HTTP is based on a client/server (C/S) architecture model, exchanging information over a reliable connection. It is a stateless request/response protocol. HTTP messages are the basis of communication between a client and a server. They consist of a series of text lines, following a specific format and structure. HTTP messages are divided into two types: request messages and response messages. An HTTP client is an application (web browser or any other client) that connects to a server with the purpose of sending one or more HTTP requests. An HTTP server is also an application (usually a web service like Nginx, Apache server, or IIS server, etc.) that receives client requests and sends HTTP response data back to the client. !(#) * * * ## Client Request Message An HTTP request message sent by a client to a server consists of the following format: request line, request headers, a blank line, and request data. The diagram below shows the general format of a request message. !(#) * **Request Line**: * **Method**: Such as GET, POST, PUT, DELETE, etc., specifying the operation to be performed. * **Request URI** (Uniform Resource Identifier): The path of the requested resource, usually including the hostname, port number (if non-default), path, and query string. * **HTTP Version**: Such as HTTP/1.1 or HTTP/2. Example format of a request line: `GET /index.html HTTP/1.1` * **Request Headers**: * Contains client environment information, the size of the request body (if any), compression types supported by the client, etc. * Common request headers include `Host`, `User-Agent`, `Accept`, `Accept-Encoding`, `Content-Length`, etc. * **Blank Line**: * The separator between the request headers and the request body, indicating the end of the headers. * **Request Body** (Optional): * For certain types of HTTP requests (like POST and PUT), the request body contains the data to be sent to the server. * * * ## Server Response Message An HTTP response also consists of four parts: status line, message headers, a blank line, and the response body. !(#) * **Status Line**: * **HTTP Version**: Matches the version from the request message. * **Status Code**: A three-digit number indicating the result of processing the request, e.g., 200 for success, 404 for resource not found. * **Status Message**: A brief description of the status code. Example format of a status line: `HTTP/1.1 200 OK` * **Response Headers**: * Contains server environment information, the size of the response body, compression types supported by the server, etc. * Common response headers include `Content-Type`, `Content-Length`, `Server`, `Set-Cookie`, etc. * **Blank Line**: * The separator between the response headers and the response body, indicating the end of the headers. * **Response Body** (Optional): * Contains the data returned by the server, such as the requested web page content, images, JSON data, etc. * * * ## Example The following example is a typical instance of using GET to pass data: Client request: GET /index.html HTTP/1.1Host: www.example.com User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Firefox/91.0Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8 Accept-Encoding: gzip, deflate Connection: keep-alive Server response: HTTP/1.1 200 OK Date: Wed,508; v=b3; q=0.318 Apr 2024 12:00:00 GMT Server: Apache/2.4.1 (Unix)Last-Modified: Wed, 18 Apr 2024 11:00:00 GMT Content-Length: 12345Content-Type: text/html; charset=UTF-8

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In practical applications, HTTP messages can contain many more header fields, and the size of the request and response bodies will vary based on the data being transmitted.