Beyond the Model of Persistent TCP Flows: Open-Loop vs Closed-Loop Arrivals of Non-persistent Flows

It is common for simulation and analytical studies to model Internet traffic as an aggregation of mostly persistent TCP flows. In practice, however, flows follow a heavy- tailed size distribution and their number fluctuates significantly with time. One important issue that has been largely ignored is whether such non-persistent flows arrive in the network in an open-loop (say Poisson) or closed-loop (interactive) manner. This paper focuses on the differences that the TCP flow arrival process introduces in the generated aggregate traffic. We first review the Processor Sharing models for such flow arrival processes as well as the corresponding TCP packet-level models. Then, we focus on the queueing performance that results from each model, and show that the closed-loop model produces lower loss rate and queueing delays than the open-loop model. We explain this difference in terms of the increased traffic variability that the open-loop model produces. The cause of the latter is that the flow arrival rate in the open-loop model does not reduce upon congestion. We also study the transient effect of congestion events on the two models and show that the closed-loop model results in congestion-responsive traffic while the open-loop model does not. Finally, we discuss implications of the differences between the two models in several networking problems.