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Re: [gpsd-dev] Clarifications needed for the time-service HOWTO

From: Greg Troxel
Subject: Re: [gpsd-dev] Clarifications needed for the time-service HOWTO
Date: Mon, 21 Oct 2013 18:43:37 -0400
User-agent: Gnus/5.130006 (Ma Gnus v0.6) Emacs/23.4 (berkeley-unix)

"Eric S. Raymond" <address@hidden> writes:

First, I think if you're writing a howto that's part of gpsd, going
into NTP details is out of scope.   gpsd's documentation should be about
how to get time from a GPS receiver and into some time sync daemon.

>      You will hear time service people speak of "Stratum 0" (the reference
>      clocks) "Stratum 1" (NTP servers directly connected to reference
>      clocks over a path with known and compensated-for latency)
>      "Stratum 2" (publicly accessible servers that get time from Stratum 1
>      over a network link.) Stratum 3 chimers redistribute time from Stratum
>      2, and so forth. There are defined higher strata up to 15, but
>      you will probably never see a public chimer higher than Stratum 3.

As Gary says, this needs a dose of "should", not "is".

There's no notion that stratum 2, or anything else, is publically
accessible.   Stratum is orthogonal to access control.

Also, the stratum of an NTP server is a dynamic property of how things
are at the moment more than a statically-configured property.  I have a
box which is supposed to be s1, with an attached TrueTime XL-DC.
However, the XL-DC is reporting time 19 years ago (March 7, 1994!!) due
to how it coped with W1K, and so my box is now s2, having tossed the
XL-DC as bad.

So there's a design stratum and a current stratum.  Basically it's a hop
count to a reference clock along the chosen-peer edges in the
synchronization graph.

>      Ordinary client computers are normally configured to get time from one
>      or more Stratum 2 (or less commonly Stratum 3) servers. With GPSD and
>      a suitable GPS, you can easily condition your clock to higher
>      accuracy than typical Stratum 2; with a little effort you can do
>      better than public Stratum 1 servers.

This sounds overblown.  Some public S1s are very good, and some aren't.
The real barrier to accuracy is having a way to read the system clock on
a PPS edge, for example a counter card that can be triggered, so it's
really reading the clock, together with using that counter for system
timekeeping.   But the point is that with gpsd and a GPS receiver with
PPS, one can do very well.

Another point which should be addressed is:

  ntpd has builtin support for reading some GPS timing receivers.  How
  should one decide to use ntpd's support vs gpsd?  Does it matter?

And I'm not sure of the answer (my XL-DC machine just uses ntpd).

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