【作者】 李曦；

【导师】 纪红；

【作者基本信息】 北京邮电大学 ， 通信与信息系统， 2010， 博士

【摘要】 随着通信技术和互联网应用的不断发展,传统应用层所采用的服务器／客户端的服务架构已经不能很好满足大量用户的访问需求,出现了许多基于P2P (Peer-to-Peer)技术的商业应用,采用对等通信的思想将网络边缘的用户和终端设备连接起来,提供点到点的直接服务。在P2P技术所构建的应用层虚拟网络中,所有终端都被映射成为对等的节点,同时兼任服务器和客户端的角色,彼此间共享文档数据、存储空间、计算能力等资源,可以实现即时通讯、影音点播、搜索资料、分布式计算等各种娱乐和科研活动,吸引了世界各地的用户加入。其中最受欢迎的是P2P文件共享系统,它允许用户在网络内数以万计的节点所建立起来的丰富资源中寻找所需要的目标文件,同时提供自己的可用资源列表,为他人提供方便,实现互相协助,文件共享。人们随时随地接入网络的需求随着无线通信技术的进步逐渐得到满足,相关的应用服务也开始由有线网络向无线网络过渡。现有P2P文件共享系统的设计实现都基于有线网络,要想在无线环境中大规模推广,还需要解决一系列问题,特别是底层传输情况的改变对于上层应用的影响,例如用户终端随机移动,无线链路不可靠,设备能量受限,节点失效风险增加,网络拓扑结构频繁变化等。对此研究人员已经展开探索,取得了大量成果,但是现有研究都集中在资源发现、安全保障、激励措施等热点领域,没有对文件共享的流程进行深入分解讨论,并且所提出的算法对适用环境有所限制,可扩展性较差。本文针对无线P2P文件共享系统所面临的挑战,紧密围绕文件共享的关键步骤展开探讨,既立足于现有研究成果,又充分考虑无线环境,兼顾有效性和可靠性,针对实现共享的主要环节进行逐步分析,提出相应的解决方法。主要的工作和创新点如下：1.提出了一种无线P2P文件共享系统的结构模型,通过分层架构对数量庞大的节点进行组织,规定了普通节点和簇头节点的功能、职责,共享文件的存储管理方式,系统初始化和运营维护的相关机制,具有良好的可扩展性和灵活性,支持多关键字和多粒度的模糊搜索。2.设计了与上述新型结构模型相配合使用的资源搜索算法,在路由表中引入远端连接,通过人为构建符合小世界特性的网络,将查询消息的转发范围从邻居簇扩大到远端簇,从而极大提高了文件查询的效率,降低了网络开销,改善了系统性能。3.提出了适用于无线P2P网络的最优文件持有节点选择标准,即始终协调最优准则,代替传统有线网络中使用的最小跳数准则。全面考虑了无线通信环境中的主要影响因素,可以设置不同的评分细则从而选出符合不同侧重要求的节点。运用模糊信息综合理论建立数学模型,并根据实际情况对算法实现流程进行了设计。该算法能够与各种P2P资源搜索协议配合使用,在文件成功传输率和文件传输时间等方面表现良好。4.设计了无线P2P文件传输中对失效路由的恢复机制,通过构建松散的聚群集合,实现对下一跳节点突然失效状况的及时发现和迅速反应。为了确定适合的备用节点,对集合中的所有节点进行周期性的信息收集和失效风险评估。为此引入了模糊认知图理论,分析无线环境下导致节点失效的可能原因和影响程度,建立模型并得到节点失效风险的衡量规则。该机制具有良好的独立性和灵活性,可以与各种传输路径选择算法配合使用,为节点之间顺利完成文件传输提供了保障。更多还原

【Abstract】 Along with the development of the communication technologies and the prosperity of Internet applications, traditional service providing pattern of Server/Client in application layer could not fulfill massive accessing requirements. Then a series of commercial services based on P2P (peer-to-peer) networks have achieved huge success with equal communicating positions among entities. Users and terminals are connected by virtual links and provided with direct end-to-end serving. They play both server and client roles in the networks to share various kinds of data, storage space, computing capacities and other resources. Depending on P2P, thousands of users all around the world could enjoy instant communication, online movie and music, files searching, distributed computing and many other entertaining and academic activities. The most popular application is P2P files sharing systems, which support looking for target files among numerous databases and encourage abundant users to offer help to others with their own resources.Wireless communication technologies make people may access to networks anywhere and anytime. Relative service demand from wireless users has also increased. Existing P2P files sharing systems are all designed for wired networks, which need further revisions to adapt to wireless environment. Underlying transmission condition changes result in obvious influence on upper layers, such as random moving of users, unreliable wireless links, energy-limited terminals, increasing failure risk of nodes and frequent topology changes. Although researchers have proposed many algorithms in wireless P2P area, it mainly focuses on resource discovering, security schemes, incentive methods and other hot fields, lacking thorough analysis on sharing procedure and scalablity. This thesis would concentrate on the key steps of files sharing in wireless P2P system, combining existing research results with wireless transmission environments, proposing effective, flexible and reliable approaches from several important respects. The main contributions of the thesis are listed as follows:1. Proposing effective network architecture for wireless P2P files sharing systems. Abundant nodes are organized by hybrid layers, with head nodes and ordinary nodes shouldering respective responsibilities. The shared files are stored in the database for fuzzy and multiple-keys lookup. Relative running and maintaining schemes are also designed for scalable and reliable requirements.2. Designing the P2P resource discovering algorithm matching the proposed network architecture. Remote links are introduced into the routing table to construct a man-made small-world network. The looking up request is forwarded not only to neighbor heads, but also to remote clusters. This may greatly improve the files discovering efficiency, decreasing overhead and enhance the system performance.3. Proposing the optimal files holder selection criteria used in wireless P2P files sharing systems, always-optimally-coordinated rule (AOC), to replace the traditional min-hops rule designed for wired networks. Based on the through consideration on wireless environment, the new AOC algorithm may adjust the evaluating stress to different cases. Fuzzy knowledge combination theory is introduced to construct the mathematic model, and the concrete procedure of the algorithm is realized. It may adapt to various P2P resource discovering protocols with good improvement on successful transmission rate and files transmission time compared to min-hop-based algorithm.4. Designing a novel path failure recovery scheme for wireless files transmission in P2P networks. Loosely-connected groups are organized to realize quick detecting and recovering for failed next-hop nodes. In order to decide the most proper backup node, periodic information collection and failure risk evaluation are carried out by all the nodes in the group. Fuzzy cognitive maps theory is introduced to set up the node failure risk model for wireless P2P files sharing and deduce the evaluating rule. It has been proved that the new scheme could work with various routing protocols to provide successful file transmission among nodes with good scalable and flexible behavior.更多还原