[ntp:questions] Setting Up NTP Subnet

Bill Myers whmyers at gmail.com
Tue Mar 7 21:22:28 UTC 2006

*Thank you for your response.  I've included some additional thoughts on my
perspective below.  As you can see, I've been thinking through this some and
have formulated some opinions and rationale behind my opinions.  Maybe I
just need to be told I'm going off the deep end!*

On 3/7/06, Danny Mayer <mayer at ntp.isc.org> wrote:
> Bill Myers wrote:
> >  I have questions regarding best practices on architecture of NTP
> subnets.
> > I've thoroughly read "Notes on Setting up a NTP subnet" from the NTP
> sites.
> >
> > Is there a need for multiple external time source TECHNOLOGIES.  That
> is, is
> > it sufficient to use multiple (8) GPS receivers with ACTS dial backup
> > and Rubidium clocks?   This is a sizeable investment for the
> infrastructure,
> > but is it the right way?  Is it necessary or desirable to have other
> sources
> > such as radio and/or Internet?
> >
> I don't think that there is any specific reason to require this. I don't
> think you gain or lose anything by having different technology sources
> or identical technology sources. Even on identical ones from the same
> manufacturer you will get slightly different results just due to the
> manufacturing process. Quality controls on the manufacturing have
> tolerances for what will be allowed but that's a range and your gizmo is
> going to be anywhere in that range.

*I'm concerned that ultimately, GPS is a single source of time that could be
a single point of failure or intentional blackout due to a perceived
security condition by government or military authorities.  I didn't really
mean different technologies as much as different, independent sources.  For
example, the documentation I have for a well known Stratum 1 NTP site shows
two GPS, two WWVB, two Loran-C, plus quartz and Cesium clocks. *

> In case you have not picked up on it, I'm talking about an appliance here.
> I find it odd that one cannot NTP peer these appliances.  This suggests to
> me that we should create a stratum 2 tier peer layer a stratum 3 tier that
> peers and serves time to the endpoints.
I don't think that's necessary. The lower the stratum the bigger the
error budget, so depending on how accurate you want your time to be you
need to limit how low you are able to go.

*The document "Notes on setting up an NTP subnet" state that one should
have at least three external time sources to each of your best stratum
servers and that the servers at this level should peer.  The GPS/clock
appliances we have cannot peer or take any other external sources.  So I
consider them as very good GPS/clock (rubidium) Stratum 0 sources that
have inadequate NTP support, and are best used as time sources to clock real
NTP Stratum 1 servers which are properly configured with multiple external
sources and internal peering with other Stratu 1 servers in the NTP subnet.*

> More dysfunction?  The plan is for the stratum 2 tier, which is also the
> > time distribution tier, to be the cache DNS for some high-volume data
> center
> > environments.
> Which should be fine. What is the time going to be used for in relation
> to the cache DNS?

*There's no relation to the cache DNS, just shared services on the same
platform.  Large server environments querying the same server for both DNS
and NTP.  With DNS being such a rich security target, could we unnecessarily
expose NTP to disruption?  Also, couldn't there be resouce contention
between the services -- mostly in the direction of intense DNS work
interfering with consistent NTP responses?*

> Danny

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