Synchronisation in TCP networks with Drop-Tail QueuesThe design of transport protocols, embedded in end-systems, and the choice of
buffer sizing strategies, within network routers, play an important role in
performance analysis of the Internet. In this paper, we take a dynamical
systems perspective on the interplay between fluid models for transport
protocols and some router buffer sizing regimes. Among the flavours of TCP, we
analyse Compound, as well as Reno and Illinois. The models for these TCP
variants are coupled with a Drop-Tail policy, currently deployed in routers, in
two limiting regimes: a small and an intermediate buffer regime. The topology
we consider has two sets of long-lived TCP flows, each passing through separate
edge routers, which merge at a common core router. Our analysis is inspired by
time delayed coupled oscillators, where we obtain analytical conditions under
which the sets of TCP flows synchronise. These conditions are made explicit in
terms of coupling strengths, which depend on protocol parameters, and on
network parameters like feedback delay, link capacity and buffer sizes. We find
that variations in the coupling strengths can lead to limit cycles in the queue
size. Packet-level simulations corroborate the analytical insights. For design,
small Drop-Tail buffers are preferable over intermediate buffers as they can
ensure both low latency and stable queues.
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