[ntp:hackers] What to do when the offset is WAYTOOBIG

Judah Levine jlevine at boulder.nist.gov
Tue Apr 17 07:20:18 PDT 2007

    I agree that having the software exit on this condition is probably
the wrong thing to do. The NIST time servers use my own LOCKCLOCK
algorithm for synchronizing the clock, and that algorithm partitions
this condition into 3 possibilities:

     1. A failure of the network or the remote host or the 
measurement process.
     2. A time step of the local clock
     3. A frequency step of the local clock

The first action is to immediately initiate a query to another time source,
if one is available. Unlike the standard NTP, my algorithm queries only
one time source on each calibration cycle and evaluates the response
based on the size of the correction that it implies for the local clock.
This checks possibility 1. Possibilities 2 and 3 are distinguished by
delaying for a short period of time and initiating a second query. I
can talk about the details if anyone is interested or you can find this
stuff in my papers. However, the point is that the software never exits.

If the program is unable to decide what to do then it sets itself to
unhealthy and waits for better times. (This is also a difference with
the standard version of NTP. My understanding is that the standard
version will never set itself to "clock unsynchronized" once it is up
and running.)

Best wishes,

Judah Levine
NIST and University of Colorado

Judah Levine
Time and Frequency Division
NIST Boulder

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