异构无线网络中 TCP Vegas 算法的研究与改进

异构无线网络是将不同接入技术、不同性能的网络融合到一起构成的单个逻辑网络.异构无线网络中,TCP 端到端的拥塞控制机制对网络的健壮性和稳定性具有非常重要的作用,因此是网络研究的一个热点问题.针对异构无线网络中移动节点发生垂直切换时传输层性能下降的特点,提出了一种基于 TCP Vegas 的传输层拥塞控制算法 B-Evegas.给出了垂直切换发生时的传输控制方法,垂直切换后拥塞窗口的恢复采用带宽估计与分段增加策略,并引入了快速恢复机制,在拥塞窗口过大时根据链路的时延指数性地减小拥塞窗口.仿真结果表明,该算法是合理的,可以有效提高垂直切换发生后 TCP 连接的吞吐量或者减小数据包的传输时延     

基于SACKTCP的提高异构网络性能的模型

随着卫星网络、无线网络等异构网络的发展和互联,在具有长时延特性和高误码率的网络环境下,当前广泛使用的传输控制协议（TCP）的性能受到极大的影响。该文利用在异构网络的边界处设置具有传输控制功能的性能增强代理（PEP）的理论模型,对TCP选择性确认（SACK）协议下PEP的拥塞控制及拥塞恢复策略进行研究,提出PEPSACK的拥塞控制算法,对具有长时延、高误码率的网络环境下TCP性能的改善具有良好的作用。     

基于TCP友好的无线网络拥塞控制机制研究

网络实时多媒体业务的广泛应用对传统传输层协议提出了新的挑战:拥塞控制机制的缺乏使得UDP严重抢占TCP应用的共享带宽,从而降低网络的公平性,甚至导致网络拥塞.针对无线网络的高误码特性,将传输延时抖动引入到TFRC控制机制中,提出了一种基于速率控制的TCP友好拥塞控制算法TFRC-JI.该算法基于传输延时抖动有效区分无线链路的拥塞和误码,并以此反馈至发送端,实现不同的速率控制机制.实验结果表明,与传统的TFRC相比,改进的TFRC-JI在保持对TCP业务友好性的同时实现了链路的高效使用,并降低了传输时延抖动,从而较好地适应多协议共存的无线网络实时业务传输.     

基于认知无线电网络的TCP协议的改进方案

首先搭建认知无线电网络环境,在该网络下分析TCP Reno、TCP Newreno、TCP Sack1、TCP Vegas、TCPWestwood协议的性能包括拥塞窗口、平均吞吐量、丢包率。针对传统TCP拥塞控制在认知无线网络中存在的问题,提出一种跨层优化TCP Westwood的改进算法,称为TCP-CR。该算法区分网络拥塞导致的超时和切换所导致的超时,同时针对不同轻度拥塞进行不同的恢复策略。ns-2仿真结果表明,TCP-CR可以减少TCP的平均慢启动次数,吞吐量比传统的TCP Westwood显著提高。增强了对认知无线网络环境的适应性,从而提高了网络的性能。     

TCP-BM:一种适用于异构网络的TCP协议改进策略

针对异构网络中的拥塞控制问题进行了研究,以传统的TCP Reno协议为基础提出一种改进算法TCP-BM。利用往返时延值将慢启动阶段分为三个部分;利用往返时延值将拥塞避免阶段分为正增长和负增长两个过程。网络发生丢包后,通过往返时延值与历史记录的比较以及估计的带宽值的比较,区分丢包原因,从而对拥塞窗口和慢启动阈值采取不同调整策略。仿真验证证明,改进后的TCP算法性能优于传统的TCP Reno协议。     

可避免数据包乱序的网络层软切换方法及其性能分析

随着异构无线网络的融合和全IP网络的推广,网络层软切换方法不断演进,用以提高垂直切换的性能.但是,现有的软切换方法在下行垂直切换过程中会造成严重的数据包乱序.为此,文中在讨论下行垂直切换时数据包乱序的原因之后,提出了一种新型网络层软切换方法——SHORDER.该方法根据移动终端与其家乡代理之间的交互信令对终端所收数据包进行适当的缓存,能有效避免移动终端因下行垂直切换接收到乱序的数据包.该方法所具有的网络层独立性使其能与各种传输层协议及其改进协议相兼容.此外,文中还从理论上定量地分析了所提方法对TCP应用的切换时延、有效吞吐量及拥塞窗口的影响,并与现有的典型网络层软切换方法进行了比较.通过比较,得出了SHORDER机制的适用条件,并基于此,进一步改进了SHORDER方法.最后,在原型系统上的实验显示出了所提机制便于部署的优点以及该机制在其适用条件下的良好性能.此外,实验结果还显示出与数值分析结果的一致性.     

