一种改进的TCP稳态流吞吐量模型及其性能分析

根据对Internet上TCP稳态数据流的实测分析,在Padhye模型的基础上,本文提出了一种改进的TCP稳态数据流模型。该模型不仅考虑了三次重复应答和超时事件对TCU稳态流吞吐量的影响,而且还充分考虑了超时后慢启动过程的影响。文中给出了改进模型与实测结果的分析比较,表明改进的模型能更好地预测实际TCP数据流的吞吐量性能。最后,本文还分析了模型的几个主要参数对TCP吞吐量性能的影响。     

考虑慢启动影响的TCP吞吐量模型

TCP吞吐量模型的研究是网络协议研究的一个重要方面,同一些其它模型相比,Padhye提出的TCP吞吐量模型比较精确地描述了TCP吞吐量与往返时间、丢包率和超时时限的关系,但在丢包率很高的场合,Padhye模型误差较大,本文分析了高网络负荷下TCP传输的性能,提出了一个高网络负荷下TCP吞吐量的改进Padhye模型,实验表明,该模型在高网络负荷环境下更接近实际情况.     

一种基于链路带宽估计的无线TCP改进算法

本文提出了一种能够通过带宽估计来判断无线链路报文段丢失原因,并采取相应拥塞控制机制的无线TCP算法-TCP_LD(TCP Loss Detection).文章采用Padhye模型的建模方法,推导了TCP_LD的稳态流量模型.理论分析和仿真结果说明,TCP_LD能够有效的区分出发生报文段丢失的原因,即拥塞丢失或者链路突发差错,从而提高系统的流量.     

T CP 中速率控制的使用

本文讨论了TCP在客户端－服务器模式中出现的TCP重起（慢启动）和慢启动忽略的情况，分析了这些情况给服务质量带来的问题，从而提出一一种能对这两种情况进行析中的方法；速率控制，并对这种方法进行了论证。     

TCP Vegas-O:一种新的基于延迟估计的TCP Vegas改进算法

针对TCP Vegas会出现慢启动过早结束、拥塞窗口过小导致带宽利用率下降的问题,以及在与Reno等基于丢包来判断拥塞的算法竞争带宽时的公平性差等问题,文中分别就慢启动和拥塞避免阶段进行了相应的改进,最后将其结合.仿真结果表明,该算法对慢启动过早结束、带宽公平性等TCP Vegas协议的缺陷有了明显的改善,特别是在高带宽时延乘积网络中.     

大带宽时延积网络TCP Vegas自适应慢启动算法

TCP Vegas具有比TCP Reno更好的带宽利用能力和稳定性,但是在带宽时延积较大的网络中,TCP Vegas会出现慢启动过早结束、拥塞窗口过小的问题,降低了传输效率.文中在分析慢启动结束的原因和条件的基础上,提出一种对临时性排队时延进行估计,将其排除后再进行慢启动结束条件判断的TCP Vegas慢启动算法,对不同网络条件有自适应能力.仿真结果表明本算法能有效避免慢启动过早结束,使TCP性能明显改善.     

编码TCP的建模与性能分析

研究了无线网络端到端分组丢失和编码纠错对TCP协议工作过程的影响机制,基于三维Markov链建模描述TCP拥塞窗口和可用窗口的变迁过程,在忽略慢启动阶段的条件下,以最大窗口尺寸、端到端分组丢失率和编码冗余系数为输入参数,通过数值计算求解编码TCP的吞吐率,据此定量分析影响编码TCP性能的因素。基于NS2的模拟实验结果表明基于Markov链的数值计算结果具有较高的准确度。     

多重慢启动TCP协议研究

多重慢启动TCP协议是为了适应高速网络的发展而提出的一类标准TCP协议的改进协议.本文首先分析了多重慢启动TCP协议的原理,然后详细阐述了近年来提出的一些典型多重慢启动TCP协议,最后对多重慢启动TCP协议下一步的工作方向进行了展望.     

一种拥塞感知的TFRC协议慢启动算法

 本文分析了TFRC(TCP-Friendly Rate Control)协议在慢启动阶段采用类似TCP协议的倍增发送速率机制存在的问题,提出了一种利用回路响应时间(Round Trip Time,RTT)来自适应调节慢启动阶段速率的算法.通过分析实际RTT值和EWMA(Exponentially Weighted Moving Average)处理后的平均RTT值来感知网络当前的拥塞状况,以调节发送速率的激进程度.仿真实验表明,该方法对TFRC协议具有明显的改进作用,减少了慢启动阶段结束时的报文丢失率,提高了协议的传输平稳度和吞吐量,从而能更有效地适应多媒体流的传输要求.     

