Port 123 supports NTP and ntp (TCP reference coverage) across distinct transports. Review each protocol before writing firewall or monitoring policy.
Clock synchronization between systems and time sources.
Servers, clients, network devices and authentication systems maintaining consistent time.
Client to server
Use trusted sources, restrict public service exposure and prevent amplification by disabling inappropriate query modes.
ntp (TCP reference coverage) is documented as a network service convention for port 123 over TCP.
Use this reference entry when identifying or validating systems documented to use 123/TCP.
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.