无线移动网络中增强TCP性能的技术综述

TCP是因特网上的主要传输协议，在数据包丢失主要是由拥塞引起的传统网络上，TCP可以充分发挥其性能，但是在无线移动网络中，TCP将无线信道比特差错和移动切换引起的数据包丢失误归于网络发生拥塞而采取拥塞控制措施，不必要地降低了端到的吞吐率，导致自身性能的下降，首先阐述了当前无线移动网络的发展状况和TCP面临的问题，在此基础上，综述和评价了各种增强TCP性能的技术方案，最后，结合无线移动通信系统的最新进展，分析和探讨了当前存在的问题和进一步的研究方向。     

A link layer adaptive pacing scheme for improving throughput of transport protocols in wireless mesh networks

In this paper, we study the performance of a static multihop wireless network, specifically that of the backhaul network of a two-tier Wireless Mesh Network (WMN) operating on IEEE 802.11 Medium Access Control (MAC) protocol. The performance of an IEEE 802.11 based backhaul network is greatly affected by the MAC contention and congestion in the network. If the sources pump data into the network than can be supported, loss rate increases due to MAC contention and congestion in the network. This also leads to the problem of unfairness among flows. In this paper, we propose a Link Layer Adaptive Pacing (LLAP) scheme that adaptively controls the offered load into the network. This improves the performance of higher layer protocols without any modifications to them. Our LLAP scheme estimates the four hop transmission delay in the network path without incurring any additional overhead (Control packets) and accordingly paces the packet transmissions to reduce MAC contentions in the network. We implement the LLAP scheme in ns-2.29 network simulator and extensively study its performance for both User Datagram Protocol (UDP) and Transmission Control Protocol (TCP) traffic in different network scenarios. In all the cases, our scheme shows a significant improvement in the performance of both UDP and TCP traffic.     

Data rate,path length and network contention trade-off in IEEE 802.11s mesh networks: A dynamic data rate selection approach

Most of the commercially available wireless routers are equipped with multi-rate support to adopt physical data rates based on the channel condition fluctuations. The recent studies in multi-rate support have shown that low data rates are more effective when the channel error rate is high. Because of the physical layer modulation and signal decoding issues, low data rates are sustainable for long transmission ranges. Therefore, for multi-hop mesh networks, low data rates may scale down the end-to-end path length towards the destination in terms of number of hops, resulting in less end-to-end forwarding delay. However, for a network with high traffic load, long transmission ranges may increase contention for channel access among the contending neighbors. This paper uses the diffusion approximation method of queuing analysis to study the trade-off among data rate, end-to-end path length and network contention in a multi-rate mesh network built over the IEEE 802.11s specifications. From the observations of the theoretical analysis, a distributed and localized rate adaptation scheme is proposed for IEEE 802.11s mesh networks, by augmenting the standard peer selection, channel access and forwarding protocols. The performance of the proposed rate adaptation protocol is evaluated and compared with existing rate adaptation protocols using simulation results.     

TCP in a wireless world

Mobile wireless is one of the more challenging environments for the Internet protocols, and for TCP in particular. One approach to supporting the wireless environment is the so-called “walled garden”. Here the transport protocol used within the mobile wireless environment is not TCP, but is instead a transport protocol that has been specifically adapted to mobile wireless. In this model, Internet applications interact with an application gateway to reach the wireless world, and the application gateway uses a wireless transport protocol and potentially a modified version of the application data to interact with the wireless device. The most common implementation of this approach is to extend a Web client into the mobile wireless device, using some form of proxy server at the boundary of the wireless network and the Internet. This is the approach adopted by the Wireless Access Protocol (WAP) Forum. An alternative is to allow mobile wireless devices to function as any other Internet-connected device. This approach requires some form of end-to-end direct IP continuity and an associated end-to-end TCP functionality, where the TCP path straddles both wired and wireless segments. Ensuring the efficient operation of TCP in this environment becomes integral to the development of the environment itself; the problem is no longer one of adjusting TCP to match the requirements of the wireless environment, but one of providing seamless interworking between the wired and wireless worlds     

