From the control theory point of view, it is reasonable to regard the TCP congestion control mechanism as a feedback control system. The TCP congestion control is complemented by the active queue management (AQM) scheme implemented in the routers, which could improve the effectiveness. Recently, an effective proportional-differential (PD) control algorithm has been proposed as a new AQM scheme for TCP congestion control. But it is still difficult to assign the parameter values of the PD-controller. This paper proposes a method for the design of an effective and stable PD-control AQM for routers in the Internet. For this purpose, a first-order plus time-delay TCP model is constructed, and a relay feedback method is employed to determine a set of stable parameter values of the model. From this set of values, and the requirements of the gain and phase margin, the specified parameter values of the PD-controller can be easily obtained by scanning the stable parameter set.
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