Research On TCP Congestion Controlalgorithm Over Satellite Networks

With the increasing development of communication networks, people on the network requirements are gradually improving. Satellite networks with its wide coverage, communication distance, without geographical restrictions, low cost data transmission, flexible networking and so on make many services that original based on Internet application and terrestrial networks can not provide application in the satellite networks. But the bandwidth of the satellite communications network relative to the increasing application services type and quantity, show obvious deficiencies. Therefore, at the same time improving satellite network link bandwidth, improving the utilization of satellite network bandwidth is particularly important. TCP protocol can provide a reliable end-to-end data transmission service and improve the utilization of network bandwidth. However, TCP is designed for a terrestrial cable network, to the satellite communication environment of long delay and high error rate can not obtain a good performance. Therefore, how to effectively applied TCP protocol to the satellite communications, has become a hot topic of research.This thesis first elaborates the TCP protocol, and then analyzes the impact of the special nature of satellite link to TCP, and introduces to improve satellite communications network in a variety of TCP performance improvement program. Comprehensive analysis of these schemes, an improved algorithm based on available bandwidth estimation of TCP Vegas is put forward.In this thesis, by introducing an acknowledgment packet interval threshold, improved the accuracy of Westwood bandwidth estimation. Then according to the characteristics of the satellite link, combined with the estimation of available bandwidth, the traditional TCP Vegas is improved, which can be well applicable to satellite communication.The main improvements are: ① According to the characteristics of satellite link bandwidth asymmetry, the thesis used delayed acknowledgment mechanism to reduce the amount of data of the reverse link. And it used the byte count window growth way, avoiding the congestion window growth too slow is due to acknowledgment packets decreases. ② According to the characteristics of satellite link round-trip delay long, Vegas window growth mechanism and the slow start stage were improved, also used large initial window and canceled the observation period. The congestion avoidance phase, combined with the available bandwidth estimation algorithm and the Vegas congestion avoidance strategy to dynamically adjust the congestion window, make the network state reaction is more sensitive. ③According to the characteristics of the satellite link high bit error rate, once the packet is lost; it can distinguish the packet loss according to the prediction of the available bandwidth and the current throughput. If it is the error packet loss, then retransmit lost data quickly; if it is the congestion packet loss, then enter the stage of fast retransmission and quick recovery.In this thesis, by using NS2 to simulate the improved protocol and the results show that compared with the traditional TCP protocol, the improved protocol in long delay, high bit error rate environment are able to achieve higher throughput. Also the improved protocol can exhibit good fairness in the competition with Reno for bandwidth.