Protocols in Detail

At A-Level you must know not just the names of key application-layer protocols, but how they work, what messages they exchange, and when to use each one. This lesson covers the most important protocols in depth.

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HTTP and HTTPS

HTTP (HyperText Transfer Protocol)

HTTP is the protocol used to request and deliver web pages and web resources (images, scripts, stylesheets). It operates on port 80 by default.

How it works (simplified):

1. The client (browser) opens a TCP connection to the web server on port 80.
2. The client sends an HTTP request (e.g. GET /index.html HTTP/1.1).
3. The server processes the request and sends an HTTP response (e.g. 200 OK, followed by the HTML content).
4. The connection may be closed or kept alive for further requests (HTTP/1.1 persistent connections).

Common HTTP Methods:

Method	Purpose
GET	Retrieve a resource
POST	Submit data to the server (e.g. form submission)
PUT	Update/replace a resource
DELETE	Remove a resource

Common HTTP Status Codes: