Re: Predictable Internet Time
2017-01-03 12:25:52
On 03/01/2017 17:42, Phillip Hallam-Baker wrote:
Agree 100%
Hence my proposal for supporting multiple time scales for different
purposes:
1) TAI: Use this and only this for any and all purposes that involve
recording the time an event took place. Including forensic and
scientific purposes.
2) PIT: Use this for inter-machine communications. It may also be used
to present TAI in a human readable form because the mapping from TAI
to PIT is fixed.
It would, in my view, be better to use TAI for all inter-machine
communications, and then allow applications that care, including
non-scientific display of time to run the PIT algorithm locally.
There was a draft that I think Yaakov Stein and I wrote in the early
days of TICTOC distinguishing transferred time from presentation time.
As far as I can see the only application that needs the PIT adapted
version of time is an astronomers wall clock, after all most humans live
with an error of an 0..2 hours between local astronomical time and the
time-zone time, and most machines would be quite happy to live with what
ever time they are using with no further leap seconds.
- Stewart
3) Local Time Zones: For human display purposes
From the conversation it seems that the best definition for PIT would be
PIT = TAI + Smear ( Lag (UTC, 50 years ))
On Tue, Jan 3, 2017 at 12:27 PM, Stewart Bryant
<stewart(_dot_)bryant(_at_)gmail(_dot_)com <mailto:stewart(_dot_)bryant(_at_)gmail(_dot_)com>> wrote:
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(_at_)isi(_dot_)edu <mailto:touch(_at_)isi(_dot_)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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- Re: Predictable Internet Time, (continued)
- Re: Predictable Internet Time, Stewart Bryant
- Re: Predictable Internet Time, Pete Resnick
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Stewart Bryant <=
- Re: Predictable Internet Time, Joe Touch
- Re: Predictable Internet Time, Stewart Bryant
- Re: Predictable Internet Time, Tony Finch
- Re: Predictable Internet Time, Joe Touch
- Re: Predictable Internet Time, Tony Finch
- Re: Predictable Internet Time, Joe Touch
- Re: Predictable Internet Time, Tony Finch
- Re: Predictable Internet Time, Joe Touch
- Re: Predictable Internet Time, Tony Finch
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