Research On Congestion Control Algorithm Based On XCP Protocol

With the further development of Internet technology and the appearance of optical fiber network and G-bit network, modern network shows new characteristic with high bandwidth delay. Traditional transmission control protocol exposed the drawbacks gradually and can not meet the requirements of network quality of service, so doing research on new network protocols with superior performance and high bandwidth delay becomes a hot issue. HSTCP(High-Speed TCP), Fast-TCP, VCP(Variable-structure congestion Control Protocol) and XCP(eXplicit Control Protocol) are representative protocols. XCP protocol in which the endpoints and intermediate routes work together has a strong fairness, fast convergence, almost no packet loss rate and shorter length of the router queue. Based on the advantages above, XCP protocol became a standard based on explicit feedback congestion control algorithm. Although the XCP protocol has many advantages mentioned above, it still has other drawbacks such as slow convergence of late start flow, setting difficulties on configure bandwidth and system instability on RTT differences. To improve the performance of XCP protocol, this paper proposed improvements on XCP protocol and achieved good results.This paper introduced some typical high-speed network congestion control protocol and presented shortcomings of XCP protocol by analyzing working process of it.Secondly,the reason of unfairness existed on late start XCP flow has been described in detail, and fast start algorithm based on late start XCP flow has been proposed. According to the characteristics of XCP header, this paper first marked the package of a new connection by the reserved bits of header before the packet has been send. Then efficiency controller reserves bandwidth for packets of late start flow by the difference of reserved bits. Finally, the fairness controller will allocate reserved bandwidth to the late start flow. Except for keeping the system stable, the algorithm reduced the time of convergence to fairness of late start flow greatly.Thirdly, XCP bandwidth compensation algorithm which is based on sliding mode control is proposed. The modern network environment is high-speed and dynamic. According to sliding mode controller has strong robustness in a dynamic environment, this paper adds XCP sliding mode controller in the route and set a reasonable output bandwidth for the XCP in dynamic network by sliding mode control theory to compensate uncertainties in the feedback. Source adjusts rate of send according to this to keep network output always maintaining high throughput and a smaller queue length.Finally, two improved algorithms are simulated in NS2. Because XCP protocol is not based on packet loss, only utilization of links and length of queue can be compared in the simulation. The results of simulation show that the improved algorithm is able to make sure the late start flows get fair bandwidth in a shorter time, reduce stable time of system to converge, guarantee high bandwidth utilization and lower routing queue length.