TCP/IP协议在卫星链路上的应用研究

因特网的发展对带宽的要求越来越高,现有的地面网络远远不能满足用户的需求,因而卫星网与IP网结合成了目前研究的热点。简要介绍了TCP/IP的协议结构和工作原理。针对卫星网的信道差错率高、传播延迟长和信道不对称性对TCP传播性能有恶化影响,按照从链路层到应用层的分类方法,分别提出了改善措施,包括:传输控制协议层的窗口控制、快速重传、快速重发、ACK控制等改进方案以及基于TCP欺骗技术和TCP分段技术的联接分段代理方案。     

Multilayer multicast congestion control in satellite environments

It is well known that long and variable link delays, link errors, and handoffs in satellite environments seriously interfere with transmission control protocol's (TCP's) congestion control mechanisms. These channel characteristics also adversely affect existing multilayer multicast congestion control schemes when they are used in satellite environments. In addition, these schemes still have problems with fairly sharing bandwidth with TCP flows, controlling the overhead of frequent grafting and pruning, and handling misbehaving receivers. In this paper, we present a new multilayer multicast congestion control scheme that is suitable for satellite environments and overcomes most of the disadvantages of existing schemes. Our scheme is not affected by the long and variable delays of satellite links. Link errors also do not decrease the performance of our scheme. Further, our scheme has very limited control overhead. In addition to these advantages specific to satellite environments, our scheme achieves good fairness in sharing bandwidth with TCP sessions and is not sensitive to misbehaving receivers.     

TCP-Jersey for wireless IP communications

Improving the performance of the transmission control protocol (TCP) in wireless Internet protocol (IP) communications has been an active research area. The performance degradation of TCP in wireless and wired-wireless hybrid networks is mainly due to its lack of the ability to differentiate the packet losses caused by network congestions from the losses caused by wireless link errors. In this paper, we propose a new TCP scheme, called TCP-Jersey, which is capable of distinguishing the wireless packet losses from the congestion packet losses, and reacting accordingly. TCP-Jersey consists of two key components, the available bandwidth estimation (ABE) algorithm and the congestion warning (CW) router configuration. ABE is a TCP sender side addition that continuously estimates the bandwidth available to the connection and guides the sender to adjust its transmission rate when the network becomes congested. CW is a configuration of network routers such that routers alert end stations by marking all packets when there is a sign of an incipient congestion. The marking of packets by the CW configured routers helps the sender of the TCP connection to effectively differentiate packet losses caused by network congestion from those caused by wireless link errors. This paper describes the design of TCP-Jersey, and presents results from experiments using the NS-2 network simulator. Results from simulations show that in a congestion free network with 1% of random wireless packet loss rate, TCP-Jersey achieves 17% and 85% improvements in goodput over TCP-Westwood and TCP-Reno, respectively; in a congested network where TCP flow competes with VoIP flows, with 1% of random wireless packet loss rate, TCP-Jersey achieves 9% and 76% improvements in goodput over TCP-Westwood and TCP-Reno, respectively. Our experiments of multiple TCP flows show that TCP-Jersey maintains the fair and friendly behavior with respect to other TCP flows.     

TCP Window Control for Variable Bandwidth in Wireless Cellular Networks

Most of TCP schemes in wireless networks assume that the bandwidth of the bottleneck link remains constant over time. However, in wireless cellular networks, to effectively manage the limited resources, the bandwidth is controlled based on radio condition over time. Such varying bandwidth can cause the networks congestion or underutilization. In this letter, we propose a new window control algorithm to improve TCP performance in wireless cellular networks with variable bandwidth. Simulation results illustrate that our proposal improves the performance of TCP in terms of fairness and link utilization     

Basestation flow control for wired to wireless networks

With the exponential growth of the internet, wireless networks such as satellite networks are becoming increasingly popular. The characteristics of satellite networks such as long latency, large delay-bandwidth product, high bit error rate over satellite links and variable round trip time, severely degrade TCP/IP performance. At the conjunction of the satellite link and the fixed link, the basestation, the difference in capacity between the satellite link and the fixed link causes the basestation to experience congestion losses that adversely impact TCP performance. We propose a technique that substantially reduces the congestion at the base station and enforces fairness among the TCP connections that are sharing the satellite link. The technique does not require any change in the TCP sender or the receiver. The stability of our algorithm is analytically proven and its performance is evaluated using ns-2 simulations. Preliminary results yield almost a null congestion loss rate, a 60% decrease in average queue length, and more than 30% increase in the throughput. Fairness is well enforced.     

