[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)

Unruh unruh-spam at physics.ubc.ca
Thu Jan 24 23:33:58 UTC 2008


"David L. Mills" <mills at udel.edu> writes:

>Guys,

>Sure, I'm stubborn as a bull. The laws of physics make me so.

I happen to be a physicist. 

>I am dismissing any comparisons between ntpd and crony or any other 
>vehicle unless the comparison includes substantially all the scenarios 
>that ntpd is designed to work with. The protocol is specifically 

That is fair enough. As I have said, these are one set of experiments. 


>designed to work over a wide spectrum including lightly loaded LANs and 
>highly congested WANs. The choice of parameters, specifically the time 
>constant and operating range, was chosen as a compromise to maximize 
>accuracy and minimize network loads under typical and extreme conditions.



>As for the SNTP restrictsions, please, please read the draft 
>specification, which explains exactly what SNTP should and should not 
>do. At the crux of the matter is the impulse response of a cascade of 
>intervening servers each with its own idiosyncratic impulse response. 
>The NTP impulse response has a controlled risetime and overshoot over a 
>wide range of time constants. Each server in the cascade must have the 
>same impulse response to avoid instabilities and possible whip effects.

????  If there were a feedback effect-- A synced to B synced to C synced to
A I could see this. I have no idea what these instabilities would be
otherwise unless you did not really want to use the term instability.

>We could have simply specified the transfer function in polynomial form 
>(it's in RFC 1305 and das Buch) and told the implementor to use that. A 
>student of digital signal processing would know how to use that 
>directly. But, we thought there would be folks like you that would not 
>believe the principles and do something evil like bring up a pool server 
>running openntp or crony and synchronized via a flaky circuit to Indonesia.

Like me? Do you really want to start ad hominem attacks? What evidence do
you have that I wanted to do anything like that? Or is libel and slander
part of your tools of argument?

It would also be useful if you actually gave a reference as to where in the
article you are refering to.



>It is easy to detect that a particular server has or has not the current 
>reference implementation. There are a number of features intrinsic to 
>the protocol design and others fiendishly crafted to do that, but I'm 
>not going to reveal them here.

Ooooooo. And do they include sacrificing goats on some altar at midnight on
Midsummer's day? 

>Dave




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