Re: [ntp:questions] can't send messages

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Brad Knowles

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Dec 13, 2004, 7:23:14 AM12/13/04
to Alaios, ques...@lists.ntp.isc.org
At 2:43 AM -0800 2004-12-13, Alaios wrote:

> Hi. I have recently subscribed to your list but all my
> messages wait for the administrator to approve them..

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Alaios

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Dec 13, 2004, 7:26:20 AM12/13/04
to ques...@lists.ntp.isc.org
Goof morning. I have an isolated network and i want to
achieve accuracy smaller than 1ms. Do u know if that
is possible? How i must configure the clients for
that?
Any suggestion?



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Richard B. Gilbert

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Dec 13, 2004, 10:52:21 AM12/13/04
to
Alaios wrote:

Without a stable source of time, I'd say that synchronization within 1ms
is not possible. The typical, unsynchronized, computer clock is neither
stable nor accurate.

You will need a hardware reference clock of some sort. A cesium
frequency standard (with clock option) is the best there is. It's also
extremely expensive. A rubidium standard is second best and
considerably cheaper but still expensive to buy and maintain. A GPS
timing receiver can be obtained for less than $500 US and is a very good
choice if you can place an antenna where it has an unobstructed view of
the entire sky. An oven controlled quartz crystal oscillator can
provide a stable source.

Configure one computer with attached hardware reference clock as the
server. Configure the clients to get their time from the server.

Don't expect much from Microsoft Windows in the way of accuracy; the
clock resolution is too low and the operating system tends to lose
interrupts (clock ticks) when it is busy doing something else. Linux
also has been known to have problems with losing interrupts.

Alaios

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Dec 13, 2004, 11:32:25 AM12/13/04
to Richard B. Gilbert, ques...@lists.ntp.isc.org
Thx really for the answer... If i have understand
correctly the problem of not having good accuracy is a
problem of my server because it always loses
interrupts.

If i connect the server to the internet under a
stratum 2 server, do u think that the server can
achieve accuracy below 1 ms? Do u think i must
configure the server in a specific way for my
requirements?

After finishing with the server then the clients can
achieve synchronization with accuracy of 1 ms or this
is still impossible? The major problem is that i want
all the clients to be synchronized and each of them
has an accuracy of 1 ms or less.
Really thank u!
Have a nice day

--- "Richard B. Gilbert" <rgilb...@comcast.net>
wrote:

> _______________________________________________
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> https://lists.ntp.isc.org/mailman/listinfo/questions
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Richard B. Gilbert

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Dec 13, 2004, 1:47:21 PM12/13/04
to
Alaios wrote:

If your server loses interrupts, I'd say you have no hope of achieving
accuracy!

Using an internet server as a time source will almost certainly not
allow you to achieve 1ms accuracy! 10ms accuracy is possible over the
internet but cannot be guaranteed.

Many many computer operating systems update their clocks at intervals of
10ms or longer. In a system updating at 10ms intervals, the time
reported at any random instant can be wrong by as much as 9.9999999999.
. . ms and, on average, will be within 5ms of the correct time.

1ms accuracy is a very high standard and quite difficult to meet. I
don't believe that Windows is capable of it. Linux may not be
either. Some flavors of Unix are capable, at least in principle. I
believe that Solaris is. I lack sufficient recent experience with other
flavors of Unix to offer an informed opinion.

Bjorn Gabrielsson

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Dec 13, 2004, 7:48:08 PM12/13/04
to
"Richard B. Gilbert" <rgilb...@comcast.net> writes:

> Using an internet server as a time source will almost certainly not
> allow you to achieve 1ms accuracy! 10ms accuracy is possible over the
> internet but cannot be guaranteed.

I get sub 1ms time from internet servers.

> 1ms accuracy is a very high standard and quite difficult to meet. I
> don't believe that Windows is capable of it. Linux may not be
> either. Some flavors of Unix are capable, at least in principle. I
> believe that Solaris is. I lack sufficient recent experience with
> other flavors of Unix to offer an informed opinion.

My Linux computers shows no problem reaching below 1ms.

