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Re: Need trusted NTP Sources
From: Andriy Bilous <andriy.bilous () gmail com>
Date: Sun, 9 Feb 2014 22:41:48 +0100

Unfortunately I don't have the book handy. May be I am wrong too. Just
checked and 4 looks to be a valid solution for 1 falseticker according to
Byzantine Generals' Problem.


On Sun, Feb 9, 2014 at 10:03 PM, Saku Ytti <saku () ytti fi> wrote:

On (2014-02-09 21:08 +0100), Andriy Bilous wrote:

Best practice is five. =) I don't remember if it's in FAQ on ntp.org or
in
David Mills' book. Your local clock is kind of gullible "push-over" which
will "vote" for the "party" providing most reasonable data. The algorithm
would filter out insane sources which run too far from the rest and then
group sane sources into 2 "parties" - your clock will follow the one
where
runners are closer to each other. That is why uneven number of
trustworthy
sources at least at start is required. With 2 sources you will blindly
follow the one which is closer to your own clock. You're also having the
the risk to degrade into this situation when you lose 1 out of 3 sources.
Four is again 2:2 and only with five you have a good chance to start
disciplining your clock into the right direction at the right pace, so
when
1 source is lost you (most probably) won't run into insanity.

I'm having bit difficulties understanding the issue with 4.

Is the implication that you have two groups which all agree with each other
reasonably well, but do not agree between the groups. Which would mean
that 4
cannot handle situation where 2 develop problem where they agree with each
other but are wrong.
But even in that case, you'd still recover from 1 of them being wrong. So

3 = correct time, no redundancy
4 = correct time, 1 can fail
5 = correct time, 2 can fail
and so forth?

But not sure here, just stabbing in the dark. For the fun of it, threw
email
to Mills, if he replies, I'll patch it back here.

--
  ++ytti




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