Differentiation Between Short and Long TCP Flows: Predictability of the Response Time

Internet measurements show that a small number of large TCP flows are responsible for the largest amount of data transferred, whereas most of the TCP sessions are made up of few packets. Several authors have invoked this property to suggest the use of scheduling algorithms which favor short jobs, such as LAS (Least Attained Service), to differentiate between short and long TCP flows. We propose a packet level stateless, threshold based scheduling mechanism for TCP flows, RuN2C. We describe an implementation of this mechanism which has the advantage of being TCP compatible and progressively deployable. We compare the behavior of RuN2C with LAS based mechanisms through analytical models and simulations of TCP in networks with tail drop buffers. In addition to studying the mean response time conditioned on given flow size, the principle performance measure used in previous studies, we also study the impact of the scheduling mechanisms on extreme values of response times as well as on the stability of the network, illustrated by the mean number of connections in the system. We show that RuN2C has a very beneficial effect on the delay of short flows, while treating large flows as the current TCP scheduling does. In contrast, we find that LAS based mechanisms can lead to pathological behavior in extreme cases