【作者】 王鹤；

【导师】 季丽萍；

【作者基本信息】 哈尔滨工业大学 ， 计算机科学与技术， 2010， 硕士

【摘要】 由于卫星网络具有全球覆盖、24小时不间断通信等特点,在通信系统领域正发挥着越来越重要的作用。卫星网络具有长时延、高误码率、高时延带宽积等特点,因此,如果将应用于地面网络的TCP协议直接应用于卫星网络,其性能会大大降低。所以,研究适用于卫星网络的传输控制机制,克服卫星网络特性对TCP性能的影响,实现卫星网络的可靠传输是非常必要的。本论文在深入分析传统TCP协议和已有改进方法的基础上,针对卫星网络的主要特性,研究了卫星网络传输协议的性能优化问题。首先,从理论的角度分析了TCP连接的平均带宽受丢包率和往返延时的影响,明确了性能优化要从拥塞控制和错误控制两个方面着手;然后,对应于不同的控制机制采用不同的策略进行优化。具体的优化策略有以下几个方面:(1)卫星网络长时延特性下TCP拥塞控制机制的研究。Peach协议是针对卫星网络传输优化问题提出的新的拥塞控制机制,本文在Peach协议的基础上通过Vegas中的时间戳机制来精确估计往返时延并基于此设计了一种新的重传算法。这种机制可以在没有第二个或者第三个重复ACK时也能检测到丢包的发生,不用等待重传定时器超时,与Reno相比能减少将近一半的定时器超时。(2)卫星链路高误码率特性下错误控制机制的研究。(a)错误检测机制。复杂的无线信道会产生大量的链路误码,如果将链路误码丢包也作为网络拥塞处理,势必会影响通信的效率。本文使用显式拥塞通告ECN选项使得TCP能区分拥塞丢包和链路丢包,减少了不必要的重传;(b)错误恢复机制。复杂的无线信道还可能使得在一个数据窗口中出现多个数据包丢失,快速检测并重传丢失的数据包,能够提高TCP的性能。本文采用选择确认SACK选项,当一个数据窗口中出现多次丢包时,能在一个RTT内重传所有丢失的包,很好地提高了TCP的性能。更多还原

【Abstract】 Satellite networks are playing a more and more important role in telecommunica-tion for their global coverage and 24-hour uninterrupted communication. Satellite net-works are characterized by long Round-Trip Time(RTT), high Bit Error Rate(BER), bigBandwidth-Delay Product(BDP). The original Transport Congestion Protocol(TCP) issuitable for wired networks, but its performance significantly degrades in satellite net-works. It is necessary to investigate the problems of improving the TCP performance insatellite networks.Based on carefully analysis on traditional TCP protocols and improved methods,the thesis mainly focuses on several key issues to improve TCP performance of satellitenetworks.First of all, the impact factors are theoretically studied for the average bandwidth ofTCP connection. Congestion control and error control are proved to be the two mainlyfactors that highly in?uence the performance. Second, several different strategies areadopted for these two types of control schemes to optimize performance, represented asfolows:(1) Research on TCP congestion control based on long RTT of satellite networks.TCP Peach is a novel congestion control scheme for satellite network. A new re-transmission scheme is designed based on Peach through accurately estimating RTT withtimestamp option in TCP Vegas. Lost packets can be detected by the new mechanismwithin the retransmission timeout, even without the second or third duplicate ACK. Thenumber of coarse-grain timeouts in Reno is also reduced by half.(2) Research on TCP error control based on high BER of satellite link.(a) Error detection mechanism.The wireless satellite channel will produce a lot of link errors. The communicationefficiency will certainly be affected if link errors are attributed to network congestion.Explicit Congestion Notification(ECN) are implemented to distinguish congestion packetloss from link error. With this method, unnecessary retransmission are reduced.(b) Error recovery mechanism.The wireless satellite channel will also causes multiple packet losses in a data win- dow. Detecting and retransmitting the lost packets will improve the TCP performance.Selective Acknowledgement(SACK) option is adopted to retransmit all the lost packetsin one RTT, and it has been observed that the proposed scheme can significantly improvethe performance.更多还原