Need trusted NTP Sources

Saku Ytti saku at ytti.fi
Sun Feb 9 08:03:46 UTC 2014


On (2014-02-08 19:43 -0500), Jay Ashworth wrote:

> In the architecture I described, though, is it really true that the odds
> of the common types of failure are higher than with only one?

I think so, lets assume arbitrarily that probability of NTP server not
starting to give incorrect time is 99% over 1 year time.
Then either of two servers not giving incorrect time is 0.99**2 i.e. 98%, so
two NTP servers would be 1% point more likely to give incorrect time than one
over 1 year time.

Obviously the chance of working is more than 99% maybe it's something like
99.999%? And is that really typical failure-mode or is typical failure-mode
complete loss of connectivity? Two NTP servers would protect from this, single
not.
However loss-of-connectivity minor impact on clients, wrong time has major
impact of client.
Maybe if loss-of-connectivity is fixed in somewhat short period of time,
single NTP always win, if loss-of-connectivity is fixed typically in very long
period of time, single NTP loses.

I don't really have exact data, but best practice is >2. Matthew said 4, which
gives the advantage that in single failure you are still operating redundantly
and do not have urgency to fix, with 3 in single failure another failure must
not occur before it is fixed.
I think 3 is enough, networks are typically designed to handle 1 arbitrary
failure at the same time and 2 arbitrary failures in most networks, when
chosen correctly, will cause SLA breaking faults (Cheaper to pay SLA
compensations than to recover from any 2 failures).
But NTP servers are cheap, so if you want to be robust and recover from n
false tickers, have 3+n.



-- 
  ++ytti



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