[ntp:questions] Remaining synced on an unsynchronised peer?

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Wed Dec 2 21:50:24 UTC 2009

michael.buetow at comsoft.de wrote:
 > It seems to me that if "prefer" was used, even a
 >  "freewheeling" ntpd that's lost all upstream reachability
 >  will still serve the time.
 > Ouch, I didn't know "prefer" would do this...

The prefer Peer
"In the prefer scheme the clustering algorithm is modified
   so that the prefer peer is never discarded"

"Ordinarily, the combining algorithm computes a weighted average of
    the survivor offsets to produce the final synchronization source.
   However, if a prefer peer is among the survivors, the combining
    algorithm is not used. Instead, the offset of the prefer peer
       is used exclusively as the final synchronization source."

Mitigation Rules
"As the selection algorithm scans the associations for selectable
   candidates, the modem driver and local driver are segregated
   for later, but only if not designated a prefer peer.  If so
   designated, a driver is included among the candidate population."


"If the prefer peer is among the survivors, it becomes the system
   peer and its clock offset and jitter are inherited by the
   corresponding system variables. Otherwise, the combining
   algorithm computes these variables from the survivor population."

The minsane Option
"The minsane option specifies the minimum number of survivors
   required to synchronized the system clock."

"minsane Specify the number of servers used by the selection
   algorithm as the minimum to set the system clock.
  The default is 1 for legacy purposes; however, for critical
   applications the value should be somewhat higher but less
   than minclock."

  That seems to cover it?

FWIW, I'm currently using tos minclock 6 minsane 4 cohort 1

"The minclock threshold is used by the clustering algorithm
   as it casts off outlyer servers until the total remaining
   is not more than this value.  At the moment, minclock
   defaults to three mostly for historic reasons.  From
   Byzantine agreement principles, it really should be four.

  The interesting threshold is minsane, which is the minimum
   number of survivors necessary to declare the client
   synchronized.  It defaults to one in the interest of fast
   synchronization, but really should be something higher
   like four, assuming that number of servers can always
   be found."

"To increase your ability to detect and eliminate
   falsetickers (especially during startup), you should use
   the following configuration command in your /etc/ntp.conf
   file: tos minclock 4 minsane 4"

Towards a resilient NTP configuration in NTP4

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