[ntp:questions] Help with reference identifier for stratum 2 version 4.

David Woolley david at ex.djwhome.demon.invalid
Sun Dec 13 00:14:48 UTC 2009

Poster Matt wrote:

>> It's an opaque hash of the identity of the server's upstream server. 
>> New code is not supposed to put any further meaning on it.
> Thanks David. But...
> Then why does RFC 2030 say the reference identifier "contains the low 
> order 32 bits of the last transmit timestamp received from the 
> synchronization source."

Version 4 NTP is still in draft and I think real version 4 servers tend 
to implement the version 3 behaviour, so RFC 2030 was somewhat premature 
in specifying version 4 behaviour.

The problem with the version 3 behaviour is that it doesn't work well 
with IPv6 addresses.  I think the insignificant bits in the timestamps 
are now randomised, so an ntpd implementation seeing a packet with a 
reference identifier that exactly matches the low order bits of a 
timestamp it recently sent can be reasonably sure that it is seeing a loop.

The current(?) draft for NTPV4 uses a hash, in the way I described: 
<http://www.ietf.org/id/draft-ietf-ntp-ntpv4-proto-13.txt>, bottom of 
page 23.  It behaves the same as NTPV3 when using IPV4 addresses.

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