Re: Predictable Internet Time

Stewart Bryant <stewart.bryant@gmail.com> Tue, 03 January 2017 17:27 UTC

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Subject: Re: Predictable Internet Time
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From: Stewart Bryant <stewart.bryant@gmail.com>
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Date: Tue, 3 Jan 2017 17:27:21 +0000
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Smearing worries me.

If you have an application where tiny fractions of a second make no 
difference, then
a slow smear is a good approximation to no leap second.

However, there are some highly accurate implementations of NTP, and some 
highly
sensitive applications that use it, and having a long term interval 
error, which is what
happens during a smear, is harmful to those applications.

It seems to me that it might be better to freeze NTP on the current leap 
second
offset. Provide the current leap second offset to the application as a 
parameter
and let the application deal with it as it chooses.

- Stewart

On 03/01/2017 14:08, 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.