Network protocols and the TCP/IP stack
A protocol is an agreed set of rules for communication. Without protocols, computers couldn't understand each other. AQA's GCSE expects you to recognise key protocols and how they fit into the TCP/IP layered model.
Why protocols?
- Compatibility — devices from different manufacturers interoperate.
- Reliability — agreed handling of errors, ordering, retransmission.
- Efficiency — known headers and structure mean fast parsing.
- Security — encrypted protocols (HTTPS) protect data.
Key protocols you must know
TCP (Transmission Control Protocol)
- Reliable, connection-oriented delivery — the receiver acknowledges every chunk.
- Splits data into packets, numbers them, retransmits lost packets, reassembles in order.
- Used by HTTP, HTTPS, FTP, SMTP, IMAP — anything where every byte matters.
IP (Internet Protocol)
- Handles addressing and routing of packets across networks.
- Each device gets an IP address.
- Two versions: IPv4 (32-bit, e.g. 192.168.1.1) and IPv6 (128-bit, e.g. 2001:db8::1) — IPv6 created because IPv4 addresses ran out.
UDP (User Datagram Protocol)
- Connectionless, no acknowledgements, no retransmission.
- Faster than TCP because no overhead.
- Used for streaming, voice/video calls, online gaming — speed matters more than perfect delivery.
HTTP / HTTPS
- HTTP (HyperText Transfer Protocol) — fetches web pages.
- HTTPS is HTTP over TLS/SSL — encrypts the data so eavesdroppers can't read it. Indicated by the padlock in browsers.
FTP (File Transfer Protocol)
- Transfers files between client and server.
- Largely replaced by HTTPS-based downloads and SFTP/SSH for security.
SMTP, IMAP / POP3
- SMTP (Simple Mail Transfer Protocol) — sending email.
- IMAP (Internet Message Access Protocol) — reading email; messages stay on the server, accessible from multiple devices.
- POP3 — older alternative; downloads emails to one device, often deleting from server.
Ethernet and Wi-Fi
- Lower-level protocols handling how data moves on a physical wire (Ethernet) or radio (Wi-Fi).
- Define frames, MAC addressing, collision avoidance.
The TCP/IP 4-layer model
A layered model separates concerns. Each layer talks to the layer below to do its job. The TCP/IP model has four layers (covered in detail in CS5.6):
- Application — what the user actually does (HTTP, HTTPS, FTP, SMTP, IMAP).
- Transport — reliable delivery between endpoints (TCP, UDP).
- Internet (or Network) — addressing and routing across networks (IP).
- Link — physical medium and local network (Ethernet, Wi-Fi).
When you load a web page, the request flows down through the layers on your device, across the network, and up through the layers on the server. The reply takes the reverse trip.
✦Worked example— Worked example — fetching a web page
You type https://example.com into your browser:
- Application layer — browser builds an HTTPS request.
- Transport layer — TCP splits the request into numbered packets.
- Internet layer — IP attaches source and destination addresses; routes packets across the internet.
- Link layer — Ethernet/Wi-Fi puts each packet on the physical wire/airwave.
- Server side — packets arrive, link layer hands up to IP, IP to TCP, TCP reassembles, application layer (web server) sees the HTTPS request.
- Reply travels back the same way.
⚠Common mistakes— Pitfalls
- Saying TCP is faster than UDP. UDP is faster because no acknowledgements; TCP is more reliable.
- Confusing IP with TCP. IP routes; TCP ensures reliable delivery.
- Treating HTTPS as a different protocol layer than HTTP. HTTPS is HTTP wrapped in TLS — same application layer.
- Forgetting why we layer. Lets us swap one layer (e.g. Wi-Fi for Ethernet) without rewriting others.
- Listing wrong layer. SMTP is application, not transport; TCP is transport, not application.
Why use TCP vs UDP?
| Scenario | Protocol | Why |
|---|---|---|
| Loading a web page | TCP/HTTP | Every byte matters |
| Streaming live video | UDP | Slight loss OK; latency must be low |
| Sending an email | TCP/SMTP | Must arrive in full |
| Online multiplayer game | UDP | Speed > perfection |
| Downloading a file | TCP/HTTPS | File must be exact |
➜Try this— Quick check
State the protocol for each task:
- Loading a website: HTTP/HTTPS.
- Sending email: SMTP.
- Reading email from server: IMAP (or POP3).
- Reliable transport: TCP.
- Streaming game audio: UDP.
- Routing across networks: IP.
AI-generated · claude-opus-4-7 · v3-deep-computer-science