Performance Characteristics of Mirror Servers on the Internet

As a growing number of web sites introduce mirrors to increase throughput, the challenge for clients becomes determining which mirror will offer the best performance when a document is to be retrieved. In this paper we present findings from measuring 9 clients scattered throughout the United States retrieving over 490,000 documents from 47 production web servers which mirror three different web sites. We have several interesting findings that may aid in the design of protocols for choosing among mirror servers. Though server performance varies widely, we have observed that a server’s performance relative to other servers is more stable and is independent of time scale. In addition, a change in an individual server’s transfer time is not a strong indicator that its performance relative to other servers has changed. Finally, we have found that clients wishing to achieve near-optimal performance may only need to consider a small number of servers rather than all mirrors of a particular site.