MANET中TCP数据包的乱序问题及其解决方案

在MANET中,节点的移动性会导致不同的TCP数据包沿不同的路径到达接收端,进而在接收端产生大量的乱序数据包,影响TCP协议的性能.提出了一种延时响应TCP协议(TCP-D),通过延时触发拥塞控制算法来提高TCP协议在MANET中的性能.对延时响应TCP协议的吞吐量分析表明,增加延时定时器后,TCP_D协议仍能保证对标准TCP协议的友好性.仿真实验表明,TCP_D算法可以明显减少乱序数据包的数量,获得较标准TCP协议更高的网络吞吐量.而在网络结构稳定、无乱序数据包的情况下,改进协议仍具有很好的公平性和友好性.     

无线网状网中自适应网络编码的MPTCP研究与设计

网络编码可以提高无线网状网的吞吐量,但是它在无线网状网中的实际应用是一个挑战性的问题。网络编码必须和TCP很好相容才能得到广泛应用。根据无线网状网的特点,研究和设计了一种自适应网络编码的多路径TCP。在提出的方案中,网络编码被加入到现有的网络系统,源节点根据目的节点编码数据块的自由度,调整发送编码的数据块,以降低多路径传输的数据报文的失序,提高网络的吞吐量,保证各个数据流之间的公平性。仿真结果表明设计的MPTCP有效。     

TCP-ACC: performance and analysis of an active congestion control algorithm for heterogeneous networks

Transmission control protocol (TCP) is a reliable transport layer protocol widely used in the Internet over decades. However, the performances of existing TCP congestion control algorithms degrade severely in modern heterogeneous networks with random packet losses, packet reordering and congestion. In this paper, we propose a novel TCP algorithm named TCP-ACC to handle all three challenges mentioned above. It integrates 1) a real-time reorder metric for calculating the probabilities of unnecessary Fast Retransmit (FRetran) and Timeouts (TO), 2) an improved RTT estimation algorithm giving more weights to packets that are sent (as opposed to received) more recently, and 3) an improved congestion control mechanism based on packet loss and reorder rate measurements. Theoretical analysis demonstrates the equilibrium throughput of TCP-ACC is much higher than traditional TCP, while maintaining good fairness with regard to other TCP algorithms in ideal network conditions. Extensive experimental results using both network emulators and real network show that the algorithm achieves significant throughput improvement in heterogeneous networks as compared with other state-of-the-art algorithms.     

一种采用流切割实现报文保序的负载均衡算法

网络链路过载或链路失效时,使用负载均衡技术可以避免网络发生拥塞。负载的分派粒度决定了负载均衡系统的均衡性能。分派粒度越细,均衡效果越理想。基于包水平粒度的负载分派可以实现理想的均衡性能,但是会造成同一TCP业务流中报文乱序;基于流水平的分派可以保证报文不乱序,但均衡效果不理想。提出了按照报文段粒度分派负载的FSLB算法。仿真实验表明,该算法可避免报文乱序并能达到较理想的均衡效果。     

Packet reordering in high-speed networks and its impact on high-speed TCP variants

Several recent Internet measurement studies show that the higher the packet sending rate, the higher the packet-reordering probability. This implies that recently proposed high-speed TCP variants are more likely to experience packet reordering than regular TCP in high-speed networks, since they are designed to achieve much higher throughput than regular TCP in these networks. In this paper, we first study the characteristics of packet reordering in high speed networks. Second, we verify the impact of packet reordering on high speed TCP variants and evaluate the effectiveness of the existing reordering-tolerant TCP enhancements using simulations. Our simulation results demonstrate that high-speed TCP variants perform poorly in the presence of packet reordering, and existing reordering-tolerant algorithms can significantly improve the performance of high-speed TCP variants.     

用于分层移动IPv6网络拥塞控制的新机制

针对在IPv6无线网络切换过程中, 短暂的链路连接断开会导致反复丢包; 同时在切换过程中, 由于带宽的改变, 在新的接入点就有可能发生丢包或资源浪费以及拥塞。提出一种以TCP协议为基础的路径损耗确认（TCP-PLACK）机制来代替TCP选择确认机制（TCP-SACK）。每当一个TCP接收方在断开或切换后而连接到一个新的接入点时, 上述的TCP-PLACK机制就会发送一个特殊的确认, 其中包含有在新接入点丢包的详细信息和可用带宽。收到这个确认, 发送方会重新发送丢失的包, 并在新接入点根据带宽的可用性调整发送速率。实验结果表明, 该机制有利于改善IPv6网络切换过程中的丢失恢复和速率控制问题。     

基于NewReno拥塞控制机制的TCP分组乱序影响分析

报文乱序是互联网中普遍存在的现象,深入理解互联网报文乱序的行为属性和影响,对于设计高性能网络协议和新型网络设备体系结构、优化网络整体效能具有重要的意义。本文建立了NewReno拥塞控制算法的工作模型,通过实验验证了模型的有效性。基于该模型,分析报文乱序对TCP连接的影响,发现乱序报文对TCP连接吞吐率的影响显著,而且乱序报文的影响与报文丢失率紧密相关:当报文丢失率较低时,报文乱序会导致TCP连接的吞吐率急剧下降;当丢失率较高时,报文乱序的影响比较小。这些结论可为报文乱序友好的网络协议和新型网络设备体系结构的设计提供指导。     

Real-time video streaming over multipath in multi-hop wireless networks

Given the limited wireless link throughput, high loss rate, and varying end-to-end delay, supporting video applications in multi-hop wireless networks becomes a challenging task. Path diversity exploits multiple routes for each session simultaneously, which achieves higher aggregated bandwidth and potentially decreases delay and packet loss. Unfortunately, for TCP-based video streaming, naive load splitting often results in inaccurate estimation of round trip time (RTT) and packet reordering. As a result, it can suffer from significant instability or even throughput reduction, which is also validated by our analysis and simulation in multi-hop wireless networks. To make real-time TCP-based streaming viable over multi-hop wireless networks, we propose a novel cross-layer design with a smart traffic split scheme, namely, multiple path retransmission (MPR). MPR differentiates the original data packets and the retransmitted packets and works with a novel QoS-aware multi-path routing protocol, QAOMDV, to distribute them separately. MPR does not suffer from the RTT underestimation and extra packet reordering, which ensures stable throughput improvement over single-path routing. Through extensive simulations, we further demonstrate that, as compared with state-of-the-art multi-path protocols, our MPR with QAOMDV noticeably enhances the TCP streaming throughput and reduces bandwidth fluctuation, with no obvious impact to fairness.     

在IP包过滤中状态TCP包的过滤研究与设计

IP包过滤防火墙是构造整体网络安全系统的必不可少的部分。传统的IP包过滤防火墙有许多的缺陷,解决方法之一是使防火墙具有状态过滤能力。以TCP为例,状态过滤机制不仅能根据ACK标志和源、目的地址及端口号进行过滤,还能根据TCP包里的序列号和窗口大小来决定对该包的操作。这样可以防止一些利用TCP滑动窗口机制的攻击。在IP包过滤里加入状态过滤机制不仅能阻止更多的恶意包通过,还能提高IP包过滤的过滤速率(这对防火墙来说是很重要的)。