This is the value of the Reference Identifier in the last
received packet defined as follows.
Reference Identifier: 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. 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.