Caching Neighborhood Protocol: a Foundation for Building Dynamic Caching Hierarchies with WWW Proxy Servers

In this paper, we propose a Caching Neighborhood Protocol (CNP) that describes an infrastructure upon which proxy servers can build dynamic caching hierarchies. Compared to the conventional schemes based on static caching hierarchies where proxy servers can only communicate with each other following predefined path configurations, the proposed dynamic scheme permits flexible communications among proxy servers. Such a scheme can decrease the response times of requests by increasing the availability of documents without compromising the currency of rendered documents. CNP is based on the concept of caching neighborhood, in which originating servers extend their spans and availability through their caching representatives, incarnated as proxy servers. The coherence of documents provided by the originating server and the cached copies on its caching representatives is maintained via employing the X tolerance coherence requirement. Some qualitative reasonings and studies on the traces of request accessing logs collected by a proxy server at NLANR [16] are presented to justify the proposed CNP. One of the most important observations is that caching the most popular documents provided by the most popular Web sites would be the most effective way for Web caching. A five day trace analysis shows that the top 20% Web sites (in terms of frequency of accesses) provide 93% of the most frequently accessed top 20% documents and such top 20% documents account for 94% documents that have been accessed more than once. We also describe how CNP exploits these access characteristics to fare against the static caching hierarchy schemes based on Internet Cache Protocol (ICP) [7], such as Harvest [2] and Squid [21].