A Study of Burstiness in TCP Flows
We study the burstiness of TCP flows at the packet level. We aggregate packets into entities we call "flights". We show, using a simple model of TCP dynamics, that delayed-acks and window dynamics would potentially cause flights at two different timescales in a TCP flow - the lower at the order of 5-10 ms (sub-RTT) and the higher at about 10 times this value (order of an RTT seen by the flow). The model suggests that flight sizes would be small at the lower timescale, regardless of the network environment. The model also predicts that the network conditions required for the occurrence of flights at the larger timescale are either large buffers or large available bandwidths - both of which result in a high bandwidth delay product environment. We argue that these two conditions indicate that the TCP flow does not operate in a congestion control region, either because the source of traffic is unaware of congestion or because there is so much bandwidth that congestion control is not required. We verify our model by passive Internet measurement. Using the trace files obtained, we collect statistics on flights at the two timescales in terms of their frequency and size. We also find the dependence of the sizes and frequency of flights on the Internet environment in which they occurred. The results concur strongly with our hypothesis on the origins of flights, leading us to the conclusion that flights are effective indicators of excess resource in the Internet.