Research On Analysis And Control Of Dynamic Behavior In Internet Congestion Control System

Since the rapid progress made in computer and communication technology, the TCP (Transmission Control Protocol) / IP (Internet Protocol)-based Internet has developed enormously. However, with the growing size, popularity and application of the Internet, congestion has become an increasing serious issue. Congestion can cause significant deterioration of network performance, including package drop probability, time delay and network throughput, and even cause the happening of congestion collapse. Network congestion control is the main method for improving the performance, increasing the robustness and strenghthening the quality of service of the whole network.At present, the congestion control strategy of Internet consists of two parts: a source algorithm carried out by TCP at hosts and a link algorithm by AQM (Active Queue Management) schemes at routers. The combination of both parts has been the main approach to solve the Internet congestion control problem and is now considered a research hotspot which intercrosses several disciplines, such as computer networks, communication and automatic control.In this dissertation, nonlinear dynamic analysis is applied to interpret the occurrence of nonlinear phenomenon (such as bifurcation and chaos) in Internet congestion control systems. Moreover, bifurcation and chaos control methods are used to eliminate nonlinear behavior in the Internet. Comparing traditional stochastic theory and queuing theory, the nonlinear dynamic analysis is expected to lead to more precise results. Thus it is important to model the Internet congestion control systems, improve the present congestion control scheme and explore new congestion control strategies for future high bandwidth-delay-product networks. Therefore, research of the Internet congestion control systems has very important theoretical significance and application values.The main contents and contributions of this dissertation can be summarized as follows:(1). Nonlinear dynamic behavior and its control problem in a discrete-time model of a simple Internet congestion control system are studied. It is shown that the system has complex dynamic behaviors, such as bifurcation and chaos, with TCP and UDP (User Data Protocol) flows at sources and RED (Random Early Detect) algorithm at router. Then, we apply standard and extended time-delayed feedback control (TDFC) approaches to control the system state or parameters in order to stabilize the chaotic behavior of average queue size of the system. Furthermore, this model is extended to systems with TCP Westwood flows and RED gateway. The existence of period-doubling bifurcation and chaotic behaviors is also indicated in the system with variation of parameters.(2). The control of dynamic behavior in stroboscopic model of Internet congestion control system is studied. A hybrid control strategy using both state feedback and parameter perturbation is applied to control the chaotic orbits embedded in the stroboscopic model in order to stabilize it to an equilibrium point. Moreover, this model is extended to a network with two bottleneck links. The interaction of two bottleneck links is investigated. The result indicates that the whole network can be stabilized when a key node is controlled.(3). The analysis and control of Hopf bifurcation in a Dual model of Internet congestion control system are studied. It is indicated that the model loses stability and a Hopf bifurcation occurs when bifurcation parameter (communication delay) passes through a critical value. Moreover, by using the perturbation method, we have analyzed the direction of Hopf bifurcation and the stability of the bifurcating periodic solutions. Then a time-delayed feedback control strategy is applied to the system for postponing onset of undesirable Hopf bifurcation and increasing stability region of the system.(4). The analysis and control of Hopf bifurcation in a Fluid-Flow model of Internet congestion control system are studied. Theoretical analysis and numerical simulations show that a Hopf bifurcation occurs in the system when communication delay passes through a critical value. This means that the state of system changes from an equilibrium point to a limit cycle. Furthermore, the direction of the Hopf bifurcation and the stability of bifurcating periodic solutions have also been investigated by applying the center manifold theorem and the normal form theory. Then a time-delay feedback controller is added to the system for controlling the Hopf bifurcation, which can increase stability region of the system.