI 10.1007/s12083-012-0132-z Characterization of community based-P2P systems and implications for traffic localization Ruben Torres · Marco Mellia · Maurizio M. Munafo · Sanjay G. Rao Received: 30 June 2011 / Accepted: 19 March 2012 © Springer Science+Business Media, LLC 2012 Abstract In this paper, we present one of the first and most extensive characterizations of closed community- based P2P systems. Such systems are organic groups of peer-to-peer (P2P) clients, which can be joined only by users belonging to a certain network (e.g., connected to a given Internet Service Provider (ISP)). A number of factors motivate the growth of these communities, such as quality of content, anonymity of transfers, and the potential for better performance that enhances user experience. Our study is conducted in two contrasting environments—a campus network and a national ISP— located in different continents. In both cases, large- scale closed communities have been found to be the predominant P2P systems in use. We shed light both on the factors motivating the growth of such communities, and present results characterizing the extensiveness of their usage, the performance achievable by the sys- tems, and the implications of such communities for network providers. While our findings are interesting in their own right, they also offer important lessons for R. Torres (B ) · S. G. Rao Purdue University, 465 Northwestern Ave, West Lafayette, IN 47907, USA e-mail: rtorresg@purdue.edu S. G. Rao e-mail: sanjay@purdue.edu M. Mellia · M. M. Munafo Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Torino, Italy M. Mellia e-mail: mellia@polito.it M. M. Munafo e-mail: munafo@polito.it ongoing research that seeks to localize traffic within ISP boundaries. In particular, our results suggest that (i) in ISPs with heterogeneous access technologies, the performance benefits to users on localizing P2P traffic is largely dependent on the degree of seed-like behavior of peers behind high-bandwidth access technologies; and (ii) while localization can reduce the traffic on Internet peering links, it has the potential to cause a sig- nificant increase in traffic on internal links of providers, potentially requiring upgrades of network links. Keywords Peer-to-peer networks · Internet measurements 1 Introduction The last decade has seen a rapid growth in pop- ularity of peer-to-peer (P2P) systems, spanning di- verse applications such as content distribution (e.g., BitTorrent, eMule, Gnutella), video streaming (e.g., PPLive,Coolstreaming), and audio conferencing (e.g., Skype). A vast majority of these systems are Internet- scale, and open to any user on the Internet. Indeed, the open nature of these systems is viewed as a key strength of P2P systems in enabling inexpensive and rapid deployment of services over the Internet. In this paper, we raise the attention of the research community to the prevalence of closed communities of P2P users, and present an extensive characterization of such communities. Membership in such communities is restricted by imposing requirements on users that join the system. We focus on an important class of closed communities, where the primary criterion for admitting users is that they must be connected to the