On careful reading the TP-Link manual, there is a security feature called FTP ALG which allows ftp traversal over NAT. Without enabling FTP ALG, ftp is destined to fail because the client provides a private IP address and a port number to the FTP server but somehow, the port number is changed by the NAT device. FTP ALG ensures that the NATed port number and the ftp data port number initiated by the client are kept in a one-to-one mapping state table.
This is another example of NAT breaking end-to-end connectivity. Similar ALGs are also needed for SIP and H.323. NAT only brings troubles to the networked Internet world and it should be dropped as soon as practicable.
