Modeling and Analyzing the Effects of Firewalls and NATs in P2P Swarming Systems

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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. This work presents a mathematical model to study the performance of a P2P swarming system in the presence of unconnectable peers. The paper quantifies the average download speeds of peers and finds 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.