卫星链路中TCP传输性能仿真与分析

卫星信道与地面有线信道具有不同的特点,当3TCP协议直接应用于卫星链路上时,其吞吐量等性能会受到影响。针对卫星链路的特点,文中分析了卫星链路中TCP协议存在的主要问题,使用NS2仿真软件,建立了仿真模型,并在给定的仿真环境下得出了一些结论。卫星链路具有时延大、误码率高等特点,仿真结果说明了当数据大小一定时,比特误码率越高,系统吞吐量越低,引起的丢包率越高;时延（往返时间）越长,系统吞吐量越低。最后,分析比较了优化TCP的运行参数、修改TCP控制机制、分割连接技术等卫星链路TCP协议的主要解决方案。     

卫星TCP/ATM传输中的明确窗口指示拥塞控制策略研究

卫星ATM是近年来卫星通信领域的一个研究热点,TCP业务在卫星ATM中的传输是其中一个重要的研究课题.由于在TCP协议中主要是通过对窗口的控制来实现拥塞控制,而卫星信道传输的长延时特性大大降低了TCP层拥塞控制的效率.本文提出一种基于UBR传输的明确窗口指示拥塞控制策略,仿真结果表明在效率、公平性、VBR背景传输下的性能等各方面,取得了比目前广泛使用的Reno TCP更好的结果.并且该算法实现简单,交换机实现零信元丢失的缓冲区很小并且与TCP源连接的数目无关.     

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

Providing reliable data communications over wireless channels is a challenging task because time-varying wireless channel characteristics often lead to bit errors. These errors result in loss of IP packets and, consequently, TCP segments encapsulated into these packets. Since TCP cannot distinguish packet losses due to bit corruption from those due to network congestion, any packet loss caused by wireless channel impairments leads to unnecessary execution of the TCP congestion control algorithms and, hence, sub-optimal performance. Automatic Repeat reQuest (ARQ) and Forward Error Correction (FEC) try to improve communication reliability and reduce packet losses by detecting and recovering corrupted bits. Most analytical models that studied the effect of ARQ and FEC on TCP performance assumed that the ARQ scheme is perfectly persistent (i.e., completely reliable), thus a frame is always successfully transmitted irrespective of the number of transmission attempts it takes. In this paper, we develop an analytical cross-layer model for a TCP connection running over a wireless channel with a semi-reliable ARQ scheme, where the amount of transmission attempts is limited by some number. The model allows to evaluate the joint effect of stochastic properties of the wireless channel characteristics and various implementation-specific parameters on TCP performance, which makes it suitable for performance optimization studies. The input parameters include the bit error rate, the value of the normalized autocorrelation function of bit error observations at lag 1, the strength of the FEC code, the persistency of ARQ, the size of protocol data units at different layers, the raw data rate of the wireless channel, and the bottleneck link buffer size.     

改善卫星融合网络随机丢包和间歇性中断的方法研究*1

当移动自组织网用户通过网关经卫星网络发送数据时,存在卫星链路的长传播时延、随机丢包以及移动终端的移动性引起链接的间歇性中断,导致TCP触发拥塞控制机制而降低传输性能降低。针对以上卫星网络的特点,提出了TCP M-Veno方法。在发送端对TCP Veno进行优化改进使得它能够缓解卫星网络中长的传播时延和随机丢包的影响。在网关中结合M-TCP算法来解决卫星融合网络移动终端间歇性中断。仿真表明,所提出的TCP M-Veno比NewReno、Veno以及M-TCP有更好的传输性能。     