TCP-Rab的吞吐量模型及实验研究

基于接收端通告的TCP(TCP-Rab,receiver advertisement based TCP)协议是我们实现的一种新的TCP协议,在文献[1]中对TCP-Rab的算法设计、实现进行了详细阐述。本文对TCP-Rab进行了少量改进,重点导出了TCP-Rab的吞吐量性能模型,并对TCP-Rab进行了试验研究。该模型采用统计的方法,在导出TCP-Rab的吞吐量性能模型的时候,不仅考虑了TCP连接的拥塞避免阶段对吞吐量的影响,也考虑了慢启动阶段对吞吐量的影响,同时还考虑了一个发送窗口内多个数据包随机丢失对吞吐量的影响,因此该模型能适用于实际的网络环境中。     

Performance Enhancement of TCP in Dynamic Bandwidth Wired and Wireless Networks

In this paper, we propose a scheme that dynamically adjusts the slow start threshold (ssthresh) of TCP. The ssthresh estimation is used to set an appropriate ssthresh. A good ssthresh would improve the transmission performance of TCP. For the congestion avoidance state, we present a mechanism that probes the available bandwidth. We adjust the congestion window size (cwnd) appropriately by observing the round trip time (RTT) and reset the ssthresh after quick retransmission or timeout using the ssthresh estimation. Then the TCP sender can enhance its performance by using the ssthresh estimation and the observed RTT. Our scheme defines what is considered an efficient transmission rate. It achieves better utilization than other TCP versions. Simulation results show that our scheme effectively improves TCP performance. For example, when the average bottleneck bandwidth is close to 30% of the whole network bandwidth, our scheme improves TCP performance by at least 10%.

QoS aware transmission control scheme for multihop WiMAX network

The IEEE 802.16j Mobile Multihop Relay (MMR) WiMAX network allows the number of hops between the end user and the base station to be more than two hops. It supports non‐real‐time Polling Service, which considers the minimum reserved rate and the maximum sustained rate as a QoS requirements. The reliability of sending the data over MMR WiMAX is achieved by using Transmission Control Protocol (TCP) in transport layer and automatic repeat request in the link layer. However, the use of automatic repeat request in the link layer makes the round trip time fluctuate rapidly, which increases the possibility of retransmission timeout (RTO) expiration. TCP performance degrades because of frequent timeout, and hence the QoS transmission rates cannot be satisfied. Therefore, this paper presents an RTO smoothing scheme and QoS aware transmission control to enhance the performance of data transmission over MMR WiMAX networks. The RTO smoothing scheme aims to reduce the frequent timeout occurrences. The slow start threshold and maximum congestion window are adjusted to satisfy the required QoS and it provides transmission rate fairness for the users at different hops. The results showed that, the proposed schemes reduce the timeout, and improve the utilization of the allocated resources and TCP throughput. Copyright © 2012 John Wiley & Sons, Ltd.     

Adding network-layer intelligence to mobile receivers for solving spurious TCP timeout during vertical handoff

With the help of mobile IP/IPv6 and soft handoff, ongoing TCP sessions can remain active and handoff packet loss can be avoided. However, TCP still faces several performance degradation issues due to the disparities in bandwidth and propagation delay between different access networks. Particularly, during vertical handoffs, some undesirable phenomena may erroneously trigger TCP congestion-control actions and thus degrade TCP performance. In this article we tackle the spurious timeout problem frequently associated with handovers from fast to slow links. We propose three network-layer schemes: fast ACK, slow ACK, and ACK delaying. These schemes require only minor modifications to the network layer of mobile receivers and no change to the TCP protocol and the TCP sender. The simulation results show that these schemes can effectively improve TCP performance during soft vertical handoffs     

A New Analytical Model for TCP Reno with Bursts Error Considered

1　IntroductionInthepastfewyears,moststudiesofTCPpro tocolhaveconcentratedonexperimentsandsimula tions[1～2 ,9～ 1 4 ] .Theprevioustheoreticalstudieson lyconsideredTCP sowncapacity ,butthecharac teristicsofwirelesslinkswerenotcaredaboutenough .Evensomeofthemconsideredwirelesslinks,butmostofthemfailedtoconsidertheeffectsofbursterror[3,1 1 ] whichisanimportantfeatureofwirelesslinks.InRef.[4],ZORZIM proposesaanalyticalmodelforTCP ,buthismodelisverycomplicated ,andalsoatsomeplacesheusesuppe…     

