Unveiling popularity of BitTorrent Darknets

BitTorrent is todays most influential peer-to-peer content distribution system. Currently BitTorrent has two very different operating models: (i) public trackers, and (ii) private trackers (a.k.a. PTs, Darknets). A PT can only be accessed by its registered users, and can provide ultrahigh downloading speed because of its effective share-ratio enforcement (SRE) incentive mechanism which stimulates the users to upload contents as much as possible. Although PTs are becoming more and more popular, they receive little attention from the research literature, possibly because they are operated underground. To understand the popularity of Darknets, the authors have traced 17 PT sites, 2 public tracker sites and 1 BitTorrent search engine for over a year. The authors investigate these PT sites from several aspects and try to understand why they are so successful in terms of attracting loyal users and providing high downloading speed. The authors then analyse the SRE mechanism and ratio free system which are commonly used by PTs. Our results unveil the reason of popularity and effectiveness of PTs. These understandings are essential to the sustainable development of future BitTorrent content distribution systems.