[ntp:questions] prefer keyword and server failover

David Woolley david at ex.djwhome.demon.co.uk.invalid
Mon Apr 28 21:21:27 UTC 2008


Danny Mayer wrote:

> It has changed. RFC 1305 describes NTPv3. You need to see Section 11.2 

As far as I can see, the intersection algorithm hasn't changed.

> of the NTPv4 draft for the current selection algorithms. Particularly:
> 
>> If the selection
>>    algorithm cannot produce a majority clique, or if it cannot produce

Note the "or" in the condition for rejecting everything; that's an "and" 
for not rejecting.  The CMIN check is done, rather later, and only if 
the absolute majority check succeeds.

As it happens, there is an undocumented option in the code of 4.2.4p4 
which allows one to force a peer to be a truechimer, even though it 
fails the intersection algorithm, but an ordinary users isn't going to 
find that option.

>>    at least CMIN survivors, the system process exits without
>>    disciplining the system clock.  If successful, the cluster algorithm
>>    selects the statistically best candidate as the system peer and its
>>    variables are inherited as the system variables.
> 
> CMIN is currently 1. Notice that it says nothing about using more than one.

All that means is that if you only have one clock that passes sanity, it 
will be used.  If you have two that pass sanity and they are 
incompatible, they will have been eliminated before it even gets to the 
CMIN test.

I'll leave it to Dave Mills to volunteer the undocumented option, if he 
thinks it is safe to use in this application.

(The use of orphan mode can also override the intersection algorithm.)




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