EHSTCP:改进的高速TCP算法

TCP在高带宽时延积网络中不能获得良好的性能,主要表现为低的吞吐量和大的窗口震荡.HSTCP算法解决了传统TCP算法在高带宽时延积网络下的性能瓶颈,但HSTCP在拥塞点时会产生大量的数据包丢失,同时当队列管理为去尾算法时,存在着严重的RTT不公平性问题.针对HSTCP算法的性能缺陷,该文提出一种在拥塞避免阶段进行拥塞避免模式切换的改进算法,称为EHSTCP.基于拥塞窗口历史值的端到端可用带宽预测方法,利用拥塞窗口历史信息来判断拥塞避免切换点.同时引入RTT公平因子,消除了HSTCP的RTT不公平性问题.NS2仿真实验验证了算法的有效性.     

HSTCP的RTT不公平现象及分析

传统的TCP在高速网络中丢失恢复时间长和丢失支持率低,不能充分利用网络带宽,难以实现大数据量传输。HSTCP（HighSpeed TCP）等高速TCP方案通过修改拥塞控制机制具有在高丢失率环境中TCP友好性和在低丢失率的环境中高扩展性等特点,但是通过实验发现存在严重的RTT（Round Trip Time）不公平性问题。对HSTCP中RTT不公平现象用模型进行了分析。     

FAST TCP公平性改进研究

FAST TCP是一种适用于高带宽长延迟的新型TCP拥塞控制协议,它能充分利用网络带宽,但存在比较严重的RTT不公平性.对FAST TCP的RTT不公平性进行了研究,通过仿真试验和数学分析,在原有算法中添加一个公平因子,从而显著改善其公平性和友好性.     

拥塞控制端算法的实验研究*

利用网络模拟软件NS-2对最近提出的端算法HSTCP、STCP、BIC-TCP、H-TCP和Fast-TCP进行了实验研究,从收敛性、稳定性、协议内公平性、RTT公平性、TCP友好性和带宽利用率等方面来比较它们的性能.     

FERED：公平性增强的RED算法

当前大多数AQM机制的实现算法都更多地强调了算法的效率和稳定性而忽视了公平性.已有研究表明,TCP的RTT不公平性问题和多拥塞链路环境下的不公平性问题广泛存在于众多的著名AQM实现算法中,比如RED,REM,PI和AVQ等.虽然FRED和Balanced RED可以解决这些不公平性问题,但它们的实现都需要在路由器上保留每流状态信息,算法可扩展性存在问题.在实际网络测量试验结果的基础上,提出了利用IP数据报头中的TTL字段信息来增强公平性的思路,并据此对RED算法进行了扩展,实现了一个公平性增强的RED算法(FERED).NS2仿真试验结果显示FERED可以显著增强公平性,同时保留了RED算法可以很好地控制队列长度的优点,而且FERED实现简单,无需在路由器保留每流状态信息.     

支持传输公平性的TCP拥塞控制机制研究

理论上 TCP窗口的和式增加积式减少的算法可以使拥塞窗口的大小收敛到一个理想的状态 ,且不同的结点可以公平共享带宽 .然而实验和分析表明 :TCP在不同的路由路径中是不会共享公平的连接的 .本文用实验验证了这种不公平性 ,并用一个算法消除了这种不公平性 .这个算法的思想就是对所有的 TCP连接能够找到一个共同的更新时间 .这样它们就可以以相同的速率去更新它们的窗口 ,从而消除了它的不公平性     

TCP New Veno:一种改进的TCP拥塞控制机制

TCP Veno协议通过对慢启动、拥塞避免和快速恢复的修改,改进了传统的TCP Reno的性能。然而,TCP的不公平性的问题仍然有待于解决。参与竞争的TCP流之间的不平衡可能造成某些通信源垄断队列空间。例如,当长RTT和短RTT流共存时,网络流量会逐渐集中于短RTT链路上。提出了一种新的TCP拥塞控制机制——TCP New Veno。其基本思路是通过引入带宽预测和动态窗口变化的思想进一步改进TCP Veno的性能,并导出其数学模型。数学分析和仿真实验都证明,改进后的算法在保证吞吐量的基础上提高了原算法的公平性。     

一种基于HSTCP的拥塞控制改进算法

针对TCP Reno在高带宽大延迟网络中带宽的利用率不高和HSTCP拥塞控制算法的公平性缺陷，该文提出了基于HSTCP的改进算法mHSTCP。当网络带宽未充分利用时，mHSTCP的窗口增长采用HSTCP模式，当网络状况趋于拥塞时，mHSTCP的窗口增长采用TCP Reno模式。测试结果表明，改进的mHSTCP算法对高速传输中不同流之间的公平性有明显的改善。     

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.     

一种基于RTT公平性的TCP慢启动算法

分析标准慢启动算法应用于包含GEO卫星链路的网络时存在的问题,提出一种基于RTT公平性的TCP慢启动改进算法。改进算法采用大初始窗口机制,慢启动初期窗口保持指数增长,慢启动后期引入窗口增长控制因子,使RTT较大的窗口增加较快,反之增加较慢。性能分析和仿真结果表明,改进算法可以在慢启动后期减缓拥塞窗口的增长速度,削弱RTT较小的TCP流竞争带宽的侵略性,在一定程度上保证不同RTT数据流共享带宽的公平性。     

一种改进HSTCP公平性的拥塞控制算法

标准传输控制协议在高速、长距离等应用条件下存在网络带宽不能充分利用的问题, 高速传输协议的提出可以解决此问题, 但在数据包往返时延上存在较大不公平性. 在分析传输公平性基础上, 通过离散事件网络模拟器进行公平性模拟验证, 提出一种通过添加公平因子改进高速传输协议公平性的算法, 经仿真验证, 该算法减轻了因RTT不同造成的不公平性.     

