[time-nuts] Recommendations for NTP server

[John Pettitt]

David Andersen wrote:
> 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
>
I'll second the soekris box - my box time.no-such-agency.net is a 4801
running FreeBSD with a GPS18LVC.    You can expect offsets in the +/-
5us range except when the box is stressed - the standard xtal in the box
is not temperature compensated so offsets will spike to as much as 250us
when the system does something compute intensive.  If you don't need the
HD consider the 4501 as it has the ability to timestamp a PPS using
internal counters rather than the DCD kludge. - Again see phk's
excellent work on the subject including the code in FreeBSD to support
PPS on a 4501.

Regarding DSL - I now have three stratum one servers here: a GPS18LVC, a
TrueTime NTS100 and TrueTime GPS XL-DC  (it is addictive isn't it) - my
ISP just installed a GPS18LVC based server one hop (on gig-e) from my
DSL concentrator so I can get a very clear picture of the error
introduced by my DSL asymmetry.  For my line (6000 down 608 up
backhauled over ATM) it's 1.5ms.  In a perfect world NTP would allow a
fudge offset to be applied to subnets (in my case 1.5ms on the default
and 0.0 on the local LAN) which would factor out the know DSL error -
alas it doesn't do that right now.

John