Research And Implementation On The Congestion Control Algorithms Of TCP Protocol

Based on the national tenth five-year 863 plan-"extensible to T bit high performance IPv4/v6 router basic flat and experiment system", this dissertation deal with the problem of implementation and evaluation of TCP protocol used in system interior communication of T bit router. Based on the analysis and comparison on the congestion control strategy about TCP protocol in existence, we propose a wholly implementation method of TCP, design the input and output processing flow of TCP protocol in detail, and lay an emphasis on the congestion control algorithms. In chapter four we give a implementation of our design. Otherwise, we propose to use the result of theoretic analysis as the reference standard of implementation about congestion control algorithms, so we introduce a TCP steady state throughput model, and improve on the lack of the model. Using the adapted model, we give the result of the protocol implementation performance as reference target. The measurements of implementation prove that our design is feasible; the performance analysis shows that the test result tally with the theoretic analysis. So our design can satisfy the requirement of T bit router.The main contribute and innovation of this paper is as follows:Firstly, we classify and compare the congestion control mechanisms in TCP protocol, and summarize the most studies in this field. We give the shortcoming and excellence of all kinds of congestion control algorithms; this not only summarizes the congestion control algorithms, but also can offer choice in reason for implementation of TCP congestion control algorithm.Secondly, based on the statistical data of TCP flow measurements and study of conventional TCP throughput model, we propose an improved analytical model of TCP steady state throughput considering the effect of impact of slow start periods after timeouts, and we present the comparison of results of the model with the live measurements, it shows that our model can predict the throughput of TCP steady state flow more precisely. The improved model simulates the TCP performance at certain condition, and we make use of the result as the reference of congestion control algorithms implementation of TCP protocol.Thirdly, we analyses the reversed effect of delayed acknowledge in slow start period, the effect of initial little windows to TCP slow start time and throughput in detail. We study the problem that congestion windows decrease because of retransmission brought with the out of sequence packets, delayed packets and unnecessary timeout retransmission, these problems reduce the efficiency throughput of TCP connection. In implementation of congestion control algorithms, we improve the conventional congestion control algorithms with dynamic delay timer, the bigger initial windows, quick acknowledge in slow start period and undo processing of decrease congestion windows, which enhance the real time performance and decrease the reverseeffect to throughput. These modifications make the design and implementation of TCP protocol more applicable for system interior communication of T bit router.Considering the problem of RTO adaptive algorithm in most TCP implementation which have predict error when the RTT decrease, and fixed RTO estimate plus and windage tradeoff aren't applicable for RTT sampling at high frequency, this paper give a new end to end retransmission timer optimize algorithm. This algorithm can avoid above problems brought by conventional adaptive algorithm. Every plus to smoothed RTT and smoothed RTT windage both change with the load of sender, and it depend on the RTT sampling rate.