Richard B. Gilbert

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Dec 13, 2004, 9:12:03 PM12/13/04
to
Bjorn Gabrielsson wrote:

>"Richard B. Gilbert" <rgilb...@comcast.net> writes:
>
>
>
>>Using an internet server as a time source will almost certainly not
>>allow you to achieve 1ms accuracy! 10ms accuracy is possible over the
>>internet but cannot be guaranteed.
>>
>>
>
>I get sub 1ms time from internet servers.
>
>

The internet servers I use tend to wander within plus/minus 5-10ms of my
GPS receiver!! I'm using the best servers I can find and I've seen far
worse than the ones I finnally settled on. Their clocks my be far
better than what I'm seeing but by the time the packets get here the
quality looks really poor. Your milage may vary.

>
>
>>1ms accuracy is a very high standard and quite difficult to meet. I
>>don't believe that Windows is capable of it. Linux may not be
>>either. Some flavors of Unix are capable, at least in principle. I
>>believe that Solaris is. I lack sufficient recent experience with
>>other flavors of Unix to offer an informed opinion.
>>
>>
>
>My Linux computers shows no problem reaching below 1ms.
>
>

My RedHat Enterprise Linux 3.0 workstation rarely runs an offset less
than 5ms. Once again your milage may vary!!

Alaios

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Dec 14, 2004, 1:09:49 AM12/14/04
to Bjorn Gabrielsson, ques...@lists.ntp.isc.org
Ok...can u tell me how u have configured your clients
and server for achievine accuracy below 1 ms?

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David J Taylor

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Dec 14, 2004, 3:02:27 AM12/14/04
to
Bjorn Gabrielsson wrote:
> "Richard B. Gilbert" <rgilb...@comcast.net> writes:
>
>> Using an internet server as a time source will almost certainly not
>> allow you to achieve 1ms accuracy! 10ms accuracy is possible over
>> the internet but cannot be guaranteed.
>
> I get sub 1ms time from internet servers.

What precise type of connection do you have to the Internet?

David


Bjorn Gabrielsson

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Dec 14, 2004, 3:33:53 AM12/14/04
to
"Richard B. Gilbert" <rgilb...@comcast.net> writes:

> Bjorn Gabrielsson wrote:
>
> >"Richard B. Gilbert" <rgilb...@comcast.net> writes:
> >
> >
> >>Using an internet server as a time source will almost certainly not
> >>allow you to achieve 1ms accuracy! 10ms accuracy is possible over the
> >>internet but cannot be guaranteed.
> >>
> >
> > I get sub 1ms time from internet servers.
>
> The internet servers I use tend to wander within plus/minus 5-10ms of
> my GPS receiver!! I'm using the best servers I can find and I've seen
> far worse than the ones I finnally settled on.

You have 5-10ms offsets. I usually have 0.1-0.5ms.

> Your milage may vary.

Exactly.

> > My Linux computers shows no problem reaching below 1ms.
> My RedHat Enterprise Linux 3.0 workstation rarely runs an offset less
> than 5ms. Once again your milage may vary!!

My FC2 x86_64 workstation very rarely sees an offset above 0.5ms. Its
getting its time from just my ntp server. Not performing better using
a server 0.3ms away compared to using public intenet servers 10-20ms
away is kind of curious...

Alaios

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Dec 14, 2004, 8:55:37 AM12/14/04
to ques...@lists.ntp.isc.org
Ok..thx a lot... buw wait a sec..and help me ask
again...

I ll make the best possible to connect permanently my
pc's on the internet...

I want to know from each client what is the offset
from my server. What command i must execute??

Then look at the following command
ntpq -c pe ripebox.grnet.gr
remote refid st t when poll reach
delay offset jitter
==============================================================================
*GPS_RIPENCC(0) .GPS. 0 l 3 64 377
0.000 -0.002 0.004

The offset is -0.002 seconds aka 2ms? How long this
offset will be valid?
Thx a lot


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Richard B. Gilbert

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Dec 14, 2004, 10:58:28 AM12/14/04
to
Alaios wrote:

That's two microseconds! It's "valid" only as of the instant it was
estimated. The estimate will be updated each time the server is polled.

Bjorn Gabrielsson

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Dec 14, 2004, 12:52:45 PM12/14/04
to

Slow assymetric ADSL. (0.5Mbit downstream , 0.768Mbit/s upstream)
The ntp servers are (today) 10 to 13 router hops away, 10 to 20 ms
"ping"-distance and 200 to 600km physical distance.

