Content-caching architectures and engines such as in the Cisco product palette deal with the challenge to deliver content reliably, efficiently, and effectively to the network edge and access layer where customers subscribe to certain content. Vice versa, they are necessary to provide sufficiently clustered server farms to feed these requests.
Historically, caching was the initial purpose of proxies and proxy chaining. Protection of expensive and rare WAN bandwidth was their prime directive. Today, with cheap bandwidth in abundance, the focus has shifted toward intelligent security, content screening, and load-balancing content and cache-engine architectures. However, these are Layer 4 through 7 issues and not the focus of this chapter. For a background on caching strategies, look at the Internet's most popular open-source proxy, squid, at http://www.squid-cache.org/, and the proxy capabilities of the Apache web server, at http://www.apache.org.
Modern proxies fall in different categories:
Transparent caching proxies
Security (intercepting) proxies
Mangling proxies (packet rewrites)