Introduction to the HTTP protocol
The protocol HTTP (HyperText Transfer Protocol) is the protocol used on the Internet since 1990. Version 0.9 was only intended to transfer data over the Internet (in particular Web pages are written in HTML ] Version 1.0 of the protocol (the most used ) now allows you to transfer messages with headers describing the content of the message using MIME- type coding.
The purpose of the HTTP protocol is to allow the transfer of files (mainly in HTML format) located using a character string called URL between a browser (the client) and a Web server (also called httpd on UNIX machines ).
Communication between browser and server
Communication between the browser and the server is done in two stages:
- The browser makes an HTTP request
- The server processes the request then sends an HTTP response
In reality, the communication takes place in more time if we consider the processing of the request by the server. Since we are only interested in the HTTP protocol, server-side processing will not be explained in the context of this article … If this subject interests you, refer to the article on CGI processing.
HTTP request
An HTTP request is a set of lines sent to the server by the browser. She understands :
- A request line: this is a line specifying the type of document requested, the method that must be applied, and the version of the protocol used. The line includes three elements that must be separated by a space:
- The method
- URL
- The version of the protocol used by the client (usually HTTP / 1.0 )
- Request header fields: this is a set of optional lines used to provide additional information on the request and/or the client (Browser, operating system, …). Each of these lines is composed of a name qualifying the type of header, followed by a colon (:) and the value of the header
- The body of the request: it is a set of optional lines which must be separated from the preceding lines by an empty line and allowing, for example, a sending of data by a POST command when sending data to the server by a form
An HTTP request therefore has the following syntax ( <crlf> means carriage return or line feed ):
METHOD URL VERSION <crlf> HEADER : Value <crlf> . . . HEADER : Value <crlf> Empty line <crlf> BODY OF THE REQUEST
Here is an example of an HTTP request:
GET [/ http://www.tops10.xyz ] 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)
Orders
| Ordered | 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 | Sending data to the program located at the specified URL |
| PUT | Sending data to the specified URL |
| DELETE | Removal of 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 with the server |
| Content-Encoding | Type of coding of the body of the request |
| Content-Language | Body language type of the request |
| Content-Length | Length of the request body |
| Content-Type | Content-type of the body of the request (for example text / HTML ). See MIME types |
| Dated | Data transfer start date |
| Forwarded | Used by intermediate machines between the browser and the server |
| From | Allows you to specify the customer’s email address |
| From | Allows you to specify that the document should be sent if it has been modified since a certain date |
| Link | Relationship between two URLs |
| Orig-URL | Request original URL |
| Refer | URL of the link from which the request was made |
| User-Agent | A string giving information about the client, such as browser name and version, of the operating system |
HTTP response
An HTTP response is a set of lines sent to the browser by the server. She understands :
-
- A status line: this is a line specifying the version of the protocol used and the status of processing the request using a code and an explanatory text. The line includes three elements that must be separated by a space:
-
- The version of the protocol used
-
- The status code
-
- The meaning of the code
-
- A status line: this is a line specifying the version of the protocol used and the status of processing the request using a code and an explanatory text. The line includes three elements that must be separated by a space:
-
- Response header fields: this is a set of optional lines used to provide additional information on the response and/or the server. Each of these lines is composed of a name qualifying the type of header, followed by a colon (:) and the value of the header
-
- The body of the response: it contains the requested document
An HTTP response, therefore, has the following syntax ( <crlf> means carriage return or line feed ):
VERSION-HTTP CODE EXPLANATION <crlf> HEADER : Value <crlf> . . . HEADER : Value <crlf> Empty line <crlf> RESPONSE BODY
Here is an example of an 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 | Response body coding type |
| Content-Language | Response body language type |
| Content-Length | Response body length |
| Content-Type | Content-type of the body of the response (for example text / Html ). See MIME types |
| Dated | Data transfer start date |
| Expires | Deadline for data consumption |
| Forwarded | Used by intermediate machines between the browser and the server |
| Rental | Redirect to a new URL associated with the document |
| Server | Characteristics of the server that sent the response |
Response codes
These are the codes you see when the browser cannot provide you with the requested page. The response code consists of three digits: the first indicates the status class and the following the exact nature of the error.
| Coded | 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 was completed correctly |
| 201 | CREATED | It follows a POST command, it indicates success, the body of the rest of the document is supposed to indicate the URL where the newly created document should be. |
| 202 | ACCEPTED | The request was granted, but the following procedure was not completed |
| 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 but there is no information to return |
| 205 | RESET CONTENT | The server tells the browser to delete the content of the fields of a form |
| 206 | PARTIAL CONTENT | It is a response to a request with the range header. The server must specify content-Range header |
| 30x | Redirection | These codes indicate that the resource is no longer in the indicated location |
| 301 | MOVED | The requested data has been transferred to a new address |
| 302 | FOUND | The requested data is at a new URL but may have been moved from … |
| 303 | METHOD | This implies that the client must try a new address, preferably trying a method other than getting |
| 304 | NOT MODIFIED | If the client has made a conditional GET command (asking if the document has been modified since the last time) and the document has not been modified, it returns this code. |
| 40x | Customer error | 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 message parameter provides specifications for acceptable forms of authorization. The customer must reformulate his request with the correct authorization data |
| 402 | PAYMENT REQUIRED | The customer must reformulate his request with the correct payment data |
| 403 | FORBIDDEN | Access to the resource is simply prohibited |
| 404 | NOT FOUND | Classic! The server could not find anything at the specified address. Left without leaving an address … 🙂 |
| 50x | Server error | These codes indicate that there was an internal server error |
| 500 | INTERNAL ERROR | The server encountered an unexpected condition which prevented it from responding to the request (such as something happening to the servers …) |
| 501 | NOT IMPLEMENTED | The server does not support the requested service (we cannot know everything …) |
| 502 | BAD GATEWAY | The server received an invalid response from the server it was trying to access by acting as a gateway or proxy |
| 503 | UNAVAILABLE SERVICE | The server cannot answer you at the moment, because the traffic is too dense (all the lines of your correspondent are busy, please call again later) |
| 504 | GATEWAY TIMEOUT | The server response was too long the time during which the gateway was prepared to wait for it (the time allotted to you is now over …) |