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.     

传输控制协议拥塞控制中往返时间公平性分析的新结论

通过平面图形和建模的方法,分析了传输控制协议(TCP)拥塞控制中的网络往返时间(RTT)公平性.对于瓶颈链路,如果采用主动队列管理(AQM),两个竞争的TCP数据流流量比值近似为RTT比的倒数,存在RTT不公平性;如果采用弃尾队列管理,只有在不发生数据包超时的情况下,才出现明显的RTT不公平性,如果出现数据包发送超时,则情况较为复杂,频繁出现RTT大的TCP数据流多占用网络带宽的现象,因此很难得到一般性的结论.最后通过NS2仿真来验证所建模型和分析方法的正确性.     

Can high-speed networks survive with DropTail queues management?

It has been observed that TCP connections that go through multiple congested links (MCL) have a smaller transmission rate than the other connections. Such TCP behavior is a result of two components (i) the cumulative packet losses that a flow experiences at each router along its path; (ii) the longer round trip times (RTTs) suffered by such flows due to non-negligible queueing delays at congested routers. This double “bias” against connections with MCLs has been shown to approximate the so-called minimum potential delay fairness principle in the current Internet. Despite the recent proliferation of new congestion control proposals for TCP in high-speed networks, it is still unclear what kind of fairness principle could be achieved with such newly proposed congestion control protocols in high-speed networks with large-delays. Studies already show that some high-speed TCP variants may cause surprisingly severe RTT unfairness in high-speed networks with DropTail routers.This paper studies the problem of unfairness in high-speed networks with some well-known high-speed TCP variants in presence of multiple congested links and highlights the severity of such unfairness when DropTail queue management is adopted.Through a simple synchronized loss model analysis, we show how synchronized losses with DropTail in high-speed networks could lead to severe RTT unfairness and drop probability (DP) unfairness; while random marking AQM schemes, which break the packet loss synchrony mitigate such unfairness dramatically by ensuring that the packet loss probability of a flow is the sum of the loss probabilities on the congested routers it crosses.Extensive simulations are carried out and the results support our findings.     

Impact of background traffic on performance of high-speed TCP variant protocols

This paper examines the effect of background traffic on the performance of existing high-speed TCP variant protocols, namely BIC-TCP, CUBIC, FAST, HSTCP, H-TCP and Scalable TCP. We demonstrate that the stability, link utilization, convergence speed and fairness of the protocols are clearly affected by the variability of flow sizes and round-trip times (RTTs), and the amount of background flows competing with high-speed flows in a bottleneck router. Our findings include: (1) the presence of background traffic with variable flow sizes and RTTs improves the fairness of most high-speed protocols, (2) all protocols except FAST and HSTCP show good intra-protocol fairness regardless of the types of background traffic, (3) HSTCP needs a larger amount of background traffic and more variable traffic than the other protocols to achieve convergence, (4) H-TCP trades stability for fairness; that is, while its fairness is good independent of background traffic types, larger variance in the flow sizes and RTTs of background flows causes the protocol to induce a higher degree of global loss synchronization among competing flows, lowering link utilization and stability, (5) FAST suffers unfairness and instability in small buffer or long delay networks regardless of background traffic types, and (6) the fairness of high-speed protocols depends more on the amount of competing background traffic rather than its rate variability. We also find that the presence of high-speed flows does not greatly reduce the bandwidth usage of background Web traffic.     

Logarithmic window increase for TCP Westwood+ for improvement in high speed,long distance networks

The majority of current Internet applications uses Transmission Control Protocol (TCP) for ensuring reliable end-to-end delivery of data over IP networks. The resulting path is, generally speaking, characterized by fairly large propagation delays (of the order of tens to hundreds of milliseconds) and increasing available bandwidth. Current TCP¹performance is far from representing an optimal solution in such operating conditions. The main reason lies in the conservative congestion control strategy employed, which does not let TCP to exploit the always increasing available path capacity. As a consequence, TCP optimization has been an active research topic in the research community over the last 25 years, boosted in the last few years by the widespread adoption of high-speed optical fiber links in the backbone and the emergence of supercomputing networked applications from one side and tremendous growth of wireless bandwidth in network access from another. This has led to the introduction of several alternative proposals for performing congestion control. Most of them focus on the effectiveness of bandwidth utilization, introducing more “aggressive” congestion control strategies. However, such approaches result often in unfairness among flows with substantially different RTTs, or do not present the inter-protocol fairness features required for incremental network deployment.In this paper, we propose TCP LogWestwood+, a TCP Westwood+ enhancement based on a logarithmic increase function, targeting adaptation to the high-speed wireless environment. The algorithm shows low sensitivity with respect to RTT value, while maintaining high network utilization in a wide range of network settings. The performance, fairness and stability properties of the proposed TCP LogWestwood+ are studied analytically, and then validated by means of an extensive set of experiments including computer simulations and wide area Internet measurements.     

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.