Cambridge 9618 · International A Level Computer Science · ~20 min read
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What is a Protocol?
A protocol is a set of agreed rules that govern how data is transmitted between devices on a network. Without agreed protocols, devices from different manufacturers could not communicate. Protocols define: message format, transmission speed, error checking, how connections are established and ended, and how devices are addressed.
The TCP/IP Model
The TCP/IP model is the standard framework for internet communication. It has four layers, each handling a different aspect of data transmission. Data passes down the layers on the sender's side (each layer adds a header) and back up on the receiver's side (each layer removes its header) — this is called encapsulation.
Application Layer 4
Protocols used by applications: HTTP, HTTPS, FTP, SSH, SMTP, DNS, POP3, IMAP. Defines how user-facing applications communicate.
Transport Layer 3
Manages end-to-end communication: TCP (reliable, connection-oriented) and UDP (fast, connectionless). Handles port numbers and flow control.
Internet Layer 2
IP addressing and routing — assigns source/destination IP addresses. Packets may travel different routes; IP handles addressing not reliability.
Link Layer 1
Physical transmission over the local network — MAC addresses, Ethernet, Wi-Fi, hardware devices. Gets the frame from one node to the next.
TCP vs UDP
TCP — Transmission Control Protocol
Connection-oriented: 3-way handshake before data
Data arrives in order (re-ordered if packets arrive out of sequence)
Acknowledgements (ACK) sent for each received packet
Lost packets are re-transmitted
Slower due to overhead
Used for: HTTP/HTTPS, email, file transfers — where accuracy matters
UDP — User Datagram Protocol
Connectionless: no handshake, sends immediately
No guarantee of delivery or ordering
No acknowledgements or re-transmission
Faster and lower overhead
Some packets may be lost and not recovered
Used for: video streaming, VoIP, online gaming, DNS — where speed matters more than perfection
TCP 3-Way Handshake
Before TCP sends any data, a connection is established using the 3-way handshake:
TCP 3-Way Handshake
Client → SYN → Server(I want to connect; here is my sequence number) Client ← SYN-ACK ← Server(Agreed; here is my sequence number) Client → ACK → Server(Acknowledged — connection established)
After this, data transmission begins. TCP then sends data in segments, each acknowledged (ACK) by the receiver. Lost segments are re-transmitted.
Key Application Layer Protocols
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HTTP
Port 80
HyperText Transfer Protocol. Used for transferring web pages. Client sends a request (GET, POST), server responds with status code + data. Data is sent in plaintext — not encrypted.
PlaintextWeb
🔒
HTTPS
Port 443
HTTP Secure. Same as HTTP but all data is encrypted using TLS (Transport Layer Security). Uses digital certificates to verify server identity. Required for login pages, payments, and any sensitive data.
SecureTLS encrypted
📁
FTP
Port 21
File Transfer Protocol. Used to upload/download files between a client and server. Uses separate control (port 21) and data (port 20) connections. SFTP adds SSH encryption for secure file transfer.
File transferControl + data
🖥️
SSH
Port 22
Secure Shell. Provides an encrypted command-line connection to a remote computer. Replaced the older (insecure) Telnet protocol. Used by developers and system administrators to manage servers remotely.
SecureEncrypted tunnel
📤
SMTP
Port 25/587
Simple Mail Transfer Protocol. Used for sending email — from a client to a mail server, or between mail servers. Does not support email retrieval (use POP3 or IMAP for that).
Sending email
📥
POP3 / IMAP
110 / 143
Post Office Protocol 3 / Internet Message Access Protocol. Used for receiving email. POP3 downloads emails and removes them from server; IMAP keeps emails on the server and syncs across devices.
Receiving email
HTTP Methods and Status Codes
HTTP defines how clients (browsers) request resources from servers:
GET — retrieve a resource (URL visible, cached, used for web pages, search queries)
POST — send data to the server (not in URL, used for forms, logins, file uploads)
The server responds with a status code:
2xx
Success
200 OK, 201 Created
3xx
Redirect
301 Moved, 302 Found
4xx
Client Error
404 Not Found, 403 Forbidden
5xx
Server Error
500 Internal Error
DNS — Domain Name System
DNS translates human-readable domain names (e.g. cszone.co.uk) into numerical IP addresses (e.g. 104.21.43.8) that computers use to communicate. Without DNS, users would need to type IP addresses to visit websites.
