Port 21 supports FTP control and ftp (UDP/SCTP reference coverage) across distinct transports. Review each protocol before writing firewall or monitoring policy.
Control channel for FTP commands and authentication.
Legacy file-transfer services and appliance integrations.
Client to server
Plain FTP exposes credentials and content. Passive mode also requires a separately configured data-port range.
ftp (UDP/SCTP reference coverage) is documented as a network service convention for port 21 over UDP + SCTP.
Use this reference entry when identifying or validating systems documented to use 21/UDP + SCTP.
Varies
Confirm the listening process and current vendor documentation before permitting traffic; a listed convention does not prove the active service or its security posture.