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Clear Command Channel (CCC)
If the Clear Control Connection (CCC) setting is enabled, the FTP client connects to the server, negotiates a secure connection, authenticates (sends user and password) and reverts back to plaintext.
To change the control connection protection in SmartFTP go to the Favorite Properties. Then go to the FTP->Connection->SSL/TLS dialog and change the Control Connection option.
NAT routers automatically examine the PORT command sent through a control channel for un-secured control connections to the default FTP port (21) of the remote FTP server. This allows them to open an incoming port for the data connection, setup the routing/forwarding and rewrite the PORT IP with the external (WAN) IP address. If a secured connection (SSL) is made to a FTP server, the NAT router cannot interpret the data correctly as everything is encrypted. Therefore it cannot translate a PORT command and open the appropriate port for the expected incoming data connection. By sending a CCC command and further reverting back to plaintext the NAT router are once again able to translate the PORT commands.
Most modern servers support this feature.
Microsoft - How NAT works
RFC 1631 - The IP Network Address Translator (NAT)