[time-nuts] Recommendations for NTP server

[David Andersen]

You'll get more than you expect -- the offset you're observing on  
ADSL is very likely wrong, because the delays your packets experience  
on adsl aren't symmetric.  NTP assumes symmetry.  So I wouldn't  
actually believe that a 1ms offset is really 1ms off, depending on  
the RTT to your ISP.

But there actually are ways to mitigate the offset spikes.  You might  
be able, for instance, to configure your gateway to prioritize NTP  
packets over everything else, which will help with half of the  
problem.  You won't be able to do the same at your ISP, of course, so  
it's not a perfect solution.

Installing a local GPS-synched server is the right answer if you  
really care.  And it's fun. :)  The Soekris boxes rock.  I assume  
you've already seen Poul-Henning Kamp's page about using his net4801  
with FreeBSD to act as a high precision timeserver?  If you want sub- 
microsecond, you'll probably have to replace the oscillator on the 4801.

And - most OSes should do the trick.  FreeBSD has a really nice  
precision timekeeping interface, though -- and it makes a marvelously  
solid time server.  I'm running it on a few Net4801s and recommend  
it.  You can very easily build an image for it using another bit of  
phk's magic called 'nanobsd' (it's in the source tree).

   -Dave

On Apr 23, 2006, at 5:00 PM, Geoff Powell wrote:

> I've just started getting interested in precision timekeeping - if you
> can call an offset from UTC that is measured in units of milliseconds
> "precision".
>
> My current timeserver is a Buffalo Linkstation, patched to run Debian
> Stable, and slaved via 2MBit ADSL to my ISPs Stratum 2 timeserver,  
> using
> NTP V4.2.0, from the Debian Stable repository. I'm seeing offsets  
> in the
> low millisecond range, but with frequent excursions up to 10 to 20
> millisec or more. These large spikes in the offset curve are  
> definitely
> correlated with large data transfers down from the Internet. Look at
> http://www.g8kbz.demon.co.uk/mrtg.html for a sample plot, derived by
> MRTG, and updated daily at midnight local time.
>
> There seems to be no prospect of mitigating these offset spikes  
> without
> taking further steps, and I intend to install a new timeserver,  
> probably
> using a Soekris NET4801 and a Garmin GPS18LVC. I hope to effect a 3
> order of magnitude improvement in observed offset, and more would be
> appreciated - I've already been bitten by the precision bug.
>
> So my question is - should I continue with Debian Stable, or would
> OpenBSD be better for sub-microsecond accuracy? Indeed, is sub-
> microsecond offset achievable with this hardware? GPSDOs and Rb or Cs
> standards are not yet practical politics.
>
> Thanks in advance for any tips,
> -- 
> Geoff Powell
>
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>
>

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