BitTorrent has been a very successful peer-to-peer (P2P) file-sharing application, and several BitTorrent-based P2P video streaming systems have been proposed in the literature. Nowadays, network address translation (NAT) has been widely used since it reduces the usage of IP addresses, but it is also considered as a factor that degrades the performance of P2P systems because NAT limits the direction of connectivity. In order to understand what impact NAT has on the performance of BitTorrent-based P2P video streaming systems, we build an analytical model which can be used to predict the average continuity index, a video streaming performance metric, when a fraction of the participating peers are behind NAT devices. A software simulator is written to validate our analytical model, and the simulation results also give some insights on the fairness issue of P2P video streaming systems in the presence of NAT devices. In this paper, both our analytical model and simulation results are presented and verified.
