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1、广西师范大学 硕士学位论文 混合网络中高误码率下TCP拥塞控制机制的研究 姓名:张军洲 申请学位级别:硕士 专业:计算机软件与理论 指导教师:陈元琰 20080401 广西师范大学硕士研究生学位论文 I 混合网络中高误码率下混合网络中高误码率下 TCP 拥塞控制机制的研究 拥塞控制机制的研究 研究生:张军洲 导师:陈元琰 教授 学科专业:计算机软件与理论 研究方向:计算机网络 年级:2005 级 摘摘 要要 在过去的十几年中,通讯网络经历了巨大的变革。包交换技术最终使传统的语音网络 和数字网络融合到一起成为统一的多媒体网络,其范围进一步扩展,将有线,无线,蜂窝 网络融合一体。有线/无线混合网络
2、已经成为一个现实,而且无线网络正在越来越多的融入 到我们的日常通信当中。一直以来,TCP 是当前最主要的网络协议,90%的网络流被 TCP 传送。而现在,蜂窝技术、广域网、局域网,无线局域网、蜂窝网等构成的综合网络的革 命使网络超越了地理和区域的限制。 在综合网络(WANLAN3G 蜂窝)系统中, 绝大多数 数据和多媒体通信传输依靠已存在的端到端网络协议构架。而在当今的网络中,TCP/IP 是 最主要的通信协议体系。现在,大多数网络运输(包括 Web accesses,E-mails,大数据传输, 远程终端等等产生的运输)都是被 TCP 完成的。因此 TCP 需要去更改它原有的有线网络的 设计
3、,以适应无线网络所带来的挑战。 如何改进无线网络的 TCP 性能一直是个很热门的研究方向。 在无线或有线无线混合网 络中,TCP 性能降低的一个重要原因是由于不能区分丢包是因为拥塞还是无线链路错误。 传统的 TCP 拥塞控制机制在有线网络中工作的很好,丢包主要是因为拥塞导致,而因为无 线错误产生的丢包被忽略不计,或即使有,每个窗口最多也不超过一个丢包。然而,在有 线无线混合网络中,高的 BER,信号衰退和中断成为影响丢包不可忽略的因素,在有线无 线混合网络中,TCP 造成吞吐量的大幅度下降。 本文中,我们首先介绍了当前混合网络中主要拥塞控制算法的研究进展,探讨了进一 步增强无线网络环境性能应该
4、注意的一些研究方向和研究趋势。并对TCPW算法在高误码 率无线环境下与各种传统中间节点算法结合的性能进行了实验分析,从仿真实验中,我们 可以看到:第一,不同的传统中间节点算法对TCPW算法的性能没有明显影响,传统的中 间节点算法在高误码率无线环境中不再适用。第二,TCPW不能区分拥塞丢包和无线丢包, 在高误码率无线环境下,TCPW算法还有很大的性能提升空间。 接着,我们分析了 RED 和 ECN 算法的包标记机制,以及当前 XCP 和 VCP 的划分拥 塞程度的原理和实现方法,在此基础上,基于它们的思想,利用 IP 头中已有的空间位,无 需修改现有的包结构,提出了一种能适用于高误码率无线环境的
5、简单有效的包标记算法 CM 算法,相对于传统的中间节点算法,它能够高效的将拥塞情况反馈给源端。仿真实 验证明新算法具有更好的性能。 广西师范大学硕士研究生学位论文 II 最后,针对高误码率无线环境下 TCP Westwood 不能区分丢包原因和性能仍显不足的 问题,结合所提出的 CM 算法,在 TCP Westwood 算法的基础上,对 TCP 发送端,接收端 做了简单改进, 将显式拥塞信息与隐式拥塞信息相结合, 共同来区分拥塞丢包和无线丢包, 设计了 TCPW-CM 算法,以提高有线无线混合网络的性能。实验证明,在高误码率混合网 络环境下,新算法能够有效区分拥塞丢包和无线丢包,在各个方面的性
6、能都得到了显著的 提高。 关键词关键词:拥塞控制;误码率;TCP Westwood; RED和ECN; XCP和VCP; 广西师范大学硕士研究生学位论文 III Research on Congestion Control Mechanism of TCP in High Bit Error Rate Wire/wireless Networks Author: Zhang Jun-zhou Tutor: Chen Yuan-yan Specialty: Computer Software and Theory Direction: Computer Network Grade: 2005 A
7、bstract Communication networks have evolved greatly in the last decade. Packet switching technologies have eventually merged the traditional voice networks and data networks together into a converged and integrated multimedia networks, it includes wired, wireless, and satellite networks. A hybrid wi
8、red and wireless networks is becoming a reality, wireless networks now is more and more important. Transmission control protocol (TCP) has become the dominant communication transmission protocol, about 90% of the Internet traffics are carried by TCP. Now, WAN, LAN, WLAN and cellular networks are int
9、egrated together, which stretch the Internet beyond the limits of geography and terrain. In integrated WAN + LAN + 3G cellular systems, most data and multimedia communications are carried end-to-end over the existing Internet protocol infrastructure. Nowadays, most of the Internet traffic is carried
10、 by TCP, including traffic generated by Web accesses, e-mails, bulk data transfers, remote terminals, etc. TCP needs to depart from its original wired networks oriented design and evolve to meet the challenges introduced by the wireless portion of the networks. How to improve the performance of TCP
11、in wireless networks has been an active research area. The performance degradation of TCP in wireless and wired-wireless hybrid networks is mainly due to its lack of the ability to differentiate the packet losses caused by networks congestions from the losses caused by wireless link errors. TCP work
12、s fairly well in wired network, where packet losses are almost always cause of networks congestions, and packet losses due to bit errors are usually negligible, or if any, not exceeding one loss per transmission window. However, in the wired/wireless heterogeneous networks, high BER, fading, and bla
13、ckout become non-negligible factors to packet losses. TCPs congestion control and avoidance mechanism based on the assumption that all packet losses are due to congestions become incapable of handling the mixed packet losses. TCP exhibits throughput degradations when used in wired/wireless heterogen
14、eous networks. In this paper, we first study the development of the new main congestion control algorithms suited to wire and wireless hybrid networks, and discuss research direction of how to further enhance performance of wireless networks. Then we do experiments about TCPW algorithm 广西师范大学硕士研究生学位
15、论文 IV working together with different traditional AQM algorithm in high bit error rate network. From the experiments, we can know that: (1) different traditional AQM algorithms have little influences on TCPW algorithm performance and make no sense in high bit error rate network. (2) TCPW lacks the a
16、bility to differentiate the packet losses caused by networks congestion from the losses caused by wireless link errors. There is still much space to improve TCPWs performance. In second part, we study the packet marking scheme and the dividing principle and method of congestion level in XCP and VCP. Based on the their ideas, we propose a new congestion marking scheme, called CM, which is simple and