What is HTTP protocol ?

Hypertext Transfer Protocol (HTTP) 

• Application Layer protocol for distributed, collaborative, hypermedia information systems
• Request-response protocol standard for client-server computing
• Stateless protocol : because each command is executed independently

FUNCTION

• A web browser acts as a client and application running on a computer hosting the web site acts as a server.
• The client submits HTTP requests to the responding server by sending messages to it.
• The server, which stores content (or resources) such as HTML files and images, or generates such content on the fly, sends messages back to the client in response.
• Resources to be accessed by HTTP are identified using URIs or URLs — using the http or https URI schemes.

HTTP versions

• HTTP/1.0 uses a separate connection to the same server for every document
• HTTP/1.1 can reuse the same connection to download, for instance, images for the just served page

HTTP session

• A sequence of network request-response transactions.
• HTTP client initiates a request.
• It establishes a TCP connection to a particular port on a host.
• HTTP server listening on that port waits for a client's request message. 
• Upon receiving the request, the server sends back a status line, such as "HTTP/1.1 200 OK", and a message of its own, the body of which is perhaps the requested resource, an error message, or some other information.

HTTPS

• A URI scheme syntactically identical to the http scheme
• used for normal HTTP connections, but signals the browser to use an added encryption layer of SSL/TLS to protect the traffic

Example

Client request

GET /index.html HTTP/1.1

Host: www.example.com

Server response

HTTP/1.1 200 OK

Date: Mon, 23 May 2005 22:38:34 GMT

Server: Apache/1.3.3.7 (Unix) (Red-Hat/Linux)

Last-Modified: Wed, 08 Jan 2003 23:11:55 GMT

Etag: "3f80f-1b6-3e1cb03b"

Accept-Ranges: bytes

Content-Length: 438

Connection: close

Content-Type: text/html; charset=UTF-8