A survey of web caching schemes for the Internet <s> Prefetching <s> As the number of World-Wide Web users grow, so does the number of connections made to servers. This increases both network load and server load. Caching can reduce both loads by migrating copies of server files closer to the clients that use those files. Caching can either be done at a client or in the network (by a proxy server or gateway). We assess the potential of proxy servers to cache documents retrieved with the HTTP protocol. We monitored traffic corresponding to three types of educational workloads over a one semester period, and used this as input to a cache simulation. Our main findings are (1) that with our workloads a proxy has a 30-50% maximum possible hit rate no matter how it is designed; (2) that when the cache is full and a document is replaced, least recently used (LRU) is a poor policy, but simple variations can dramatically improve hit rate and reduce cache size; (3) that a proxy server really functions as a second level cache, and its hit rate may tend to decline with time after initial loading given a more or less constant set of users; and (4) that certain tuning configuration parameters for a cache may have little benefit. <s> BIB001 </s> A survey of web caching schemes for the Internet <s> Prefetching <s> Caching in the World Wide Web is based on two critical assumptions: that a significant fraction of requests reaccess resources that have already been retrieved; and that those resources do not change between accesses. ::: ::: We tested the validity of these assumptions, and their dependence on characteristics of Web resources, including access rate, age at time of reference, content type, resource size, and Internet top-level domain. We also measured the rate at which resources change, and the prevalence of duplicate copies in the Web. ::: ::: We quantified the potential benefit of a shared proxy-caching server in a large environment by using traces that were collected at the Internet connection points for two large corporations, representing significant numbers of references. Only 22% of the resources referenced in the traces we analyzed were accessed more than once, but about half of the references were to those multiply-referenced resources. Of this half, 13% were to a resource that had been modified since the previous traced reference to it. ::: ::: We found that the content type and rate of access have a strong influence on these metrics, the domain has a moderate influence, and size has little effect. In addition, we studied other aspects of the rate of change, including semantic differences such as the insertion or deletion of anchors, phone numbers, and email addresses. <s> BIB002 </s> A survey of web caching schemes for the Internet <s> Prefetching <s> Prefetching and caching are techniques commonly used in I/O systems to reduce latency. Many researchers have advocated the use of caching and prefetching to reduce latency in the Web. We derive several bounds on the performance improvements seen from these techniques, and then use traces of Web proxy activity taken at Digital Equipment Corporation to quantify these bounds. ::: ::: We found that for these traces, local proxy caching could reduce latency by at best 26%, prefetching could reduce latency by at best 57%, and a combined caching and prefetching proxy could provide at best a 60% latency reduction. Furthermore, we found that how far in advance a prefetching algorithmwas able to prefetch an object was a significant factor in its ability to reduce latency. We note that the latency reduction from caching is significantly limited by the rapid changes of objects in the Web. We conclude that for the workload studied caching offers moderate assistance in reducing latency. Prefetching can offer more than twice the improvement of caching but is still limited in its ability to reduce latency. <s> BIB003
Although Web performance is improved by caching documents at proxies, the benefit from this technique is limited BIB002 BIB003 . Previous research has shown that the maximum cache hit rate can be achieved by any caching algorithm is usually no more than 40 % to 50%. In other words, regardless of the caching scheme in use, one out of two documents can not be found in cache BIB001 . One way to further increase the cache hit rate is to anticipate future document requests and preload or prefetch these documents in a local cache. Prefetching can be applied in three ways in the Web contexts: 1. Between browser clients and Web servers. 2. Between proxies and Web servers. 3. Between browser clients and proxies.