MANET中TCP性能改进研究

TCP是为有线网络所设计的,其拥塞控制机制中,假设丢包是由网络拥塞造成的这一结论在MANET中不再适用。MANET中的高信道误码率,路由频繁中断等因素都会造成丢包。TCP错误的将所有丢包事件都当作拥塞处理,造成了传输性能的极大下降。文中首先总结了MANET中导致TCP传输性能下降的主要原因,然后对现有的一些典型TCP改进方案进行了讨论,最后对这些技术方案进行了比较,并指出今后研究的重点。     

卫星网络传输控制协议研究

标准传输控制协议（TCP）直接用于卫星网络,由于其不能够区分丢包和慢启动的长时间需求等导致协议的非适应性。在分析卫星网络影响协议性能因素的基础上,总结了当前针对这些因素造成的不利影响所增加的额外技术,并对现有提高卫星网络TCP性能的方法进行了分类总结,比较了这些TCP的特点,最后指出了卫星网络TCP的未来研究策略和发展趋势。     

A cross-layer congestion and contention window control scheme for TCP performance improvement in wireless LANs

Neither the current TCP protocol nor the standard backoff algorithm of IEEE 802.11 protocol is able to distinguish corruption loss from congestion or collision loss. Hence, high transmission errors and a varying latency inherent in wireless channel would have a seriously adverse effect on the performance of TCP. In this paper, we propose a novel and pragmatic cross-layer approach with joint congestion and contention window control scheme to improve the performance of TCP in IEEE 802.11 wireless environments. In addition to theoretical analysis, simulations are conducted to evaluate the proposed scheme. As it turns out, our design indeed provides a more efficient solution for frequent transmission loss and enables TCP to distinguish between congestion loses and transmission errors, thus to take proper remedial actions.     

Retransmission loss recovery by duplicate acknowledgment counting

The performance of transmission control protocol (TCP) is largely dependent upon its loss recovery. Therefore, it is a very important issue whether the packet losses may be recovered without retransmission timeout (RTO) or not. TCP always evokes RTO if a retransmitted packet is lost again. In order to alleviate this problem, we propose an algorithm called duplicate acknowledgment counting (DAC). We use a stochastic model to evaluate the performance of DAC, and compare it with TCP NewReno. Numerical results evaluated by simulations show that DAC can improve TCP loss recovery significantly in presence of random losses.     

Comprehensive performance analysis of a TCP session over a wireless fading link with queueing

A link model-driven approach toward transmission control protocol (TCP) performance over a wireless link is presented. TCP packet loss behavior is derived from an underlying two-state continuous time Markov model. The approach presented here is (to our knowledge) the first that simultaneously considers (1) variability of the round-trip delay due to buffer queueing; (2) independent and nonindependent (bursty) link errors; (3) TCP packet loss due to both buffer overflow and channel errors; and (4) the two modes of TCP packet loss detection (duplicate acknowledgments and timeouts). The analytical results are validated against simulations using the ns-2 simulator for a wide range of parameters; slow and fast fading links; small and large link bandwidth-delay products. For channels with memory, an empirical rule is presented for categorizing the impact of channel dynamics (fading rate) on TCP performance.     

An Adaptive Delayed Acknowledgment Strategy to Improve TCP Performance in Multi-hop Wireless Networks

In multi-hop wireless networks, transmission control protocol (TCP) suffers from performance deterioration due to poor wireless channel characteristics. Earlier studies have shown that the small TCP acknowledgments consume as much wireless resources as the long TCP data packets. Moreover, generating an acknowledgment (ACK) for each incoming data packet reduces the performance of TCP. The main factor affecting TCP performance in multi-hop wireless networks is the contention and collision between ACK and data packets that share the same path. Thus, lowering the number of ACKs using the delayed acknowledgment option defined in IETF RFC 1122 will improve TCP performance. However, large cumulative ACKs will induce packet loss due to retransmission time-out at the sender side of TCP. Motivated by this understanding, we propose a new TCP receiver with an adaptive delayed ACK strategy to improve TCP performance in multi-hop wireless networks. Extensive simulations have been done to prove and evaluate our strategy over different topologies. The simulation results demonstrate that our strategy can improve TCP performance significantly.     

移动Ad Hoc网络中TCP性能的分析和改进

分别采用静态链状拓扑和动态网络拓扑,通过仿真分析和比较了不同TCP版本在移动AdHoc网络中的性能。文中还提出了一种改进的选择性应答算法(SSACK),以解决移动AdHoc网络中由于高丢包率和路由重建引起的TCP接收窗的"缺口(gap)"问题。仿真结果显示,改进的SSACK算法在一定程度上改善了移动AdHoc网络中的TCP性能。     

Improving Throughput in Lossy Wired/Wireless Networks

Wireless communication is more prone to random loss than wired communication because of noise and mobility. Over years researchers have developed TCP variants that do not decrease the send window when random loss arises. Years ago it was introduced TCP CERL algorithm that proved to present a high performance compared to other protocols. Here, we test CERL assuming two-way transmission of relatively heavy load and compare with TCP BIC, TCP NewReno, TCP Westwood+, TCP NewJersey and TCP Illinois. Simulation Results show that TCP CERL gains a 145%, 137%, 120%, 97% and 125% throughput improvement over New Reno, Bic, Westwood+, New Jersey and Illinois, respectively.

