Network Congestion Control, Active Queue Management Algorithm

Congestion control in current Internet, which plays an significant role in guaranteeing sound Quality of Service, has stimulated researchers to compensate defects in transmission protocols on both end-points and intermediate switchers. Though dozens of effective scenarios aiming to improve TCP performances have been put forwarded in recent years, the congestion predication ability on routers should be enhanced.In this thesis, several new AQM algorithms are proposed in terms of control theory.We validiate their performance through a number of computer-based simulations.(1) Propose a robust PI controller with position internal feedback compensation (PIP). The new algorithm integrates the advantage of PI controller and internal position feedback to produce stable drop probability. We configurate parameters in terms of ITAE metrics which improves transient performance and simplifies the configuration process as well. Simulations show that I-PIP controller is obviously superior to other classic AQM controllers, especially in long delay and heavy load environment.(2) A low pass filter is placed after the differential portion of PID & PD controller on purpose of smoothing the instinct sensitivity of differential portion against error signal disturbance. Compared with traditional PID (PD) controllers, partial differential PID (PD) controllers perform better when coping with burst traffic.(3) Adaptively adjust the target queue length (TQL) based on the instantaneous load which is estimated by a low pass filter. Reducing TQL when traffic is light and increasing it when traffic grows maximizes buffer's efficiency to absorb burst traffic and maintain high link utilization as well. It also helps to improve the throughput of each connection and reduce unnecessary loss. A fact reflects from ATQL indicates that oscillation in queue length is not always caused by unstable drop probability, but results from inherent burst traffic. Though ATQL is inserted before PI controller, it can be transplanted to other similarly algorithms which engage TQL to constrain instant queue length.