Re: [Ntp] Antw: Re: Antw: Re: Antw: [EXT] NTPv5 Loop Detection without Stratum

[Heiko Gerstung <[e-mail]>]

Am 31.08.22, 10:41 schrieb "ntp im Auftrag von Miroslav Lichvar" <[e-mail] im Auftrag von [e-mail]>:

>  On Wed, Aug 31, 2022 at 09:52:01AM +0200, Ulrich Windl wrote:
> > I think that is not sufficient: It would be a loop if the server's clock
> > (actually the error estimate) gets worse again, and then the process continues
> > (clock is getting more inaccurate over time). However it it's "smoothed out"
> > it's not a loop IMHO.

> The impact of the loop depends on how much weight the source has,
> polling intervals, number of servers in the loop, and other things.
> Even if the clocks can stay close to the true time, there is still a
> negative impact on stability.

I agree. A timing loop is by (my) definition a situation where a clock servo uses a reference which directly or indirectly is using the output of said clock servo. Even if the input source is only one of multiple sources and even if - in a weighted scenario - it is providing a small contribution. I agree with the statement that, no matter how small the contribution is, it is going to have a negative impact. 

> When the conditions are right, the loop can amplify oscillations. In
> this example (which I have shown before) there are 4 ntpd servers
> using the same source and also synchronizing in a A->B->C->D->A loop.

> https://fedorapeople.org/~mlichvar/ntpresponse/sync-loop.png

> Do you blame the user for misconfiguration? Or would you prefer if the
> protocol enabled the servers to detect the loop?

Yes, it can happen that something changes upstream which is not under the control of the server admin for "A", maybe the admin for "C" decided to switch sources and use "D" now. You can blame both for not talking to each other, but I still see value in helping users to avoid this scenario in an automatic, self-healing way. 


Heiko

> > > In the graph theory, it's a loop in a directed graph.
> > 
> > The problem is the "line" (correpsonding to time): In NTP it's a combination
> > of several sources (lines).

> Which means the loops can be ignored?


>  -- 
> Miroslav Lichvar

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