     

一种异构网络TCP拥塞控制算法

针对互联网中端对端带宽、时延和丢包率等的差异性日益加剧,导致TCP传输性能严重退化,该文提出一种链路自适应TCP拥塞控制算法(INVS)。INVS在拥塞避免阶段初期采用基于指数函数的凸窗口增长函数,以提高链路利用率;在窗口增长函数中引入了自适应增长因子实现窗口增长速率与链路状态相匹配;采用了自适应队列门限的丢包区分策略以提高无线环境下TCP的性能。性能分析和评估表明,INVS提高了TCP拥塞控制算法的吞吐量、公平性、链路利用率和RTT公平性。     

ATM cell delay and loss for best-effort TCP in the presence ofisochronous traffic

This paper reports the findings of a simulation study of the queueing behavior of “best-effort” traffic in the presence of constant bit-rate and variable bit-rate isochronous traffic. In this study, best-effort traffic refers to ATM cells that support communications between host end systems executing various applications and exchanging information using TCP/IP. The performance measures considered are TCP cell loss, TCP packet loss, mean cell queueing delay, and mean cell queue length. Our simulation results show that, under certain conditions, best-effort TCP traffic may experience as much as 2% cell loss. Our results also show that the probability of cell and packet loss decreases logarithmically with increased buffer size     

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     

解决FAST TCP缓存溢出相关问题的改进pacing technique算法和α参数调整算法

分析FAST TCP在缓存溢出发生时的性能,发现在缓存溢出场景中,收敛中的FAST TCP流经历严重的报文段丢失。相反,已经收敛了的FAST TCP流维持着高吞吐量和低报文段丢失概率。这种不公平是由FAST TCP缩减其窗口时的零传输率导致的。通过修改FAST TCP pacing算法,可以解决此问题。文中提出的α-adjusting算法,通过动态调整FAST TCP协议中的α参数来避免频繁的缓存溢出。通过分析ns2仿真结果,证明该算法在公平性和稳定性方面可获得令人满意的性能。     

Cross‐layer design to improve elastic traffic performance in WLANs

In this paper, we are interested in improving TCP flow performance when a short loss of 802.11 signal leads to losing segments and triggers inappropriately TCP congestion control mechanisms. A set of measurements in a common wireless environment with signal losses due to mobility or interference is made to highlight the distinct MAC and TCP loss recovery levels and the lack of interactions between them. Initially, we demonstrate the interest of adapting the 802.11 MAC layer Retry Limit parameter in the case of signal losses due to distance or obstacles (mobility). Thus, a first‐level loss differentiation algorithm (LDA) acting at the MAC layer is proposed to improve TCP flow performance in the case of segment losses due to mobility. Hence, for a signal failure, the MAC layer reacts consequently by dynamically adapting the Retry Limit parameter. This adaptation allows avoiding a costly end‐to‐end TCP loss recovery. Segment losses due to interference are differentiated from those due to congestion through the use of a second‐level LDA. The latter is a cross‐layer LDA acting at the TCP layer but using a specific 802.11 parameter, the AckFailureCount, to realize the targeted loss differentiation. The TCP NewReno version is then adapted in order to integrate the cross‐layer LDA results and to avoid reducing the TCP congestion window unsuitably. The efficiency and completeness of a solution integrating both LDA schemes is then discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

Syndrome: a light‐weight approach to improving TCP performance in mobile wireless networks

It is well known that the performance of TCP deteriorates in a mobile wireless environment. This is due to the fact that although the majority of packet losses are results of transmission errors over the wireless links, TCP senders still take packet loss as an indication of congestion, and adjust their congestion windows according to the additive increase and multiplicative decrease (AIMD) algorithm. As a result, the throughput attained by TCP connections in the wireless environment is much less than it should be. The key problem that leads to the performance degradation is that TCP senders are unable to distinguish whether packet loss is a result of congestion in the wireline network or transmission errors on the wireless links. In this paper, we propose a light‐weight approach, called syndrome, to improving TCP performance in mobile wireless environments. In syndrome, the BS simply counts, for each TCP connection, the number of packets that it relays to the destination host so far, and attaches this number in the TCP header. Based on the combination of the TCP sequence number and the BS‐attached number and a solid theoretical base, the destination host will be able to tell where (on the wireline or wireless networks) packet loss (if any) occurs, and notify TCP senders (via explicit loss notification, ELN) to take appropriate actions. If packet loss is a result of transmission errors on the wireless link, the sender does not have to reduce its congestion window. Syndrome is grounded on a rigorous, analytic foundation, does not require the base station to buffer packets or keep an enormous amount of states, and can be easily incorporated into the current protocol stack as a software patch. Through simulation studies in ns‐2 (UCB, LBNL, VINT network simulator, http://www‐mash.cs.berkeley.edu/ns/ ), we also show that syndrome significantly improves the TCP performance in wireless environments and the performance gain is comparable to the heavy‐weight SNOOP approach (either with local retransmission or with ELN) that requires the base station to buffer, in the worst case, a window worth of packets or states. Copyright © 2001 John Wiley & Sons, Ltd.     

