[ntp:questions] NTP roadmap (was Re: Poul-Henning Kamp and re-write of NTP)

William Unruh unruh at invalid.ca
Tue Dec 9 05:24:22 UTC 2014

On 2014-12-09, skillzero at gmail.com <skillzero at gmail.com> wrote:
> On Sunday, December 7, 2014 7:25:01 PM UTC-8, Harlan Stenn wrote:
>> Rob writes:
>> > David Woolley <david at ex.djwhome.demon.invalid> wrote:
>> > > It starts off a bit badly by talking about the size of nptd, when, in 
>> > > fact, very little of ntpd is actually involved in the core algorithm. 
>> > > Most of it, I guess, is reference clock drivers, plus configuration and 
>> > > monitoring code.
>> > 
>> > It has been discussed before that reference clock drivers should be
>> > loadable modules or even separate processes.
>> A complete rewrite of the full NTP software is slated for post-4.2.8.
>> We're expecting (the names might be changed):
>> - tsc - Time Sync Client: a leaf-client.  PHK is working on this.
> Are there any plans to incorporate ideas from RADclock? The PLL that PHK mentions on his site implies a feedback model. I'm wondering if any thought has been given to a feed forward design (e.g. determine frequency based on raw ticks). Or was that model considered and rejected?

Why would one want feedforward? You surely want the system to correct
the errors in the local clock, and those are not constant or
predictable. Temperature, crystal aging, maybe even computer load and
voltages will all affect the clock in undetermined and undeterminable

It would seem to me that the key parameter is the reach of the
feedback-- does one only use the current error to correct the system, or
does one use a longer stretch of errors to correct the system. The
latter allow one at least in part to separate the random influences from
the uniform errors-- the random errors get knocked down by the averaging
of the fluctuations. Exactly which is best for time control still needs
more study, but as mentioned, in the comparison of ntpd (PLL) and chrony
(regression) it seems as if chrony wins in its speed of response to
outside influences and in its ability to control the clock to track UTC.

But I guess we all have our favourites.

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