Re: [Ntp] Leap second draft

[Martin Burnicki <[e-mail]>]

Kurt Roeckx wrote:
> One question I have that I can't seem to find the answer for is
> which second do you announce during the leap second in NTP.
> 
> There was a leap second at 3692217600. Do you send:
> 3692217599
> 3692217599 (leap second)
> 3692217600
> 
> Or:
> 
> 3692217599
> 3692217600 (leap second)
> 3692217600
> 
> I can see arguments for both ways, and if it's not specified
> somewhere yet, I think we need to specify it.

NTP implementations I've seen just pick up the current system time, so
which timestamps are returned during a leap second depends on the way
the particular operating system handles the leap second, and e.g. ntpd
running on one type of operating system can behave differently than ntpd
running on a different type of operating system.

So the discussion which way is the "better" one had to be done with the
maintainers of the OS kernel clock implementation. ;-)

Letting an NTP daemon decide what kind of timestamp to return would
require that it

1.) uses the adjtimex()/ntp_adjtime() API calls to retrieve a timestamp
including some status information, and then modifies the time stamp in a
preferred way

2.) maintains its own clock beside the system clock so it could handle
the leap second in a different way than the kernel clock

Solution 1.) is normally not used because adjtimex()/ntp_adjtime() take
longer to execute than a simple clock_gettime()

Solution 2.) can cause additional problems if the daemon's internal leap
second handling differs from the leap second handling observed by
applications running on the particular system.


Martin
-- 
Martin Burnicki

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