Re: [Ntp] Smearing

[Miroslav Lichvar <[e-mail]>]

On Tue, Oct 19, 2021 at 05:15:16PM +0100, Tony Finch wrote:
>   * 24h smear, not too long but long enough that NTP clients will notice
>     the rate change even with a 1024 second polling interval, and the rate
>     change is small enough that NTP clients are willing and able to follow it

If there are stricter requirement on maximum offset between different
clients, a longer interval might be needed.
> 
>   * smear from midday to midday around the leap second, to keep the phase
>     error below 0.5 seconds

A disadvantage of this approach is that it relies on the leap second
to be known at least 12h before the leap. With some reference clocks
it's known only much closer to the leap and it takes time to be
distributed by NTP for a stratum >=2 server.
> 
>   * use a linear smear, because NTP clients are better at following
>     constant rates (a cosine or other smoothing means the rate changes
>     continuously during the smear, which NTP does not handle so well)

A linear smear is easy to work with for humans, but not for NTP. A
cosine smear, or even better a quadratic smear, reduces the maximum
time error of the client's clock as it can more closely follow the
frequency offset.

-- 
Miroslav Lichvar