Packet Recycling and Delayed ACK for Improving the Performance of TCP over MANETs

Most of the schemes that were proposed to improve the performance of transmission control protocol (TCP) over mobile ad hoc networks (MANETs) are based on a feedback from the network, which can be expensive (require extra bandwidth) and unreliable. Moreover, most of these schemes consider only one cause of packet loss. They also resume operation based on the same stand-by parameters that might vary in the new route. Therefore, we propose two techniques for improving the performance of TCP over MANETs. The first one, called TCP with packet recycling (TCP-PR), allows the nodes to recycle the packets instead of dropping them after reaching the retransmission limit at the MAC layer. In the second technique, which is called TCP with adaptive delay window (TCP-ADW), the receiver delays sending TCP ACK for a certain time that is dynamically changed according to the congestion window and the trip time of the received packet. TCP-PR and TCP-ADW are simple, easy to implement, do not require network feedback, compatible with the standard TCP, and do not require distinguishing between the causes of packet loss. Our thorough simulations show that the integration of our two techniques improves the performance of TCP over MANETs.     

Reducing the impact of false time out on TCP performance in TCP over OBS networks

Random burst contention losses plague the performance of Optical Burst Switched networks. Such random losses occur even in low load network condition due to the analogous behavior of wavelength and routing algorithms. Since a burst may carry many packets from many TCP sources, its loss can trick the TCP sources to conclude/infer that the underlying (optical) network is congested. Accordingly, TCP reduces sending rate and switches over to either fast retransmission or slow start state. This reaction by TCP is uncalled-for in TCP over OBS networks as the optical network may not be congested during such random burst contention losses. Hence, these losses are to be addressed in order to improve the performance of TCP over OBS networks. Existing work in the literature achieves the above laid objective at the cost of violating the semantics of OBS and/or TCP. Several other works make delay inducing assumptions. In our work, we introduce a new layer, called Adaptation Layer, in between TCP and OBS layers. This layer uses burst retransmission to mitigate the effect of burst loss due to contention on TCP by leveraging the difference between round trip times of TCP and OBS. We achieve our objective with the added advantage of maintaining the semantics of the layers intact.     

在Ad Hoc网络中TCP-SACK性能研究及改进

在Ad Hoc网中,经常发生链路失效和路由变化。TCP把数据包的丢失归结为链路拥塞,所以TCP在Ad Hoc网中表现很差。在本文中通过应用一种新的方法,基于传输包乱序检测与响应方法,研究TCP在Ad Hoc网中的性能。通过实验结果表明,此算法解决了,提高了网络的吞吐量。     

Bandwidth estimation schemes for TCP over wireless networks

The use of enhanced bandwidth estimation procedures within the congestion control scheme of TCP was proposed recently as a way of improving TCP performance over links affected by random loss. This paper first analyzes the problems faced by every bandwidth estimation algorithm implemented at the sender side of a TCP connection. Some proposed estimation algorithms are then reviewed, analyzing and comparing their estimation accuracy and performance. As existing algorithms are poor in bandwidth estimation, and in sharing network resources fairly, we propose TIBET (time intervals based bandwidth estimation technique). This is a new bandwidth estimation scheme that can be implemented within the TCP congestion control procedure, modifying only the sender-side of a connection. The use of TIBET enhances TCP source performance over wireless links. The performance of TIBET is analyzed and compared with other schemes. Moreover, by studying TCP behavior with an ideal bandwidth estimation, we provide an upper bound to the performance of all possible schemes based on different bandwidth estimates.     

Tuning radio resource in an overlay cognitive radio network for TCP: greed isn't good [Accepted from Open Call]

In this article we illustrate the performance of Transmission Control Protocol in an overlay cognitive radio network under dynamic spectrum access. We show that the performance of TCP in overlay CR networks that implement DSA to be quite different from its performance in conventional networks, which do not allow DSA. The key difference is that secondary users in an overlay CR network have to cope with a new type of loss called service interruption loss, due to the existence of primary users. We demonstrate on an NS2 simulation testbed the surprising result: Excessive radio resource usage leads to a decrease in aggregate TCP throughput. This behavior is in contrast to the behavior of TCP in conventional networks, where throughput increases monotonically with the available radio resource.