Introduction to the HTTP protocol

Since 1990 HTTP protocol (HyperText Transfer Protocol) has been the most widely used protocol on the Internet. Version 0.9 was only intended to transfer data over the Internet (in particular Web pages written in HTML. Version 1.0 of the protocol (the most used) now allows the transfer of messages with headers describing the content of the message by using MIME type coding.

The aim of HTTP protocol is to allow transfer of files (essentially in HTML format) between a browser (the client) and a Web server (called among other things httpd on UNIX machines) located using a character string called a URL.

Communication between browser and server

Communication between the browser and server takes place in two stages:

• The navigator makes a HTTP request
• The server processes the request then sends a HTTP response

HTTP Request

A HTTP request is a collection of lines sent to the server by the browser. It includes:

• A request line: This is a line specifying the type of document requested, the method which must be applied, and the version of the protocol used. The line is made up of three elements which must be separated by a space:
  • The method
  • The URL
  • The version of the protocol used by the client (generally HTTP/1.0)
• The request header fields: This is a collection of optional lines allowing additional information about the request and/or the client to be given (browser, operating system, etc.). Each of these lines is composed of a name describing the header type, followed by a colon (:) and the value of the header
• The body of the request: This is a collection of optional lines which must be separated from preceding lines by an empty line and for example allowing data to be sent by a POST command during the sending of data to the server using a form

A HTTP request therefore has the following syntax (<crlf> meaning carriage return and line feed):

METHOD URL VERSION<crlf>
HEADER: Value<crlf>
.
.
.
HEADER: Value<crlf>
Empty line <crlf>
BODY OF THE REQUEST

Here is an example of a HTTP request:

GET http://en.kioskea.net/ HTTP/1.0
Accept: text/html
If-Modified-Since: Saturday, 15-January-2000 14:37:11 GMT
User-Agent: Mozilla/4.0 (compatible; MSIE 5.0; Windows 95)

Commands

Headers

HTTP Response

A HTTP response is a collection of lines sent to the server by the browser. It includes:

• A status line: this is a line specifying the protocol version used and the status of the request being processed using a code and explanatory text. The line is made up of three elements which must be separated by a space:
  • The version of the protocol used
  • The status code:
  • The meaning of the code
• The response header fields: This is a collection of optional lines allowing additional information about the response and/or the client to be given (browser, operating system, etc.). Each of these lines is composed of a name describing the header type, followed by a colon (:) and the value of the header
• The body of the response: contains the requested document

A HTTP response therefore has the following syntax (<crlf> meaning carriage return and line feed):

VERSION-HTTP CODE EXPLANATION<crlf>
HEADER: Value<crlf>
.
.
.
HEADER: Value<crlf>
Empty line <crlf>
BODY OF THE RESPONSE

Here is an example of a HTTP response:

HTTP/1.0 200 OK
Date: Sat, 15 Jan 2000 14:37:12 GMT
Server: Microsoft-IIS/2.0
Content-Type: text/HTML
Content-Length: 1245
Last-Modified: Fri, 14 Jan 2000 08:25:13 GMT

Response headers

The response codes

These are the codes that you see when the browser cannot display the requested page. The response code is made up of three digits: the first indicates the status and the following two digits explain the exact nature of the error.

More information

For more information on HTTP protocol, please refer to RFC1945 which explains the protocol in detail:

• RFC 1945 - Hypertext Transfer Protocol -- HTTP/1.0 (French translation)
• RFC 1945 - Hypertext Transfer Protocol -- HTTP/1.0 (original version)
• RFC 2616 - Hypertext Transfer Protocol -- HTTP/1.1 (original version)
• Cookies