改进的网络拥塞控制策略算法研究

研究网络拥塞优化控制问题.针对网络承载量的不断增加,使得网络传输效率降低.传统网络拥塞控制算法要求系统根据无线网络的容量实时编号,动态调整TCP拥塞窗口的大小,难于建立准确的数学模型,从而导致网络带宽利用率低,网络拥塞严重.为了降低网络拥塞的概率,提出了一种改进的无线TCP拥塞控制算法.算法主要是集中解决在TCP拥塞窗口的大小调整问题上,首先利用BP神经网络对参数进行训练,有效地解决了TCP拥塞窗口大小的调整,从而实现了拥塞避免、快速重传和快速恢复机制,改善网络性能.实验结果表明改进的算法提高了网络平均吞吐量,带宽利用率更高,有效避免了网络拥塞.     

MPSSF:一种低失序的缓存转发移动切换方案

全IP无线移动网络的微移动协议在无线接入网采用快速切换技术降低移动切换时延，同时采用了数据包缓存转发技术来解决切换过程中的丢包。多流转发方案存在较多的失序，使上层的TCP协议不适当地启动拥塞控制机制而降低吞吐量。单流转发方案虽然没有失序，但是会占用较多的网络资源并增加数据包的时延。提出一种多径单流转发方案MPSSF，较好地解决了移动切换过程中数据包的丢失与失序问题，同时网络资源消耗以及数据包时延也比单流转发方案显著减小。网络模拟实验表明，MPSSF在移动切换时避免了数据包失序，保持了TCP的拥塞窗口，对TCP性能的改善效果优于多流转发方案及单流转发方案。     

TCP协议在无线网络中的一种改进方法

近年来,无线网络飞速发展,但由于其物理特性的影响,使其不如有线网络那么可靠,无线网络上的丢包率大大高于有线网络.当无线网络中出现丢包后,传统TCP协议会启动其拥塞控制机制,从而造成TCP性能的极大下降.对此提出了一种在传输层上改进wired-cum-wireless网络上TCP协议的方法--WirelessACK响应机制,力图在不改变有线网络的基础上提高TCP的性能.     

无线网络中一种智能控制的组播拥塞控制机制

针对现有组播拥塞控制算法应用到无线网络中存在的性能下降问题,提出一种基于新的智能组播拥塞控制机制ECMCC。ECMCC机制根据网络相对队列时延和数据包丢失检测网络的拥塞状态,采用代表集合机制反馈信息,利用专家控制器的推理判断区分丢包原因和当前的网络状态,进而采取不同的控制策略调节组播源端发送速率。仿真结果表明,ECMCC机制收敛速度快、灵敏性好、速率变化平滑,在有线网络中具有良好的TCP友好性。同时,ECMCC能有效区分网络拥塞和随机差错,提高了网络的吞吐量,适用于无线网络环境,且在无线网络较低误码率时具有一定的TCP友好性。     

网络拥塞控制模型Hopf分岔的PD控制

网络拥塞会导致信息丢失,时延增加,甚至系统崩溃。由于无线接入网络中的时变衰落和分组错误率,使得TCP协议在网络拥塞控制更加复杂。TCP Westwood是专门为高速无线网络设计的,大大提高了网络带宽的利用率,改善了网络性能。TCP Westwood/AQM拥塞控制的连续流体流模型被引用,源端采用TCP Westwood拥塞控制协议,路由器端采用主动队列管理(AQM)机制中的随机早期检测(RED)算法。为了延迟无线接入网络拥塞控制模型中霍普夫(Hopf)分岔现象的发生,采用比例微分(PD)控制器,通过选择通信延迟作为分岔参数,分析无线网络系统中的Hopf分岔行为,并由理论分析得知当分岔参数超过临界值时系统发生Hopf分岔。利用中心流形和规范型理论,推导得出系统发生Hopf分岔的条件和反映Hopf分岔性质,方向和周期的参数,数值仿真验证理论分析的准确性,表明PD控制器的有效性。     

