Professional Ajax, 2nd Edition

2.1. HTTP Primer

Central to a good grasp of Ajax techniques is hypertext transmission protocol (HTTP), the protocol to transmit web pages, images, and other types of files over the Internet to your web browser and back. Whenever you type a URL into the browser, an "http://" is prepended to the address, indicating that you will be using HTTP to access the information at the given location. (Most browsers support a number of different protocols as well, most notably FTP.)

Note that this section covers only those aspects of HTTP that are of interest to Ajax developers. It does not constitute an HTTP reference guide or tutorial.

HTTP consists of two parts: a request and a response. When you type a URL in a web browser, the browser creates and sends a request on your behalf. This request contains the URL that you typed in as well as some information about the browser itself. The server receives this request and sends back a response. The response contains information about the request as well as the data located at the URL (if any). It's up to the browser to interpret the response and display the web page (or other resource).

2.1.1. HTTP Requests

The format of an HTTP request is:

<request-line>
<headers>
<blank line>
[<request-body>]

In an HTTP request, the first line must be a request line indicating the type of request, the resource to access, and the version of HTTP being used. Next, a section of headers indicate additional information that may be of use to the server. After the ...