原始的檔案傳輸方式，是採用伺服器架構(client / server)，因為是單一伺服器，若此伺服器發生問題，則整個系統無法運作；另外，服務數量和服務速度，都有著限制。然而，人們提出了新的點對點(peer to peer)架構，以此架構，每一個使用者(client)，都同時扮演著伺服器(server)和使用者(client)的角色，在下載檔案的同時，也把自己擁有的檔案分享給其他的使用者，同一時間有許多使用者在作上傳的動作，也就是多點平行下載(multi-source parallel downloading)的概念，可以讓下載檔案的速度加快，也解決了單一伺服器的問題。
為了使這個方式實現，我們把一個大的檔案，切成一塊塊檔案片段，當使用者擁有一塊檔案片段時，他就可以扮演伺服器的角色，把此片段傳送給其他的使用者。然而，這個方式其實有一個潛在的問題，稱為斷頭問題(last block problem)。如果擁有所有檔案片段的提供者，離開了這個網路；因為剩下的使用者都只有一些檔案片段，而沒有全部的檔案片段；若所有使用者，都缺乏某個檔案片段，而導致這個檔案永遠無法下載完成，則斷頭問題就發生了；另外，在不穩定的點對點網路，使用者都動態加入和動態離開時，這種問題會更容易發生。
這篇論文，是針對在Bittorrent網路中，加入一個新的稀有檔案片段下載的機制(RPDM)，以這個機制，希望達成上述第一個方向的目標；以增加使用者之間的合作，也增加系統的檔案下載成功機率，來解決斷頭的問題。

BitTorrent as a peer-to-peer based file distribution system has received great attention recently. A file is divided into pieces for facilitating parallel downloading. However, peer dynamics usually causes the last block problem, where a file may never be completed because some pieces are missing in the network. This paper suggests a piece selection strategy to explicitly acquire rare pieces in order to avoid the last block problem. The proposed mechanism allows peers to be notified of the existence of rare pieces and thus these pieces can be easily retrieved. A hierarchical BitTorrent structure is also proposed to mitigate the potential overhead of this algorithm. Simulation results show that our scheme approximates the performance of the optimal globally rarest first piece-selecting algorithm and effectively enhances service availability. In addition, the hierarchical structure successfully bounds the message overhead and performs well at the forepart of simulations.

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