Two-degree-of-freedom Congestion Control Strategy Against Time Delay and Disturbance

Most of the existing congestion control schemes cannot guarantee the quality of service in wide-area networks with large round trip time (RTT). Aiming at the above problem, a two-degree-of-freedom congestion control strategy (named TCC) is proposed for improving the stability and robustness of the TCP/AQM system. TCC employs a modified Smith predictor with two additional controllers to compensate for the RTT delay. The feedback controller is designed by the internal model control theory for fast set-point tracking. The disturbance rejection controller is derived from frequency-domain analysis to reject external disturbance. Simulation results in NS2 demonstrate that TCC can effectively overcome the negative influence cased by time delay and disturbance. Compared with other congestion control schemes, our proposed method is superior in stabilizing the queue length with small jitters and rejecting disturbances.