BitTorrent是目前點對點（peer-to-peer）模式的網路應用最受歡迎的系統。在BitTorrent系統中，一個大型檔案被劃分為許多片段（piece）做為最小的傳輸單位，當一個節點擁有部份檔案片段時，即可與其他節點進行分散式分享，取代了傳統主從式（client-server）架構的集中式下載。然而在點對點自主分散式的分享模式下，節點間資源使用的不公平現象，會減低使用者分享資源的意願。例如自私節點（free-rider）僅分享少量或不分享任何檔案資源，卻由系統中得到他人分享的檔案資源而完成下載。為了達到互惠的合作，以及避免BitTorrent所產生之不公平現象減低使用者分享資源的意願，資源分享的公平性是一個相當重要的關鍵。尤其在許多未來行動通訊網路架構中，將採用以資料流量計費的方式，因此資源分享公平性成為使用者願意使用系統的考量的一個重要因素。
此篇論文主要焦點是放在BitTorrent系統中，不再跟傳統的BitTorrent一樣沒有任何條件的使用optimistic unchoking機制，而是只有在有必要的情形才使用，因此設計一個利用調整BitTorrent中optimistic unchoking機制來達到節點間資源分享公平性的方法。主要概念為透過對每一個節點上下載量之即時監控，設定開啟或關閉optimistic unchoking的使用，達到每一個別節點其總上傳量與其總下載量平衡的效果。最後由模擬測試結果，可知我們所提出的方法，可僅對傳統的BitTorrent進行極小的修改，並在維持相同總體效能的條件下，有效地達到節點間資源分享的公平性。

BitTorrent is a popular file distribution system based on peer-to-peer networking model. Files are partitioned into pieces which can be downloaded from multiple peers in parallel so as not to contend for limited server resources. A peer uploads pieces to other peers while it simultaneously retrieves incomplete pieces from other peers at the same time. However, the heterogeneous of peer capacities and optimistic unchoking algorithm of BitTorrent results into unfairness problem regarding the volume of data that each peer serves, thus discouraging users to share their resources.
In this thesis, we suggest to ensure fairness by keeping peers with similar capacity to cooperate and restricting the use of unbalanced optimistic unchoking. Instead of aggressively hunting for better peers, a peer tries to find similar peers and keeps cooperating with those peers as long as they have pieces to reciprocate each other. Optimistic unchoking is restricted to be used only when a peer predicts the unavailability of cooperating peers. Ensuring a peer to cooperate with similar peers easily achieves fairness and provides incentives for a peer to open resources in order to cooperate with better peers. Simulation results show that the proposed scheme effectively achieves fairness among peers in terms of upload and download ratio, while maintaining equivalent download performance as original BitTorrent.

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