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L5.2 — Application Layer Protocols by Dr Erisa Karafili. This lecture covers the top-most layer of the network stack — the application layer — and the key protocols that enable core internet functionality. It covers SMTP (sending email), IMAP vs POP3 (receiving/managing email), HTTP (web browsing — requests, responses, status codes, headers), HTTPS (HTTP over TLS for security), HTTP/2 and HTTP/3, CoAP (HTTP-like protocol for IoT/constrained environments), QUIC (fast UDP-based transport used by HTTP/3), RTSP (real-time streaming control), and RTP (actual delivery of real-time audio/video). Security weaknesses (plain-text HTTP, unauthenticated original SMTP) and the confidentiality benefit of HTTPS and IMAP authentication are highlighted throughout.

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📋 At a Glance

• [ ] Know that the application layer is the top-most layer — it interfaces directly between user applications and the network
• [ ] Understand that application layer protocols define how data is formatted, transmitted, and interpreted so applications can process it
• [ ] Know that the application layer interacts with the transport layer, not directly with the physical layer
• [ ] Explain SMTP (Simple Mail Transfer Protocol) — sends emails from client to server and server to server; does not handle retrieval
• [ ] Describe the SMTP handshake: TCP connection → 220 Ready → EHLO → 250 OK → mail transactions → QUIT → 221 Goodbye
• [ ] Know SMTP uses port 25 and is a text-based protocol
• [ ] Know original SMTP had no authentication — allowing open relays to be used for spam; authentication is now standard
• [ ] Explain IMAP (Internet Message Access Protocol) — retrieves and manages emails stored on a server
• [ ] Know IMAP ports: 143 (standard), 993 (encrypted/TLS)
• [ ] Explain IMAP synchronisation — emails stay on the server; all devices stay in sync via persistent TCP connection
• [ ] Contrast IMAP with POP3 — POP3 downloads emails to the device and removes them from the server (no sync)
• [ ] Know that IMAP authentication ensures confidentiality — only you can access your emails
• [ ] Explain HTTP — used for web browsing; browser connects to web server via TCP on port 80 (HTTP) or 443 (HTTPS)
• [ ] Know the main HTTP methods: GET (fetch resource), HEAD (like GET but no body), POST (send/create data), PUT (update resource), DELETE (remove resource)
• [ ] Describe the HTTP request/response cycle: TCP open → HTTP request → server processes → HTTP response → browser displays
• [ ] Know the HTTP status code families: 1xx info, 2xx success (e.g. 200 OK), 3xx redirect (e.g. 301), 4xx client error (e.g. 403, 404), 5xx server error (e.g. 500)
• [ ] Know that HTTP responses have: status line, response headers, blank line, body