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

Poster Matt postermatt at no_spam_for_me.org
Sun Dec 13 15:06:39 UTC 2009

Thanks for the clarification David, it's appreciated.

One quick thing if you have a moment to clarify it for me. Was I right in 
writing that in the transmit timestamp (in a NTP message being sent to the 
server) that the "low order unused bits" get randomized. 2 questions about 
that; is that just version 4 not 3, and are the "low order unused bits" of 
that timestamp just the final byte?

Thanks again. Regards,


David Woolley wrote:
> 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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