Network Time Protocol (NTP), as specified in RFC 5905, uses port 123 even for modes where a fixed port number is not required, which makes it easier for remote attackers to conduct off-path attacks.
https://tools.ietf.org/html/draft-gont-ntp-port-randomization-00
https://support.f5.com/csp/article/K09940637?utm_source=f5support&%3Butm_medium=RSS