[ntp:questions] Number of Stratum 1 & Stratum 2 Peers

William Unruh unruh at invalid.ca
Tue Dec 9 18:17:08 UTC 2014


On 2014-12-09, Mike Cook <michael.cook at sfr.fr> wrote:
><snip>
>> 
>> 
>>> 
>>>> Three are fine, as long as only one dies or goes nuts.
>>> 
>>> Again, define "goes nuts". You don't seem to like the term 
>>> "falseticker", so how do you define "goes nuts"? If one "goes nuts" or 
>>> even goes offline, if the remaining two do not agree then it is like 
>>> having no server at all.
>> 
>> No, it is like having two, with one being out. 
>> falseticker is a term with a very specific internal definition. Thus a
>> server whose time is right on UTC could be a falseticker, because the
>> other two servers were both exactly 3 days out, with tiny jitter estimates. 
>> I would say then that you had two servers going nuts, and one good, even
>> though ntpd would say there were two good and one false ticker.
>
> In fact this does not happen. I just tested the hypothesis.
> What happens depends on how the two wayward get there exaggerated offset:
> a) someone,something resets the date:
>    result: ntp on both those servers crashes due to the panic_stop limit.
>
>  So in this case  the client has only one reference and continues using that. It is not flagged as a falsticker.
>  That is normal.

I gave an example. The server is set up with the localclock driver, and
its gps puck goes down. It gaily keeps delivering the time which drifts
off further and further. It goes nuts in other words.

>
> b) someone restarts ntp on the servers with the wrong date. Here the servers ntpd has no way of knowing that it has bad time and so continues serving normally. 
>    On the client. The running ntp sees immediately a huge offset and huge jitter.
>
> Tue Dec  9 13:15:04 CET 2014
>     remote           refid      st t when poll reach   delay   offset  jitter
>==============================================================================
> *192.168.1.15    .GPS1.           1 u  320   64  360    0.549    0.040   0.037
> +192.168.1.16    .GPS2.           1 u   37   64  377    0.606    0.006   0.028
> +192.168.1.17    .GPS1.           1 u  309   64  360    0.576    0.027   0.025
> Tue Dec  9 13:16:08 CET 2014
>     remote           refid      st t when poll reach   delay   offset  jitter
>==============================================================================
> 192.168.1.15    .GPS1.           1 u   55   64  341    0.565    0.042 9660780
> *192.168.1.16    .GPS2.           1 u   37   64  377    0.606    0.006   0.024
> 192.168.1.17    .GPS1.           1 u   42   64  341    0.579    0.041 9660773
>
> After 5 mins the client is unable to resolve this and declares all clock falsetickers and then panics. I did not have ntpd in debug mode here, but it is reasonable to assume that it panics due to the selected clock being too far out and hitting the panic limit.

Ie, clockhopping. It cannot decide. But while the server clock slowly drifts
away it gives a terrible time. If you want microsecond accuracy, 128ms
is a HUGE distance off, without triggering a complete crisis in the
client. 




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