HTTP Headers - Part 4 of Chapter 3 from HTTP: The Definitive Guide

Headers

[The following is the conclusion of our series of excerpts from chapter 3 of the O'Reilly title, HTTP: The Definitive Guide.]

Headers and methods work together to determine what clients and servers do. This section quickly sketches the purposes of the standard HTTP headers and some headers that are not explicitly defined in the HTTP/1.1 specification (RFC 2616). Appendix C summarizes all these headers in more detail.

There are headers that are specific for each type of message and headers that are more general in purpose, providing information in both request and response messages. Headers fall into five main classes:

General headers

These are generic headers used by both clients and servers. They serve general purposes that are useful for clients, servers, and other applications to supply to one another. For example, the Date header is a general-purpose header that allows both sides to indicate the time and date at which the message was constructed:

Date: Tue, 3 Oct 1974 02:16:00 GMT

Request headers

As the name implies, request headers are specific to request messages. They provide extra information to servers, such as what type of data the client is willing to receive. For example, the following Accept header tells the server that the client will accept any media type that matches its request:

Accept: */*

Response headers

Response messages have their own set of headers that provide information to the client (e.g., what type of server the client is talking to). For example, the following Server header tells the client that it is talking to a Version 1.0 Tiki-Hut server:

Server: Tiki-Hut/1.0

Entity headers

Entity headers refer to headers that deal with the entity body. For instance, entity headers can tell the type of the data in the entity body. For example, the following Content-Type header lets the application know that the data is an HTML document in the iso-latin-1 character set:

Content-Type: text/html; charset=iso-latin-1

Extension headers

Extension headers are nonstandard headers that have been created by application developers but not yet added to the sanctioned HTTP specification. HTTP programs need to tolerate and forward extension headers, even if they don't know what the headers mean.