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

David L. Mills mills at udel.edu
Thu Jan 24 21:07:41 UTC 2008


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

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 
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.

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.

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.


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