面向卫星网络的TCP传输性能的研究

卫星信道的长时延、高误码率、大延时带宽积等的特性使TCP协议的性能受到了很大的影响.为了提高卫星网络下TCP的性能,在不破坏TCP端到端语义的前提下,文中利用在异构网络的边界处设置具有传输控制功能的性能增强代理(Performance Enhancing Proxies,PEP)的理论模型,在SACK TCP协议下对该模型实现了一种卫星网络的拥塞控制和拥塞恢复策略.仿真实验表明,新模型能够充分利用卫星网络的带宽,达到获得最佳有效吞吐量的目的.     

Improving link layer performance on satellite channels with shadowing via delayed two-copy selective repeat ARQ

This paper focuses on improving performance of land mobile satellite channels (LMSCs) at high band (Ka-band or EHF band), where shadowing is the primary impediment to reliable data transmission. Compared with multipath fading, shadowing exists on a longer time scale; hence, interleaving to combat shadowing introduces unacceptably large decoding delay. We use Lutz's model to investigate bit-error rate/packet-error rate (BER/PER) performance of interleaving with various forward error correction (FEC) coding as a function of different channel parameters to demonstrate its limited effectiveness for combatting burst errors whose mean duration significantly exceed a link layer (LL) packet. We propose a delayed two-copy selective repeat ARQ (DTC-SR-ARQ) scheme, whereby two copies of a packet are sent-the second with a delay relative to the first-in every transmission or retransmission. Closed-form expressions for mean transmission time, success probability, and residual loss probability are provided and simulations used to validate the analysis. Furthermore, the issue of optimum delay is addressed as well, and a simple yet effective strategy is suggested to support transmission control protocol (TCP) traffic over this data link layer. DTC-SR-ARQ is shown to achieve much shorter additional delay than interleaving and compared with normal SR-ARQ, reduces mean transmission time at expense of a small increase in residual packet loss probability. Furthermore, ns2 simulation results show that for TCP traffic, DTC-SR-ARQ acquires higher end-to-end throughput than normal SR-ARQ.     

适用于卫星网络的TCP跨层改进机制

该文提出基于跨层信息交互,将链路层ARQ重传状态信息通知TCP的机制,避免了链路层重传引起的时延变化对TCP的不利影响。该机制使用完全可靠选择性重传ARQ为TCP提供可靠的链路,避免卫星链路上发生丢包,并且不必要求链路层保证包按序递交,消除了重排序的等待时延,适合带宽时延积较大的卫星网络。仿真结果表明,能显著提高TCP在卫星网中的性能,特别是在误帧率较高条件下。     

A study on a receiver‐based management scheme of access link resources for QoS‐controllable TCP connections

Although the bandwidth of access networks is rapidly increasing with the latest techniques such as DSL and FTTH, the access link bandwidth remains a bottleneck, especially when users activate multiple network applications simultaneously. Furthermore, since the throughput of a standard TCP connection is dependent on various network parameters, including round‐trip time and packet loss ratio, the access link bandwidth is not shared among the network applications according to the user's demands. In this thesis, we present a new management scheme of access link resources for effective utilization of the access link bandwidth and control of the TCP connection's throughput. Our proposed scheme adjusts the total amount of the receive socket buffer assigned to TCP connections to avoid congestion at the access network, and assigns it to each TCP connection according to characteristics in consideration of QoS. The control objectives of our scheme are (1) to protect short‐lived TCP connections from the bandwidth occupation by long‐lived TCP connections, and (2) to differentiate the throughput of the long‐lived TCP connections according to the upper‐layer application's demands. One of the results obtained from the simulation experiments is that our proposed scheme can reduce the delay of short‐lived document transfer perceived by the receiver host by up to about 90%, while a high utilization of access link bandwidth is maintained. Copyright © 2006 John Wiley & Sons, Ltd.     

