| The total number of Internet users has exceeded one billion at the end of December 2008, so the application of the Internet has developed by leaps and bounds. Internet business has increased rapidly with the sharp increase of the network users and network applications, so there appears network congestion when transmitting the data in the network. At a certain time, the network load caused by the user needs goes beyond the capacity of network resources and node processing capability, leading to the decrease of the network performance indicators, this phenomenon is what we call network congestion. The network congestion can easily cause decline in performance such as data transmission delay and the throughput, which seriously affects the utilization rate of network resources. So, the study to the network congestion control is important and practical.Many scholars at home and abroad has done a lot of research and proposed a number of congestion control algorithms to deal with the increasing network congestion. All these congestion control algorithms can be divided into two major categories according to the different realized layer: one is the source congestion control realized in transport layer, the congestion control that has been widely used is TCP congestion control. Another is communication sub-network congestion control realized in network layer, well known as IP congestion control.The earliest TCP congestion control algorithm was proposed by Jacobson in his famous paper in 1988, who proposed a simple TCP send rate modeling with the AIMD algorithm in congestion window adjustment mechanism. AIMD algorithm is widely used to adjust the rate no matter what kind of modeling type adopted by TCP send rate. The TCP congestion control mechanism which is widely used currently adopts the AIMD algorithm with the parametersα= 1,β= 0.5. The main advantage of this algorithm is to make use of the useful resources fast in the networks, lowering the data send rate drastically, reducing the network congestion fast when the network congestion is increasingly bad. It is for this reason, the data send rate changes greatly, and the average sending rate of the network is not very high, and the supporting to some applications of the Internet is poor. The purpose of this paper is to improve the performance of TCP. The innovative works of this paper are as follows:1. First of all, this paper makes research on the TCP full send rate modeling based on FGAIMD algorithm which was proposed in reference[14]. Besides, taking into account the TCP-friendly and constraints such as practical network features, the parameter setting relationship of AIMD algorithm is studied, then deriving the setting method of the parameters (α,β) in AIMD algorithm that can make the send rate is TCP-Friendly under two different conditions. After that, the paper compares the typical values obtained from two setting equations with the ones in comparison test experiment, and acquires the better one. Then the paper designs the simulation experiments according to the most similar AIMD algorithm equations, and sets the parameters in AIMD algorithm according to the corresponding typical values to obtain the AIMD algorithms of different parameter values, and regulates the send rate by getting the send rate values under different algorithms via comparation simulations. Finally, validates the approximation degree of send rate under different typical values. So, it is able to set one value adaptively according to another value in parameter setting equations.2. Considering the existing defects of AIMD algorithm, proposes an adaptive parameter setting AIMD algorithm in the paper according to the setting relationship of parametersαandβunder this algorithm. First, the network operation condition is divided into different congestion states according to the change of network loop round-trip time, the parametersαandβis regulated adaptively under different congestion states. Besides, the TCP send rate is regulated adaptively with different congestion control mechanism to make the network running in a good state. Then, makes the realization under the simple parameters in the A-AIMD algorithm in NS2, and does the simulation comparisons between the states in A-AIMD algorithm and the send rate in TCP Reno algorithm with different queue management algorithms to verify the correctness of parameter values in AIMD algorithm under the states above via the behavioral characteristics of send rate. Finally, the paper makes the simulation comparisons of TCP send rates between A-AIMD algorithm and Reno algorithm under different queue management algorithms respectively. The simulation results show that: when the network is running at a steady state, using A-AIMD algorithm can improve the average sending rate of TCP flow and reduce the fluctuation of sending flow; When there are resources available in the network, using A-AIMD algorithm can occupy available resources quickly, at the same time, it can maintain a certain degree of TCP-friendly with the data flows using Reno algorithm. |
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