[ntp:questions] prefer keyword and server failover
mayer at ntp.isc.org
Mon Apr 28 11:38:16 UTC 2008
David Woolley wrote:
> 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).
It has changed. RFC 1305 describes NTPv3. You need to see Section 11.2
of the NTPv4 draft for the current selection algorithms. Particularly:
> If the selection
> algorithm cannot produce a majority clique, or if it cannot produce
> 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.
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