基金国家自然科学基金项目（60773058,60672051） P2P content distribution, represented by BitTorrent, has become an important Internet application in recent years. BitTorrent is studied by many groups and organizations. Some researchers employ simulation and mathematics models to analyze the efficiency of BitTorrent. They prove that BitTorrent is high-efficient on the assumption that bandwidths among peers are homogenous. In our opinion, these works just give the maximum performance of BitTorrent in theory. However, they cannot well predicate the practical performance of the BitTorrent system. Others analyze the incentive mechanism of BitTorrent and found that altruism has a very important effect on the overall efficiency of the system. We believe that an in-depth analysis from the perspective of techniques is demanded to understand the reasons why BitTorrent is popular with users and content providers, while complained by ISPs. Only if all the advantages and the disadvantages of BitTorrent are figured out, it is possible to design a better P2P content distribution protocol. We quantify BitTorrent＇s key mechanisms, such as neighbor selection choking/unchoking and piece selection, and point out the some flaws which impair system performance. These analyses help our design of a new P2P content distribution protocol named ShareStorm. Our work is supported by the National Natural Science Foundation of China （60773058 and 60672051）.