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

David L. Mills mills at udel.edu
Sun Feb 3 20:43:48 UTC 2008


Unruh,

My message to David probably is most relevant.

Dave

Unruh wrote:

> "David L. Mills" <mills at udel.edu> writes:
> 
> 
>>Unrug,
> 
> 
>>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).
> 
> 
> 
>>Davde
> 
> 
>>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 
>>>>>>probably
>>>>>>more typical than your set-up.
>>>>>>
>>>>>>Brian Utterback
>>>>>
>>>>>Brian,
>>>>>
>>>>>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.
>>>>>
>>>>>Cheers,
>>>>>David 
>>>>>
>>>>>
>>>
>>>
>>>>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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