How does the HTTP protocol work?

The HTTP (Hypertext Transfer Protocol) is an application layer protocol used for transferring hypermedia documents, such as HTML. It operates on a "client-server" principle.

Main stages:

Establishing a connection: The client (e.g., browser) establishes a TCP/IP connection with the server.
Sending a request: The client sends an HTTP request to the server. The request consists of:
- Request line: Method (e.g., GET, POST), resource path, and protocol version (e.g., HTTP/1.1).
- Headers: Additional information for the server (e.g., client type, supported languages).
- Request body: Data sent by the client (used with methods like POST).
```
GET /index.html HTTP/1.1 // Request line
Host: www.example.com // Header
User-Agent: Mozilla/5.0 // Header

// Request body (absent for GET)
```
Processing the request: The server receives and processes the request.
Sending a response: The server sends an HTTP response to the client. The response includes:
- Status line: Protocol version, status code (e.g., 200 OK, 404 Not Found), and status text.
- Headers: Information about the server, content, caching, etc.
- Response body: The requested resource (e.g., HTML code, image).
```
HTTP/1.1 200 OK // Status line
Content-Type: text/html // Header
Content-Length: 1234 // Header

<!DOCTYPE html> // Response body
<html>
<body>
    <h1>Hello, World!</h1>
</body>
</html>
```
Closing the connection: After sending the response, the connection can be closed (HTTP/1.0) or kept alive for subsequent requests (HTTP/1.1 with keep-alive, HTTP/2).

Key features:

Stateless: Each request is processed independently of previous ones. User state is maintained using mechanisms like cookies or URL parameters.
Request methods: Define actions the client wants to perform on the resource (GET, POST, PUT, DELETE, HEAD, OPTIONS, PATCH).
Status codes: Three-digit numbers indicating the result of request processing (1xx - informational, 2xx - success, 3xx - redirection, 4xx - client errors, 5xx - server errors).

Status Code	Description
200	OK
404	Not Found
500	Internal Server Error

Modern versions of the protocol, such as HTTP/2 and HTTP/3, improve performance through request multiplexing, header compression, and the use of UDP (HTTP/3).