DNS resolution process
1
User types cszone.co.uk in browser. Browser checks its local DNS cache for the IP address.
2
If not cached, the device queries the recursive resolver (usually provided by the ISP).
3
The resolver queries a root nameserver, which directs it to the .uk TLD nameserver.
4
The TLD server directs to the authoritative nameserver for cszone.co.uk.
5
The authoritative server returns the IP address. The resolver caches it and returns it to the browser.
6
Browser connects to the IP address — the web page loads.
Port Numbers
A port number identifies a specific application or service on a computer. IP addresses identify the machine; port numbers identify the service on that machine. The combination is a socket.
Protocol
Port
Purpose
HTTP
80
Web pages (unencrypted)
HTTPS
443
Secure web pages
FTP
20, 21
File transfer (data/control)
SSH
22
Secure remote shell
SMTP
25/587
Sending email
POP3
110
Receiving email (downloads)
IMAP
143
Receiving email (synced)
DNS
53
Domain name resolution
Cambridge 9618 exam tip: Know the four TCP/IP layers and at least two protocols per layer. Be able to describe the TCP 3-way handshake (SYN → SYN-ACK → ACK). Distinguish TCP (reliable, in-order, acknowledged, connection-oriented) from UDP (fast, connectionless, no guarantee — used for streaming/gaming). Know what DNS does (translates domain names to IP addresses). Know HTTPS uses TLS encryption and port 443; HTTP uses port 80 and is unencrypted.
⚠️ Common Mistakes
Confusing TCP and UDP — TCP has acknowledgements and re-transmission; UDP does not
Saying HTTPS "uses port 80" — HTTPS uses port 443; HTTP uses port 80
Saying DNS stores web pages — DNS only stores IP address records for domain names, not the pages themselves
Confusing FTP with HTTP — FTP is for file transfers between client and server; HTTP is for loading web pages
Saying SSH is only for file transfers — SSH provides a secure command-line shell; SFTP (SSH + FTP) handles secure file transfers
Confusing SMTP and POP3/IMAP — SMTP sends email; POP3 and IMAP are for receiving/retrieving email
Forgetting POP3 vs IMAP distinction: POP3 downloads and deletes from server; IMAP syncs and keeps on server
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Worksheet — 3.2.1 Communication Protocols
8 questions · Cambridge 9618 standard
Q1Name the four layers of the TCP/IP model and give one protocol that operates at each layer.[4]
✅ Mark scheme
Application layer — any of: HTTP, HTTPS, FTP, SSH, SMTP, DNS [1]; Transport layer — TCP or UDP [1]; Internet layer — IP [1]; Link layer — Ethernet, Wi-Fi [1]. Award 1 per correct layer+protocol pair.
Q2Describe the TCP 3-way handshake and explain why it is needed before data transmission begins.[4]
✅ Mark scheme
Client sends SYN to server (requests connection with sequence number) [1]; server responds with SYN-ACK (acknowledges client SYN and sends its own sequence number) [1]; client sends ACK (acknowledges server SYN — connection now established) [1]; needed to ensure both sides are ready to communicate, agree on sequence numbers, and confirm the connection exists before data is sent [1].
Q3Give two reasons why UDP might be preferred over TCP for video streaming.[2]
✅ Mark scheme
Any two from: UDP has lower overhead (no acknowledgements) so data is sent faster [1]; lost packets are not re-transmitted, so a late/re-transmitted video frame would just cause a momentary glitch rather than a delay — speed is more important than perfection [1]; no 3-way handshake means connection is immediate [1].
Q4A user logs in to their bank account using HTTPS. State two ways HTTPS differs from HTTP and explain why both are important for online banking.[4]
✅ Mark scheme
HTTPS encrypts all data using TLS — important because login credentials and financial details cannot be intercepted and read by a third party during transmission [1+1]; HTTPS uses digital certificates to verify the server's identity (authentication) — important because it confirms the user is communicating with the real bank, not a fraudulent copy [1+1]. Accept port 443 vs port 80 as a difference.
