[ntp:questions] prefer keyword and server failover

David Woolley david at ex.djwhome.demon.co.uk.invalid
Mon Apr 28 06:39:03 UTC 2008

Danny Mayer wrote:
> David Woolley wrote:
>> Richard B. Gilbert wrote:
>>> Sort of!  Two servers is generally a poor choice.  If they differ, as 
>>> they inevitably will, which one should you believe?  'Prefer' will tell 
>> If they differ enough for voting to apply, both will be rejected.  If 
>> they are mutually true chimers and you don't use prefer, ntpd will use a 
>> combination of the time from both of them.
> No, it will choose one and use it until it decides that it is worse than 
> another one or they are all declared bad. It never combines both. The 
> real problem here is that it has no idea whether or not one is better 
> than the other when you only have two of them.

For RFC 1305, and I have no reason to believe this has changed, given m 
  servers that pass the basic sanity check, it will start with another 
parameter, f, at 0 and try to find m -f intersecting servers.  It will 
try with successively greater values of f until it finds a mutually 
intersecting set of servers. However, if f equals or exceeds m/2, it 
will stop with all servers marked as falsetickers.  m/2 = 1 in this 
case, so the termination condition will be met after one iteration, and 
it will fail to find any truechimers (the low end of the range will be 
set to the low end of the second server tolerance (search is done 
forwards) and the high end of the range will be set to the high end of 
the first (search is done backwards).

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