改进FAST TCP模型参数选择方案

针对FAST TCP协议参数选择指导方案存在的缺陷,提出了一种改进FAST TCP模型选择协议参数指导方案。建立了包含窗口更新周期参数和历史窗口值的非线性、时滞FAST TCP网络拥塞控制模型,在平衡点线性化后,采用pade近似式处理其时滞项,应用劳思-郝尔维兹稳定判据获得了确保该模型稳定的相关协议参数和网络参数应满足的条件。根据该稳定性条件,提出了一种根据控制器增益参数、窗口更新周期和网络相关参数选择合适协议参数的指导方案。NS2仿真结果表明,该方案得到更不保守的协议参数设置范围和更广泛的应用范围。     

基于认知无线电的频谱感知及TCP跨层设计

目前针对认知无线电网络中TCP协议的研究大多假设次用户的感知是完美的,且未综合考虑TCP协议参数和感知时间等因素对TCP性能的影响。针对上述问题,在TCP Westwood协议的基础上,提出一种基于传输预判的改进TCP协议,建立基于认知无线电网络的TCP吞吐量跨层模型。采用部分可观测马尔可夫决策过程对有感知误差的次用户频谱感知和接入过程进行建模,将其转换为信念状态马尔可夫决策过程,使用Sarsa(λ)算法对其进行求解,以在最大化TCP吞吐量的同时得到最优感知时间。仿真结果表明,与TCP Reno和TCP Newreno协议相比,使用该方案所得的TCP拥塞窗口值分别提高约42%和27%,平均吞吐量分别提高约5.7%和5.5%,当感知时间为0.2s时,所得的TCP平均吞吐量为最大值。     

一种Ad Hoc网络端到端的TCP拥塞控制改进方案

传统的TCP协议是为有线网络而设计的,它假定数据包的丢失是由网络拥塞引起的,然而在Ad Hoc网络中,除拥塞丢包外,其它非拥塞因素也会引起数据包的丢失。分析Ad Hoc网络影响TCP性能的主要因素,在原有拥塞控制方案MMJI的基础上,提出了一种端到端的TCP拥塞控制改进方案（Imp MMJI）。该方案能根据前向路径跳数自适应调整拥塞窗口的大小,防止拥塞窗口过快增长,当发生路由改变或链路中断时,重新计算拥塞窗口cwnd和ssthresh的值,以确保路由重建前后TCP连接负载率的一致性;并在ACK应答包的TCP首部增加了状态标志位,结合多个度量参数联合判断网络状态,提高网络状态识别的准确性,使发送端实时采取相应的措施。仿真结果表明,该方案能使网络吞吐量得到明显的提高,改善了TCP的性能。     

基于无线自组织网络的TCP Freeze-Probing改进协议

传统的TCP协议在有线网络中能够良好地工作,但用于无线自组织网络时则性能有所下降.其原因在于,传统的TCP协议无法分辨网络丢包原因,如网络拥塞、链路断开、信道错误或者链路改变.为了提高TCP协议在无线自组织网络中的性能,提出了一种TCP协议的改进方案TCP Freeze-Probing.该方案是一种端到端方法,不需要网络中间节点的反馈合作同时,提出了一种基于TCP Freeze-Probing的吞吐量模型并利用仿真对模型进行了验证.分析和仿真结果表明,该方案能够有效地改进TCP在无线自组织网络的性能.     

无线自组网络中TCP流公平性的分析与改进

研究了TCP(transmission control protocol)流在多跳无线自组网络中的公平性问题,发现IEEE802.11DCF协议在此环境下会导致严重的不公平性,即部分节点垄断了网络带宽而其他节点被饿死.首先,通过仿真分析了产生TCP流不公平性的原因,指出其根源在于MAC(media access and control)协议的不公平性,同时,TCP的超时机制加剧了不公平性的产生;然后,利用概率模型定量分析了TCP不公平性与MAC协议参数之间的关系,发现TCP流的公平性与TCP报文长度直接相关,并且增加MAC协议初始竞争窗口的大小能够有效提高公平性.据此,提出了一种根据TCP报文长度动态调节初始回退窗口大小的自适应回退MAC协议改进算法.理论分析和仿真表明,该算法在很大程度上可以有效缓解不公平性问题的产生,并且不会引起网络吞吐量的严重降低.     

FAST TCP稳定性研究

拥塞控制机制是一种分布式算法,本文分析和证明了一种在高速度高延时的TCP协议(FAST TCP)的拥塞控制机制的稳定性.在研究中,使用了一种基于时钟自同步分离时间模型.通过试验证明和理论分析,证明了FAST TCP在单瓶颈链路中拥有的反馈延时可以通过调整参数来达到稳定.     

