傳統的檔案分享機制多為client-server模式，他們採用中央伺服器來管理所有的資源，使用者需透過網路連上伺服器，向其要求提供服務。這樣單一伺服器獨自運作的機制，可能因負荷過重引起當機，而有嚴重損失。

於是，一種新的網路機制因應而生：點對對網路(Peer-to-Peer Network)。每個使用者同時扮演client 和 server的角色，他們有相同的能力，可以直接和其他人溝通、交換資訊。Gnutella就是一個極有名P2P協定。當一個使用者想要尋找檔案時，它使用廣播(flood)的方式將詢問訊息，傳送給與它相連的使用者，如果有符合要求的結果，就將結果循原路送回。如果沒有，則繼續將詢問訊息廣播出去，讓系統內的其他使用者能收到此要求訊息。然而，這種方式產生大量重複的訊息，嚴重浪費了網路的頻寬，而造成網路的擁塞。

因此，我的碩士論文提出了效率高的搜尋方法，去改善Gnutella搜尋上的缺失。此方法是建構在結構化的點對點網路上，我們經由基層結構取得豐富的路徑選擇資訊(routing information)，經由這些資訊，每個使用者都可造出屬於他們自己的spanning-tree架構。欲搜尋檔案時，使用者透過此spanning-tree結構去發佈訊息。在搜尋過程中，每個使用者都會選擇適合的、先前不曾傳送過的使用者去傳遞訊息，保證不會產生多餘且重複的詢問訊息，大大的減少traffic的數量。我們透過實驗去分析我們提出的方法的效能，藉以映證此論文的優點與貢獻。

Peer-to-peer (P2P) computing has received a lot of attention in recent years. Gnutella is one popular P2P system that allows users to share and exchange MP3 music. It relies on flooding to search for desired data, which causes very serious traffic problem. The large traffic results in network and resource overhead. In this thesis, we propose to use an efficient underlay for Gnutella to reduce the network traffic in the search process. This underlying infrastructure is based on the Tornado P2P system which provides sufficient routing information. A spanning tree structure can be constructed over Tornado, through which the search messages can be distributed to the nodes in the Gnutella network without duplicated messages. We discuss different ways of using the spanning tree. The performance of the proposed scheme is evaluated with simulation. Through the simulation, the advantages of our proposed scheme are confirmed.

[1] Napster website. http://www.napster.com.
[2] The Gnutella Protocol Specification v0.41 Document Revision 1.2. http://dss.clip2.com/GnutellaProtocol04.pdf.
[3] A. Rowstron and P. Druschel. “Pastry: Scalable, Decentralized Object Location and Routing for Large-Scale Peer-to-Peer Systems,” in Proceedings of the IFIP/ACM International Conference on Distributed Systems Platforms (Middleware 2001), November 2001.
[4] B. Y. Zhao, J. D. Kubiatowicz, and A. D. Joseph. “Tapestry: An Infrastructure for Fault-Tolerant Wide-Area Location and Routing,” U.C. Berkeley Technical Report UCB//CSD-01-1141, U. C. Berkeley, April 2001.
[5] S. Ratnasamy, P. Francis, M. Handley, R. Karp, and S. Shenker, “A Scalable Content-Addressable Network,” in Proceedings of ACM SIGCOMM, San Diego, August 2001, pp. 149–160.
[6] I. Stoica, R. Morris, D. Karger, M. F. Kaashoek, and H. Balakrishnan. “Chord: A Scalable Peer-to-Peer Lookup Service for Internet Applications,” in ACM SIGCOMM, pages 149-160, August 2001.
[7] H.C. Hsiao and C.T. King. “Tornado: Capability-Aware Peer-to-Peer Storage Networks,” in IEEE International Conference on Parallel and Distributed Processing Symposium (IPDPS 2003), April 2003.
[8] A. Crespo and H. Garcia-Molina, “Routing Indices For Peer-to Peer systems,” in Proceedings of the 22nd IEEE International Conference on Distributed Computing Systems (ICDCS), Vienna, Austria, July 2002.
[9] B. Yang and H. Garcia-Molina, “Efficient Search in Peer-to-peer Networks,” in Proceedings of the 22nd International Conference on Distributed Computing Systems (ICDCS), Vienna, Austria, July 2002.
[10] Q. Lv, et al., “Search and Replication in Unstructured Peer-to-Peer Networks,” in Proceedings of ACM International Conference on Supercomputing, 2002.
[11] Web Caching and Content Delivery Resources. http://www.web-caching.com/.
[12] ClarkNet-HTTP trace. http://ita.ee.lbl.gov/html/contrib/ClarkNet-HTTP.html.
[13] LimeWire website. http://www.limewire.com.
[14] I. Clarke, O. Sandberg, B. Wiley, and T. W. Hong, “Freenet: A Distributed Anonymous Information Storage and Retrieval System,” Lecture Notes in Computer Science, vol. 2009, pp. 46+, 2001.
[15] Morpheus software website. http://www.morpheussoftware.net/.
[16] Shareaza website. http://www.shareaza.com/.
[17] Gnucleus website. http://www.gnucleus.com/Gnucleus/.
[18] Bearshare website. http://www.bearshare.com/.
[19] Z. Zhuang, Y. Liu, L. Xiao and L. M. Ni, “Hybrid Periodical Flooding in Unstructured Peer-to-Peer Networks”, in Proceedings of 2003 International Conference on Parallel Processing (ICPP 2003), Kaohsiung, Taiwan, China, October 6-9, 2003.
[20] S. Jiang, L. Guo and X. Zhang, ”LightFlood: An Efficient Flooding Scheme for File Search in Unstructured Peer-to-Peer Systems”, in Proceedings of 2003 International Conference on Parallel Processing (ICPP 2003), Kaohsiung, Taiwan, China, October 6-9, 2003.
[21] Peer-to-Peer Research Group, IRTF. http://www.irtf.org/charters/p2prg.html
[22] IEEE distributed systems ONLINE, Expert-authored articles and resources. http://dsonline.computer.org/os/related/p2p/index.htm
[23] White Paper: A Survey of Peer-to-Peer File Sharing Technologies.
http://www.eltrun.aueb.gr/whitepapers/p2p_2002.pdf