Re: Predictable Internet Time

Michael Breuer <michael.breuer@ilsf.de> Tue, 28 March 2017 19:09 UTC

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Subject: Re: Predictable Internet Time
To: Joe Touch <touch@isi.edu>, =?UTF-8?B?UGF0cmlrIEbDpGx0c3Ryw7Zt?= <paf@frobbit.se>
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From: Michael Breuer <michael.breuer@ilsf.de>
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Hej Joe

On 03/28/17 19:47, Joe Touch wrote:
> Hi, Patrik,
> 
> 
> On 3/27/2017 11:37 PM, Patrik Fältström wrote:
>> Joe,
>> 
>> I have read your I-D and like it! Let me start there :-)
>> 
>> What I think is not clear enough is the problem with POSIX, and it
>> should be more clear in some place, maybe section 6.1, that POSIX
>> definition which is in use in quite a number of systems do not
>> handle leap second very well. Too many do believe the time_t
>> definitions include the number of seconds since epoch when in
>> reality it does not (as you note in the definitions).
> 
> I can make that more clear, but AFAICT POSIX time is *defined* as 
> seconds since the Unix epoch *not counting* leap seconds at all.
> 
>> One could even question whether it is Continuous as two seconds
>> will have the same number since epoch around the addition of a leap
>> second
> It is Continuous by definition.
> 
> When UTC adds a leap second, nothing different happens to POSIX
> time.
> 

That's a quite complex topic to find precise wording. If the last
statement would be right, wouldn't this be a contradiction to your
earlier statement, POSIX time be defined *not counting* leap seconds?
AFAIK POSIX time is continuous and not counting leap seconds. So when
UTC adds a leap second, something quite different happens to POSIX time.
It will change the leap-second-offset between UTC and Unix time.

To my opinion, any time system utilising leap seconds isn't continuos at
all. There are continuous time systems like TAI or GPS system time (or
any other satellite navigation system time) and POSIX time, which (by
design) shift away from UTC over time.

Michael

> ...
>> There are some people that have suggested a change, for example
>> <http://www.cl.cam.ac.uk/~mgk25/time/c/> but I have not seen any
>> movement. Maybe you know more than me on this?
> AFAICT, that's just attempting to redefine the <time.c> interface as 
> returning UTC rather than POSIX time.
> 
> Which works *IF* your machine has access to updated leap-second 
> information, e.g., via NTP.
> 
> Joe
> 
>> On 27 Mar 2017, at 21:34, Joe Touch wrote:
>> 
>>> Hi, all,
>>> 
>>> I've submitted the time frame discussion intended to resolve this
>>> issue, which also recently arouse on another mailing list.
>>> Further discussion on this draft will occur on the ART mailing
>>> list (art@ietf.org).
>>> 
>>> Joe
>>> 
>>> -----------
>>> 
>>> A new version of I-D, draft-touch-time-01.txt has been
>>> successfully submitted by Joe Touch and posted to the IETF
>>> repository.
>>> 
>>> Name:		draft-touch-time Revision:	01 Title:		Resolving Multiple
>>> Time Scales in the Internet Document date:	2017-03-27 Group:
>>> Individual Submission Pages:		17 URL:
>>> https://www.ietf.org/internet-drafts/draft-touch-time-01.txt
>>> Status:
>>> https://datatracker.ietf.org/doc/draft-touch-time/ Htmlized:
>>> https://tools.ietf.org/html/draft-touch-time-01 Htmlized:
>>> https://datatracker.ietf.org/doc/html/draft-touch-time-01 Diff:
>>> https://www.ietf.org/rfcdiff?url2=draft-touch-time-01
>>> 
>>> Abstract: Internet systems use a variety of time scales, which
>>> can complicate   time comparisons and calculations. This document
>>> explains these   various ways of indicating time and explains how
>>> they can be used   together safely. This document is intended as
>>> a companion to   Internet time as discussed in RFC 3339.
> 
>