CAIE 9618 · Paper 3 · Topic 3.2.1

Communication
Protocols

TCP/IP Suite · OSI Model · HTTP/HTTPS · FTP · SMTP · BitTorrent

CSZone Cambridge International AS & A Level Computer Science 9618

What is a Protocol?

Agreed Rules for Communication

A protocol is a set of agreed rules that define how data is transmitted between devices on a network. Without protocols, devices from different manufacturers could not communicate. Protocols specify format, timing, error checking, and sequencing of data.

WHY PROTOCOLS ARE NEEDED

Different hardware/software must be able to communicate

Define how data is packaged, addressed, transmitted

Handle errors, retransmission, acknowledgements

Enable interoperability between manufacturers and OS

LAYERED APPROACH BENEFITS

Each layer has a specific task — separation of concerns

Can update one layer without changing others

Different manufacturers implement the same layer identically

Simplifies development and troubleshooting

TCP/IP Suite

The Internet's 4-Layer Model

TCP/IP is the protocol suite that underpins the internet. It has 4 layers, each building on the one below:

4

Application — user-facing protocols: HTTP, HTTPS, FTP, SMTP, DNS. Handles what data to send.

3

Transport — TCP (reliable, ordered) or UDP (unreliable, fast). Segmentation, ports, error correction. TCP three-way handshake (SYN → SYN-ACK → ACK).

2

Internet — IP addressing, routing packets across networks. Source/destination IP addresses added here. Best-effort delivery — no guarantee of order.

1

Link (Network Access) — physical transmission on local network. MAC addresses, Ethernet, Wi-Fi frames.

Encapsulation: Data passed down the stack — each layer adds its own header (and sometimes trailer) before passing to the layer below. Reversed on arrival.

Application Layer Protocols

HTTP/S, FTP, SMTP, IMAP, DNS

Protocol	Full Name	Port	Purpose
HTTP	HyperText Transfer Protocol	80	Transfer web pages (HTML, images) — plaintext, not encrypted
HTTPS	HTTP Secure	443	Encrypted HTTP using TLS/SSL — secure web browsing, online banking
FTP	File Transfer Protocol	20/21	Upload/download files between client and server; two channels (control + data)
SMTP	Simple Mail Transfer Protocol	25	Send email from client to server, and between mail servers
IMAP	Internet Message Access Protocol	143	Retrieve email from server — emails stay on server, synced across devices
POP3	Post Office Protocol 3	110	Download email to client — emails removed from server after download
DNS	Domain Name System	53	Translates domain names (cszone.co.uk) to IP addresses

TCP vs UDP

Reliable vs Fast Transport

TCP

RELIABLE, CONNECTION-ORIENTED

3-way handshake before data sent (SYN, SYN-ACK, ACK)

Segments numbered; receiver sends acknowledgements

Lost segments retransmitted automatically

Data arrives in order (reordered if needed)

Flow control and congestion control

Used for: HTTP, HTTPS, FTP, SMTP, SSH

UDP

FAST, CONNECTIONLESS, UNRELIABLE

No handshake — data sent immediately

No acknowledgements or retransmission

Packets may arrive out of order or be lost

Low latency — minimal overhead

Supports broadcast and multicast

Used for: video streaming, VoIP, online gaming, DNS

Exam Practice

Cambridge-style questions

Question 1

A user streams live video over the internet. Explain why UDP is more suitable than TCP for live video streaming. [3]

1

UDP has lower latency because there is no connection setup (three-way handshake) and no acknowledgement mechanism — data is sent immediately.

1

In live video, if a packet is lost, there is no point retransmitting it — the moment has passed. TCP would retransmit lost packets, causing delays and freezing the live stream.

1

UDP is "good enough" because the human eye can tolerate occasional dropped frames or slight quality reduction without it affecting the viewing experience significantly.

Common Mistakes

Don't lose easy marks

1

Saying "UDP doesn't check for errors at all" — UDP does have a checksum field for basic error detection, but it doesn't guarantee delivery or retransmit. The key difference from TCP is: no guaranteed delivery, no retransmission, no ordering.

2

Confusing IMAP and POP3 — IMAP keeps emails on the server and syncs across devices; POP3 downloads and removes from server. Exams often ask which is better for multi-device use — always IMAP.

3

Mixing up OSI and TCP/IP layers — TCP/IP has 4 layers, OSI has 7. CAIE 9618 Paper 3 focuses on TCP/IP (4 layers). Don't describe 7 OSI layers in a TCP/IP question — you'll describe the wrong model.

Topic Summary — 3.2.1

What You Need to Know

TCP/IP — 4 LAYERS (bottom to top)

1. Link — MAC, Ethernet frames
2. Internet — IP addresses, routing
3. Transport — TCP/UDP, ports, segments
4. Application — HTTP, FTP, SMTP, DNS

KEY PORTS

HTTP:80 · HTTPS:443 · FTP:20/21 · SMTP:25 · IMAP:143 · DNS:53

TCP vs UDP

TCP — reliable, ordered, ACK, 3-way handshake, retransmission. Use: web, email, file transfer.
UDP — fast, no ACK, no reorder, no retransmit, low latency. Use: video streaming, VoIP, gaming, DNS.

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3.2.2

Circuit & Packet Switching

Dedicated paths vs dynamic routing · Comparison

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