Modeling and analyzing the effects of firewalls and NATs in P2P swarming systems

Many P2P systems have been designed without taking into account an important factor: a large fraction of Internet users nowadays are located behind a network address translator (NAT) or a firewall, making them unable to accept incoming connections (i.e. unconnectable). Peers suffering from this limitation cannot fully enjoy the advantages offered by the P2P architecture and thus they are likely to get a poor performance. In this work, we present a mathematical model to study the performance of a P2P swarming system in the presence of unconnectable peers.We quantify the average download speeds of peers and find that unconnectable peers achieve a lower average download speed compared to connectable peers, and this difference increases hyperbolically as the percentage of unconnectable peers grows. More interestingly, we notice that connectable peers actually benefit from the existence of peers behind NATs/firewalls, since they alone can enjoy the bandwidth that those peers offer to the system. Inspired by these observations, we propose a new policy for the allocation of the system's bandwidth that can mitigate the performance issues of unconnectable peers. In doing so, we also find an intrinsic limitation in the speed improvement that they can possibly achieve.