The data transfer process (DTP) for FTP establishes and manages the data connection. The DTP can be passive or active. Passive, or PASV, is a command that requests that the server "listen" on a data port (which is not its default data port) and to wait for a connection rather than initiate one upon receipt of a transfer command. Certain FTP servers support both active and passive transfers while other FTP servers only allow active transfers. * [Adapted from RFC 959]
See also FTP.