System supports for protocol and application adaptation in vertical handoffs

In overlay networks, the network characteristics before and after a vertical handoff would be drastically different. Consequently, in this paper, we propose an end‐to‐end based scheme to support protocol and application adaptation in vertical handoffs. First, we proposed a Vertical‐handoff Aware TCP, called VA‐TCP. VA‐TCP can identify the packet losses caused by vertical handoffs. If segments losses are due to vertical handoffs, VA‐TCP only retransmits the missing segments but does not invoke the congestion control procedure. Moreover, VA‐TCP dynamically estimates the bandwidth and round‐trip time in a new network. Based on the estimated bandwidth and round‐trip time, VA‐TCP adjusts its parameters to respond to the new network environment. Second, during a vertical handoff, applications also need to be adapted accordingly. Therefore, we design a programming interface that allows applications to be notified upon and adapt to changing network environments. To support our interface, we utilize the signal mechanism to achieve kernel‐to‐user notification. Nevertheless, signals cannot carry information. Thus, we implement the shared memory mechanism between applications and the kernel to facilitate parameters exchange. Finally, we also provide a handoff‐aware CPU scheduler so that tasks that are interested in the vertical‐handoff event are given preference over other processes to attain a prompt response for new network conditions. We have implemented a prototype system on the Linux kernel 2.6. From the experimental results, our proposed protocol and application adaptation mechanisms are shown to effectively improve the performance of TCP and applications during vertical handoffs. Copyright © 2008 John Wiley & Sons, Ltd.     

Improving TCP performance in heterogeneous mobile environments by exploiting the explicit cooperation between server and mobile host

In recent years, many different kinds of wireless access networks have been deployed and become inseparable parts of the Internet. But TCP, the most widely used transport protocol of the Internet, was designed for stationery hosts. It faces severe challenges when user moves around in these networks and handoff occurs frequently. In this paper, we investigate the potential benefits of bringing explicit cooperation between TCP server and mobile host.For this purpose, TCP HandOff (TCP-HO), a practical end-to-end mechanism, is designed for improving TCP performance in heterogeneous mobile environments. TCP-HO assumes that a mobile host is able to detect the completion of handoff immediately and has a coarse estimation of new wireless link’s bandwidth. When a mobile host detects handoff completion, it will immediately notify the server through two duplicate ACKs, whose TCP option also carries the bandwidth of new wireless link. After receiving this notification, the server begins to transmit immediately and keeps updating ssthresh according to the bandwidth from mobile host and its new RTT samples. This update will end after four RTT samples or after congestion is detected.TCP-HO has been implemented in FreeBSD 5.4. Experimental results indicate that in heterogeneous mobile environments, TCP-HO can improve TCP performance a lot without adversely affecting cross traffic even when mobile host only has a coarse estimation of new wireless link’s bandwidth. Considering that more and more users are accessing the Internet through heterogeneous wireless networks and mobile host could have a coarse estimation of wireless link’s bandwidth, it should be worthwhile to change both server and mobile host for improving TCP performance.     

An Efficient Handoff Decision Algorithm for Vertical Handoff Between WWAN and WLAN

Vertical handoff is one significant challenge for mobility management in heterogeneous wireless networks. Compared with horizontal handoff, vertical handoff involves different wireless network technologies varying widely in terms of bandwidth, delay, coverage area, power consumption, etc. In this paper, we analyze the signal strength model of mobile node and present a new vertical handoff decision algorithm. This algorithm can adapt to the change of mobile node＇s velocity and improve the handoff efficiency significantly. We analyze the algorithm＇s performance and the effect of different parameters on handoff triggering. In addition, we propose three performance evaluation models and verify the algorithm＇s feasibility and effectiveness in simulations.     

