On designing a novel PI controller for AQM routers supporting TCP flows

Active Queue Management (AQM) is an effective method to enhance congestion control, and to achieve tradeoff between link utilization and delay. The de facto standard, Random Early Detection (RED), and most of its variants use queue length as congestion indicator to trigger packet dropping. As an extension of RED, a novel AQM algorithm, called NPI-RED, is proposed in this paper. The NPI-RED is based on a novel proportional and integral controller, which not only considers the average queue length at the current time slot, but also takes into consideration the past average queue lengths within a round trip time. We provide a guideline for the selection of the feedback gains for TCP/RED system to stabilize the dynamics, make the queue length converge at a certain target and improve the network performance. We present the condition of asymptotic stability for the model in terms of the average queue length, by using a method, in which we construct a Routh table associated with the characteristic polynomial. Based on the stability condition and control gains selection method, the extensive simulation results by ns2 demonstrate that the NPI-RED algorithm outperforms than the existed AQM schemes in robustness, drop probability and stability.