【作者】 任伟；

【导师】 金海；

【作者基本信息】 华中科技大学 ， 计算机软件与理论， 2006， 博士

【摘要】 传输控制协议TCP是最重要的网络协议之一,但TCP协议在无线移动Ad Hoc网络中性能较差,如吞吐率较低,稳定性和公平性较差,有安全隐患等。这是无线移动Ad Hoc网络环境中特有的问题造成的,如传输通道的易错性和丢包率、媒体介质层的竞争和冲突、节点的移动性、网络拥塞和安全问题等。当前无线移动Ad Hoc网络中TCP的改进算法通常针对某个问题,尚无一种适用较广泛的方案。考虑到异构环境的应用需求,保持端到端语义的TCP增强方案应做为研究的重点。为解决TCP在多跳无线网络中吞吐率的性能下降问题,可采用一种端到端的组合方案TCP-CD,即限制发送窗口最大值和使用延迟确认机制,并建议发送窗口限制为2个最大发送包为宜。模拟实验表明,TCP-CD优于使用单一方案。考虑到路由协议和移动模型对TCP性能的影响,为解决TCP在移动无线网络中的吞吐率的性能降低问题,可采用TCP-CDR方案,即在TCP-CD基础上增加TCP最大重传重试时间的限制。在多种移动模型中如随机移动(Random Waypoint)和社会网络(Social Network)模型中与当前的方案,如FeW、TCP-AP进行比较。模拟实验表明,TCP-CDR中均优于其他方案。对于TCP在多跳无线网络中的稳定性问题,分析发现TCP在经过多跳网络路径传输时,往往会因为隐藏终端导致大量MAC层冲突丢包,导致MAC层包重传重试超过相应限制导致丢包,使得TCP数据包或确认包丢失,TCP反复进入慢启动过程,从而最终破坏TCP稳定性。为解决稳定性问题,仅仅限制TCP发送窗口大小是不够的,延迟确认策略应该同时使用。稳定性问题的形式化描述和隐藏终端问题对TCP稳定性影响的定量分析进一步揭示TCP稳定性问题。对于TCP在有线网络和无线网络混合的异种网络中的公平性问题,可采用使用限制TCP发送窗口大小来提高上行和下行TCP流之间的公平性的方案。对于TCP在移动Ad hoc网络中的不公平性问题,分析其原因并对TCP公平性产生影响的场景进行分类。实验发现,重叠TCP流的公平性好于非重叠TCP流,对于非重叠TCP流,只有当TCP流中所有节点在干涉范围内时才能保持公平性。如果TCP流发送节点是其他TCP流发送节点的隐藏终端,公平性将很差,可使用802.11e设定单跳TCP流的优先值为一个较小的值来提高TCP公平性。使用累计公平指数和瞬时公平指数来衡量移动Ad Hoc网络中TCP的公平性较为合理。性能问题还需要考虑安全性,移动Ad Hoc网络中基于网络拥塞的降低TCP性能的RoQ(Reduction of Quality)分布式拒绝服务攻击方式,包括脉冲攻击,循环攻击,自消耗攻击和泛洪攻击。防御机制包括检测和响应机制。检测信号包括RTS/CTS包频率、信号干涉频率以及包重传次数。响应机制则依据显式拥塞通告标记和通知。更多还原

【Abstract】 Transmission Control Protocol - TCP is one of the most important network protocols, however, the performance of TCP over wireless mobile Ad Hoc networks (MANET) is damaged, such as goodput, stability, fairness and security. The reasons of performance decline come from some problems in MANET, such as the bit error rate and loss rate of the channel, the contention and collision of the media access layer, the mobility of nodes, the congestion and security etc. The state of art protocol improvement schemes for TCP over MANET always focus on one problem, so the general scheme is still on research. For the application in heterogeneous networks, the end-to-end TCP enhancement scheme is more significant to research.For the improvement of the TCP goodput performance over multihops wireless networks, an end-to-end combination scheme - TCP-CD, including the limit of sending window and delayed ACK mechanism is adoptable. The sending window size is recommended to set to 2 MSS (Maximized Size Segment). The simulation results show it outperforms single scheme. Considering the impact of routing protocol and mobility model to the TCP performance, for the improvement of TCP performance over mobile ad hoc networks, TCP-CDR scheme is proposed, which includes TCP-CD and the maximized retransmission timeout limit. After the comparisons with the state of art end-to-end TCP improvement scheme, such as FeW and TCP-AP, TCP-CDR has better performance than other schemes both in the Random Waypoint mobile scenarios and Social Network mobile scenarios.For the TCP stability over MANET, the analysis find that when TCP over multihops wireless network, the hidden terminal will lead to a large amount of MAC packet dropping owing to the collision. It results in the exceeding of retransmission retry times in MAC layer and packet dropping, so that the TCP DATA or ACK packet will be loss. TCP sender always goes into Slow Start phase and the stability is declined. To deal with this problem, it is not effective only to limit sending window size. Delayed Ack scheme is also required. The general discussions including formal description of TCP stability problem and quantitive analysis of hidden terminal points out the reason of TCP stability.For the TCP fairness over wireless cum wired networks, the scheme of setting limit of TCP sending window size is proposed and evaluated to improve the TCP fairness between upload and download flows in wired cum wireless networks. For the TCP fairness over MAENT, the analysis of the reason of TCP unfairness and the classification of possible scenarios that have impacts to the fairness are also given. Fairness is better between overlapped TCP flows. For none-overlapped TCP flows, only all nodes in TCP flows are in the interference range can maintain TCP fairness. If one TCP sending node is the hidden terminal of the sending node in another TCP flow, the unfairness will be greatest. To resolve this problem, 802.11e is recommended to improve the TCP fairness for that the priority of the one hop TCP flow can be set to a low value. The accumulative fairness index and instant fairness index are the metrics of TCP fairness in MANET.Performance is always with the consideration of security, congestion targeted TCP RoQ (Reduction of Quality) DDoS attacking schemes are classified into four categories: Pulsing attacking, Round Robin attacking, Self-whisper attacking and Flooding attacking. The defense scheme includes detection and response. Detection signals include RTS/CTS packets receiving frequency, channel sensing busy (signal interference) frequency and RTS/DATA retransmission times. The response scheme includes ECN (Explicit Congestion Notification) marking mechanism.更多还原