The HTTP protocol
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
| 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:
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