Re: Predictable Internet Time

Tony Finch <dot@dotat.at> Tue, 03 January 2017 14:08 UTC

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Date: Tue, 3 Jan 2017 14:08:21 +0000
From: Tony Finch <dot@dotat.at>
To: Joe Touch <touch@isi.edu>
Subject: Re: Predictable Internet Time
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Joe Touch <touch@isi.edu>; wrote:
>
> Smearing leads to differing interpretations of elapsed time for two reasons:
>
> 1) smearing isn't unambiguously specified
> 2) smearing doesn't match the clock standards set by the ITU (who
> defines UTC)

Since leap smear is becoming more popular, it would be sensible to try to
get a consensus on the best way to do it if you do it. Clearly
organizations that do leap smear think (2) leap seconds are too much
trouble so it's better to diverge from official time in a controlled
manner.

To clear up (1) there are a few technical choices on which people seem to
be working towards some kind of agreement...

* If you centre the smear period over the leap second, your maximum error
  from UTC is 0.5s, which seems to be preferable to starting or ending the
  smear period on the leap second

* Linear smear works better than sigmoid smear, since it minimizes the
  rate divergence for a given smear period, and NTP's algorithms react
  better

* Longer smear periods are better, because they give NTP more time to
  react to the rate change, and they minimize the rate difference

It looks to me like a 24h leap smear from 12:00 UTC before the leap to
12:00 UTC after the leap has a good chance of becoming more popular than
other leap smear models.

Tony.
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