In large multiplexers with many TCP flows, the aggregate traffic flow behaves predictably; this is a basis for the fluid model of Misra, Gong and Towsley and for a growing literature on fluid models of congestion control. I will explain how different buffer-sizing rules lead to different fluid models, using various techniques from queueing theory.
I will also describe the behaviour of these fluid models (on a single bottleneck link, for a collection of identical long-lived flows). For what parameter regimes is the fluid model stable, and when it is unstable what is the size of oscillations and the impact on goodput? This analysis lets us decide which buffer-sizing rule to use.