基于卫星信道的TCP/IP传输性能研究

本文通过介绍卫星传输以及TCP/IP传输协议的各自优势,对两者相结合的TCP/IP over SatNet通信方式存在的链路时延过长以及误码率过高等问题进行了分析,并从TCP算法层和链路层分别寻求了多种解决方案,使得Internet通过卫星传输能够较好发挥优势.     

无线自组织网络TCP速率联合调制

TCP在无线自组织网络中受到信道竞争和队列拥塞的双重影响,因而不能准确地调节数据发送速率从而降低了传输性能。本文针对该问题提出一种基于对信道竞争和排队拥塞联合检测的TCP速率调制策略。实验结果显示,本策略在吞吐量和端到端时延上均明显提高了TCP的性能。     

Corruption‐aware adaptive increase and adaptive decrease algorithm for TCP error and congestion controls in wireless networks

The conventional TCP tends to suffer from performance degradation due to packet corruptions in the wireless lossy channels, since any corruption event is regarded as an indication of network congestion. This paper proposes a TCP error and congestion control scheme using corruption‐aware adaptive increase and adaptive decrease algorithm to improve TCP performance over wireless networks. In the proposed scheme, the available network bandwidth is estimated based on the amount of the received integral data as well as the received corrupted data. The slow start threshold is updated only when a lost but not corrupted segment is detected by sender, since the corrupted packets still arrive at the TCP receiver. In the proposed scheme, the duplicated ACKs are processed differently by sender depending on whether there are any lost but not corrupted segments at present. Simulation results show that the proposed scheme could significantly improve TCP throughput over the heterogeneous wired and wireless networks with a high bit error rate, compared with the existing TCP and its variants. Copyright © 2008 John Wiley & Sons, Ltd.     

基于星上处理的卫星ATM中ABR 业务拥塞控制算法研究

卫星ATM是近年来通信领域的研究热点,拥塞控制是其中很关键的一个问题.本文主要研究基于星上处理的卫星ATM网的ABR流量控制,它通过对流量的长时预测来达到控制目的.由于卫星信道的大延时,无法实现精确的长时预测,因而将预测结果直接用于ABR的拥塞控制效果不甚理想.将长时预测同动态利用因子控制的方法结合之后,在链路效率、信元丢失率、不同连接数目和暂态响应等方面都取得很好的结果.     

REFWA: An Efficient and Fair Congestion Control Scheme for LEO Satellite Networks

This paper examines some issues that affect the efficiency and fairness of the Transmission Control Protocol (TCP), the backbone of Internet protocol communication, in multi-hops satellite network systems. It proposes a scheme that allows satellite systems to automatically adapt to any change in the number of active TCP flows due to handover occurrence, the free buffer size, and the bandwidth–delay product of the network. The proposed scheme has two major design goals: increasing the system efficiency, and improving its fairness. The system efficiency is controlled by matching the aggregate traffic rate to the sum of the link capacity and total buffer size. On the other hand, the system min-max fairness is achieved by allocating bandwidth among individual flows in proportion with their RTTs. The proposed scheme is dubbed Recursive, Explicit, and Fair Window Adjustment (REFWA). Simulation results elucidate that the REFWA scheme substantially improves the system fairness, reduces the number of packet drops, and makes better utilization of the bottleneck link. The results demonstrate also that the proposed scheme works properly in more complicated environments where connections traverse multiple bottlenecks and the available bandwidth may change over data transmission time.     

多个TCP连接的拥塞丢包模型

本文提出了有关TCP连接的拥塞丢包分析模型.网络瓶颈一般承载许多TCP连接,瓶颈处不可避免的拥塞和缓存溢出,是导致网上丢包的主要原因.网络瓶颈处的行为很大程度上左右了网络性能.本文的模型估计了存在大量持续TCP连接时,网络瓶颈的丢包概率和网络传输中断概率,给出了对实际网络的良好近似.这对于研究TCP对网络性能的影响,提出改善网络性能的新算法,以及分析(从长远来看)TCP还应做哪些改进,都是非常有用的.     

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.