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:

Code	Meaning
200	OK — request succeeded
301	Moved Permanently — resource has a new URL
404	Not Found — resource does not exist
403	Forbidden — access denied
500	Internal Server Error

HTTPS (HTTP Secure)

HTTPS is HTTP with encryption provided by TLS (Transport Layer Security, the successor to SSL). It operates on port 443.

• All data between client and server is encrypted — preventing eavesdropping.
• The server's identity is verified using a digital certificate issued by a Certificate Authority (CA).
• Prevents man-in-the-middle attacks — an attacker cannot read or modify the data in transit.
• The browser shows a padlock icon when HTTPS is active.

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FTP (File Transfer Protocol)

FTP is used to transfer files between a client and a server. It uses two TCP connections:

Connection	Port	Purpose
Control	21	Sends commands (login, directory listing, file operations)
Data	20 (or dynamic)	Transfers file data

Key points:

• FTP sends credentials (username and password) in plaintext — insecure.
• SFTP (SSH File Transfer Protocol) and FTPS (FTP Secure) are secure alternatives.
• FTP supports both ASCII and binary transfer modes.

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SMTP (Simple Mail Transfer Protocol)

SMTP is used to send email from a client to a mail server, or between mail servers. It operates on port 25 (or 587 for authenticated submission).

How email sending works:

1. User composes an email in their mail client.
2. The client connects to the outgoing mail server (SMTP server) and sends the email.
3. The SMTP server looks up the recipient's mail server using DNS (MX records).
4. The email is forwarded to the recipient's mail server.
5. The recipient retrieves the email using POP3 or IMAP.

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POP3 and IMAP (Retrieving Email)

Protocol	Port	Behaviour
POP3	110	Downloads emails to the client and (by default) deletes them from the server. Simple, good for single-device access.
IMAP	143	Emails stay on the server and are synchronised across multiple devices. Better for users who check email on phone, laptop and desktop.

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