Measurements on a high-performance switched Ethernet system are presented that reveal new insights into the statistical nature of file server and web server traffic. Both file sizes and data requested from the web server are shown to match well a truncated Cauchy distribution. This is a distribution with heavy tails similar in nature to the commonly used Pareto distribution but with a much better fit over smaller file/request sizes. We observe self similar characteristics in the traffic at both servers and also at a CPU server elsewhere on the network. Traffic from this server is predominantly targeted at the file and web servers, suggesting that self-similar properties at one point on a network are being propagated to other points.
A simple simulation model of an isolated server is presented with Poisson arrivals and service (packet transmission) demands with the same Cauchy distribution as we observed. The departure process is shown to follow a power law and the corresponding power spectrum is shown to match extremely well that of the observed traffic. This supports the suggested link between file/request size distribution and self-similarity. The resulting implication that self similarity and heavy tails are primarily due to server-nodes, rather than being inherent in offered traffic, leads to the possibility of using conventional queueing network models of performance. This idea is further supported by an additional simulation experiment and suitable models are proposed.
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