Q5Explain the role of DNS in accessing a website, including what would happen without DNS.[3]
✅ Mark scheme
DNS translates a human-readable domain name (e.g. cszone.co.uk) into an IP address (e.g. 104.21.43.8) that can be used to route network traffic to the correct server [1]; the browser queries the DNS resolver, which contacts nameservers in a hierarchy until the IP address is returned and cached [1]; without DNS, users would need to type the numerical IP address of every website they visit, which is impractical as IP addresses are hard to remember and can change [1].
Q6A developer wants to access and manage a company's web server remotely over the internet. Which protocol should they use and why? What protocol would they use to transfer files to the server securely?[4]
✅ Mark scheme
SSH (Secure Shell) — provides an encrypted command-line connection to the remote server [1]; all data (including commands and credentials) is encrypted, so it cannot be intercepted [1]; for secure file transfer they should use SFTP (SSH File Transfer Protocol) or FTPS [1]; standard FTP should be avoided as it sends data in plaintext and is insecure over the internet [1].
Q7Describe the role of TCP and IP in transmitting data across the internet. Explain how TCP ensures reliable delivery that IP alone does not provide, referring to acknowledgements, sequence numbers, and retransmission.[5]
✅ Mark scheme
IP handles addressing and routing — it forwards packets from source to destination based on IP addresses but provides no guarantee of delivery or ordering [1]; TCP provides a reliable, connection-oriented service on top of IP [1]; TCP assigns sequence numbers to segments so the receiver can reorder them correctly if they arrive out of order [1]; the receiver sends ACK (acknowledgement) packets to confirm receipt of each segment [1]; if an ACK is not received within a timeout period, the sender retransmits the missing segment, ensuring all data arrives intact [1].
Q8Explain the purpose of the application layer protocols HTTP, FTP, and SMTP. For each, state one scenario where it would be used and identify whether it typically uses TCP or UDP as its transport protocol, giving a reason.[6]
✅ Mark scheme
HTTP: transfers web pages (HTML, images, etc.) between web server and browser [1]; uses TCP — web pages must arrive complete and in order for correct display [1]; FTP: transfers files between a client and a file server [1]; uses TCP — file integrity is critical; any corruption would make the file unusable [1]; SMTP: sends email messages from a mail client to a mail server or between mail servers [1]; uses TCP — email must be delivered reliably; loss of message data is unacceptable [1]. Award max 6.
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Mini Test — 3.2.1 Communication Protocols
10 questions · 10 marks · 10 minutes
⏱ 10:00
Section A — Multiple Choice [5 marks]
Q1Which port number does HTTPS use?
Q2Which protocol is connection-oriented and guarantees delivery?
Q3What is the purpose of DNS?
Q4Which email protocol downloads emails and typically removes them from the server?
Q5Which layer of the TCP/IP model handles IP addressing and routing?
Section B — Short Answer [5 marks]
Q6Describe one advantage and one disadvantage of using IMAP instead of POP3.
Mark schemeAdvantage: IMAP keeps emails on the server and syncs across all devices — you can access the same emails on your phone, laptop, and tablet [1]; Disadvantage: requires an active internet connection to access email; POP3 works offline once downloaded [1].
Q7State the three steps of the TCP 3-way handshake.
Mark schemeSYN — client sends synchronise request with its sequence number [1]; SYN-ACK — server acknowledges and sends its own sequence number [1]; ACK — client acknowledges server's synchronise [1]. Award 1 per correct step.
Q8Explain why a video call application would use UDP rather than TCP.
Mark schemeUDP has lower overhead (no acknowledgements/re-transmission) so data is transmitted faster with lower latency [1]; in a live video call, a brief moment of glitchy video due to a lost packet is acceptable — but TCP re-transmission would cause a noticeable delay/freeze as the call waits for re-sent data, which is worse for real-time communication [1].
Q9State the port numbers used by HTTP, HTTPS, and SSH.
Mark schemeHTTP: port 80 [1]; HTTPS: port 443 [1]; SSH: port 22 [1]. Award 1 per correct pair. If all three correct but one wrong, max 2.
Q10Give two differences between HTTP and HTTPS.
Mark schemeAny two from: HTTPS encrypts data using TLS; HTTP sends data in plaintext [1]; HTTPS uses port 443; HTTP uses port 80 [1]; HTTPS uses digital certificates to verify server identity; HTTP has no authentication [1].