點對點(Peer-to-Peer, P2P)網路的許多優點，如服務可得性、容錯性、自我組織性和匿名性，使得其在學術研究上吸引了很多注意。但由於點對點網路是建構在疊加網路(Overlay Network)上，使得點對點網路的連結會和其下的實體網路(Physical IP Network)的連結產生拓撲的不匹配(Topology Mismatch)的現象。拓撲不匹配會使得同一條實體連結不斷有著重覆的信息經過，進而傷害到點對點網路的效能。因此在本篇論文中，我們提出了一個具有節點區域性的兩階層點對點的網路架構(Two-Tier Locality-Aware P2P Architecture)來減少點對點網路和實體網路拓撲的不匹配。我們所提出來的架構主要的概念是將在實體上相近的節點組成一個叢集。在這們的架構中，上階層(Upper Tier)是由Superpeer組成一個結構式的點對點網路(Structured P2P Network)，而下階層(Lower Tier)則是由Normal Peer所組成。在下階層的Normal Peer會連結鄰近的Superpeer形成一個叢集。透過模擬的結果，我們的架構確實可以減少疊加網路和實體網路間的拓撲的不匹配，進而增進點對點網路的效能。

Peer-to-peer (P2P) networks have attracted significant attention due to their advantages on service availability, fault tolerance, self-organization, and provision of anonymity. P2P networks are built as overlays on top of physical IP network. However, peers randomly join and leave a P2P network resulting in topology mismatch between P2P overlay and physical IP network, which degrades the performance of P2P systems due to redundant and duplicate messages are transmitted over physical IP links. For matching the P2P overlay and the physical IP network, the location information of peers should be taken into account on selecting neighbors. This thesis proposed a two-tier locality-aware P2P architecture to alleviate the problem of topology mismatch in the P2P network by employing location information of peers to group nearby peers into clusters, which are led by superpeers. The upper tier is a Pastry [6] overlay constructed by superpeers, and the lower tier consists of clusters formed by normal peers that only connect to nearby superpeers. The simulation result shows that our proposed two-tier locality-aware P2P architecture could effectively improve the performance of P2P networks in term of average neighbor distance and search latency.

[1] Gnutella. http://Gnutella.wego.com.
[2] Kazaa. http://www.kazza.com.
[3] J.Ritter, “Why Gnutella Can't Scale. No, Really,” http://www.tch.org/Gnutella.html, 2001.
[4] Ion Stoica, Robert Morris, David Karger, M. Frans Kaashoek, and Hari Balakrishnan, “Chord: A Scalable Peertopeer Lookup Service for Internet Applications,” in Proceedings of SIGCOMM, 2001.
[5] Sylvia Ratnasamy, Paul Francis, Mark Handley, Richard Karp, and Scott Shenker, “A Scalable Content-Addressable Network,” in Proceedings of SIGCOMM,2001
[6] Antony Rowstron1 and Peter Druschel, “Pastry: Scalable, decentralized object location and routing for large-scale peer-to-peer systems,” in Proceedings of the 18th IFIP/ACM International Conference on Distributed Systems Platforms (Middleware 2001). Heidelberg, Germany, November 2001.
[7] Sylvia Ratnasamy, Mark Handley, Richard Karp, and Scott Shenker, “Topologically-Aware Overlay Construction and Server Selection,” in Proceedings of IEEE INFOCOM, 2002.
[8] Yunhao Liu, Li Xiao, Xiaomei Liu, Lionel M. Ni, and Xiaodong Zhang, “Location Awareness in Unstructured Peer to Peer Systems,” IEEE Transactions on Parallel and Distributed Systems, Vol. 16, No. 2, February 2005, Pages 163-174.
[9] Y. Liu, Z. Zhuang, L. Xiao, and L. M. Ni, “A Distributed Approach to Solving Overlay Mismatching Problem,” in Proceedings of the 24th International Conference on Distributed Computing Systems (ICDCS), 2004.
[10] Ronaldo A. Ferreira, Suresh Jagannathan, and Ananth Grama, “Enhancing Locality in Structured Peer-to-Peer Networks,” in Proceedings of International Conference on Parallel and Distributed Systems(ICPADS), 2004.
[11] Xin Yan Zhang, Qian Zhang, Zhensheng Zhang, Gang Song, and Wenwu Zhu, “A Construction of Locality-Aware Overlay Network: mOverlay and Its Performance,” IEEE Journal on Selected Areas in Communications, Vol. 22, No. 1, January 2004.
[12] Miguel Castro , Michael B. Jones, Anne-Marie Kermarrec, Antony Rowstron, Marvin Theimer, Helen Wang, and Alec Wolman, “An Evaluation of Scalable Application-tier Multicast Built Using Peer-to-peer Overlay,” in Proceedings of IEEE INFOCOM, 2003.
[13] Beverly Yang and Hector Garcia-Molina, “Designing a Superpeer Network,” in Proceedings of the 19th International Conference on Data Engineering (ICDE), 2003.
[14] K. Sripanidkulchai, B. Maggs, and H. Zhang, “Efficient content location using interest-based locality in peer-to-peer systems,” in Proceedings of IEEE INFOCOM, 2003.
[15] Shansi Ren, Lei Guo, Song Jiang, and Xiaodong Zhang, “SAT-Match: A Self-Adaptive Topology Matching Method to Achieve Low Lookup Latency in Structured P2P Overlay Networks,” in Proceedings of the 18th International Parallel and Distributed Processing Symposium (IPDPS), 2004.
[16] Lakshmish Ramaswamy, Bugra Gedik, and Ling Liu, “Connectivity Based Peer Clustering in Decentralized Peer-to-Peer Networks,” in Proceedings of the Third International Conference on Peer-to-Peer Computing, 2003.
[17] Marcelo Werneck Barbosa, Melissa Morgado Costa, Jussara M. Almeida, and Virgflio A. F. Almeida, “Using Locality of Reference to Improve Performance of Peer-to-Peer Applications,” in Proceedings of Workshop on Software and Performance (WOSP), 2004.
[18] Yatin Chawathe, Sylvia Ratnasamy, and Lee Breslau, “Making Gnutella-like P2P Systems Scalable,” in Proceedings of SIGCOMM, 2003.
[19] Brighten Godfrey, Karthik Lakshminarayanan, Sonesh Surana, Richard Karp, and Ion Stoica, “Load Balancing in Dynamic Structured P2P Systems,” in Proceedings of IEEE INFOCOM, 2004.
[20] Zhiyong Xu, Rui Min, and Yiming Hu, “HIERAS: A DHT Based Hierarchical P2P Routing Algorithm,” in Proceedings of International Conference on Parallel Processing (ICPP), 2003.
[21] Adriana Iamnitchi, Matei Ripeanu, and Ian Foster, “Small-World File-Sharing Communities,” in Proceedings of IEEE INFOCOM, 2004.