[ntp:questions] NTP vs RADclock?

unruh unruh at invalid.ca
Wed Jun 6 16:26:54 UTC 2012


On 2012-06-06, Julien Ridoux <julien at synclab.org> wrote:
> On Wednesday, June 6, 2012 4:41:59 PM UTC+10, unruh wrote:
>> On 2012-06-06, Harlan Stenn <stenn at ntp.org> wrote:
> Dear unruh (Dr. William Unruh?),
>
> It seems there is a little misunderstanding, I hope you won't mind some clarification.

Of course not. 
>
> The article you keep quoting is a high level description of the work, and we have worked with ACM Queue editor and reviewers to provide an overview easy to grasp for the non-expert. This was what ACM Queue was after. Please view this article that way and not as a scientific paper. I believe you are well aware of synchronisation issues, and I understand your reaction, but again, you were not the targeted audience (and for what it is worth, yes, we also have cheap GPS receivers on our office window that took less than 15mn to set up).

But high level descriptions should not be puff pieces. 

>
> I would encourage you to read the scientific papers we published. I am confident you will see the level of hype being much lower. The description of the algorithm you are requesting can be found in several papers. Repeating myself, the main one being : ""Robust Synchronization of Absolute and Difference Clocks over Networks". It does describe the algorithm in more details. It may not deliver all the details you are after (or be slightly outdated), but the code publicly available is also there for you to read. It is the ultimate reference after all.
>

It would be nice to have a link to that publication, in particular not
not one where you have to buy it. 
And asking people to read the code is a cop out. I have tried hard to
read the ntpd code for example, and the chrony code. While the latter is
slightly (only sightly) better than the former, both are a real pain to
figure out what is going on. Reading code is always a case of far fr too
many trees for to see the forest. 

For others, I have found a copy at 
http://www.cubinlab.ee.unimelb.edu.au/~darryl/Publications/synch_ToN.pdf



> It was not my intention to describe the entire algorithm in my previous message (it has been done in publicly available papers), but instead give some pointers and general answers to some of the questions raised earlier. My message did not intend to sparkle a heated discussion but instead try to provide honest answers to this group.

I am very confused by your graph comparing ntpd to radclock. The ntp has
huge oscillations while from what I have read it is critically damped
while what I see is a pretty high Q (of the order of 10 or so) in the
graphs. Things I have not seen in my looks at ntpd. (Ntpd does have
problems which I have spent time pointing out in the comparison with
chrony) but oscillations like that I have not seen. Is this really ntpd
or some stripped down testbed?

 
>
> I am happy to receive criticism from people who have interest in improving computer clock synchronisation. Please accept my apologies if my message offended you, I am sure we can pursue this discussion over a more private channel in a constructive way.



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