Re: [chrony-users] Root delay and Root dispersion values in `chronyc ntpdata'

[Chris Perl <cperl@xxxxxxxxxxxxxx>]

On Mon, Jun 26, 2017 at 4:37 AM, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
> The root delay and dispersion fields printed by the ntpdata command
> are the values from the received packet. They should be the same as
> printed by tcpdump. Can you post tcpdump -v -x output?

I guess I must have confused myself somehow.  Looking at tcpdump again
does indeed show that the values coming from the server to the client
are 15us for both "Root delay" and "Root dispersion."

> The reason why they are always 15 microseconds is that the fields have
> a 32-bit fixed-point format with ~15 microsecond resolution and
> chronyd as a server rounds them up. So, if it calculates its delay and
> dispersion as 1 microsecond, they will still be rounded to 15
> microseconds. It's a limitation of the NTPv4 protocol. I'd like to
> improve this in NTPv5 when the NTP WG starts working on a new version.
>
> If you wanted to get a more accurate root distance on the client, you
> could set the delay of the SHM refclock on the server to 0 and add
> half of the delay to the precision instead. I think it might have a
> small effect on the timekeeping performance though.

That is very helpful.  Thanks!

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