TCP Westwood: End-to-End Congestion Control for Wired/Wireless Networks

TCP Westwood (TCPW) is a sender-side modification of the TCP congestion window algorithm that improves upon the performance of TCP Reno in wired as well as wireless networks. The improvement is most significant in wireless networks with lossy links. In fact, TCPW performance is not very sensitive to random errors, while TCP Reno is equally sensitive to random loss and congestion loss and cannot discriminate between them. Hence, the tendency of TCP Reno to overreact to errors. An important distinguishing feature of TCP Westwood with respect to previous wireless TCP extensions is that it does not require inspection and/or interception of TCP packets at intermediate (proxy) nodes. Rather, TCPW fully complies with the end-to-end TCP design principle. The key innovative idea is to continuously measure at the TCP sender side the bandwidth used by the connection via monitoring the rate of returning ACKs. The estimate is then used to compute congestion window and slow start threshold after a congestion episode, that is, after three duplicate acknowledgments or after a timeout. The rationale of this strategy is simple: in contrast with TCP Reno which blindly halves the congestion window after three duplicate ACKs, TCP Westwood attempts to select a slow start threshold and a congestion window which are consistent with the effective bandwidth used at the time congestion is experienced. We call this mechanism faster recovery. The proposed mechanism is particularly effective over wireless links where sporadic losses due to radio channel problems are often misinterpreted as a symptom of congestion by current TCP schemes and thus lead to an unnecessary window reduction. Experimental studies reveal improvements in throughput performance, as well as in fairness. In addition, friendliness with TCP Reno was observed in a set of experiments showing that TCP Reno connections are not starved by TCPW connections. Most importantly, TCPW is extremely effective in mixed wired and wireless networks where throughput improvements of up to 550% are observed. Finally, TCPW performs almost as well as localized link layer approaches such as the popular Snoop scheme, without incurring the overhead of a specialized link layer protocol.     

Modeling TCP SACK performance over wireless channels with completely reliable ARQ/FEC

In this paper, we propose an analytical cross‐layer model for a Transmission Control Protocol (TCP) connection running over a covariance‐stationary wireless channel with a completely reliable Automatic Repeat reQuest scheme combined with Forward Error Correction (FEC) coding. Since backbone networks today are highly overprovisioned, we assume that the wireless channel is the only one bottleneck in the system which causes packets to be buffered at the wired/wireless interface and dropped as a result of buffer overflow. We develop the model in two steps. At the first step, we consider the service process of the wireless channel and derive the probability distribution of the time required to successfully transmit an IP packet over the wireless channel. This distribution is used at the next step of the modeling, where we derive expressions for the TCP long‐term steady‐state throughput, the mean round‐trip time, and the spurious timeout probability. The developed model allows to quantify the joint effect of many implementation‐specific parameters on the TCP performance over both correlated and non‐correlated wireless channels. We also demonstrate that TCP spurious timeouts, reported in some empirical studies, do not occur when wireless channel conditions are covariance‐stationary and their presence in those measurements should be attributed to non‐stationary behavior of the wireless channel characteristics. Copyright © 2011 John Wiley & Sons, Ltd.     

Comparative performance analysis of versions of TCP in a localnetwork with a lossy link

We use a stochastic model to study the throughput performance of various transport control protocol (TCP) versions (Tahoe (including its older version that we call OldTahoe), Reno, and NewReno) in the presence of random losses on a wireless link in a local network. We model the cyclic evolution of TCP, each cycle starting at the epoch at which recovery starts from the losses in the previous cycle. TCP throughput is computed as the reward rate in a certain Markov renewal-reward process. Our model allows us to study the performance implications of various protocol features, such as fast retransmit and fast recovery. We show the impact of coarse timeouts. In the local network environment the key issue is to avoid a coarse timeout after a loss occurs. We show the effect of reducing the number of duplicate acknowledgements (ACKs) for triggering a fast retransmit. A large coarse timeout granularity seriously affects the performance of TCP, and the various protocol versions differ in their ability to avoid a coarse timeout when random loss occurs; we quantify these differences. We show that, for large packet-loss probabilities, TCP-Reno performs no better, or worse, than TCP-Tahoe. TCP-NewReno is a considerable improvement over TCP-Tahoe, and reducing the fast-retransmit threshold from three to one yields a large gain in throughput; this is similar to one of the modifications in the TCP-Vegas proposal. We explain some of these observations in terms of the variation of fast-recovery probabilities with packet-loss probability. The results of our analysis compare well with a simulation that uses actual TCP code     

Analysis of Internet services in IP over ATM networks

This article presents a trace-driven analysis of IP over ATM services from an ATM-centric standpoint. We provide a characterization of TCP flows as VBR streams with burstiness (MBS) and throughput (SCR). On the other hand, a macroscopic analysis comprising percentage of flows and bytes per service, TCP average transaction duration, and bytes transferred both ways is also presented. The ATM link under analysis concentrates traffic from a large population of 1500 hosts from the Public University of Navarra campus network, that produce 1,700,000 TCP connections approximately in the measurement period of one week. The results obtained from such a wealth of data indicate that the sustained throughput of Web connections does not grow beyond 80 kb/s with 70 percent probability in the data transfer phase (i.e., in the established state), and we observe a strong influence of the connection establishment phase on the user-perceived throughput. On the other hand, the burstiness of individual TCP connections is rather small; namely, TCP connections do not produce bursts according to the geometric law given by slow start and commonly assumed in previously published studies