Ports and Protocols

Item	Purpose
UDP 123	Network Time Protocol (NTP).
PPS	Local pulse-per-second timing signal.
GPS Serial	NMEA data from the GPS receiver.
SOCK	Preferred Chrony/GPSD interface.
SHM	Legacy shared-memory interface.

When troubleshooting, verify that UDP port 123 is permitted between client and server and that no firewall is blocking responses.

Test-NetConnection server.example.com -Port 123

Microsoft reference: Windows Time networking documentation.