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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.)
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"