[ntp:questions] high precision tracking: trying to understand sudden jumps

Heiko Gerstung heiko_removeme_.gerstung at meinberg.de
Wed Apr 2 14:32:12 UTC 2008

David Woolley schrieb:
> Heiko Gerstung wrote:
>> time has passed without the signal coming back. This results in the 
>> time server replying with stratum 12 (for example) after a while and 
>> ensures that everybody has the same time, although it might be wrong. 
>> If a user does not want that, they can simply set the local clock 
>> stratum to 15 and the server will not be accepted anymore.
>> Can you please let me know why you consider this a "bad implementation"?
> Because the protocol fails to signal the loss of the time source 
> properly when one has a local clock configured.  As such, I believe that 
> enabling a local clock should always be an opt in choice.  Basically, 
> when it falls back to the local clock, root dispersion goes to zero, 
> when the true situation is that root dispersion is growing without bound.

The signal is the higher stratum level, at least for a lot of SNTP 
implementations. Almost noone is looking at the root dispersion value when it 
comes to SNTP ...

In our web interface you can disable the use of the local clock reference 
completely. I always recommend to keep it active but set its stratum to 15, 
which should result in being rejected by any standards compliant client.

Running without the local clock ref means the server signals itself as being 
synchronized by a stratum 0 source (e.g. GPS) and only the root dispersion value 
is increasing. As I said, most embedded/SNTP-only software checks for the SYNC 
status and (sometimes) stratum level.

> Things can go seriously wrong if there is more than one local clock 
> source on a network, as it becomes possible for them to outvote the real 
> time.

Yes, but I would not go that far to say that offering the end user the choice to 
enable the local clock driver in his NTP appliance is a "bad implementation". I 
however can fully agree that there are a number of things that could go wrong 
when you use it (something that applies to a number of configuration options 
like tinker or restrict ...).


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