Re: Need trusted NTP Sources

[Brett Frankenberger <rbf+nanog () panix com>]

On Sun, Feb 09, 2014 at 03:45:19PM -0500, Jay Ashworth wrote:
> ----- Original Message -----
> > From: "Saku Ytti" <saku () ytti fi>
>
> > > That's only true if the two devices have common failure modes,
> > > though, is it not?
> >
> > No, we can assume arbitrary fault which causes NTP to output bad time. With
> > two NTP servers it's more likely that any one of them will start doing
> > that than with one alone. And if any of the two start doing it, you don't
> > know which one.
>
> Hey, waitaminnit!  I saw you palm that card.  :-)
>
> If I'm locked to 2 coherent upstreams and one goes insane, I'm going to
> know which one it is, because the other one will still match what I already
> have running, no?

If it suddenly goes insane as a step function?  Sure.  But if the one
you've selected for synchronization starts drifting off true time very
slowly, it will take your clock with it, and then ultimately the other
one (that is actually the good clock) will appear to be insane clock.

     -- Brett