Modeling HTTP service times

The service time distribution is a fundamental parameter in analysis and simulation models. However, the distribution of the service times of Web servers and caches is not well understood. As a consequence, in many studies on Web server scheduling and task assignment, the service times have been assumed as directly proportional to the size of the file transferred. But Web transfers of the same size may experience different service times due to dynamic characteristics such as server load, network traffic and client connectivity. In this paper we present models for both the service time and the file size of Web cache transfers. Our goal is to compare both empirical distributions, exploring the correlation between them. Our results are based on recent logs from NLANR. We show that both variables may be modeled by the log-normal distribution but with different parameters.