[LEAPSECS] A standard for leap second smearing

Michael Shields mshields at google.com
Tue Sep 27 19:51:16 EDT 2016

Thanks for organizing the discussion on this.

On Tue, Sep 27, 2016 at 3:45 PM, Stephen Colebourne
<scolebourne at joda.org> wrote:
> 1) Linear or Other?
> All current known smears are linear. Google previously had a different
> approach, but changed to linear. Are there any major arguments against
> linear? It would seem to be easier to specify and code that way.

Yes, the linear smear is much simpler.  It also minimizes the maximum
frequency change.

> 2) When to smear?
> Some smear up to midnight, some smear after midnight, some smear both
> sides. What are the arguments for/against each?

A good argument in favor of a symmetrical smear is that it minimizes
the offset to UTC at any point.  With a symmetrical smear, the clock
is always within <= 0.5 s of UTC.

> 3) Speed of smearing?
> The existing approaches have two broad groups - fast (under an hour -
> Bloomberg/UTC-SLS) and slow (20 hours or more - Google/Amazon) with
> QTnet an outlier towards the fast end.

I expect a smear of 2000 s or less would be challenging for NTP
clients, because of the 500 ppm max slew rate, which would be entirely
consumed by tracking the smear.  Clients that have backed off to 1024
s polling would also have trouble noticing that a smear was happening
when the smear is only 1000 s or 2000 s long.

Besides being easier for NTP clients to track, a slow smear has the
advantage that it limits the smear-induced error in interval
measurements to 11-14 ppm.  We know this is a reasonable amount,
because it's within the manufacturing and thermal errors of most
machines' quartz oscillators, and well under the 500 ppm slew rate
promised by NTP.

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