Free-Riding, Fairness, and Firewalls in P2P File-Sharing

Peer-to-peer file-sharing networks depend on peers uploading data to each other. Some peers, called free-riders, will not upload data unless there is an incentive to do so. Algorithms designed to prevent free-riding typically assume that connectivity is not a problem. However, on the Internet, a large fraction of the peers resides behind a firewall or NAT, making them unable to accept incoming connections. In this paper, we will prove that it is impossible to prevent free-riding when more than half of the peers are firewalled, and we will provide bounds on the sharing ratios (defined as the number of bytes uploaded divided by the number of bytes downloaded) of both firewalled and non-firewalled peers. Firewall puncturing techniques are complex but can be used to connect two firewalled peers; we will provide a bound on their required effectiveness in order to achieve fairness.We confirm our theory by simulating individual BitTorrent swarms (sets of peers that download the same file), and show that the theoretical bounds can be met in systems with many firewalled peers. We have also collected statistics covering thousands of BitTorrent swarms in several communities, both open and closed; the latter ban peers if their sharing ratios drop below a certain treshhold. We found 45% of the peers to be firewalled in the closed communities, as opposed to 66% in the open communities, which correlates with our theory that to obtain fair sharing ratios for all peers, at most half of them can be behind firewalls.