Re: [LEAPSECS] An immodest proposal

From: M. Warner Losh <imp_at_BSDIMP.COM>
Date: Tue, 14 Feb 2006 15:44:14 -0700 (MST)

In message: <20060214222446.GA8989_at_feynman>
            Neal McBurnett <neal_at_bcn.boulder.co.us> writes:
: > UTC time stamps in NTP are ambiguous. TAI ones are not. UTC time
: > stamps do not convey enough information to properly implement things
: > like intervals, while TAI ones do. The NTPNG stuff that I've seen
: > appears to consider these problems as worthy of needing a solution and
: > they plan on solving them. It isn't rocket science, but one has to
: > divorce ones self from the chauvinistic view that UTC is always best.
: > For time exchange, it is not the best, and has many problems around
: > the edges.
:
: Yes it is messy, but the tradeoffs are complex. I don't think the
: latest drafts specify NTP timestamps. I suspect they still rely on
: the leap second bit to disambiguate the timestamp on a leap second,
: but I haven't checked recently. They are all linked to from the
: charter page I noted above.

But ntp time stamps to not have a leap second bit. To be precice, it
is a leap indicator that's in the packet. This makes it impossible to
represent times that straddle the leap event, since there are 4
different time stamps involved... I guess in those cases, the packets
should be discarded and tried again later...

But you are right. I did misremeber that it used TAI time. The
drafts still specify time in UTC. I could have sworn I saw something
that said otherwise, but I can't seem to find it now.

: > Doing NTP with TAI (and the implied requirement for DTAI) doesn't
: > change what time is displayed for users. It does make it *MUCH*
: > easier to get leap seconds right for those users that need them.
: > Anything else is madness. UTC is a display convention, not a sane[*]
: > counting convention.
:
: I think that changing to a different over-the-wire timestamp epoch
: would just add to the confusion, not make things simpler in practice.

TAI is the same epoch as UTC. It is just not adjusted for leap
events. However, your point that the second numbering would be
different and might cause confusion is still valid.

: People still need to know UTC, and transmitting the leap second table
: info, especially via autokey, is much more complex than the basic
: protocol. But at least this is a standards process conducted in the
: open, where you can just get involved directly if you have something
: to add.

Autokey is interesting, but it is way too complex for something as
simple as leap second propigation....

Warner
Received on Tue Feb 14 2006 - 14:48:30 PST