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Summary
Although Microsoft's marketing emphasis with ISA Server 2004 is on its firewall and VPN gateway functionality, it also provides companies with a viable Web caching solution that can save hundreds or thousands of dollars that would have to be spent for a separate caching product if you implemented competing firewall products that don't include caching functionality (and that's most of them).
ISA Server 2004's caching capabilities enhance your network's productivity by providing acceleration of access to external Web sites by your internal users, via the forward caching feature. It can also accelerate the access of external users who connect to your internal Web sites, via the reverse caching feature.
In larger, more complex network environments, multiple ISA Server 2004 computers can be used in distributed or hierarchical caching arrangements to provide for the best possible performance. Distributed caching distributes, or spreads, the cached Web objects across two or more caching servers. These servers are all on the same level on the network. In a hierarchical caching setup, caching servers are placed at different levels on the network. Upstream caching servers communicate with downstream proxies. For example, a caching server is placed at each branch office. These servers communicate with the caching array at the main office. Requests are serviced first from the local cache, then from a centralized cache before going out to the Internet server for the request.
ISA Server 2004 uses the Cache Array Routing Protocol (CARP), for communications between Web caching servers. CARP is a hash-based protocol that allows multiple caching proxies to be arrayed as a single logical cache and uses a hash function to ascertain to which cache a request should be sent. ISA Server 2004 uses cache rules to allow you to customize what types of content will be stored in the cache and exactly how that content will be handled when a request is made for objects stored in cache.
ISA Server 2004 can act as a combined firewall and Web-caching server, or as a dedicated Web-caching server, in addition to its default configuration (firewall only). In this chapter, you learned about the concepts of Web caching and how to configure an ISA Server 2004 computer to perform caching for your organization.
