FTP over TLS (RFC 4217)

The formal specification for securing FTP connections with TLS, covering both the control and data channels. RFC 4217 defines explicit security negotiation using the AUTH TLS command on port 21 to upgrade a plaintext FTP connection, and the PROT command to specify whether the data channel should also be encrypted. This is the standards-track basis for FTPS explicit mode and is supported by most modern FTP servers and clients.