[ntp:questions] LOCL clock reachability not 377?

William Unruh unruh at invalid.ca
Thu Jul 31 15:27:57 UTC 2014


On 2014-07-31, Martin Burnicki <martin.burnicki at meinberg.de> wrote:
>
> Unlike otherwise stated in this thread I don't think it's a good idea to 
> leave the 1 PPS signal alone disciplining the time of the NTP server. 
> This can easily yield unforeseen problems, similarly as if you use an 
> IRIG time reference which only provides day-of-year and time-of-day, but 
> no year number. If you don't take care then such signal can be accepted 
> and provide a "valid" time which is off by an integral number of years.

My point is that most of the internal clocks on computers are well able
to maintain the time to better than a second for a long time, even if
they were freewheeling, and if disciplined by a PPS, they are able to
maintain the time forever (well, until the next power failure anyway). 
Now, it is certainly true that that provides no redundancy and there are
situations in which you could be let astray, but that is also true even
if you have 5 time sources.  Ie, a PPS source on its own is quite
capable of disciplining the system to microsecond accuracy even if all
other time sources disappear, as long as they are there to deliver the
seconds initially. To have PPS be discarded just because the supplier of
seconds goes offline seems far far too extreme to me. Note that the
supplier of seconds could even be a human setting the clock by hand
intially. (one could do that to within certianly less than half a
second). 

>
> I don't think this would be acceptable for environments where an 
> accurate precise time is most important.

Of course, the greater the redundnacy, the better.

Anyway, your proposals seem very sensible to me, and encompass my use
case above. 

>
>
> Since the NTP daemon is unable to know the precision and stability of a 
> PPS source just from the PPS slopes the concept of a trust time could 
> help fine-tuning a configuration in which several PPS sources of 
> different quality are available.
>
>
> Beside the implementation of such a flexible concept in ntpd it would 
> have to be discussed how this can easily be configured. With NTP's basic 
> configuration syntax in mind a possible way could be something like this:
>
> # a refclock with PPS signal but no good oscillator
> server 127.127.8.0
> server 127.127.22.0 ref 127.127.8.0
>
> # a refclock with PPS signal and good oscillator
> server 127.127.8.1
> server 127.127.22.1 ref 127.127.8.1 trust 3600
>
> # a PPS source relying on the usual system peer to
> # provide absolute time
> server 127.127.22.2 ref sys_peer
>
> # a PPS source which should be trusted always
> server 127.127.22.3 trust always
>
>
> Martin



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