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HTTP-in-Depth.md

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Hypertext Transfer Protocol (HTTP):

  • An application-layer protocol
  • Stateless protocol, meaning that the server does not keep any data (state) between two requests.
  • Hypertext to enable web browsers to retrieve and display web pages, images, videos, and other resources from web servers. eg. Hyperlinks, Non-linear Navigation1, Interconnected Information.
  • image

HTTP Messages:

  • There are two types of HTTP messages ie. Requests and responses.

HTTP Request: Client requests to Server for WebPages.

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Example:

GET /example-page HTTP/1.1
Host: developer.mozila.org
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.45 Safari/537.36
Accept: text/html,application/xhtml+xml

Requests consist of the following elements:

  • Method: Specifies the HTTP method being used. eg. "GET" method. The GET method is used to retrieve data from the server, specifically, to request a specific resource identified by the URL.
  • URI (Uniform Resource Identifier): represents the path to the resource being requested. In this case, it is /example-page & the URI is relative to the host (www.example.com).
  • HTTP Version: the HTTP version being used is specified. Here, it is "HTTP/1.1." HTTP/1.1 is a widely used version of the HTTP protocol
  • Host: specifies the domain name of the server to which the request is being sent. eg. developer.mozila.org
  • User-Agent: This header provides information about the client making the request.
  • Accept: indicates the media types (content types) that the client can understand or accept as a response.

HTTP Response: Server responses to Client with Webpage.

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Example:

HTTP/1.1 200 OK
Date: Wed, 21 Jul 2023 12:00:00 GMT
Server: Apache/2.4.41 (Unix)
Content-Length: 182
Content-Type: text/html; charset=UTF-8

<!DOCTYPE html>
<html>
<head>
    <title>Welcome to Example Website</title>
</head>
<body>
    <h1>Hello, World!</h1>
    <p>Welcome to our example website.</p>
</body>
</html>

Responses consist of the following elements:

  • Status Line: includes the HTTP version, status code, and status message. Here, the status line is HTTP/1.1 200 OK. It means HTTP version is 1.1, the status code is 200 (OK), and the status message is "OK".
  • Headers: provide additional information about the response. eg.
    • Date: The date and time of response was generated.
    • Server: The type and version of the server software used (Apache/2.4.41 in this case).
    • Content-Length: The size of the response body in bytes (182 bytes in this example).
    • Content-Type: The MIME type of the response body (text/html; charset=UTF-8), indicating that the response contains HTML content encoded with UTF-8.
  • Blank Line: separates the headers from the response body.
  • Response Body: contains the actual content sent back by the server. In this example, it is an HTML document.

HTTP response status code: More

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Footnotes

  1. Imagine reading a book, where you start at the first page and read through it page by page until you reach the end. That's a linear way of reading. Now, think of a web page with clickable links. When you click on a link, it takes you to another web page or a different section of the same page. You can then click on other links and move around the web in a non-linear fashion, not following a fixed order like in a book. ↩