[ntp:questions] SNTP server + ntpd 4.2.4 client

Noob root at localhost.invalid
Mon Mar 17 11:23:08 UTC 2008


David Woolley wrote:

> Noob wrote:
> 
>> I've been running ntpd 4.2.4 to synchronize my system clock using 
>> remote stratum 2 servers as a reference. (The RTT to these servers is 
>> in the 30-50 ms range.) The accuracy is in the 1-2 ms range, based on 
>> the reported offset.
> 
> Offset doesn't tell you the accuracy, it only gives you an idea of the 
> variability of the error.  Theoretically, the error could be as much as 
> 15 to 25ms, plus the error from the stratum one to the stratum 2.

What metric should I consider to determine accuracy?

# ntpq -crv
assID=0 status=06f4 leap_none, sync_ntp, 15 events, event_peer/strat_chg,
version="ntpd 4.2.4p0 at 1.1472 Fri Mar 16 10:45:43 UTC 2007 (1)",
processor="i686", system="Linux/2.6.22.1-rt9", leap=00, stratum=2,
precision=-20, rootdelay=46.299, rootdispersion=17.260, peer=18760,
refid=134.226.81.3,
reftime=cb88c64c.edb1a310  Mon, Mar 17 2008 10:28:28.928, poll=8,
clock=cb88c77e.ef7df269  Mon, Mar 17 2008 10:33:34.935, state=4,
offset=-0.295, frequency=-6.442, jitter=0.291, noise=0.248,
stability=0.001

09:07:02: offset -0.000288 sec freq -6.433 ppm error 0.000453 poll 8
09:14:02: offset -0.000712 sec freq -6.436 ppm error 0.000431 poll 8
09:21:02: offset -0.000551 sec freq -6.437 ppm error 0.000418 poll 8
09:28:02: offset -0.000551 sec freq -6.437 ppm error 0.000422 poll 8
09:35:02: offset -0.000539 sec freq -6.438 ppm error 0.000415 poll 8
09:42:02: offset -0.000347 sec freq -6.439 ppm error 0.000856 poll 8
09:49:02: offset -0.000389 sec freq -6.440 ppm error 0.000597 poll 8
09:56:02: offset -0.000389 sec freq -6.440 ppm error 0.000874 poll 8
10:03:02: offset -0.000039 sec freq -6.441 ppm error 0.000793 poll 8
10:10:02: offset -0.000039 sec freq -6.441 ppm error 0.000402 poll 8
10:17:02: offset -0.000039 sec freq -6.441 ppm error 0.000402 poll 8
10:24:02: offset -0.000039 sec freq -6.441 ppm error 0.000368 poll 8
10:31:02: offset -0.000295 sec freq -6.442 ppm error 0.000291 poll 8
10:38:02: offset -0.000295 sec freq -6.442 ppm error 0.000167 poll 8
10:45:02: offset -0.000295 sec freq -6.442 ppm error 0.000417 poll 8
10:52:02: offset -0.000313 sec freq -6.443 ppm error 0.000349 poll 8
10:59:02: offset -0.000103 sec freq -6.444 ppm error 0.000312 poll 8
11:06:02: offset -0.000103 sec freq -6.444 ppm error 0.000285 poll 8
11:13:02: offset -0.000103 sec freq -6.444 ppm error 0.000255 poll 8
11:20:02: offset -0.000103 sec freq -6.444 ppm error 0.000340 poll 8

>> I've been asked to evaluate the following time server, in order to 
>> reach a better accuracy than what the current setup provides.
>> 
>> http://www.heoldesign.com/index.php?id=58
>> 
>> (AFAIU, this time server implements SNTP, not the full NTP.)
> 
> There are many network timeservers that implement full NTP, so I'm not 
> sure what they are doing with just SNTP; maybe it is aimed at the 
> Windows w32time market.

Maybe they just need a clue? :-)

> Also, you can use a GPS timing receiver, with 1 pps output, directly.

My system is running a Linux kernel patched with real-time support.
I don't feel confident applying the PPS support patch on top of it.

>> The idea would be to put this (stratum 1) time server in the same LAN 
>> as my box, and add it my ntp.conf. (The RTT on the LAN is on the order 
>> of 100 µs.)
>>
>> Will it work?
> 
> It's a violation of NTP, so the result will only be compliant as an 
> SNTP client.

What is a violation of NTP?

> It may or may not work, depending on whether or not early 
> w32time implementations conformed to SNTP.  Early versions of w32time 
> didn't set enough of the response fields to sensible values to guarantee 
> that ntpd would work as a client.

Why do you mention w32time?

Do you think the HEOL-T101 is running Windows?

>> What kind of accuracy may I expect?
> 
> If it works, you can probably expect most of the errors to be within 
> your box.

Does this mean it is reasonable to expect 100 µs accuracy?

Regards.




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