Recap: HTTP and the Application Layer

One meaning across three versions, and a transport underneath that decides what a lost packet costs.

The application layer is where bytes get their meaning, and HTTP is its most common language.

Application protocols each do one job: HTTP for documents and APIs, SMTP to send mail, IMAP to read it, SSH for remote shells, SFTP for files.
An HTTP/1.1 message is a start line, headers, an empty line and a body whose length is given.
Stateless means each request stands alone; connections are still reused for many requests.
HTTP/2 interleaves many streams on one TCP connection; a lost packet still stalls them all.
HTTP/3 runs on QUIC over UDP, with ordering per stream, TLS built in and connections that survive an address change.
FTP, FTPS and SFTP are three protocols; SFTP runs inside SSH.

The three versions of HTTP and the transports under them

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