[ntp:questions] Re: peering question

Richard B. Gilbert rgilbert88 at comcast.net
Thu Mar 9 04:25:05 UTC 2006

Joshua Coombs wrote:
> While I wait for my 4.2.1-RC1 test box to go haywire, I have a question 
> regarding peered servers.
> I have 4 ntp servers that peer with each other.  Two in one site, two in 
> another.  I have a selection of geographically and ip wise nearby 
> servers that they all query.  I then have an additional subset spread 
> across the 4 servers, such that each remote server is only conf'd on one 
> of my units.
That's the right way to do it; each peer has at least one unique source.

> If my understanding of peering is correct, each clock sees the other's 
> data for the server's it's tracking.  So, in theory I could get 
> identical time quality by just having any given remote server on only 
> one peer?  Or should I continue having duplication across my peers of 
> remote servers?
> Joshua Coombs

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.

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