[...]
FTP command issued as part of setting up a transfer with the connection connection direction being in the same direction as the command channel (i.e. the FTP server creates a temporary server socket and it is up to the client to connect to it.)
As of February 2003, tcllib's ftp implements PASV, but ftpd does not. It's often wiser at the system level just to rely on HTTP.
http://compnetworking.about.com/library/glossary/bldef-pasv.htm
RFC 478 [L1 ]: "Server-Server Interaction"
RFC 1579 [L2 ]: "Firewall-friendly FTP"
RFC 959 [L3 ] "File Transfer Protocol"