Real time network applications require effective mechanisms for managing Internet traffic in order to avoid or at least limit the level of congestion. We propose, design and analyze an Active Queue Manager (AQM) for controlling jitter using robust control theory. Here, we use an output feedback controller and this controller is based on discrete-time model of TCP. The natural properties of robust controller help us decreasing effect of model uncertainties and undesirable flows on performance of traffic control. The proposed Robust Discrete-Time Controller (RDTC) ensures the maintenance of the router's queue length at the desired value. We used ns2 for simulation of controller performance and the results obtained verify the validity of the algorithm.