AQA 7517 · Paper 2 · 4.9.3b

TCP/IP Model
TCP vs UDP

The 4-layer TCP/IP stack · protocols at each layer · TCP vs UDP

WHAT YOU'LL LEARN

TCP/IP 4-layer model · protocols · TCP vs UDP · encapsulation

AQA SPEC LINK

4.9.3 — TCP/IP, layers, TCP vs UDP, HTTP, FTP, SMTP, IMAP, SSH

TCP/IP Model

The 4-Layer TCP/IP Model

APPLICATION LAYER

HTTP, HTTPS, FTP, SMTP, IMAP, SSH, DNS

Layer 4

TRANSPORT LAYER

TCP, UDP — end-to-end communication, ports

Layer 3

INTERNET LAYER

IP — addressing and routing packets across networks

Layer 2

LINK LAYER

Ethernet, WiFi — physical transmission on local network

Layer 1

Application Layer

Application Layer Protocols

HTTP/HTTPS — HyperText Transfer Protocol; web pages; HTTPS = encrypted with TLS

FTP — File Transfer Protocol; transferring files between client and server

SMTP — Simple Mail Transfer Protocol; sending emails from client to mail server

IMAP — Internet Message Access Protocol; reading emails stored on mail server (emails stay on server)

SSH — Secure Shell; encrypted remote access to servers/devices via command line

DNS — Domain Name System; translates hostnames to IP addresses (operates at application layer)

TCP

TCP — Transmission Control Protocol

TCP is a connection-oriented protocol. It guarantees that all packets arrive, in order, without errors.

Three-way handshake: SYN → SYN-ACK → ACK establishes connection before data transfer

Receiver sends acknowledgements (ACKs); sender retransmits any unacknowledged packets

Flow control (window size) prevents sender overwhelming receiver

Used for: HTTP/HTTPS, FTP, SMTP, IMAP — anything where data integrity matters

UDP

UDP — User Datagram Protocol

UDP is a connectionless protocol. It sends packets without establishing a connection, without ACKs, and without guaranteeing delivery or order.

Much faster than TCP — no handshake, no ACKs, no retransmission overhead

Used for: video streaming, online gaming, VoIP, DNS queries, live broadcast

A dropped frame in a video call is preferable to waiting for retransmission (which would cause worse lag)

Application layer can implement its own error handling if needed

TCP vs UDP

TCP vs UDP Comparison

Feature	TCP	UDP
Connection	Connection-oriented (handshake)	Connectionless
Delivery	Guaranteed, in order	Not guaranteed
Speed	Slower (overhead)	Faster (low overhead)
Use cases	Web, email, file transfer	Streaming, gaming, VoIP

Encapsulation

Encapsulation & Decapsulation

As data passes down the TCP/IP layers, each layer adds its own header (encapsulation). As data passes up at the receiver, headers are stripped off (decapsulation).

Application creates message (e.g. HTTP request)

Transport adds TCP/UDP header (source/dest port, sequence no.)

Internet adds IP header (source/dest IP)

Link adds frame header/trailer (MAC addresses) for local delivery

Port Numbers

Ports identify which application on a host should receive data. An IP address gets the data to the right machine; a port gets it to the right application.

Protocol	Port	Use
HTTP	80	Web (unencrypted)
HTTPS	443	Web (encrypted)
FTP	21	File transfer
SMTP	25/587	Send email
SSH	22	Secure remote access

AQA Exam Style

Practice Question

AQA 7517 — Paper 2 Style

(a) State the FOUR layers of the TCP/IP model and name ONE protocol at each layer. [4]
(b) Explain why UDP is used for video conferencing rather than TCP. [3]
(c) What is meant by encapsulation? [2]

[9 marks]

4 marks

(a) Application — HTTP/FTP/SMTP/SSH [1]; Transport — TCP/UDP [1]; Internet — IP [1]; Link — Ethernet/WiFi [1]

3 marks

(b) UDP has no connection overhead so is faster [1]; dropped packets cause a brief glitch which is acceptable [1]; waiting for TCP retransmission would cause significant delay/freezing which is worse than a dropped frame [1]

2 marks

(c) Each layer adds its own header to the data as it passes down the stack [1]; creating a layered structure that allows each layer at the receiver to strip and read its own header [1]

Summary

Key Points to Remember

TCP/IP has 4 layers: Application, Transport, Internet, Link

TCP — reliable, ordered, connection-oriented; used for HTTP, FTP, email

UDP — fast, connectionless, no guaranteed delivery; used for streaming, gaming, VoIP

Encapsulation — headers added going down stack; stripped going up (decapsulation)

Know key port numbers: HTTP=80, HTTPS=443, FTP=21, SMTP=25, SSH=22

🎉 Lesson complete — move to the quiz!