无线网络中优化TCP性能的网络编码方法研究

网络编码的出现为改进网络的传输性能提供了新的方法.J.K.Sundararajan等人将网络编码技术与传输控制协议相结合,提出了基于网络编码的TCP/NC协议,其在改进无线网络中传统TCP的性能方面取得了明显效果.但该协议及其改进协议存在的数据传榆和解码操作同步性问题会严重影响TCP/NC性能.提出的改进协议TCP/NCW根据解码时间调节解码窗口,以确保数据传输和解码操作同步,从而获得更好性能.运用排队论分析了TCP/NCW最优解码窗口的存在性.在NS2中的仿真结果表明,在静态场景和动态场景下TCP/NCW的吞吐率比TCP/Vegas和TCP/NC有显著提高,同时也具有较好的公平性.     

基于不变流形的FAST TCP参数选择方法

在各路径FAST TCP连接到达服从泊松过程、传送文件长度服从指数分布和瓶颈链路处于重流量服务强度的条件下,利用含有各路径FAST TCP连接平均逗留时间的Lyapunov函数,证明了若处于不变流形状态,则各路径活跃FAST TCP连接平均逗留时间最短,并且在有限的时间内系统可到达该不变流形.针对FAST TCP采用静态映射表确定协议参数α的缺陷,按照降阶的不变流形中连接数和协议参数的关系,提出了一种在慢时间尺度比例下根据连接数期望值和不变流形迭代搜索协议参数的方法.NS-2仿真结果表明该方法是有效的.     

TCP Vegas在异构网络环境中的优化研究

研究TCP Vegas和TCP Reno中可用带宽分配均匀性问题。观察在异构网络环境中两种协议的拥塞窗口大小的变化。探讨TCP Vegas在异构网络环境中公平分配可用带宽资源的改进问题,通过NS-2进行仿真,提出改进方案,结果表明方法可行。     

基于改进慢启动算法的大文件快速传输

针对传统TCP在当前网络环境下传输大文件性能较低的问题,对TCP传输协议中的慢启动算法部分进行了相应的研究与优化。根据标准慢启动算法存在的问题并结合高速网络以及大文件传输的性能特点,提出了一个具有网络状态感知能力的慢启动改进算法。改进算法主要优化了TCP拥塞窗口的增长策略,它实时地监测文件在传输过程中TCP报文段的往返时间（RTT）,并根据RTT的变化情况采用不同的窗口增长方式更新拥塞窗口;将改进算法部署在Linux网络模块中并分别在模拟网络环境和实际网络中进行测试。实验结果显示,改进算法能使发送窗口一直保持在一个较高的水平,实际数据传输速率和吞吐量均有了明显的提高。     

非线性时变时滞FAST TCP系统低保守全局稳定性

研究了非线性、时变时滞的快速传输控制协议(FAST TCP)系统的全局稳定性. 首先, 利用其时滞变化与源端发送窗口变化成比例关系和其非线性函数是严格单调递减函数这两个特性, 计算每个振荡周期的解轨迹界, 建立计算轨迹界的改进型迭代公式, 得到了比已有结论更低保守的全局稳定性条件. 最后, 通过网络模拟器2(NS-2)仿真实例验证了改进的全局稳定性条件的有效性.     

Global asymptotic stability of FAST TCP in the presence of link dynamics

We propose a continuous-time model to describe a single-link single-source network with the FAST TCP source. The proposed model explicitly includes both the queuing delay dynamics for the link dynamics and the time-varying network feedback delay. Based on the proposed model, we establish a sufficient condition for the global asymptotic stability of the FAST TCP network. We prove the sufficient condition by constructing two sequences that represent the variations of the lower and upper bound of the source’s congestion window with respect to time, and by showing that the two sequences converge to the equilibrium point of the congestion window. The simulation results illustrate the validity of the sufficient condition for the global asymptotic stability.     

Unreliable transport protocol using congestion control for high-speed networks

Currently there is no control for the real-time traffic of multimedia applications using UDP (User Datagram Protocol) in high-speed networks. Therefore, although a number of high-speed TCP (Transmission Control Protocol) protocols have been developed for gigabit-speed (or faster) links, the real-time traffic could also congest the network and result in unfairness and throughput degradation of TCP traffic. In this paper, a new unreliable transport protocol, FAST DCCP, is presented for the real-time traffic in high-speed networks. FAST DCCP is based on the DCCP protocol and adopts the FAST scheme to realize congestion control. Some modifications have been made to the mechanisms inherited from DCCP so as to let the proposed protocol can efficiently operate under a large size window. In addition, an enhanced protocol, EEFAST DCCP, using the measurements of one-way delay to dynamically adjust the window size is also proposed to improve the throughput of FAST DCCP with the effect of reverse traffic. Simulation results show that FAST DCCP not only can satisfy the requirements of real-time data delivery, but also perform well in bandwidth utilization and fairness in high-speed networks. Meanwhile, EEFAST DCCP is able to effectively conquer the throughput degradation caused by the reverse traffic.     

