Right buffer sizing matters: stability and queuing dynamics in TCP.
Debayani Ghosh, Krishna Jagannathan, Gaurav Raina · arXiv (Cornell University) · 2016
Motivated by recent concerns that queuing delays in the Internet are on the rise, we conduct a performance evaluation of Compound TCP (C-TCP) in two topologies: a single bottleneck and a multi-bottleneck topology. The first topology consists of a single core router, and the second consists of two distinct sets of TCP flows, regulated by two edge routers, feeding into a common core router. For both topologies, we develop fluid models and conduct a detailed local stability analysis in the small buffer regime, and obtain necessary and sufficient conditions for local stability. Further, we show that the underlying non-linear models undergo a Hopf bifurcation as the stability conditions just get violated. Using a combination of analysis and packet-level simulations, we emphasise that larger buffer thresholds, in addition to increasing latency, are prone to inducing limit cycles. These limit cycles in turn cause synchronisation among the TCP flows, and also result in a loss of link utilisation. For the single bottleneck topology, we empirically analyse some statistical properties of the bottleneck queue. We highlight that in a high bandwidth-delay product and a small buffer regime, and with a large number of long-lived flows, the bottleneck queue may be modelled as an M/M/1/B or an M/D/1/B queue. The combination of the dynamical and the statistical properties explored in this paper could have important implications for quality of service in the Internet.