基于RTT的TCPW拥塞控制算法的改进

无线网络受传输介质的限制,传输过程中会受到较大干扰,产生抖动,这种抖动会产生零星丢失而被拥塞控制机制误认为拥塞发生.TCP Westwood 协议(简称TCPW)就是这样不能区分拥塞丢失和无线抖动丢失,导致拥塞机制过于敏感,降低了带宽利用率.基于此,根据RTT值的估计对TCPW协议进行修改--称之为TCPW BR.该算法以测得的平滑RTT值并依据加权平均思想划分拥塞等级,判断拥塞丢失和无线抖动丢失.仿真结果表明,TCPW BR算法增强了无线网络对拥塞和随机差错的判断处理能力,提高了带宽利用率和吞吐量,并保持良好的公平性与友好性.     

一种改进的TCP拥塞控制算法及仿真

TCP协议提供面向连接、可靠的服务,但应用于时延敏感的实时网络时,并不能保证实时性。当网络负载过大时,会出现拥塞、传输延迟和丢包等问题。为了降低网络拥塞概率,提出了一种改进的TCP拥塞控制算法TCP-EB。该算法根据确认数据包的速率估计网络可用带宽,调整拥塞窗口的大小,提高带宽利用率。出现拥塞时,对窗口衰减速度进行限制,保证传输的优先级高于其他数据流。最后将TCP-EB与传统拥塞控制算法TCP Reno、TCP Vegas进行比较,结果表明,提高了网络吞吐量和网络传输的平滑性。     

Achieving high throughput and TCP Reno fairness in delay-based TCP over large networks

The transport control protocol (TCP) has been widely used in wired and wireless Internet applications such as FTP, email and http. Numerous congestion avoidance algorithms have been proposed to improve the performance of TCP in various scenarios, especially for high speed and wireless networks. Although different algorithms may achieve different performance improvements under different network conditions, designing a congestion algorithm that can perform well across a wide spectrum of network conditions remains a great challenge. Delay-based TCP has a potential to overcome above challenges. However, the unfairness problem of delay-based TCP with TCP Reno blocks widely the deployment of delay-based TCP over wide area networks. In this paper, we proposed a novel delay-based congestion control algorithm, named FAST-FIT, which could perform gracefully in both ultra high speed networks and wide area networks, as well as keep graceful fairness with widely deployed TCP Reno hosts. FAST-FIT uses queuing delay as a primary input for controlling TCP congestion window. Packet loss is used as a secondary signal to adaptively adjust parameters of primary control process. Theoretical analysis and experimental results show that the performance of the algorithm is significantly improved as compared to other state-of-the-art algorithms, while maintaining good fairness.     

用于分层移动IPv6网络拥塞控制的新机制

针对在IPv6无线网络切换过程中, 短暂的链路连接断开会导致反复丢包; 同时在切换过程中, 由于带宽的改变, 在新的接入点就有可能发生丢包或资源浪费以及拥塞。提出一种以TCP协议为基础的路径损耗确认（TCP-PLACK）机制来代替TCP选择确认机制（TCP-SACK）。每当一个TCP接收方在断开或切换后而连接到一个新的接入点时, 上述的TCP-PLACK机制就会发送一个特殊的确认, 其中包含有在新接入点丢包的详细信息和可用带宽。收到这个确认, 发送方会重新发送丢失的包, 并在新接入点根据带宽的可用性调整发送速率。实验结果表明, 该机制有利于改善IPv6网络切换过程中的丢失恢复和速率控制问题。     

面向噪声丢包感知的无线网络拥塞算法

针对无线网络链路干扰大、误码率高等特点，以及TCP Westwood算法（TCPW）存在估算带宽时过度依赖包的反馈，缺乏区分传输过程中丢包类型的缺点等问题，提出一种TCPW拥塞控制优化算法--TCPW-F。该算法利用发送速率等构建拥塞因子[F]作为判断丢包类型的依据，同时对判定发生噪声丢包时的拥塞窗口进一步调整，避免噪声丢包引起的窗口下降，提高该情况下窗口的发送效率。仿真结果表明，TCPW-F算法在时延性能方面表现更优，单位时间抖动趋于稳定的速度更快。在同一信道带宽下增大包生成速率，改进算法的实时吞吐量明显高于原算法，具备一定的噪声丢包感知能力，无线网络的TCP传输质量获得较大改善。