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:
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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