无线自组织网络中TCP稳定性的分析及改进

在无线自组织网络中,基于IEEE 802.11的TCP流存在严重的不稳定性,其原因与MAC协议、路由协议和TCP本身均有一定的关系,但最根本之处在于MAC协议的不公平性以及假的链路失效消息导致了不必要的耗时的路由发现过程.结合IEEE802.11的MAC协议和DSR路由协议,对这些原因进行了深入的理论分析和仿真实验,并提出了针对MAC协议和路由协议的改进算法.仿真结果证明,提出的改进算法不仅能基本上避免TCP流的不稳定性,还能够极大地提高TCP流的平均吞吐量.     

Revealing the problems with 802.11 medium access control protocol in multi-hop wireless ad hoc networks

The IEEE 802.11 medium access control (MAC) protocol is a standard for wireless LANs, it is also widely used in almost all test beds and simulations for the research in wireless mobile multi-hop ad hoc networks. However, this protocol was not designed for multi-hop networks. Although it can support some ad hoc network architecture, it is not intended to support the wireless mobile ad hoc network, in which multi-hop connectivity is one of the most prominent features. In this paper, we focus on the following question: can IEEE 802.11 MAC protocol function well in multi-hop networks? By presenting several serious problems encountered in transmission control protocol (TCP) connections in an IEEE 802.11 based multi-hop network, we show that the current TCP protocol does not work well above the current 802.11 MAC layer. The relevant problems include the TCP instability problem found in this kind of network, the severe unfairness problem, and the incompatibility problem. We illustrate that all these problems are rooted in the MAC layer. Furthermore, by revealing the in-depth cause of these problems, we conclude that the current version of this wireless LAN protocol does not function well in multi-hop ad hoc networks. We thus doubt whether the current WaveLAN based system is workable as a mobile multi-hop ad hoc test bed. All the results shown in this paper are based on NS2 simulations, and are compatible with the results from the OPNET simulations.     

Using traffic asymmetry to enhance TCP performance

A wealth of recent work has gone into optimizing the performance of Transmission Control Protocol (TCP) on the downlink channel of wireless networks such as for example, honing its congestion awareness mechanism so that it is minimally affected by random wireless losses, and optimizing achieved fairness of the end-to-end TCP rates. Other work has gone into balancing the allocation of a shared resource between the downlink and uplink in order to optimize TCP performance. We build on such previous research by proposing a cross-layer algorithm for resource allocation in OFDMA systems aiming not only to achieve optimal throughput for competing TCP flows but also to allocate resources appropriately between the downlink and uplink. This is important due to the increasing number of Internet applications where the mobile terminal is the TCP sender (social networking, peer-to-peer, etc.). Therefore, our scheme makes use of the asymmetry in the traffic and by defining the boundary between downlink and uplink capacity dynamically, enhance the TCP performance. Through numerical investigations we show the performance of the proposed scheme in terms of achieved fairness to the receivers and efficient allocation of downlink to uplink ratios based on the TCP traffic.     

基于无线TCP的簇生丢失重传协议

为提高TCP在无线网络中的传输性能,提出一种局部数据链路层重传协议,簇生丢失重传协议CLRP(Clustered-Loss Retransmission Protoc01)．针对无线链路上突发丢失性强、分组丢失率高的特点,CLRP协议与移动主机端TCP相结合,一方面为重发提供明确的分组丢失种类和高效的无线多分组丢失信息;另一方面提出了更为完善的无线丢失重发控制机制．此外,本协议无需对固定主机端TCP做任何改动．     

