[ntp:questions] Isolated Network Drift Problem

Richard B. Gilbert rgilbert88 at comcast.net
Wed Nov 26 01:06:17 UTC 2008


Cal Webster 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. 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. 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.
> 
> 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.
> 
> 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. 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.
> 
> Thanks! :-)
> 
> ./Cal

The most important thing to do or attempt is to get a primary source of 
time.  This can be an atomic clock (extremely expensive, extremely 
stable, extremely accurate), or something directly connected to an 
atomic clock.  A GPS timing receiver will capture signals from four 
satellites and solve a system of four equations in four unknowns: time, 
latitude, longitude, and elevation.  Each of the satellites carries an 
atomic clock!  The GPS timing receivers sell for ~$100 and up.  If you 
can site such a receiver so that it has an unobstructed view of the sky, 
this is a very good source of time.

If it is not possible for you to have your own stratum 1 time server, 
there are a fair number of them on the internet.  Stratum 2 servers are 
even more plentiful.  Stratum one servers have direct access to an 
atomic clock or equivalent; e.g. GPS receiver.  Stratum two servers get 
their time from stratum 1 servers.  Stratum 3 gets time from stratum 2. 
. . .   If you can't use internet servers and you can't set up your own 
stratum one server, you are SOL.

It is possible to "fake it" using a server that has a good clock but 
performance will not be as good as having an atomic clock at the root of 
the tree.  That rock solid beat of one second per second is easy to get 
in step with and easy to keep step with.

An unsynchronized clock WILL drift and will be more difficult to get in 
step with and keep step with.

I can tell you from experience that a GPS receiver will give you rock 
solid time and that clients will synchronize easily with it.  So will an 
atomic clock but that costs $50,000 or $100,000 more than the GPS receiver!




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