[ntp:questions] strange behaviour of ntp peerstats entries.

Unruh unruh-spam at physics.ubc.ca
Sun Feb 3 16:11:02 UTC 2008

"David L. Mills" <mills at udel.edu> writes:


>You are not accurately describing the ntpd clock filter. An accurate 
>desciption would surely help the point you are making.

You are undoubtedly correct. I was reading the source code again loast
night and getting myself confused again. I do not think my comments about
the aging are correct.

I think that the process is --look though the last 8 samples for the one
with the smallest delay, If that is the the latest one, use it and go to
the clock selection process. If not do not go to clock selection (ie ignore
the latest measurement).


>Unruh wrote:

>> Grian Utterback <brian.utterback at sun.com> writes:
>>>David J Taylor wrote:
>>>>Brian Utterback wrote:
>>>>>Which is why NTP prefers the source with the smallest delay. The
>>>>>system I am using has servers whose delays are 51ms to 94. I can't
>>>>>find any closer. On my company LAN, the delays range from 16ms to
>>>>>87ms. The offsets of all these servers agree to within 9ms. Sure, I
>>>>>am not going to get sub-millisecond from that, but I think it is 
>>>>>more typical than your set-up.
>>>>>Brian Utterback
>>>>I know this is supposed to happen, but I recall seeing behaviour some time 
>>>>ago where a server with a little more jitter and much more delay was 
>>>>preferred, almost as if it was the jitter/offset which was the measure 
>>>>rather than offset itself.  I ended up adding a "prefer" qualifier.
>>>I mis-spoke. It is actually the case that NTP prefers the sample of
>>>the eight from any one single source that has the least delay. After
>>>the sample is chosen, the samples from all the servers are then
>>>subjected to different criteria to determine which will be the final
>>>choice. Jitter and stratum both factor in.
>> Worse than that . Only if the latest sample is the one with the min delay
>> is it chosen Otherwise it is not. You can go for 16 or more samples never
>> using any of thembefor one fits the criteion. (actually the samples are
>> aged as well-- ie the delay is increased as they get olderbut that seems to
>> have little effect.)
>> )

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