HTTP: The Definitive Guide

Chapter 20. Redirection and Load Balancing

HTTP does not walk the Web alone. The data in an HTTP message is governed by many protocols on its journey. HTTP cares only about the endpoints of the journey—the sender and the receiver—but in a world with mirrored servers, web proxies, and caches, the destination of an HTTP message is not necessarily straightforward.

This chapter is about redirection technologies—network tools, techniques, and protocols that determine the final destination of an HTTP message. Redirection technologies usually determine whether the message ends up at a proxy, a cache, or a particular web server in a server farm. Redirection technologies may send your messages to places a client didn’t explicitly request.

In this chapter, we’ll take a look at the following redirection techniques, how they work, and what their load-balancing capabilities are (if any):

• HTTP redirection

• DNS redirection

• Anycast routing

• Policy routing

• IP MAC forwarding

• IP address forwarding

• The Web Cache Coordination Protocol (WCCP)

• The Intercache Communication Protocol (ICP)

• The Hyper Text Caching Protocol (HTCP)

• The Network Element Control Protocol (NECP)

• The Cache Array Routing Protocol (CARP)

• The Web Proxy Autodiscovery Protocol (WPAD)