Value,Description,Reference
0,Reserved,[RFC8972]
1,NTP,[RFC8972]
2,PTP,[RFC8972]
3,SSU/BITS,[RFC8972]
4,GPS/GLONASS/LORAN-C/BDS/Galileo,[RFC8972]
5,Local free-running,[RFC8972]
6-239,Unassigned,
240-249,Reserved for Experimental Use,[RFC8972]
250-254,Reserved for Private Use,[RFC8972]
255,Reserved,[RFC8972]