Impact of Locality-awareness on Tit-for-Tat-based P2P File Distribution

Periodic update of software is one of preventive measures against malicious attacks. When the software is used by many users, e.g., Operating System (OS), the distribution server for update tends to be a bottleneck. To tackle this problem, several systems, e.g., Windows update, recently apply Peer-to-Peer (P2P) file distribution where clients called peers upload retrieved fragments of the whole content, i.e., pieces, to other peers. However, some peers will not be willing to upload pieces to others, which are called free riders, due to communication overhead. Tit-for-Tat (TFT) strategy in game theory can alleviate such free riding behavior by encouraging equivalent exchange of pieces among each pair of peers. In recent years, the optimality of P2P file distribution under strict TFT constraint has been analyzed. In this paper, considering the fact that the communication overhead inside a group, e.g., LAN or AS, is much less than that between different groups, we consider the locality-aware TFT-based P2P file distribution where the TFT constraint is relaxed for intra-group communication. We find that the minimization of average file download time in the relaxed TFT-based P2P file distribution can also be modeled as Integer Linear Programming (ILP), as in the existing work. Through numerical results, we show that the relaxed TFT model contributes to shortening the average file download time among peers by about 18.3% compared with the strict TFT-based model.