An Improved Link Model for Window Flow Control and Its Application to FAST TCP

This paper presents a link model which captures the queue dynamics in response to a change in a transmission control protocol (TCP) source's congestion window. By considering both self-clocking and the link integrator effect, the model generalizes existing models and is shown to be more accurate by both open loop and closed loop packet level simulations. It reduces to the known static link model when flows' round trip delays are identical, and approximates the standard integrator link model when there is significant cross traffic. We apply this model to the stability analysis of fast active queue management scalable TCP (FAST TCP) including its filter dynamics. Under this model, the FAST control law is linearly stable for a single bottleneck link with an arbitrary distribution of round trip delays. This result resolves the notable discrepancy between empirical observations and previous theoretical predictions. The analysis highlights the critical role of self-clocking in TCP stability, and the proof technique is new and less conservative than existing ones.      

Internet primal–dual congestion control: Stability and applications

This paper studies the stability of the primal–dual algorithm for Internet congestion control with round-trip communication delays. The model is formulated as a time-delayed multivariable control system for a general network, and then is studied by using time-delayed control theory. A necessary and sufficient condition is established for a network with a general topology, and then is further discussed for the representative dumbbell network. The stability condition is applied to the design of FAST TCP and adaptive virtual queue (AVQ) scheme as guideline, which is validated by simulation results.     

改进的TCP/RED模型的稳定性分析

TCP自同步特性可提高网络稳定性,TCP/RED拥塞控制模型忽略了TCP自同步特性。针对该问题,提出包含TCP自同步特性更接近实际网络的改进TCP/RED拥塞控制模型。应用Nyquist稳定性判据给出当改进模型在复平面上属于某一不包含-1+j0点的下半平面时的稳定性条件,得到比TCP/RED模型更宽松的RED参数设置范围。NS2仿真验证了该稳定性条件的有效性。     

Delay-based TCP congestion avoidance: A network calculus interpretation and performance improvements

In delay-based TCP congestion avoidance mechanisms, a source adjusts its window size to adapt to changes in network conditions as measured through changing queueing delays. Although network calculus (NC) has been used to study window flow control and determine performance bounds, there is a lack of a bridge between NC theory and the practical issues of delay-based TCP congestion avoidance. In this paper, we use an NC-based approach to derive ideal congestion controllers for representative delay-based window flow control models with time-variant feedback delays. We show that the basic delay-based TCP congestion avoidance mechanisms in TCP Vegas, Enhanced TCP Vegas, and FAST TCP can be viewed as different approaches to approximating a certain NC controller. Moreover, we derive another NC controller that is explicitly constructed to address the throughput degradation of the current delay-based methods due to delayed acknowledgement (ACK) packets caused by network traffic in the reverse path (ACK packet) direction. The approximation of this F-model NC-based controller is shown to provide better throughput and fairness over the existing delay-based methods for a variety of network topologies in ns-2 simulations.     

对FASTTCP网络拥塞控制优缺点的分析

随着互联网规模与应用的扩大，网络环境变得复杂，传统的拥塞控制已经不能适应高速网络，FASTTCP因此应运而生。通过仿真实验与传统TCPReno进行对比，发现FASTTCP确实有很大的提高，但也存在问题需要改进。     

TCP的RTT不公平性问题模型分析与仿真

传输控制协议(TCP)在保证当前网络的稳定运行上起着至关霞要的作用.然而,TCP的和式增加积式减小的拥塞窗口更新策略也存在一定的问题:具有不同往返时延(RTT)的TCP数据流在竞争瓶颈带宽时将引发带宽不公平分配问题.Chiu利用平面图形的方法对两个数据流间的RTr不公平性问题进行了直观的分析,并在网络研究领域内被广泛认可.通过建立TCP拥塞窗口更新策略的简化的模型,从理论上更加深刻地分析了TCP所存在的RTT不公平性问题,得到了更具普遍性的结论.同时,通过在NS2网络仿真平台下的试验验证了该模型的合理性.     

单链路多源网络中FAST TCP的稳定性

FAST TCP是面向未来高带宽延迟积网络提出的新一代传输控制协议.在不同的网络环境中分析其稳定性是目前研究的热点之一.建立了单链路多源网络中,FAST TCP和路由器组成的闭环系统的非线性、延迟微分方程模型.根据模型得到系统的平衡点,在平衡点附近将非线性模型进行线性化,采用现代控制理论方法证明了系统局部稳定的充分条件,解除了稳定性条件与网络参数之间的耦合.大量的仿真实验验证了稳定性结论的正确性.