HTTP: The Definitive Guide

Content-Negotiation Techniques

There are three distinct methods for deciding which page at a server is the right one for a client: present the choice to the client, decide automatically at the server, or ask an intermediary to select. These three techniques are called client-driven negotiation, server-driven negotiation, and transparent negotiation, respectively (see Table 17-1). In this chapter, we will look at the mechanics of each technique as well as their advantages and disadvantages.

Table 17-1. Summary of content-negotiation techniques

Technique	How it works	Advantages	Drawbacks
Client-driven	Client makes a request, server sends list of choices to client, client chooses.	Easiest to implement at server side. Client can make best choice.	Adds latency: at least two requests are needed to get the correct content.
Server-driven	Server examines client’s request headers and decides what version to serve.	Quicker than client-driven negotiation. HTTP provides a q-value mechanism to allow servers to make approximate matches and a Vary header for servers to tell downstream devices how to evaluate requests.	If the decision is not obvious (headers don’t match up), the server must guess.
Transparent	An intermediate device (usually a proxy cache) does the request negotiation on the client’s behalf.	Offloads the negotiation from the web server. Quicker than client-driven negotiation.	No formal specifications for how to do transparent negotiation.