The file transfer protocol (FTP) was created in 1971. Its final RFC is 959. If you're curious, you can read more about it at  https://tools.ietf.org/html/rfc959.
Being an old protocol, a few commands don't have clear specifications, so some alternative specifications (that are more or less official) have been written in order to fill those blanks. We'll go back to them when writing the server.
Another important point to note is that FTP uses TCP connections.
Now that we've quickly introduced you to FTP, let's see how it works.
