Cost-effectivenesss of highly-accurate clocks for NTP

Leo Bicknell bicknell at ufp.org
Mon May 16 14:27:50 UTC 2016


In a message written on Fri, May 13, 2016 at 03:39:27PM -0400, Eric S. Raymond wrote:
> According to RFC 5095 expected accuracy of NTP time is "several tens
> of milliseconds." User expectations seem to evolved to on the close
> order of 10ms.  I think it's not by coincidence this is pretty close
> to the jitter in ping times I see when I bounce ICMP off a
> well-provisioned site like (say) google.com through my Verizon FIOS
> connection.

For a typical site, there are two distinct desires from the same
NTP process.

First, syncronization with the rest of the world, which is generally
over the WAN and the topic that was well discussed in your post.
I agree completely that the largest factor is WAN jitter.

The other is local syncronization, insuring multiple servers have
the same time for log analysis purposes and such.  This typically
takes place across a lightly loaded ethernet fabric, perhaps with
small latency (across a compus).  Jitter here is much, much smaller.

Does the limitation of accuracy remain jitter in the low jitter
LAN/Campus enviornment, or does that expose other issues like the
quality of oscellators, OS scheduling, etc?  Or perhaps another
way, is it possible to get say 10's or 100's of nanosecond accuracy
in the lan/campus?

-- 
Leo Bicknell - bicknell at ufp.org
PGP keys at http://www.ufp.org/~bicknell/
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