/Björn

Bjorn Gabrielsson

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Dec 14, 2004, 1:02:49 PM12/14/04
to
"Richard B. Gilbert" <rgilb...@comcast.net> writes:

> Alaios wrote:
> >ntpq -c pe ripebox.grnet.gr
> > remote refid st t when poll reach delay offset
> > jitter
> >==============================================================================
> > *GPS_RIPENCC(0) .GPS. 0 l 3 64 377 0.000 -0.002
> > 0.004
> >

> That's two microseconds! It's "valid" only as of the instant it was
> estimated. The estimate will be updated each time the server is
> polled.

No he was asking 'ripebox.grnet.gr' good its _local_ time is wrt the time it
receives from its Trimble GPS receiver.

David J Taylor

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Dec 14, 2004, 1:14:27 PM12/14/04
to

The number of hops surprises me. Ping distance sounds not bad. I'd
welcome comments from others on the accuracy achievable with such a
connection, but I would have thought that better than 10ms should be
achievable (providing your server running NTP isn't heavily loaded), but
it might be limited if you are saturating either the uplink or the
downlink to the Internet - with a sustained upload or download, for
example. Perhaps you can give NTP packets higher priority?

I would investigate whether your first or second hop (i.e. the first piece
of the ISP's kit offers NTP. You may find it is only 10ms away.

Cheers,
David


Bjorn Gabrielsson

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Dec 14, 2004, 6:26:52 PM12/14/04
to
Bjorn Gabrielsson <b...@lysator.liu.se> writes:

Hmmm should not be posting when in a hurry... Insert a 'how' at suitable
place to make previous post parseable. *Sorry*

Richard B. Gilbert

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Dec 14, 2004, 11:35:31 PM12/14/04
to
Bjorn Gabrielsson wrote:

And the local clock is two microseconds off.

Alaios

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Dec 15, 2004, 6:20:30 AM12/15/04
to Richard B. Gilbert, ques...@lists.ntp.isc.org
Thx a lot... I will ask my final question.. What isthe
accuracy that can be achieved in an isolated network?
I must say something to my boss. Why the accuracy is
so small when we have a Fast Ethernet with a
full-duplex switch for all the clients?

Also the accuracy what is the accuracy that the
internet provides? What should i answer him... We have
no GPS os other hardware can we achieve accuracy below
the 1ms....?? I suppose no


--- "Richard B. Gilbert" <rgilb...@comcast.net>
wrote:

> Bjorn Gabrielsson wrote:

Richard B. Gilbert

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Dec 15, 2004, 9:27:22 AM12/15/04
to
Alaios wrote:

Definitions first.

Accuracy: The magnitude of the difference between your clock and the
the "correct" UTC time. You can't know this without knowing what UTC
is and, in an isolated network with no hardware reference clock, you
have no way of knowing what the correct time is. With good luck and
good reflexes you might be able to set the clock on your server to
within plus/minus one second using your wristwatch or your cellular
phone (the best source)

Precision: The granularity with which you can set or read the clock.
How much time does the least significant digit (bit) represent?
Determined by the hardware and the O/S.

Closeness or tightness of synchronization: the standard deviation of
the clients with respect to the server.

The closeness of synchronization is the only thing that you have much
control of. That will depend primarily on the stability of your
server's clock. The typical unsynchronized computer clock does not have
good stability. It depends primarily on the temperature of the clock
circuitry which, in turn, depends on the room temperature. There are
also dependencies on the supply voltage, the age of the crystal, etc.

I have seen ten unsynchronized PC clocks develop a spread of twelve
hours over the course of a year!!! These PCs were all the same make
and model and were originally set to the correct time (from my wrist
watch) when I installed them. The PCs were running Solaris 8 Intel
Platform Edition. When I configured NTP using a server on the local
network (a DEC Alpha server 4000 running VMS V7.2-1), which in turn was
synchronized via the internet, they synched up to within 100
microseconds of each other and the server. The server was usually
within 10 milliseconds of the correct time!

If your server is not synchronized you are, in effect, standing on a
randomly moving platform and shooting at a randomly moving target. I
don't know if anyone has ever tried to make a scientific study of the
behavior of such a system. I'd say that an accuracy of 1ms could be
achieved only by random chance! Synchronization within 1 ms might be
achievable; you could try it and see.

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