Stability of TCP-RED Congestion Control

We present a method for deriving necessary and sufficient conditions for the local stability of the TCP-RED congestion control. The analysis is based on a differential equation model of the system. One equation describes the dynamics of the TCP source population operating in the congestion avoidance phase. The TCP dynamics are driven by another differential equation representing the RED controlled queue, which determines, in the form of a packet loss function, the feedback to the TCP population. The stability of the system is characterized in terms of certain stability parameters and we illustrate how the physical RED parameters affect the stability. Finally, different stability conditions (sufficient vs. necessary) are compared by analyzing a further simplified TCP model, where the queue feedback is a packet loss function depending only on the aggregate traffic rate of the TCP population.