[time-nuts] An embedded NTP server

Rob Kimberley robkimberley at btinternet.com
Wed Dec 26 10:29:36 UTC 2012


Which version of Meinberg's NTP build did you install? I've had problems
with high jitter content on one of them.


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Michael Tharp
Sent: 25 December 2012 20:44
To: time-nuts at febo.com
Subject: Re: [time-nuts] An embedded NTP server

On 12/25/2012 02:51 PM, David J Taylor wrote:
> I think you should be able to do better on the jitter as your 
> algorithms develop.

Yes, for starters something is causing a silly amount of extra latency hence
the 2.4ms round-trip. I managed to cut that in half by changing compiler
options but talking to someone using the same stack I should be able to get
it down to ~ 400us round-trip, at worst. I have some ideas on how to achieve
that but I want to get all the functionality working before I start

The main lack of functionality, which may or may not be the cause of the
jitter, is that I haven't actually integrated the PLL/disciplining stuff
yet. I just have a timer that resets with each GPS pulse, and runs at
whatever nominal frequency the oscillator has. Not great but it succeeded at
getting results fast. Not up to time-nuts standards though. 
Now I'm going back and adding the PLL.

> "SNTP" does imply something else, so perhaps avoid calling it that, 
> although if it doesn't respond to all the NTP management calls I'm 
> unsure what it should be called!

I suppose that I could call it NTP and still sleep at night since, while not
a traditional NTP implementation, it does still feature control loops
similar to those found in purely software-based NTP. I have not yet looked
at what administrative/query functionality I could implement. 
Some of it like listing peers does not make sense but perhaps other commands
could be mapped to make it more complete. I would also like to expose some
of the internal statistics via SNMP or some other means for graphing.

> Oh, and your Windows box is either poorly configured or 
> broken - they can work much, much better than yours illustrates as I 
> expect you know.

Yes, it's not healthy. I installed Meinberg's NTP build and configured it
using defaults other than the server list but it seems to not be tracking
well. Right now it's "only" 30ms off but as shown in the forum snippet it
makes excursions into the hundreds. Still an improvement over default
Windows timekeeping but I'd love for it to work properly.

-- m. tharp

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