[tz] Google public NTP servers to 'smear' leap seconds.

John Hawkinson jhawk at MIT.EDU
Mon Dec 5 07:49:33 UTC 2016

Paul Eggert <eggert at cs.ucla.edu> wrote on Sun,  4 Dec 2016
at 23:33:05 -0800 in <5915ef83-dbde-9e26-814a-fe4bc7192d06 at cs.ucla.edu>:

> Although it's pretty common, we indeed don't have to say "often".

I'm not sure how we get to common. Even if we accept that the majority
of cloud service providers now perform some sort of leap smearing
(a point I doubt), that doesn't make it actually "common" in the Internet.
What is the gain from our publishing seemingly authorative information
from this experimental practice?

Anyhow, your patch only changes the Makefile, not the tz-link.htm file,
which is probably more important because HTML gets indexed and viewed
more. Would you like me to take a stab at that?

> >>as this is more accurate than strict POSIX time
> >
> >I don't see how the accuracy or precision
> With leap smearing, the ideal clock is at most 0.5 s off UTC, as
> opposed to being at most 1.0 s off with strict POSIX time.

Err...I think we would say that with a leap smearing, the clock is
KNOWN to be inaccurate, by up to 0.5 seconds, and a POSIX UTC clock is
exactly correct, except maybe during the one second of the positive
leap. Therefore smearing is less accurate, not more accurate. I guess
it might be more precise because POSIX's leap behavior might be deemed
as inaccuracy, but I don't know. These words are not well suited to this
(and therefore it's best not to use them).

> Given the popularity of leap smearing, and the fact that this
> Makefile section advises users whether to use "posix" or "right"
> time, the section should say something -- otherwise people are more
> likely to incorrectly choose "right" time on a system that uses leap
> smearing.

If people are choosing "right" on the basis of what's in there,
something is probably terribly wrong, and we make it too easy for them
to foot themselves in the shoot.

> >The biggest problem we have with leap smearing is that smeared time
> >is not clearly traceable.
> Yes, that would be a nice problem to fix somehow. Perhaps append
> "-A" to the time zone indication when the clock is using AWS-style
> leap smearing? And we could append "-P" for strict POSIX time (hmm,
> but that's the default, so perhaps not....). Just thinking out loud.

Paul Goyette would, I believe, ask me to say something here that asks
how the tz maintainers feel about making up abbreviations.

I don't think there is much merit to a shorter more cryptic suffix
versus a longer more verbose/explanatory one. Although the trick
would be to only print this extra information when the smearing was
in effect. Maybe that's too ambitious.

By the way, I hesitate to mention this, but Martin Burnicki from
Meinberg did a writeup comparing the effects of different smears
on ntp that may be of interest:
(Discussion of this on the leapsecs list, rather than here, please.)

Except I don't think we should really be influencing the smear
experiment at all. Maybe I'm an outlier here.


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