[time-nuts] Recommendations for NTP server
David Andersen
dga+ at cs.cmu.edu
Sun Apr 23 21:16:40 UTC 2006
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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