Research On Performance Improvement Of TCP Protocol In Wireless And Wired Networks

With the development of computer network and communication technology, new wireless and wired network environment continues to appear, which not only provides us with convenient communication but also brings up great impact on the performance of TCP connections. As the traditional TCP Reno protocol is proposed based on early network environment, the sender detects the packet loss according to duplicate ACK, interprets this loss as network congestion and adjusts its congestion window through AIMD algorithm. The above interpretation and judgement gradually lapsed through the transformation of network environment and brings up more and more problems.Compared with wired network environment, the communication in wireless has to face the high BER (Bit Error Rate) problem and the data frames that have been sent to wireless cable may be affected by collision and signal-to-noise. The current wide-used 802.11 protocol provides verified mechanism in data link layer to ensure the correctness of data transformation, but it can't effectively eliminate the effection of packet error. Because the transport layer protocols like TCP could not distinguish wireless losses from congestion losses, the congestion window and packet sending rate always decrease due to the high BER on the wireless channel. It leads to the impairment of performance and the waste of network resources.At the same time, the improvement of wired network scope and environment causes the augment of packet loss rate and network transmission delay, and increases the probability of packet loss. The higher RTT and packet loss rate also impairs the performance of TCP connections, and reduces the utilization of current network.This dissertation summarizes the difficulties and challenges that TCP protocol encountered in the new network environment, analyses the current TCP Researchs, and simulates the impact of different network parameters on the performance of TCP result. The results of analysis and testing show that current researches have the shortcomings of deployment and promotion, and the mechanism under the transport layer protocol will be the best idea.Secondly, considering the demand of wireless TCP, a Dynamic-Segment method (DS-MAC) is proposed to improve the performance of TCP over high BER wireless networks. This approach uses several checksums to verify a packet, which improves this packet's probability of successful transmission. It estimates BER, and fixes the data length of each segment to accommodate BER of the wireless channel. This dissertation extends the 802.11 module in NS-2 to validate the improvements brought by such approach. The results of the simulations shows that this approach is adaptive to variable BER of wireless channel and throughput can be greatly improved.After that, in order to meet the demands of remote access users, especially for VPN users, Tunnel Retransmission Protocol (TRP) on VPN Gateway is designed to reduce the probability of packet loss. Through TRP, a reliable transmission link is provided to TCP protocol. Experiment shows that TRP mechanism can reduce the probability of packet loss and improve the performance of TCP connections.Finally, to the problem of TRP which exist in environments with high RTT and high packet loss rate, further mechanism named TCP-Proxy is proposed based on TRP protocol. Through the modification of acknowledge number field and window field in TCP header, VPN gateway shields TCP sender from the perception of packet loss, and thus prevents the unnecessary reduction of congestion window. Experiment and analysis show that TCP-Proxy mechanism can effectively eliminate the influence of high packet loss rate, thereby improve the utilization of VPN network and remote TCP connection speed.