FTP Data Connection Management
The control channel created between the server-PI and the user-PI using the FTP connection establishment and authentication process is maintained throughout the FTP session. Over the control channel, the protocol interpreters exchange commands and replies, but not data.
Each time files or other data need to be sent between the server and user FTP processes, a data connection must be created. The data connection links the user-DTP with the server-DTP. This connection is required both for explicit file transfer actions (getting or receiving a file) and for implicit data transfers, such as requesting a list of files from a directory on the server.
The FTP standard specifies two different ways of creating a data connection, ...