Modeling Unconnectable Peers in Private BitTorrent Communities

In a typical BitTorrent swarm, a large proportion of the peers are behind firewalls or NATs. These peers are called unconnectable. When developing P2P applications, a main requirement is to handle unconnectable peers appropriately. One important aspect of this problem, which has not been emphasized so far, is understanding the difference between the attributes of unconnectable peers and peers in the open Internet. For example, if unconnectable peers spend much less time online, or if they download significantly more, exploiting these facts helps to optimize the implementation, and ignoring these facts can even lead to severe performance problems. Comparing open and unconnectable peers is not easy because most traces contain no information about connect ability. Here we study two large traces collected in two private BitTorrent communities: FileList.org and BitSoup.org, both of which contain the connect ability attribute. From these traces we extract several attributes of individual online sessions, swarms, and users. We compare the distributions of these attributes over unconnectable and open peers. We find that there are some potentially important differences, e.g., unconnectable users tend to have a lot more sessions, and they tend to spend slightly more time online. Some of our findings are in contradiction with previous results that were based on a different trace collection methodology.