OTRASFER COSTS IPEER-TO-PEER SYSTEMS: MODELIG AD OPTIMIZATIO

Peer-to-Peer (P2P) systems have become extremely popular in the Internet. One of the most important aspects of P2P systems is that the traffic produced by various P2P systems (e.g. BitTorrent) has been overloading the Internet. According to many works it would be decisive to reduce this load simultaneously maintaining the performance of P2P systems at acceptable level. Therefore, in this paper we take a detailed look at the problem of flows modeling in P2P systems with a special focus on aspects related to transfer costs. We formulate an optimization model of a P2P network. To solve this problem we apply an exact algorithm and heuristic approaches developed akin to real P2P systems. Optimal results produced by the exact algorithm can be applied as an effective benchmark to evaluate quality of heuristic approaches. Extensive numerical study is provided analyzing P2P networks in terms of transfer costs. To our best survey, this is the first study that provides optimal results of flow optimization in P2P systems. 1. ITRODUCTIO� The concept of Peer-to-Peer (P2P) has gained much attention recently in the research community. Many applications including file-sharing, distributed computing, Internet based telephony (e.g. Skype), Internet television (e.g. Joost) were successfully implemented using P2P techniques. BitTorrent and other P2P systems generate more than 50% of consumer Internet IP traffic (1). Consequently, a great number of research challenges in the area of P2P is still open (13), (18), (20). The problem that this paper investigates, namely, modeling and optimizing of network flows in P2P systems, is one that arises naturally from the need to optimize performance of P2P systems from the network perspective. We propose an offline model that enables optimization of network flows in P2P systems. We are aware of the fact that P2P systems are very dynamic and stochastic, so offline modeling of such systems can seem incorrect. However, it is a common research approach that difficult and dynamic systems are reduced to simpler cases to enable creation of models that can be solved optimally in reasonable time. Having Linear Program of Integer Program formulations, network design problems can be solved by standard methods, and optimal or near optimal solutions can be found. Thus, we can obtain benchmarks that allow us to estimate performance of real systems. Finally, our work is a theoretical one and we hope that methodical analysis of the model we propose and obtained results will produce some inspirations in evaluation of various issues regarding P2P systems. Our focus on transfer costs in P2P networks follows from the fact that currently used P2P systems do not take into account locality information. For instance, current implementations of BitTorrent ignore the underlying Internet topology or ISP (Internet Service Providers) link costs, and set up