[ntp:questions] NTPD in Broadcst mode

Charles Elliott elliott.ch at verizon.net
Wed Oct 17 12:29:09 UTC 2012


Hello:

                There is a way of hugely improving the accuracy of NTPD when
it is run in broadcast mode.  It is, in short, to increase the rate  of
broadcast and reception of NTPD time packets.

 

                My configuration is one server (platform is 3 GHz Intel
Pentium4/Server 2003 doing nothing but DNS and NTPD (version 4.2.7p273)
connected to 9 Stratum 2 external time servers) and four clients.  The
server has this line in ntp.conf:

 

broadcast 224.0.1.1 minpoll 4 maxpoll 4

 

The client runs Windows 7 on a Core i7 @3.24 GHz and NTPD (version 4.2.7p287
from Dave Hart's website)  in broadcast mode and has this line in ntp.conf:

 

multicastclient 224.0.1.1 minpoll 4 maxpoll 4

 

               Every client has two Fast Ethernet network interfaces
(192.168.1.NNN (no Internet access and for data only) and 192.168.0.NNN
(with internet access)).  So, the NTPD client must be seeing two NTP packets
every 16 ms.  Also, all this client does for work is process Seti at Home work
units on two GPUs; CPU utilization is between 0 and 2%.

 

                This client experiences offsets from true time in the range
of about +- 5 milliseconds (ms), but most often the offset is +- 2 ms from
true time.  The client's offset from the server is usually sub-millisecond;
presently the instantaneous offset is  -0.032 ms, but I have seen
client/server offsets as high as 4.672 ms.  The minpoll and maxpoll key
words in the client's  ntp.conf  may  cause an error message to be sent to
the Windows Event Log, but NTP protocol packets are still processed every 16
ms.  The client's offset from true time depends heavily on what transmission
delay between server and client NTPD experiences on startup.  This
transmission delay will be  lowest if the system waits until the machine is
quiescent after a reboot before starting NTPD.

 

Charles Elliott

 



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