[ntp:questions] tool to collect statistics on round trip delay offset?

Richard B. Gilbert rgilbert88 at comcast.net
Thu Oct 26 01:30:00 UTC 2006

joe blow wrote:

> Are there any monitoring tools that allow you to collect statistics on the 
> round trip delay, and offset without using the data.
> On a small, self contained network, (using no routers, just cisco switches), 
> I have multiple servers that I want to be in sync to  < 1ms (200-300 usec 
> ideally). It is not so important how well these match to GMT, but it is very 
> important that they agree with each other.  They are used to time tag event 
> data that needs to be correlated across servers.
> I want to take some statistics on the measurement jitter (both offset and 
> round trip delay) of a typical NTP packet.  I envision a utility that would 
> send the UPD NTP packets, and be able to calculate delay and offset from a 
> specific server (but not use the data), but just collect them for analysis 
> later.
> I'm trying to assess how much variation there is in the NTP measurements.
> The standard out of the box NTP takes way too long to stabalize to 1 ms 
> offset.  I suspect that this is due to the time constants used to be able to 
> sync to typical NTP servers on the internet. Since my network will be closed 
> (and small - all in the same room), with only 2 NTP servers (one for 

This configuration is just begging for problems.  With but one server 
there is no question which to rely on!  With two, there is no way for 
ntpd to determine which of the two is more nearly correct.  Use either 
one server, four servers or five servers.  Four are necessary and 
sufficient to unambiguously vote out one bad server.  Five are necessary 
and sufficient to vote out two bad servers.

If you are going to measure jitter, it seems to me that you need a 
better clock than the server you are trying to measure else how would 
you know what you are measuring!!

Ntpd is a little slow to pull into tight synchronization.  You should, 
however, only have to do this once.  Unless, of course, you are 
rebooting your systems every day. . . .

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