RFC 959 File Transfer Protocol specifies that the DTP (Data Transfer Process), the process that establishes and manages the data connection, can be passive or active. The PASSIVE (PASV) command requests the server-DTP to "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.

Currently, the FTPURLConnection class supports active FTP only. We would need passive FTP support in the presence of firewalls.

Note: This feature is part of the J2SE Reference implementation.
However, it does not add any APIs to the J2SE specification,
is not covered by the TCK, and is not a required part of the
J2SE platform.