[ntp:hackers] GPS weirdness with ntp-dev-4.2.0a-20050303

Greg Dowd GDowd at symmetricom.com
Tue Mar 8 13:37:22 PST 2005


Thanks, that makes sense.   


Greg Dowd
gdowd at symmetricom dot com (antispam format)
Technologist, TT&M Div.
Symmetricom, Inc. 
www.symmetricom.com




-----Original Message-----
From: hackers-bounces at lists.ntp.isc.org [mailto:hackers-bounces at support.ntp.org] On Behalf Of David L. Mills
Sent: Tuesday, March 08, 2005 1:35 PM
Cc: hackers at support.ntp.org
Subject: Re: [ntp:hackers] GPS weirdness with ntp-dev-4.2.0a-20050303

Greg,

See lines around 2022 in ntp_proto.c. It is not a strict sort, the stratum is multiplied by MAXDISTANCE (1.0) and the dispersion over a day is about 1.3 s, so a nearby low-stratum server could well hijack the clock at least for some portion of the day.

My design assumption is, if you had a low-stratum nearby host, then why run up phone charges with ACTS? In the normal mode of operation you would use ACTS only if all other sources fail. I tested this configuration and it works as intended. For everything else there's tinker.

Dave

Greg Dowd wrote:

>That's the part I don't quite understand.  If it is sorted first by 
>stratum and THEN by sync distance, why would it drop the preferred 
>stratum 0 refclock for the stratum 1 network server. The dispersion 
>rate of increase can be tinkered
>
>
>Greg Dowd
>gdowd at symmetricom dot com (antispam format) Technologist, TT&M Div.
>Symmetricom, Inc. 
>www.symmetricom.com
>
>
>
>
>-----Original Message-----
>From: hackers-bounces at support.ntp.org 
>[mailto:hackers-bounces at support.ntp.org] On Behalf Of David L. Mills
>Sent: Monday, March 07, 2005 7:27 PM
>Cc: hackers at support.ntp.org
>Subject: Re: [ntp:hackers] GPS weirdness with ntp-dev-4.2.0a-20050303
>
>Greg,
>
>If the correctness intervals of two severs don't overlap, a majority clique does not exist and both are falseticker. If for any reason the prefer peer is considered a falseticker, the clustering algorithm doesn't get to see it. However, if there are only two servers, the clusting algorithm will not throw either of them away. It may however determine which one wins. The list is sorted first by stratum, then by synchronization distance.
>
>Remember, the synchronization distance can get rather large at the typically long poll intervals. If your s2 server is nearby, that might be the reason.
>
>Dave
>
>Greg Dowd wrote:
>
>
>>I'll echo that.  With only 2 lines in the config file, ntpd now selects s2 server over my ACTS refclock even with the prefer keyword set on the ACTS driver.  I know 2 entries are bad but I'm still surprised. 
>>
>>
>>Greg Dowd
>>gdowd at symmetricom dot com (antispam format) Technologist, TT&M Div.
>>Symmetricom, Inc. 
>>www.symmetricom.com
>>
>>
>>
>>
>>-----Original Message-----
>>From: hackers-bounces at support.ntp.org 
>>[mailto:hackers-bounces at support.ntp.org] On Behalf Of Mark Martinec
>>Sent: Monday, March 07, 2005 6:07 AM
>>To: hackers at support.ntp.org
>>Cc: mills at udel.edu
>>Subject: Re: [ntp:hackers] GPS weirdness with ntp-dev-4.2.0a-20050303
>>
>>Bjorn Gabrielsson writes:
>>
>>
>>>Hi John,
>>>I have seen something like your case. Internet time servers was 
>>>prefered over a local Trimble Accutime (Palisade). It looked 
>>>_something_ like this. ...
>>>Using "ntpd 4.2.0a at 1.1190-r" on "Linux/2.4.21-NANO".
>>>Will try to document it properly next time I see it.
>>>br,  Björn
>>>
>>>John Pettitt <jpp at cloudview.com> writes:
>>>
>>>
>>>>I just updated gatekeeper one of my FreeBSD servers from
>>>>ntp-dev-4.2.0a-20050206 to ntp-dev-4.2.0a-20050303 and it's started 
>>>>selecting clocks other than the attached GPS.
>>>>
>>>>Its started selecting solar (another box on the same lan daisy 
>>>>chained to the same Garmin GPS18) rather than the local GPS.  Any 
>>>>ideas on what changed between snapshots to make this happen? (there 
>>>>are so many diffs it's hard to tell). ...
>>>>
>>>>
>>I'm quite certain I've seen such behaviour with our GPS clock too with recent versions (but I wasn't careful enough to be able to point to a particular version). I had to add the option 'true' along with 'prefer'
>>to the GPS refclock 'server' line to make it stable again.
>>
>>My guess is the changed behaviour coincides with the introduction of 'tos mindist' or option 'true'. It certainly never happened before.
>>
>> Mark
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>>
>
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