End-to-end maxmin fairness in multihop wireless networks: Theory and protocol

To promote commercial deployment of multihop wireless networks, the research/industry communities must develop new theories and protocols for flexible traffic engineering in these networks in order to support diverse user applications. This paper studies an important traffic engineering problem–how to support fair bandwidth allocation among all end-to-end flows in a multihop wireless network–which, in a more precise term, is to achieve the global maxmin fairness objective in bandwidth allocation. There exists no distributed algorithm for this problem in multihop wireless networks using IEEE 802.11 DCF. We have two major contributions. The first contribution is to develop a novel theory that maps the global maxmin objective to four local conditions and prove their equivalence. The second contribution is to design a distributed rate adjustment protocol based on those local conditions to achieve the global maxmin objective through fully distributed operations. Comparing with the prior art, our protocol has a number of advantages. It is designed for the popular IEEE 802.11 DCF. It replaces per-flow queueing with per-destination queueing. It achieves far better fairness (or weighted fairness) among end-to-end flows.     

Improving Throughput of Transmission Control Protocol Using Cross Layer Approach

Most of the internet users connect through wireless networks. Major part of internet traffic is carried by Transmission Control Protocol (TCP). It has some design constraints while operated across wireless networks. TCP is the traditional predominant protocol designed for wired networks. To control congestion in the network, TCP used acknowledgment to delivery of packets by the end host. In wired network, packet loss signals congestion in the network. But rather in wireless networks, loss is mainly because of the wireless characteristics such as fading, signal strength etc. When a packet travels across wired and wireless networks, TCP congestion control theory faces problem during handshake between them. This paper focuses on finding this misinterpretation of the losses using cross layer approach. This paper focuses on increasing bandwidth usage by improving TCP throughput in wireless environments using cross layer approach and hence named the proposed system as CRLTCP. TCP misinterprets wireless loss as congestion loss and unnecessarily reduces congestion window size. Using the signal strength and frame error rate, the type of loss is identified and accordingly the response of TCP is modified. The results show that there is a significant improvement in the throughput of proposed TCP upon which bandwidth usage is increased.     

Overcoming performance pitfalls in rate-diverse high speed WLANs

Recent developments on the IEEE 802.11 family of standards promise significant increases in speed by incorporating multiple enhancements at the physical layer. These high modulation speeds apply to the data portion of the transmitted frames, while headers must remain at lower speeds; this has motivated the use of frame aggregation to increase data payloads in the newer standards. However, this simple method may still utterly fail to deliver the promised speeds, due to a series of cross-layer effects involving the transport and multiple access layers: the downward equalization of throughputs imposed by TCP under physical rate diversity, the excessive impact of the TCP ACK stream, or the unreasonable fraction of access opportunities taken by uplink flows when competing with the more numerous downlink connections. A first contribution of this paper is to demonstrate these impediments and isolate their causes through a series of experiments with the ns3 packet simulator, on the 802.11n and 802.11ac protocol versions.     

基于混合无线网状网协议的改进算法研究*

HWMP提供了两种路由模式,分别是以树状路由(TBR)为基础的先应路由模式和以AODV为基础的按需路由模式.然而这个协议存在着许多问题,如可扩展性较差、效率较低、根是网络故障点,当网络业务繁重时,数据包的碰撞和丢失增加.提出一种混合无线网状网协议的改进算法来解决以上问题.在Mesh网络内部.改进后的协议能够使源节点快速...     

无线自组织网络中的IEEE802．11 MAC协议的研究

IEEES02．11 DCF作为无线局域网的MAC层规范被广泛应用于无线自组织网络的仿真和测试中。通过研究发现即使在拓扑不变的条件下，使用表驱动路由协议DSDV，TCP流仍然存在不稳定和不公平现象，本文分析了问题产生的原因，提出了基于网络负载的解决问题方法。     

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.     

