[ntp:questions] Tricking NTP's driftfile

Steve Kostecke kostecke at ntp.org
Wed Jun 25 01:45:52 UTC 2008

On 2008-06-24, J. Cochran <jdc at smof.fiawol.org> wrote:

> Steve Kostecke <kostecke at ntp.org> wrote:
>>On 2008-06-24, J. Cochran <jdc at smof.fiawol.org> wrote:
>>> Steven <sed_facebook at vt.edu> wrote:
>>> And then for the slave servers, I'd suggest that server
>>> Also be replaced with fudge stratum 12
>>The Undisciplined Local Clock contributes nothing the quality of
>>the client systems' clocks and, in fact, can cause problems if
>>The Undisciplined Local Clock is _not_ a "back-up"; it is merely a
>>hack which allows an ntpd to pretend to be synced even when no real
>>time sources are reachable.
> I know that, you know that.

There is no reason to use the Undisciplined Local Clock on a leaf node.

> But in the OPs case, he really isn't concerned with having accurate
> time that's attributable to an authorative source.

The OP has demonstrated a classic misunderstanding of how NTP works.

> He merely wants an isolated network of several macros to all have a
> consistent time between them.

NTP is overkill for this application.

> You may have noticed that he's *already* using an undiscoplined local
> clock.

I noticed that the OP is using the Undisciplined Local Clock on all
nodes (both master and slave) on his time island. This observation is
what led to my reply to your article.

The fact is that the Undisciplined Local Clock does only _one_ thing: it
allows ntpd to serve time to others even when it is not synced to a real
time source. That's all. The Undiscplined Local Clock does not provide
any sort of time base, or other sync information, to ntpd.

> My suggestion is to simply reduce the polling time to reduce
> his initial startup time (which he's complaining about)

You're not the only person who has suggested this.

> and to fudge the stratum to a high enough number that *if* for some
> reason his unconnected network finds itself on the public internet,
> then no one will accidently use him as a time reference. Yet, a low
> enough stratum that he's not going to have issues with exceeding
> stratum 15.

The default stratum for the Undisciplined Local Clock is 5.

> Yes, making a recommendation to the OP that he should get a couple of
> GPS receivers (the Garmen 18 LVC is a nice one)

Please re-read my original article. I suggested _one_ GPS for his
"master" server.

> and that he should then use the PPS interface, etc., etc., etc. to
> make a NTP server that's accurate to within a few micro seconds and is
> attributable to a national time standard would also solve his problem.

You have missed the point.

This is not an issue of using a time source traceable to a national
standards laboratory. It _is_ a matter of providing ntpd with a stable
time base.

> However, given his requirements, it would be more than a little
> overkill.

Given the OP's requirements, anything beyond rdate is overkill.

Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/

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