Re: [Ntp] Timescales

[Miroslav Lichvar <[e-mail]>]

On Wed, Dec 09, 2020 at 02:18:30AM -0800, Hal Murray wrote:
> Given TAI, there are 2 pieces of information that a clients needs to handle 
> UTC and its leap seconds.  The first is the current TAI-UTC offset.  The 
> second is when the next leap second happens.  That data changes very 
> infrequently and with plenty of warning.  There is no need to clutter up 
> on-the-wire protocols with it.  A client could get it from a leap-seconds file 
> distributed by their distro.

Relying on a separate source like the leap-seconds file is not always
possible. The device may not have an up-to-date OS, or actually have
any writable memory to store that information.

There is definitely not a plenty of warning. Some reference clocks
give the leap announcement only an hour before it happens (e.g.
DCF77). Even delaying setting of the kernel leap status to the clock
update can be a problem if you are polling at 1024 seconds and have
more than two measurements dropped in a row in the clock filter.

-- 
Miroslav Lichvar