【作者】 黄家玮；

【导师】 王建新；

【作者基本信息】 中南大学 ， 计算机应用技术， 2008， 博士

【摘要】 随着无线技术的不断发展,无线网络(无线接入网络、卫星网络、移动自组网、传感器网络等)已经融入到Internet,成为了目前互联网络中重要的组成部分。虽然网络带宽在以吉尔德定律飞速增长,但仍然无法满足各种网络应用对网络资源的需求,因此必须对有限的网络资源进行公平和高效的分配。特别在目前有线/无线互联网络中,由于无线信道具有低带宽、高延时和高差错率等特点,传统的基于有线网络的拥塞控制机制暴露出严重的不公平和网络效率低下等问题。本文主要研究有线/无线网络中TCP拥塞控制的公平性,以改善网络整体的传输性能。本文的主要工作包括以下几个方面:1、在多速率无线网络中,由于低速率信道节点挤占高速率信道节点的信道时间,造成网络整体效率低下。本文提出了一种多速率无线网络中时间公平的主动队列管理算法TFRED。在接入节点上,TFRED依据各流的无线信道数据发送速率进行丢包率的计算,实现了保证信道时间公平的拥塞控制机制。理论分析和仿真实验证明了该算法相对于吞吐率公平的算法有效提高了多速率无线网络的总吞吐率。2、通过仿真实验分析发现上下行TCP流不公平问题的直接原因是ACK包在接入节点缓存中的侵占性,并基于此结论提出了限制缓存大小的算法MBA。MBA算法基于上下行TCP流的不公平比例和缓存大小的关系,自适应地调节ACK包的最大缓存排队数量。实验结果表明MBA算法不但能实现上下行TCP流公平,还能通过减少无线信道ACK包传输概率提高网络总有效吞吐率。3、针对TCP在无线网络中接入有线网络时出现的上行流之间不公平、上下行流不公平和时间不公平问题,提出了上下行时间公平的调度算法UDTFLAS。UDTFLAS算法依据TCP流信道发送速率和上下行TCP流的方向实施分组调度,以保证上下行TCP流占用相等的无线信道时间。实验结果表明UDTFLAS算法可以保护单流的吞吐率,实现上下行TCP流的时间公平,最终提高网络总吞吐量。4、针对有线/无线网络中接入节点的拥塞问题,提出了基于ECN的拥塞控制算法APCC。APCC结合缓存队列长度和无线信道负载来检测拥塞,依据联合的拥塞测度来实施拥塞控制,在保证低丢包率和低排队延时的同时得到高信道利用率和网络吞吐率;利用ECN显式反馈机制,APCC对通过接入节点的上行和下行TCP DATA和ACK分组实施ECN标记,实现了上下行TCP流公平的双向拥塞控制;同时在多速率信道环境下,依据各流的无线信道速率来合理调节ECN的标记概率,实现不同无线信道速率TCP流之间的时间公平,改进网络的总体效率。论文对有线/无线网络中TCP拥塞控制的公平性进行了深入的研究,提出了时间公平的主动队列管理算法、上下行TCP流公平的缓存限制算法、上下行TCP时间公平的调度算法和基于ECN的拥塞控制算法,这些研究成果解决了目前混合网络拥塞控制中存在的多种不公平性问题,在下一代互联网络中具有良好的应用前景。更多还原

【Abstract】 With the rapid development of wireless technology, wireless networks including wireless access network, satellite network, mobile ad hoc network and sensor network have become the important part of the Internet. Though increasing under Gilder’s law, the network bandwidth can not meet the requirement of the network applications. It is indispensable to allocate the network resources in fair and effective manners. Especially, because of the low bandwidth, high delay and high bit error rate in wireless channel, the traditional congestion control mechanisms suffer great unfairness and inefficiency in wired/wireless hybrid networks. Considering these problems in the wired/wireless hybrid networks, this thesis proposes enhanced congestion control mechanisms to improve network performances. The main works include:1. In multi-rate wireless network, when the wireless node with lower data rate dominates channel usage time, the throughput experienced by other nodes transmitting at higher data rates will be drastically reduced. While most fair AQM (active queue management) algorithms are based on throughput fairness, we propose an AQM algorithm based on time fairness—TFRED (Time Fair RED), which works on AP (Access Point). To guarantee equal channel usage time for each wireless node, TFRED sets different drop probability for each flow going through the AP according to the transmission rate of each flow. Analysis and simulation results show that, compared with throughput fair congestion control algorithms, TFRED achieves time fairness and increase in total throughput.2. Through the extensive network simulations, we find that the direct cause of the TCP unfairness problem is the TCP-unfriendly nature of TCP ACK packet. According to this consideration, a buffer sizing algorithm, called MBA (Maximum Buffer for ACKs), is proposed. Based on the relation between up/down TCP unfairness index and buffer sizing, MBA algorithm adjusts the maximum size of buffer allocated for the ACK packets. Analysis and simulation results show that, MBA not only improves the fairness greatly by limiting the buffer for ACK packets, but also achieves higher total goodput by reducing the channel occupying by ACK packets.3. A new scheduling algorithm for up/down TCP flows, which is denoted as UDTFLAS (Up/Down Time Fair LAS) is proposed. Taking into account the transmission rate of each flow, UDTFLAS sets higher transmission probability for flows with higher wireless rate. In this way, UDTFLAS guarantees equal channel time for each up/down TCP flow. Analysis and simulation results show that UDTFLAS achieves per-flow throughput protection, up/down time fairness, and increase in total throughput.4. To resolve the congestion problem in AP, an ECN-based congestion control scheme called APCC (AP congestion control) is proposed. The main properties of APCC are: (1) Using both wireless channel load and buffer length as congestion indicators, APCC brings low loss rate and low queue delay while keeping high link utilization and network goodput; (2) APCC achieves the up/down TCP fairness by marking the ECN bit in TCP DATA and ACK packets; and (3) Taking into account the wireless channel rate of each TCP flow, APCC sets different ECN marking probability for each flow to guarantee the time fairness of each flow and higher total goodput.The dissertation focuses on the fairness of TCP congestion control in wired/wireless networks. It proposes a time fair AQM algorithm, a buffer sizing algorithm to achieve the up/down TCP fairness, a scheduling algorithm for up/down time fairness, and an ECN-based congestion control scheme. These algorithms slove the unfairness problems of TCP congestion control and can be used in the next generation networks.更多还原