In this paper, recently proposed active queue management (AQM) algorithms for supporting end-to-end transmission control protocol (TCP) congestion control is revisited. We focus recently developed theoretic results on design and analysis for the AQM based TCP congestion control dynamics. In this context, the existing fluid model of the TCP/AQM network is discussed. Moreover, an improved fluid model is addressed, taking time delay in inner feedback loop into account, which is neglected in the modeling process of fluid model. The stabilization of the fluid model is investigated, which has shown that the stabilizing region of PID congestion controller for the conventional fluid model moves 1/k and T1/k along the kp and kd axes respectively compared with that for the improved fluid model, ie. the actual stability region. The improved fluid model has a great potential in analyzing and designing various network congestion control algorithms.