[ntp:questions] Re: peering question

Joshua Coombs jcoombs at gwi.net
Tue Mar 14 19:40:53 UTC 2006

"Richard B. Gilbert" <rgilbert88 at comcast.net> wrote in message 
news:waCdnRpEKui8M5LZRVn-gQ at comcast.com...

> Duplication is not particularly desirable but it's damned difficult 
> to avoid!  Each peer needs a minimum of four time sources. 
> Providing four unique servers to each peer would require sixteen 
> servers.  It can be difficult to find even four really good servers 
> for a site.  There are many good servers out there but "good at the 
> server" is not the same as "good, as delivered here".  The Internet 
> can really mess up the time as delivered at your site so the problem 
> becomes one of finding servers with low latency and stable round 
> trip delays.
> Peers with more than one server do not just "track" one server.  One 
> server is selected as the primary source and the remaining "usable" 
> servers act as an "advisory committee" having some small influence 
> on the clock.  See the RFC or Dave's "slide show" for the math; I'm 
> sure the math is much more precise than my fumbling English 
> translation thereof.

Ok, so I've taken a kind of aggressive approach to getting an 'ideal' 
setup.  I used the list of public stratum 1 and stratum 2 clocks, and 
pulled all US entries that are A) Open and B) don't request 
notification.  I've setup a dedicated box in my lab that does 
aggressive (burst, low interval) polling of my 4 existing ntp time 
sources, using them as a baseline.  I've conf'd all the servers in the 
list I generated, setting min/max poll to 12 (34mins) so I don't abuse 
anyone.  I've got a perl script going through and recording the 
observed offsets, latency, and jitter using RRDTool and plan to try 
and find 16 solid sources from that list.

I only have about 24 hours worth of data, but I've already spotted 
three trends.  First are servers that tend to report offsets very 
close to what I'm getting as 'true' time from my 4 existing ntp 
servers, one of which is tracking ACTS.  The second is a group of 
servers that all seem to have a systemic offset, but are otherwise 
quite stable.  The third are all over the map.

I know the third group is garbage, but what about the systemic offset 
servers?  I would think they would be usable to at least get frequency 
info from despite the offset, or should I avoid them as well and start 
contacting remote servers if I can't fill out my 16 slots using first 
pick servers?

Joshua Coombs 

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