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1、太原理工大学 硕士学位论文 无线网络TCP拥塞控制机制研究 姓名：杨新芳 申请学位级别：硕士 专业： 指导教师：王庆生 太原理工大学硕士研究生学位论文 I 无线网络 TCP 拥塞控制机制研究 摘 要 随着 Internet 的飞速发展，网络用户数日益增多，网络中的通信量急剧 增大，随之而来的是网络拥塞的频繁发生，网络拥塞已经成为制约网络发 展和应用的一个瓶颈。为了使网络能更好的运行，这就需要采取一定的策 略来避免和控制网络拥塞。目前，在 Internet 中普遍采用的拥塞控制算法是 TCP 拥塞控制算法。 随着通信网络技术的发展，无线通信和移动计算的需求持续增长，无 线网络在未来 Intern
2、et 中无疑将扮演极为重要的角色。与有线网络相比，无 线网络具有误码率高、带宽低、时延大以及移动频繁等特性，这使得无线 链路有很高的差错率。由于无线网络呈现的固有特性使得传统的 TCP 拥塞 控制机制由于设计本身的局限，无法良好地运行于无线网络中。传统 TCP 拥塞控制的前提是网络拥塞是数据丢失的唯一原因，每当检测到丢包时， 都会相应地减小发送窗口的大小，以降低 TCP 的发送速率。然而对于无线 网络来说，除了拥塞丢包，还存在随机丢包，而传统的 TCP 协议无法区分 丢包原因， 网络中一旦发生丢包， 仍然直接启动传统的 TCP 拥塞控制机制， 将数据发送速率进行不必要的降低，造成链路空闲，导致
3、系统性能下降。 因此，传统 TCP 拥塞控制技术已无法适应链路质量相对较差的无线环境。 这样，在无线网络中如何有效的提高 TCP 性能自然成了一个重要的研究课 题。 基于无线网络的固有特点，本文分析了无线网络丢包的原因，指出传 统 TCP 拥塞控制机制应用于无线网络中存在的问题；分析了经典的 AIMD 算法以及 TCP 拥塞控制的原理，并介绍了几种常见的拥塞控制算法 Tahoe，Reno，NewReno，Sack 和 Vegas。针对传统 TCP 拥塞控制机制在 无线网络中存在的问题，提出了基于 Reno 的无线 TCP 拥塞控制机制 （WReno） 。 WReno 主要包括三个方面： 1.网
4、络状态细分算法 NSSA。 该算法利用对 RTT 值的实时测量来对网络 状态进行细化，针对不同的网络状态，采用不同的窗口调节参数。 太原理工大学硕士研究生学位论文 II 2. 非线性 AIMD 算法 NLAIMD。TCP Reno 是基于 AIMD 机制进行拥 塞控制的。但传统的 AIMD 算法有较大的抖动性，数据流传输的平滑性和 稳定性很差。NLAIMD 算法在网络状态细分的基础上，根据网络的实际负 载情况，动态计算窗口加性因子和乘性因子，从而是使得拥塞窗口的变化 更加平滑和稳定。 3. 无线网络丢包区分算法 WLDA。该算法通过借用 TCP Vegas 算法中 两个吞吐量的概念（期望吞吐量
5、和实际吞吐量）来区分网络丢包的原因。 当网络中发生数据包丢失的时候，先进行数据包丢失原因的判断，根据不 同的丢包原因，采用不同的窗口调节机制。 最后本文利用NS2仿真工具对提出的WReno算法进行了仿真和性能分 析。实验表明，WReno 比 TCP Reno 具有更高的吞吐量，拥塞窗口的变化 更加平滑和稳定，能有效提高系统的性能。 关键字：无线网络，拥塞控制，TCP Reno，AIMD，WReno 太原理工大学硕士研究生学位论文 III RESEARCH OF TCP CONGESTION CONTROL MECHANISM OF WIRELESS NETWORK ABSTRACT With
6、the rapid development of Internet, Internet users and traffic increase rapidly, followed by the frequent occurrence of network congestion, it has become a bottleneck constrained the development and application of network. In order to enhance the network operation, we must take the necessary strategy
7、 to prevent and control network congestion. Currently, the congestion control algorithm of Internet is TCP congestion control algorithms commonly. With the development of communication network technology, the demand of wireless communications and mobile technology are growing continuously. Wireless
8、networks will undoubtedly play a very important role in the future Internet. Compared with the wired network, wireless network has some inherent characteristics including high bit error rate, low bandwidth and the frequent movement, which makes wireless links to represent high error rate. Because of
9、 the inherent characteristics of wireless network and the limitations of the traditional TCP congestion control mechanism，the traditional TCP congestion control mechanism can not be good to run on wireless networks. The traditional TCP congestion control mechanism based the premise that data loss is
10、 the only reason when network congestion happened. When packet loss is detected, it will correspondingly reduce the sending window size and the TCP sending rate. However, for the wireless environment, there also exists random packet loss apart from congestion packet loss. Because the traditional TCP
11、 protocol can not distinguish the reasons of packet loss, it is still directly activate the traditional TCP congestion control mechanism when the network occur packet loss, resulting in the data sent rate reducing unnecessarily, wasting network 太原理工大学硕士研究生学位论文 IV bandwidth, and decreasing system per
12、formance. Therefore, the traditional TCP congestion control technology has been unable to adapt to the wireless environment which has the poor link. In this way, in the wireless environment, how to effectively improve the performance of TCP naturally became an important research topic. Based on the
13、inherent characteristics of wireless networks, this paper analyses the causes of packet loss in wireless networks, points out the problems when the traditional TCP congestion control mechanism applies in the wireless network; analyses classic AIMD algorithm and the TCP congestion control theory, and
14、 introduces several common congestion control algorithm -Tahoe, Reno, NewReno, Sack and Vegas. For the problem when traditional TCP congestion control mechanism runs in the wireless networks, this paper put forward the wireless TCP congestion control mechanism based Reno (WReno). WReno mainly includ
15、e three aspects: 1. Network state subdivision arithmetic (NSSA). The algorithm makes use of the real-time measurement value about RTT to refine on the network status, with different network status, using different congestion window parameters. 2. Nonlinear AIMD (NLAIMD). TCP Reno bases on the AIMD.
16、However, the traditional AIMD algorithm has larger jitter, and the smoothness and stability of data stream transmission is very poor. NLAIMD based on NSSA and the actual network load conditions dynamically calculates the additive and multiplicative factor, which is to make the changes of congestion window more smooth and stable. 3. Wireless loss-package distinguishing arithmetic (WLDA).The algorithm by borrowing the tw
