[ntp:questions] [ntpwg] Idea to improve ntpd accuracy

Miroslav Lichvar mlichvar at redhat.com
Fri Feb 26 15:19:26 UTC 2016


On Fri, Feb 26, 2016 at 02:21:47PM +0100, Martin Burnicki wrote:
> Tal Mizrahi wrote:
> > Sounds a bit like interleave mode, right (a bit similar to PTP two-step clocks)?
> 
> But the problem is still that (again, AFAIK) NTP interleave mode works
> only in broadcast mode.

It works also in the symmetric mode.

> In broadcast mode ntpd only tries to measure the packet delay when the
> association is first created, but the network route etc. can change at
> any time, so IMO interleave mode provides only limited benefit.

It probably wouldn't be difficult to modify ntpd to run the delay
measurement periodically.

A bigger problem is that the delay is measured using the client/server
mode packets, for which there is no interleaved mode. If the delay is
inaccurate, the offset is inaccurate too, even if the tx timestamp in
the broadcast packet was accurate.

Broadcast clients would need to create ephemeral symmetric
associations with the server in order to measure the delay accurately.

-- 
Miroslav Lichvar


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