[ntp:questions] NTP behaviour when server is not synchronized

Richard B. Gilbert rgilbert88 at comcast.net
Fri Jun 29 19:12:19 UTC 2007

Geir G wrote:
> Hello,
> NTP configuration:
> Client: ntpd version 4.2.2 at 1.1532 running on Windows XP
> Servers: 2 identical stratum 1 servers with GPS antennas directly
> connected
> I want comments on the following observations. Unfortunately, the
> system is currently not available, hence I cannot supply ntpq
> diagnostics. I know that 2 servers is the worst possible
> configuration, it is not necessary to comment that.
> For some reason one NTP server lost its GPS synch. It switched to
> internal clock started reporting stratum 12 when this happened. The
> other server was perfectly synchronized and reported stratum 1.
> Problem: ntpd rejects both servers and considers itself not
> synchronized.
> Questions:
> 1. Does the NTP server behave correctly when changing its stratum to
> 12? 

Yes it does.  I'm assuming that the server was set up to use its local 
clock as a "clock of last resort.

> Should it perform different/other actions when it considers itself
> not synchronized?

What would you like it to do?  Set fire to itself?

> 2. Does ntpd behave correctly? Is a server's stratum used in any way
> in ntpd's clock selection process?

Yes it is.  Other things being equal, a stratum one server will be 
preferred over a stratum two server or a stratum three server.  A server 
at stratum 12 will NOT be used by any system that has a better server 
available; this is why a stratum of 10 or 12 is conventional for an 
unsynchronized local clock.  Note that a stratum one server can have 
extremely poor quality of time; a server synchronized to the WWV 
broadcast can be off by many milliseconds thanks to the vagaries of HF 
radio propagation.

Ntpd should not have rejected the server that remained synchronized at 
stratum 1 unless there was some other problem with it that you have not 
described.  The log files of the clients may reveal why the remaining 
stratum 1 server was rejected.

Situations such as you have described are the reason that a minimum of 
four servers is recommended.  Not all of the four need a hardware 
reference clock unless you insist on extremely accurate time; internet 
servers at stratum 1 or 2 will serve quite well.  OTOH, GPS timing 
receivers can be purchased for less than $100 US each so you can easily 
have as many stratum one servers as you think you will need.

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