FTP Connection Modes (Active vs. Passive) :: WinSCP

FTP may operate in an active or a passive mode, which determines how a data connection is established. In both cases, a client creates a TCP control connection to an FTP server command port 21. This is a standard outgoing connection, as with any other file transfer protocol (SFTP, SCP, WebDAV) or any other TCP client application (e.g. web browser). So, usually there are no problems when opening the control connection.

Where FTP protocol is more complicated comparing to the other file transfer protocols are file transfers. While the other protocols use the same connection for both session control and file (data) transfers, the FTP protocol uses a separate connection for the file transfers and directory listings.