[chrony-users] Searching for reasoning on design decisions

[Julian Steinmann <jsteinmann@xxxxxxx>]

Hi,

I'm a bachelor student at ETH Zurich and am currently working on my 
bachelor thesis, which is about helping in implementing a new clock 
synchronisation approach called G-SINC.
G-SINC (https://github.com/marcfrei/scion-time, paper linked in README) 
focuses especially on byzantine fault-tolerance and builds on a new 
internet architecture called SCION.

Right now I am focusing on implementing the sample offset interpolation 
algorithm. We have already implemented a PLL-based algorithm, taken from 
Ntimed by Poul-Henning Kamp and are exploring an approach using the 
Theil-Sen estimator, but are also looking at re-implementing the 
existing algorithms in chrony and ntpd.
In all of this, chrony is basically our "gold standard" when it comes to 
accuracy and design approaches.
However, I haven't found much reasoning why e.g. weighted linear 
regression is used in chrony for NTP samples, whilst a robust linear 
regression (Least Absolute Deviations) is used for RTC samples and 
manual input (I hope I'm not mistaken in my reading of the code).

Is there some place where these design decisions are justified? For 
ntpd, there is of course the Computer Network Time Synchronisation book, 
but David L. Mills seems to reach different conclusions than chrony does...
I hope I haven't overlooked some obvious place to search; otherwise, I'm 
very sorry for bothering the list.

Best regards,
Julian


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