[ntp:questions] Remaining synced on an unsynchronised peer?

unruh unruh at wormhole.physics.ubc.ca
Mon Nov 30 23:08:15 UTC 2009

On 2009-11-30, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
> Michael Butow wrote:
>> P.S. Detail that I omitted: we use minpoll=maxpoll=5 (for historic 
>> reasons in our system).
> "Historical Reasons" is a pretty feeble excuse.  NTPD will, if permitted 
> to do so, adjust that polling interval as needed to get the most 
> accurate time.  By pegging minpoll and maxpoll you are crippling the 
> ability of NTPD to get the greatest possible accuracy.

That of course has nothing to do with his question, and if furthermore
wrong. It depends on what they want to optimize as to what the optimal
poll level is. The poll6-10 of the stock ntp is a "one sixe fits all"
which is not necessarily optimal for any particular situation. Whether
or not they have the ability to figure out what is optimal, or even what
optimal means is another question, but "historical reasons" could well
be " sometime in the past someone who really knew what they were doing
optimised the system at poll level 5". It could mean many other things
as well, but you have no idea what it means, and your attack is
Note that poll level 11 is almost certainly NOT the ideal poll level to
minize offset from true time while connected to the net. It is also
level which guarentees that daily temp fluctuations will drive the
system mad. (poll level 11 means that the ntp system has a time scale for
responding to changes of about 8 hours, while daily temp fluctuations
are closer to 1 hr. time scale. )
Now perhaps we can get back to answering his question. 
I suspect but do not know, that part of the problem is the level. The
machines will always have a level one higher than the system they are
getting their time from, and they do not like to get time from a system
at a higher level than they are. Thus the first one to get its time from
the other will up its level to one higher than the other, and the other
will then see the first as having too high a level(stratum) to sync to.
(or the two will walk each other up to the highest level of 15 and see
each other as unuseable sources)

I might be wrong in that analysis but that is my suspicion.

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