Reference Identifier, notes:
This is a 32-bit bitstring identifying the particular reference source.
In the case of NTP Version 3 or Version 4 stratum-0 (unspecified) or
stratum-1 (primary) servers, this is a four-character ASCII string,
left justified and zero padded to 32 bits. NTP primary (stratum 1)
servers should set this field to a code identifying the external reference
source according to the following list. If the external reference is one
of those listed, the associated code should be used. Codes for sources not
listed can be contrived as appropriate.
Code External Reference Source
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LOCL uncalibrated local clock used as a primary reference for
a subnet without external means of synchronization
PPS atomic clock or other pulse-per-second source
individually calibrated to national standards
DCF Mainflingen (Germany) Radio 77.5 kHz
MSF Rugby (UK) Radio 60 kHz
GPS Global Positioning Service
In NTP Version 3 secondary servers, this is the 32-bit IPv4 address of the
reference source.
In NTP Version 4 secondary servers, this is the low order 32 bits of the
latest transmit timestamp of the reference source.