[ntp:questions] NTP vs RADclock?

Julien Ridoux julien at synclab.org
Sat Jun 9 04:11:23 UTC 2012

On Friday, June 8, 2012 12:43:12 PM UTC+10, David L. Mills wrote:
> Julian,
> Thanks for the paper reference. Your ideas on feed-forward are similar 
> to the ideas in Greg Troxel's MIT dissertation. These ideas were 
> partially implemented in NTPv3 a very long time ago.
> There are some minor misinterptretations in the paper. The NTP 
> discipline loop is not critically damped; it is purposely underdamped 
> with a minor overshoot of a few percent in order to improve the 
> transient response. The impulse response was slavishly copied in the 
> microkernel and nanokernel code that left here over a decade ago. The 
> microkernel survives in Solaris and the nanokernel in most BSD systems 
> with varying degrees of fidelity; however, many systems have elected to 
> modify the original BSD tickadj semantics, which result in an extra 
> pole. The result is a moderate instability at the longer poll intervals, 
> especially if the step threshold is increased or eliminated. In any 
> case, the response has no serious misbehavior as the paper described. 
> Note that in no case are the daemon and kernel algorithms cascaded as 
> the paper implies. Either one or the other is used, but not both.
> The system behavior with multiple servers is indeed as the paper 
> suggests, but there is considerable algorithm horsepower to diminish the 
> effects, including the cluster  and combine algorithms, plus the 
> anti-clockhop and prefer peer mechanisms. These provisions were first 
> implementd in the Internet of twenty years ago when the congestion on 
> overseas links frequently reached over one second. Perhaps today these 
> algorithms could be more carefully tuned for LANs and even wifi netorks.
> As the paper describes, NTP algorithms are designed for traditional 
> mathematical analysis, but with both linear and nonlinear components. 
> However, the FLL algorithm is based on a model described by Levine as 
> predictive. The model  in the documentation describes both the PLL and 
> FLL in predictive terms, but that doesn't change the conclusions in the 
> paper.
> The paper suggests possible improvements in data filtering and analysis. 
> The clock filter and popcorn spike suppressor algorithms in NTP 
> represent one approach. A persistent observation is that NTP does not 
> effectively use offset/delay samples other than at the apex of the 
> scattergram. While it does indeed do that for the huff-n'-puff fiilter, 
> the possible improvement in other cases is problematic. The paper does 
> not mention the implications of roundtrip delay in the maximum error 
> statistic, such as in Cristian's model, as used by NTP. It is a natural 
> error bound for asymmetric paths such as mentioned in the paper.
> In summary, the NTP algorithms have evolved over thiry years in response 
> to major changes in Internet service models and surely could use some 
> further evolution. I am glad there is continuing interest in improvements.
> Dave

Dear David,

Thanks a lot for mentioning Greg Troxel's dissertation, I was not aware of
it and will read it avidly.

I really appreciate your comments and details you provided on ntpd design.
I believe this is valuable information for readers interested in our paper, and
I am willing to add your comment to our webpage where the paper is 
referenced. Would you agree to this?


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