Re: Predictable Internet Time

Joe Touch <touch@isi.edu> Tue, 03 January 2017 18:26 UTC

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Subject: Re: Predictable Internet Time
To: Tony Finch <dot@dotat.at>
References: <CAMm+LwgfQJ8aG5wB=d3fRbbeje3J9o7Z4_DCuP8DL88ouDeKzw@mail.gmail.com> <504e2cea0d1668c31486b05fec0a967a4446aefe@webmail.weijax.net> <CAMm+Lwi_jU6gjdtdM6a2n_9_89tUvWBNXxnMtSjTEA++h1D4Ew@mail.gmail.com> <e0a43370-751f-808c-3719-9716f9cd57d1@isi.edu> <alpine.DEB.2.11.1701031348430.7102@grey.csi.cam.ac.uk>
From: Joe Touch <touch@isi.edu>
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Date: Tue, 3 Jan 2017 10:25:48 -0800
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We really need to separate the frames of reference of time - there's no
need for smear for internal "seconds since epoch" time. Smear only
renders UTC values incorrect,, which is an invitation to have financial
and legal transactions fail and report local events (e.g., rates, etc.)
incorrectly.

We shouldn't propagate an incorrect solution.

Joe


On 1/3/2017 6:08 AM, Tony Finch wrote:
> 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.