The impact of NAT on BitTorrent-like P2P systems

BitTorrent nowadays is one of the most popular peer-to-peer (P2P) applications on the Internet; on the other hand, network address translation (NAT) has become pervasive in almost all networking scenarios. Despite the effort of NAT traversal, it is still very likely that P2P applications cannot receive incoming connection requests properly if they are behind NAT. Although this phenomenon has been widely observed, so far there is no quantitative study in the literature examining the impact of NAT on P2P applications. In this paper, we build analytical models to capture the performance of BitTorrent-like P2P systems with the presence of homogeneous and heterogeneous NAT peers. We further propose biased optimistic unchoke strategies in order to improve the overall system performance considerably. The analytical models have been validated by simulation results, which also reveal some interesting facts about the coexistence of NAT and public peers in P2P systems.