[ntp:questions] Isolated Network Drift Problem

Steve Kostecke kostecke at ntp.org
Wed Nov 26 04:16:38 UTC 2008

On 2008-11-25, Cal Webster <cwebster at ec.rr.com> wrote:

> On Tue, 2008-11-25 at 22:23 +0000, David Woolley wrote:
>> Cal Webster wrote:
>> > None of our client machines use the local clock. The servers
>> > configured like this are the other 3 "peer" NTP servers. Don't
>> > they need some type of time reference if the master server becomes
>> > unreachable?
>> No. PCs keep time even without NTP installed.
> Okay, I understand that the "PC" (Intel Linux host) will keep time
> on its own, albeit at some unpredictable level of accuracy.

The system clock will be pushed around by environmental changes and
system loading.

> What I'm not getting is whether the NTP service running on that host
> without a local clock reference will be able to provide time to NTP
> clients.

"local clock reference" is a misleading term.

An ntpd must by synced to a time source before it can serve time to
others. This time source can be: another ntpd, a locally attached
ref-clock, the Undisciplined Local Clock, or Orphan Mode.

> If so, will it be more accurate or less accurate if all the NTP peers
> each use their own local clock as a time reference as opposed to
> having no reference at all.

An ntpd "synced" to the Undisciplined Local Clock (or in Orphan Mode) is
not really synced to anything. It is just reporting the system's notion
of the correct time.

These peers will eventually drift apart unless they are following one

> When I tried removing the "server" and "fudge" lines for the local
> clock from ntp.conf but leaving the server lines naming the sister
> peers, ntpdq showed nothing in the "peer" or "association" listings.

That is to be expected.

> After communicating with Steve Kostecke and reading the NTP
> Association page at ntp.org I'm thinking that a combination of a
> master server using a fairly accurate local clock with several Orphan
> mode servers will work best.
>> ntpd disciplines the local clock. If you use the local clock as your
>> reference, it will always read exactly the same as the local clock
>> being disciplined, so no corrections will ever be made. You have
>> fallen for an urban legend!
>> The Local Clock driver is a fiction used to allow an unsynchronised
>> machine to appear synchronised to downstream machines.
> Isn't having an "apparently" synchronized NTP server better than
> having no NTP server at all? Since we can't all have GPS reference
> clocks or Internet connections, some of us have to make use of what we
> do have.
> I'm just looking for the best way to get the most accurate time
> possible distributed to a couple hundred machines spread across
> 15 networks in 3 buildings.

If time synchronization is important to your organization they should be
willing to pay for it.

> Maybe that's asking a lot but that's
> what I'm aiming to do. Anything you can do to help would be greatly
> appreciated.

Your time servers need an accurate reference standard. This can be
(amonr other things):

1. A POTS modem which periodically contacts NIST

2. A network connected remote time server

3. A GPS timing receiver

4. A WWV, WWVB, CHU, or other HF receiver

5. A rubidium stabilized PPS source

As a worst case you could set up a dedicated time server box in a stable
environment (perhaps an insulated cabinet) and use the knobs
documented on the driver1.html page to tweak the Undisciplined Local
Clock over the course of several weeks.

Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/

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