We have measured the performance of two commercial content distribution net
works (CDNs), one operated by Akamai and one operated by Digital Island. Al
though there are differences in the implementation of these services, both
CDNs redirect requests by using DNS. In this paper, we describe our simple
measurement technique for a DNS-based CDN, our data for the two commercial
services, and our interpretation of that data. Our main conclusion is that
CDNs provide a valuable service, but that neither Akamai nor Digital Island
can consistently pick the best server of those available. Contrary to some
widely disseminated marketing messages, we argue that CDNs succeed not so
much by choosing an "optimal" server as by avoiding notably bad servers. (C
) 2001 Published by Elsevier Science B.V.