Measuring BitTorrent swarms beyond reach

BitTorrent is one of the most popular P2P file sharing applications in the world. Each BitTorrent network is called a swarm and millions of peers may join multiple swarms. However, there are many unreachable peers (NATed, Fire-Walled, or inactive at the time of the measurement) in each swarm. Due to this unreachable peers problem, the existing work can measure only a part of the entire peers in a swarm. In this paper, we propose an improved measurement method for BitTorrent swarms that many unreachable peers. In a nutshell, our crawler obtains peers behind NAT and firewalls by letting them connect to our crawlers through actively advertising our crawlers addresses to them. The evaluation result shows that our proposed method increases the number of unique contacted peers by 112 % compared to the conventional method. The proposed method also increases the total volume of downloaded pieces by 66 %. We then investigate the sampling bias among our proposed method and conventional methods, and find that different measurement methods can lead to significantly different measurement results.