The Undismissible Rats: How Do Short Connections Affect Long-Lived TCP Flows under Moderate Traffic Load?

摘要

It is well-known that the distribution of file sizes in the Internet has a long tail, and that the traffic mainly consists of small flows 1. "the mice", while a large portion of the bytes are sent by large flows 2. "the elephants". However, it is not yet well understood regarding how the mice and the elephants interact with each other when they share a common link. A simplified modelling assumption is that the mice appear as an uncontrolled and random background load while the elephants take the remaining link bandwidth. In this paper, we find that under moderate traffic load, short connections, particularly large-sized mice whom we refer to as "rats", have a significant impact on the throughput of long-lived TCP flows. The occurrence of rats, who are in the slow-start phase of the TCP protocol, impair the ability of long-lived TCP flows to grab a link's available bandwidth. Our analysis shows that the throughput of the long-lived TCP flows is a function of the rats' arrival rate; and since the file sizes in the Internet are Pareto-distributed, this arrival rate is actually a function of the traffic's shape parameter.