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]
