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OCR H446 · Component 1 · 1.3.5

Application Layer:
HTTP, FTP, SMTP & IMAP

OCR A Level Computer Science · cszone.co.uk
H446 SpecA Level
Learning Objectives

By the end of this topic you will be able to:

Describe the purpose and operation of HTTP and HTTPS
Explain FTP and when it is used
Describe SMTP and IMAP for email and explain the difference
Identify which protocol is appropriate for a given scenario including port numbers
Protocol Table

Key Application Layer Protocols

ProtocolPortPurposeTransport
HTTP80Web page transfer (unencrypted)TCP
HTTPS443Encrypted web (HTTP + TLS)TCP
FTP20/21File transfer between hostsTCP
SMTP25/587Sending email (client → server)TCP
IMAP143/993Retrieving email (leaves on server)TCP
DNS53Domain name to IP resolutionUDP
SSH22Secure remote shell accessTCP
HTTP / HTTPS

HTTP and HTTPS

HTTP (HyperText Transfer Protocol): a request-response protocol. The client (browser) sends an HTTP request (GET, POST, PUT, DELETE) to the server, which responds with a status code and the requested resource (HTML, CSS, JSON, etc.).
Common HTTP methods: GET (retrieve data) · POST (submit data) · PUT (update resource) · DELETE (remove resource)
Status codes: 200 OK · 301 Moved Permanently · 404 Not Found · 500 Internal Server Error
HTTPS = HTTP + TLS encryption. Port 443. All data is encrypted in transit so an eavesdropper cannot read the content. The TLS certificate authenticates the server's identity. Required for any site handling sensitive data.
Email Protocols

SMTP, IMAP and FTP

SMTP (Simple Mail Transfer Protocol)
Used to send email from a client to a mail server, and between mail servers. Push protocol — the client pushes mail to the server. Port 25 (server-server) or 587 (client-server submission).
IMAP (Internet Message Access Protocol)
Used to retrieve email. Messages remain stored on the server — the client downloads copies for display. Allows access from multiple devices in sync. Port 143 (unencrypted) or 993 (encrypted). Preferred over POP3 for multi-device use.
FTP (File Transfer Protocol): transfers files between a client and server. Uses two channels: port 21 (control) for commands and port 20 (data) for file transfer. Unencrypted by default — SFTP (SSH FTP) or FTPS (FTP + TLS) add security.
Exam Practice
OCR H446 Style · 4 marks
Explain the difference between SMTP and IMAP in an email system. Include what happens when a user sends an email and when they retrieve one.
[4 marks]
2
Sending (SMTP): when the user sends an email, their email client uses SMTP to push the message to their mail server (port 587). The sending server then uses SMTP to forward the email to the recipient's mail server (port 25).
2
Retrieving (IMAP): when the recipient checks their email, their client uses IMAP (port 143/993) to retrieve the message from the server. The email remains stored on the server so it can be accessed from multiple devices in synchronised state.
Common Mistakes

Don't Lose Marks

!
Saying IMAP downloads and deletes emails from the server — that is POP3 behaviour. IMAP keeps emails on the server and synchronises state across devices. Know the distinction as OCR regularly tests IMAP vs POP3.
!
Confusing SMTP with receiving email — SMTP is only for sending. You cannot use SMTP to retrieve emails. Always pair SMTP (send) with IMAP or POP3 (receive) in your descriptions of email systems.
!
Getting port numbers wrong — learn the key ports: HTTP=80, HTTPS=443, FTP=20/21, SMTP=25/587, IMAP=143/993, DNS=53. Port numbers can be directly tested in H446 exam questions.
1.3.5b Complete
Well done! ✓
Application Layer: HTTP, HTTPS, FTP, SMTP & IMAP
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