[[...]]


[FTP] command issued as part of setting up a transfer with the 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.)  Why do this?
Generally--very generally--it's to get through a [firewall].  Why not
just use [HTTP]?  It's not clear.  There's folklore about FTP being faster, in some sense,
and there are the usual legacy reasons, too.



As of February 2003, [tcllib]'s ftp implements PASV, but [ftpd] does not.
----
http://compnetworking.about.com/library/glossary/bldef-pasv.htm

"The PASV ... verbs" 
[http://cr.yp.to/ftp/retr.html]


RFC 478
[http://www.faqs.org/rfcs/rfc478.html]:
"Server-Server Interaction"

RFC 1579
[http://www.faqs.org/rfcs/rfc1579.html]:
"Firewall-friendly FTP"

RFC 959
[http://www.faqs.org/rfcs/rfc959.html]:
"File Transfer Protocol"

"The difference between
PASV FTP and Normal FTP"
[http://war.jgaa.com/ftp/?cmd=show_page&ID=ftp_pasv]