HTTP: The Definitive Guide

Proxy Interoperation

Clients, servers, and proxies are built by multiple vendors, to different versions of the HTTP specification. They support various features and have different bugs. Proxy servers need to intermediate between client-side and server-side devices, which may implement different protocols and have troublesome quirks.

Handling Unsupported Headers and Methods

The proxy server may not understand all the header fields that pass through it. Some headers may be newer than the proxy itself; others may be customized header fields unique to a particular application. Proxies must forward unrecognized header fields and must maintain the relative order of header fields with the same name.^[17] Similarly, if a proxy is unfamiliar with a method, it should try to forward the message to the next hop, if possible.

Proxies that cannot tunnel unsupported methods may not be viable in most networks today, because Hotmail access through Microsoft Outlook makes extensive use of HTTP extension methods.

OPTIONS: Discovering Optional Feature Support

The HTTP OPTIONS method lets a client (or proxy) discover the supported functionality (for example, supported methods) of a web server or of a particular resource on a web server (Figure 6-26). Clients can use OPTIONS to determine a server’s capabilities before interacting with the server, making it easier to interoperate with proxies and servers of different feature levels.

Figure 6-26. Using OPTIONS to find a server’s supported methods

If the URI ...