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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