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. Commands and replies are exchanged between the protocol interpreters over this channel, 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 also 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, though it doesn't really explain them in a way that is very easy to understand. That's my job. The two methods differ primarily in which device, the client or the server, initiates the connection. This may at first seem like a trivial matter, but as we'll see shortly, it is actually quite important.