[ntp:questions] frequency adjusting only

maxime louvel m.louvel at gmail.com
Mon Apr 21 08:50:09 UTC 2008


Thanks for your answer,

I can't have a step on the clock because that would screw up my
applications.
However if I keep the load within a certain range I should fine, don't I ?
I am synchronising one node to several public NTP servers, and the others
nodes are synchronised to the first one.
There are 2 to 24 nodes in my sub net.
Do you think that should be feasible ?

Maxime

On Sat, Apr 19, 2008 at 10:34 AM, David Woolley
<david at ex.djwhome.demon.co.uk.invalid> wrote:

> Unruh wrote:
> > Harlan Stenn <stenn at ntp.org> writes:
> >
> >>>>> In article <
> dda529d0804160332l3bcb4e43l5623478bcd8f4fa at mail.gmail.com>,
> m.louvel at gmail.com (maxime louvel) writes:
> >
> >> maxime> Hi, I would like to know if NTP (and particularly ntpd) is able
> to
> >> maxime> synchronise a clock, only playing with the frequency.  I want
> to
> >> maxime> avoid any step in the clock that would probably screw up my
> network
> >> maxime> appli.
> >
> >> It's *possible* and it can be a really bad idea.
> >
> >> For most folks, only steps *backward* are a problem.
> >
> > Actually, ntp does synchronize the clock by "playing with the frequency
> > only" unless the step size is so large (128ms by default but up to
> 600sec
> > by option) that it will step.
>
> There are two supported settings for this limit, 128ms and 600,000ms,
> but you can set other values, including infinite, with an option that
> comes with severe health warnings.  If you set a value of more than
> 500ms on systems that implement part of the clock discipline in the
> kernel, you will force them not to use that mechanism, and therefore get
> lower quality time.
>
> There are also options, unfortunately also with health warnings, that
> can effectively eliminate the main legitimate cause of this, which is
> high asymmetric traffic loads on medium speed internet connections which
> have not been traffic shaped for NTP.  If you suffer large steps for
> other reasons, you should fix the underlying cause.  Fixing asymmetric
> load can have commercial implications, as only high value ISP accounts
> are likely to support the necessary traffic shaping.
>
> (Another source of steps is lost clock ticks, but that generally only
> gives the, relatively benign, positive steps.)
>
> Systems which suffer large steps and aren't allowed to step have been
> reported to hunt (in the control theory sense) rather badly.
>
> Note, the subject is a little confusing as it could be read as meaning
> that steps are allowed to remain uncorrected.  That would be
> unacceptable for a protocol where every node is both client and server.
>  That would be like if you had to drive 30 miles and insisted that you
> drove at 30 mph, but you were delayed by a traffic jam, and still
> declared the journey complete after exactly one hour.
>
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>



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Maxime Louvel
0044 7964 5555 80
43 Allen road
Whitemore reans
WV60AW Wolverhampton
United Kingdom



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