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What is the difference between Implicit TLS and Explicit TLS?
Explicit security requires that the FTP client issues a specific command to the FTP server after establishing a connection to establish the SSL link. In explicit TLS the FTP client needs to send an explicit command (i.e. "AUTH TLS") to the FTP server to initiate a secure control connection. The default FTP server port is used. This formal method is documented in RFC 4217.
Implicit security is a mechanism by which security is automatically turned on as soon as the FTP client makes a connection to an FTP server. In this case, the FTP server defines a specific port for the client (990) to be used for secure connections. FTP over TLS Implicit has been deprecated and should no longer be used.