[ntp:questions] 2xGPS and 4x peers setup - disturbed by 5th and 6th peer

David Lord snews at lordynet.org
Mon Nov 3 23:52:15 UTC 2014


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> Martin Burnicki wrote:
>> We have had a case where a customer had one local
>> computing center and 2 ones at different remote locations.
>> In each of the computing centers were 2 GPS
>> controlled LANTIME NTP servers installed.
>>
>> In the local computing center there was also a Linux server
>> running ntpd which had all 6 LANTIMEs configured as time sources.
>>
>> Unfortunately the internet connection of the local
>> computing center seemed to have an asymmetry in the
>> packet delay, so from the Linux client's point of view
>> all 4 LANTIMEs in the remote locations seemed to have a
>> time offset in the same range, a few milliseconds,
>>  compared to the 2 local LANTIME.
>>
>> Even though all the 4 remote servers showed much more
>>  jitter due to the long network path they were preferred
>>  by the Linux client, and the 2 local LANTIMEs were marked
>>  as "falsetickers" even though they showed much less jitter.
>>
>> If I remember correctly then the Linux client was
>>  running 4.2.6p?, and a test with a -dev version of ntpd
>>  showed that the newer ntpd preferred the 2 local
>>  LANTIMEs over the 4 remote ones. This seems to indicate
>>  that the weight put on different criteria
>>  in the selection algorithm has changed over versions,
>>  and the newer versions of ntpd act more like you'd expect.
> 
> TOS MinDist affects this.
> 
> e.g. in the case of serial nema and pps, sometimes the mindist
>  needs to be increased from 1ms to perhaps 20ms,
>  and I've seen as much as 400ms (fairly often);
>  {which makes me wonder if the PPS is inverted?}

Hi

In my case that's been a figure large enough to span the range
of offset variation (both Garmin and Sure have been used). For
a while Garmin weren't even usable until a firmware fix was
released, as the range of offsets exceeded one second.


David



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