Non-linear oscillations in TCP networks with Drop-Tail buffers

Our focus is on a non-linear, time delayed, fluid model for Compound TCP (C-TCP) which is widely implemented in the current Internet. We couple this model with a Drop-Tail queue policy, currently deployed in routers, in two different limiting regimes: a small buffer and an intermediate buffer regime. We consider a topology where two distinct sets of long lived Compound flows, each regulated by separate edge routers, merge at a common core network router. For both the small and the intermediate router buffer regimes, we derive explicit analytical conditions under which synchronisation would occur between the two competing sets of TCP flows. The conditions are made explicit in terms of a coupling strength, which depends on Compound parameters and on network parameters like the feedback delay, link capacity and router buffer sizes. Variations in the coupling strength can lead to the emergence of deterministic, nonlinear, oscillations in the form of limit cycles in the queue size. Our analytical results are corroborated with packet-level simulations. With Drop-Tail, small buffers were found to be preferable over intermediate buffers; they can ensure both low latency and stable queues, at least in high speed networks.