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. As an analytical model, we use a two level priority processor sharing PS + PS. In the PS + PS system, a connection is classified as high or low priority depending on the amount of service it has obtained. We show that PS + PS reduces the mean response time in comparison with standard processor sharing when the hazard rate of the file size distribution is decreasing. By simulations we study the impact of RuN2C on extreme values of response times and the mean number of connections in the system. Both simulations and analytical results show that RuN2C has a very beneficial effect on the delay of short flows, while treating large flows as the current TCP implementation does. In contrast, we find that LAS based mechanisms can lead to pathological behavior in extreme cases.