Show understanding of protocols (HTTP, FTP, POP3, IMAP, SMTP, BitTorrent) and their purposes

14.1 Protocols 📡

HTTP – HyperText Transfer Protocol

Think of HTTP as a waiter in a restaurant. You (the client) place an order (a request) and the waiter brings back your food (the response). The waiter follows a strict script: you say what you want, the waiter writes it down, goes to the kitchen, and brings it back.

  • Stateless: each request is independent.
  • Uses GET to fetch data, POST to send data.
  • Runs over TCP port 80 (or 443 for HTTPS).
Exam Tip: Remember that HTTP is stateless – the server does not remember previous requests unless you use cookies or sessions.

FTP – File Transfer Protocol

FTP is like moving boxes between two houses. You (client) tell the moving company (server) which boxes to pick up and where to drop them. FTP can be active or passive depending on who opens the door.

  • Uses two connections: control (port 21) and data (dynamic port).
  • Supports upload (STOR) and download (RETR).
  • Can be secured with FTPS or SFTP.
Exam Tip: Distinguish between active and passive FTP modes – which side opens the data connection?

POP3 – Post Office Protocol 3

POP3 is like a mailbox that you empty. When you check your mail, you pick up all letters and they disappear from the post office.

  • Downloads messages to the client.
  • Messages are usually deleted from the server after download.
  • Runs over TCP port 110.
Exam Tip: POP3 deletes messages by default – remember this when comparing to IMAP.

IMAP – Internet Message Access Protocol

IMAP is like a shared folder in the cloud. You can open the folder from any device, and all devices see the same contents.

  • Messages stay on the server.
  • Supports folders, flags, and search.
  • Runs over TCP port 143 (or 993 for IMAPS).
Exam Tip: IMAP does not delete messages unless you explicitly do so; it is ideal for multi-device access.

SMTP – Simple Mail Transfer Protocol

SMTP is the postman who delivers your letter to the recipient’s mailbox. It only knows how to send, not how to receive.

  • Handles sending email from client to server or between servers.
  • Uses TCP port 25 (or 587 for submission).
  • Can be secured with STARTTLS.
Exam Tip: SMTP is unidirectional – it only sends. Receiving is handled by POP3 or IMAP.

BitTorrent – Peer‑to‑Peer File Sharing

BitTorrent is like a group of friends sharing a huge pizza. Instead of everyone waiting for the pizza to be delivered, each friend cuts a slice and shares it with others.

  • Files are split into pieces; peers download pieces from each other.
  • Reduces load on any single server.
  • Uses a tracker to coordinate peers.
Exam Tip: Remember that BitTorrent relies on peer‑to‑peer sharing; the more peers, the faster the download.

Protocol Comparison Table

Protocol Primary Use Typical Port Key Feature
HTTP Web pages 80 / 443 Stateless, request/response
FTP File transfer 21 / 20 Control + data channels
POP3 Email retrieval 110 Downloads & deletes
IMAP Email retrieval 143 / 993 Keeps messages on server
SMTP Email sending 25 / 587 Unidirectional send
BitTorrent P2P file sharing Varies (usually 6881‑6889) Decentralised distribution

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