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

[Gary E. Miller]

Yo Eric!

On Sat, 19 Oct 2013 22:35:47 -0400 (EDT)
address@hidden (Eric S. Raymond) wrote:

> This thing is coming together nicely.  But, Gary, I need to extract
> more information from your head (and from other founts of expertise
> such as Hal and Harlan) before it will it will be anywhere near
> complete.

Understood.  I whould have movre frre time this week than last.

> Is it really true that most public NTP servers are Stratum 2, or
> are there more layers in normal use?

Maybe most, but you'll see a lof of 1 and 3's.

  When I look in my system's
> config file, I see one server: ntp.ubuntu.com.  Stratum 2 or higher?
> Is there any way I can tell? 

# ntptrace ntp.ubuntu.com
91.189.89.199: timed out, nothing received
***Request timed out

They seem to be down.

> More generally: what can I discover about the quality of the chimers
> I listen to?

Just compare several.  You should have at least 2, more likely 5 in
at least one of your ntp.conf.  Then the bad (to you) ones will just
stand out.  Some are just bad, some will not have a good network
connection to you and will appear bad.

> How specific can we be about time jitter?  Is this a topic for the
> HOWTO at all?

We can describe it, but since it is the error part, it will be 
specific to chimers, time sources, networks and clients.

>    Those hotplug devices will, however, may be able to use plain,
>    non-kernel PPS. gpsd tries to automatically fall back to this when
>    absence of root permissions makes KPPS unavailable. This fallback
> is complicated by the fact that gpsd needs to communicate to ntpd in
>    a different way in root and non-root mode.  This complicates the
>    configuration in ways beyond the scope of this document and is
> strongly discouraged in practice.
> 
> This paragraph troubles me. I'm not sure, but I think it may be
> conflating two different issues and two sets of constraints. 

Yes, two related issues.  KPPS to PPS fallback, and the problems of
fallback to non-root.  In general we should just discourage non-root 
and say it is bad, do not do that.

> Which set of ntpd segments GPSD can use is constrained by whether
> it started up as root or not.

Worse, by whether it is root or not when initialized, which may be at
hot plug time.

>  But whether it can use KPPS is
> controlled by whether it still has root *at the time the PPS source
> is opened*. 

Yes.  But is you consider 'opening' and 'initialization' as similar
the you can see it they are really the same issue.

> The following scenarios are possible:
> 
> 1) GPSD started as root; device path passed on command line.  GPSD
> will use privileged ntpd segments 0 and 1, KPPS will work. Best case.

Yes.

> 2) GPSD started as root; device is hotplugged. GPSD
> will use privileged ntpd segments 0 and 1,

No.  It will use units 2 and 3.  Which is likely not what is in ntp.conf
and in practive is not a fail.

> but KPPS will fail
> (and fall back to plain PPS) because privileges were dropped 
> before the device was opened.

Yes.

> 3) GPSD started as non-root; device path either passed on command line
> *or* hotplugged.  GPSD will use privileged ntpd segments 2 and 3; KPPS
> will not work but plain PPS will.

Sort of, the ntp.conf mmust be changed to use units 2 and 3.
 
> The fallback to plain PPS has nothing to do with the ntpd
> communication mode; this is shown by the fact that 1 and 2 aren't
> the same case with the same outcome

Yes, they are similar in that non-root operation is bad, but bad for
different reason and with diffent outcomes.  So just say: bad, do not do it!
 
> I think everything after the first sentence in this 'graph should
> be removed.  But maybe I'm missing something.

The problem with just keeping the first sentence is the user is not
left with an idea of the severity of the problems he will encounter.  We
have seen that in the past where users try to run as non-root and
have not  understood the instructions to run as non-root are incomplete
and problematic.  So if you keep the first sentence, then say if you are
not root (hot plug or initialization) that is bad, unsupported and out
side the scope, that could work.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

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