Li Fan, Pei Cao and Jussara Almeida
Andrei Z. Broder
Department of Computer Science
Systems Research Center
University of Wisconsin-Madison
Digital Equipment Corporation
The sharing of caches among Web proxies is an important technique to reduce Web traffic and alleviate network bottlenecks. Nevertheless it is not widely deployed due to the overhead of existing protocols. In this paper we demonstrate the benefits of cache sharing, measure the overhead of the existing protocols, and propose a new protocol called ``Summary Cache''. In this new protocol, each proxy keeps a summary of the cache directory of each participating proxy, and checks these summaries for potential hits before sending any queries. Two factors contribute to our protocol's low overhead: the summaries are updated only periodically, and the directory representations are very economical, as low as 8 bits per entry. Using trace-driven simulations and a prototype implementation, we show that, compared to existing protocols such as the Internet Cache Protocol (ICP), Summary Cache reduces the number of inter-cache protocol messages by a factor of 25 to 60, reduces the bandwidth consumption by over 50%, eliminates 75% and 95% of the protocol CPU overhead, all while maintaining almost the same cache hit ratio as ICP. Hence Summary Cache scales to a large number of proxies.