The WICKPro protocol with the Packet Delivery Ratio metric

Wireless mesh networks (WMNs) with chain topologies are very useful in road and railroad transportation or in tunnel and mine applications. The proposed protocols for WMNs usually support best-effort traffic or some kind of Quality of Service (QoS). However, some applications such as remote-controlled machines in industrial control networks have hard real-time (HRT) requirements, i.e., strict deadlines. For this reason, this paper incorporates the Packet Delivery Ratio (PDR) metric into the WICKPro (WIreless Chain networK Protocol) protocol, a HRT protocol for WMNs with chain topologies which uses a token-passing approach. WICKPro is a Medium Access Control (MAC) protocol based on the ideas of the Timed-token protocol and the cyclic executive. The incorporation of the PDR metric lets calculate the needed time to be reserved where packet retransmissions can be accommodated while satisfying HRT traffic constraints. Moreover, the feasibility of using the PDR metric in a token-passing protocol is shown. Since WICKPro has been designed to work on top of IEEE 802.11, we made a testbed using commercial 802.11 wireless cards and compared the WICKPro’s performance with the 802.11 protocol and three specific protocols for WMNs with chain topologies.     

网络通信协议的分析与实现

网络通信协议是网络通信的基础。TCP/IP协议族是应用最为广泛的一种网络通信协议,无论在局域网、广域网还是Internet,无论是Unix系统、Windows平台都支持TCP/IP协议,是计算机世界的一个通用"语言"。讨论了TCP/IP协议族中传输控制协议TCP、用户数据报协议UDP和网际控制报文协议ICMP等原理和应用。     

加密环境中的无线TCP性能优化

TCP是一种广泛使用的可靠传输协议,它的设计基于拥塞是引起丢包主要原因的假设。但在无线网络中,丢包通常是传输差错的缘故,这导致了TCP传输性能的急剧下降。现有不少无线TCP传输性能优化方法要求中间节点能够获得TCP连接信息,因此不适用于数据流被加密的情况。文中提出的丢包识别机制可应用于加密环境,它通过TCP发送端判断丢包原因并采取相应措施来提高传输速率,仿真结果证明该方法是有效的、鲁棒的,在局域网和广域网环境中都能明显提高TCP端到端传输性能。     

Analyzing the effective throughput in multi-hop IEEE 802.11n networks

In this paper we characterize the effective throughput for multi-hop paths in IEEE 802.11n based wireless mesh networks. We derive an analytical model capturing the effects of frame aggregation and block acknowledgements, features found in the new IEEE 802.11n standard. We describe the throughput at MAC layer as a function of physical data rate, error rate, aggregation level and path length. While being mathematically tractable, the proposed model is flexible enough to account for complex and realistic error characteristics of the wireless channel, such as long-term fluctuations and burstiness. We further show how to integrate the well-known Gilbert-Elliot channel model into our model and evaluate both models in our indoor wireless testbed.     

Maximal Throughput of Wireless Access to the Internet: Its Estimation

A star-like regional wireless network under the IEEE 802.11 protocol is studied: its end stations are hidden from each other and linked only with the base station—the point of access to outer networks. It provides access to the Internet to users of cable LANs connected to its end stations. Transfer of large files through TCP/IP connections forms the main bulk of the wireless traffic. Methods for estimating the maximal network throughput for transmission of TCP-level information are designed.     

改进的无线Mesh网路由判据算法及其路由协议

为了改善802.11s无线Mesh网传输过程中因重传次数过大而造成的丢包问题,根据实际无线传输中的网络重传次数特性,提出一种基于最大重传次数的空中传播时间改进路由判据算法.同时为了缓解因单根节点造成的网络拥塞,提高实际场景中边缘节点的传输性能,并保证网络中所有节点使用无线信道资源的公平性,基于树的寻径机制从拓扑结构的角度,提出一种改进的多根节点的混合无线Mesh网路由协议(M-HWMP).理论及仿真结果表明,改进的路由判据算法及路由协议优化了无线Mesh网传输不同数据流时的吞吐量、时延特性,并在一定程度上避免了网络拥塞.