HTTP: The Definitive Guide

Cache Array Routing Protocol

Proxy servers greatly reduce traffic to the Internet by intercepting requests from individual users and serving cached copies of the requested web objects. However, as the number of users grows, a high volume of traffic can overload the proxy servers themselves.

One solution to this problem is to use multiple proxy servers to distribute the load to a collection of servers. The Cache Array Routing Protocol (CARP) is a standard proposed by Microsoft Corporation and Netscape Communication Corporation to administer a collection of proxy servers such that an array of proxy servers appears to clients as one logical cache.

CARP is an alternative to ICP. Both CARP and ICP allow administrators to improve performance by using multiple proxy servers. This section discusses how CARP differs from ICP, the advantages and disadvantages of using CARP over ICP, and the technical details of how the CARP protocol is implemented.

Upon a cache miss in ICP, the proxy server queries neighboring caches using an ICP message format to determine the availability of the web object. The neighboring caches respond with either a “HIT” or a “MISS,” and the requesting proxy server uses these responses to select the most appropriate location from which to retrieve the object. If the ICP proxy servers were arranged in a hierarchical fashion, a miss would be elevated to the parent. Figure 20-13 diagrammatically shows how hits and misses are resolved using ICP.

Figure 20-13. ICP queries ...