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The HTTP protocol

Latest update on Thursday 16 October 2008 à 09:43 by Jean-François Pillou.

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:

Communication between browser and server

  • The navigator makes a HTTP request
  • The server processes the request then sends a HTTP response
In reality, the communication is conducted in more stages if you consider the processing of the request by the server. Given that we are only concerned with HTTP protocol, server side processing will not be explained as part of this article.

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

Command Description
GET Request for the resource located at the specified URL
HEAD Request for the header of the resource located at the specified URL
POST Sends data to the program located at the specified URL
PUT Sends data to the specified URL
DELETE Deletes the resource located at the specified URL

Headers

Header name Description
Accept Type of content accepted by the browser (for example text/html). See MIME types
Accept-Charset Character set expected by the browser
Accept-Encoding Data coding accepted by the browser
Accept-Language Language expected by the browser (English by default)
Authorization Identification of the browser to the server
Content-Encoding Type of coding for the body of the request
Content-Language Type of language in the body of the request
Content-Length Length of the body of the request:
Content-Type Type of content of the body of the request (for example text/html). See MIME types
Date Date data transfer starts.
Forwarded Used by intermediary machines between the browser and server
From Allows the client email address to be specified
From Makes it possible to specify that the document must be sent if it has been modified since a certain date.
Link Link between two URLs
Orig-URL URL from which the request originated
Referer Link URL from which the request has been made
User-Agent String giving information about the client, such as the name and version of the browser and the operating system

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

Header name Description
Content-Encoding Type of coding for the body of the response
Content-Language Type of language in the body of the response
Content-Length Length of the body of the response
Content-Type Type of content of the body of the response (for example text/html). See MIME types
Date Date data transfer starts.
Expires Data use by date
Forwarded Used by intermediary machines between the browser and server
Location Redirection to a new URL associated with the document
Server Features of the server having sent the response

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.

Code Message Description
10x Information message These codes are not used in version 1.0 of the protocol
20x Success These codes indicate the smooth running of the transaction
200 OK The request has been accomplished correctly
201 CREATED This follows a POST command and indicates success, the remaining body of the document indicates the URL where the newly created document must be located.
202 ACCEPTED The request has been accepted, the procedure which follows has not been accomplished
203 PARTIAL INFORMATION When this code is received in response to a GET command it indicates that the response is not complete.
204 NO RESPONSE The server has received the request by there is no information to send back
205 RESET CONTENT The server tells the browser to delete the content in the fields of a form
206 PARTIAL CONTENT This is a response to a request consisting of the header range. The server must indicate the header content-Range
30x Redirection These codes indicate that resource is no longer in the location specified
301 MOVED The requested data has been transferred to a new address
302 FOUND The requested data is at a new URL, but has however maybe been moved since...
303 METHOD This means that the client must try a new address, preferably by trying another method to GET
304 NOT MODIFIED If the client has carried out a conditional GET command (by requesting if the document has been modified since the last time) and the document has not been modified it sends back this code.
40x Error due to the client These codes indicate that the request is incorrect
400 BAD REQUEST The syntax of the request is badly formulated or is impossible to satisfy
401 UNAUTHORIZED The parameters of the message give specifications of unacceptable forms of authorization. The client must reformulate its request with the correct authorization data
402 PAYMENT REQUIRED The client must reformulate its request with the correct payment data
403 FORBIDDEN Access to the resource is quite simply denied
404 NOT FOUND Classic! The server has not found anything at the specified address. Left without leaving a forwarding address....:)
50x Error due to the server These codes indicate that there is an internal error in the server
500 INTERNAL ERROR The server has encountered an unexpected condition which prevented it from following up the request (just one of those things that happen to servers...)
501 NOT IMPLEMENTED The server does not support the service requested (it cannot know everything...)
502 BAD GATEWAY The server has received an invalid response from the server which it is trying to access by acting as a gateway or proxy
503 SERVICE UNAVAILABLE The server cannot respond to you at the present time since it is too busy (all your communication lines are busy, please try again later)
504 GATEWAY TIMEOUT The response from the server has taken too long in relation to the time for which the gateway had been prepared to way (the time that was assigned to you has now passed...)

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)
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