Application-layer protocols
The protocols apps speak
- The application layer defines the protocols that user programs use.
- Each common task — web, files, email — has its own protocol.
- This is where a programmer most often works.
Web and file transfer
- HTTP — browsers fetch web pages from servers (over TCP, port 80).
- HTTPS — HTTP over TLS: encrypted, port 443. Use it whenever data is sensitive.
- FTP — transfers files between a client and a server.
Practice
HTTPS is:
HTTPS runs HTTP over TLS encryption (port 443), protecting the data in transit.
- SMTP — sends email (client → server, and server → server).
- POP3 — downloads email and usually deletes it from the server.
- IMAP — leaves email on the server and syncs across devices, so the same inbox appears everywhere.
Practice
Which protocol is used to SEND email?
SMTP sends mail; POP3 and IMAP are used to receive/read it.
Practice
A key difference between IMAP and POP3 is that IMAP:
IMAP keeps mail on the server and syncs every device; POP3 typically downloads and deletes.
Practice
Match each protocol to its use.
HTTP = web, FTP = files, SMTP = sending email (POP3/IMAP receive it).
BitTorrent
- BitTorrent is a peer-to-peer protocol.
- A file is split into pieces, downloaded from many peers in parallel.
- No single server carries all the load, so popular files distribute efficiently.
Practice
BitTorrent reduces load on any single server because it:
As a peer-to-peer protocol, each peer shares pieces, so no single server carries the whole load.
You've got it
Key idea
- HTTP (web, port 80); HTTPS = HTTP over TLS (encrypted, port 443); FTP (files)
- SMTP sends email; POP3 downloads (and deletes); IMAP leaves it on the server and syncs
- BitTorrent is peer-to-peer — file pieces shared among many peers