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CAIE 9618 · Paper 1 · Topic 1.2.4

Internet
Protocols

HTTP · HTTPS · FTP · SMTP · POP3 · IMAP · SSH

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 govern how data is transmitted between devices. Without protocols, devices from different manufacturers could not communicate."
PROTOCOL DEFINES
Data format and structure
Error detection methods
Speed and timing of transmission
WHY NEEDED
Interoperability between devices
Universal internet communication
Standardised data exchange
Web & File Transfer Protocols

HTTP, HTTPS, and FTP

ProtocolPortPurposeKey Feature
HTTP80Transfer web pagesUnencrypted — data visible in transit
HTTPS443Secure web transferEncrypted with TLS — secure
FTP20/21Transfer files between systemsSeparate control and data connections
SSH22Secure remote accessEncrypted command-line access
HTTPS = HTTP + TLS: The connection is encrypted using TLS. The browser shows a padlock icon. Essential for any site handling personal data.
Email Protocols

SMTP, POP3, and IMAP

ProtocolPortDirectionKey Behaviour
SMTP25/587Sending emailSends email from client to server, server to server
POP3110Receiving emailDownloads and deletes from server — offline access
IMAP143Receiving emailSyncs with server — email stays on server
POP3
Downloads to device, removes from server. Good for one device access only.
IMAP
Email stays on server. Syncs across multiple devices. Better for modern use.
Exam Practice

Cambridge-style questions

Question 1
Describe the difference between POP3 and IMAP for receiving email. State one advantage of IMAP over POP3.
3 marks
✓ MARK SCHEME
1 mark
POP3: downloads email to device and (typically) deletes from server
1 mark
IMAP: email remains on server and is synchronised across all devices
1 mark
IMAP advantage: email accessible from multiple devices / if one device is lost, email is not lost
Exam Practice
Question 2
State what is meant by a protocol. Give one reason why protocols are needed for internet communication.
2 marks
1 mark
A protocol is a set of agreed rules/standards that govern how data is transmitted between devices
1 mark
Protocols enable devices from different manufacturers to communicate with each other (interoperability)
Question 3
Explain why HTTPS is more suitable than HTTP for online banking.
2 marks
1 mark
HTTPS encrypts data using TLS — financial/personal data cannot be read if intercepted
1 mark
HTTP transmits data in plaintext — sensitive data such as passwords and account numbers would be visible
Common Mistakes

Don't lose easy marks

1
Saying SMTP is for receiving email — SMTP is for sending only. POP3 and IMAP are for receiving. Remember: SMTP = Send My Typical Protocol.
2
Saying HTTPS "compresses" data — HTTPS does NOT compress; it encrypts using TLS. Encryption and compression are entirely different operations.
3
Confusing port numbers — while you don't always need to memorise them, HTTP=80, HTTPS=443, SMTP=25, FTP=20/21 are commonly tested in "state the port" style questions at A-Level.
Topic Summary — 1.2.4

What You Need to Know

WEB PROTOCOLS
HTTP (80): web pages, unencrypted
HTTPS (443): web pages, TLS encrypted
FTP (20/21): file transfer
SSH (22): secure remote command line
EMAIL PROTOCOLS
SMTP (25): send email
POP3 (110): receive, download, delete from server
IMAP (143): receive, sync, stays on server
KEY PRINCIPLE
Protocol: agreed rules for data transmission
Enable interoperability between devices
Each protocol has a specific purpose and port
IMAP preferred over POP3 for multiple devices
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Next Video

1.2.5
TCP/IP Model
Four Layers · Encapsulation · Decapsulation
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