Layer is where users interact with the network through application protocols.
Common protocols:
- HTTP/HTTPS: web browsing
- SMTP/IMAP: email
- DNS: domain resolution
- FTP: file transfer
- SSH: secure shell
Layer doesn't mean applications themselves. Your browser is an application; HTTP is the Layer protocol it uses.
A error is Layer . The network worked. TCP delivered bytes. But HTTP returned an error. Distinguishing "network broken" from "app rejected request" matters.