What data flows through ftp port 20?

Question

Can you please tell me specifically what kind of data flows through which port during an FTP connection?

To be specific, I'd like to know whether contents of the directory and the server response codes flow through port 20.

Answer 1

FTP uses two types of connections: (1) the control connection (default port 21), which is used to send commands to the server and receive status codes back, and (2) the data connection (default port 20), which is used to transfer the content requested from the server: the content of a file or a directory listing, for example. I recommend to use a network protocol analyzer to see it with your own eyes. FTP specification RFC959 is written in an easily understandable manner, don't afraid to have a look at it.