[ntp:questions] SNTP server + ntpd 4.2.4 client
unruh-spam at physics.ubc.ca
Sat Mar 15 18:07:23 UTC 2008
David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:
>> I've been running ntpd 4.2.4 to synchronize my system clock using remot=
>> stratum 2 servers as a reference. (The RTT to these servers is in the=20
>> 30-50 ms range.) The accuracy is in the 1-2 ms range, based on the=20
>> reported offset.
>Offset doesn't tell you the accuracy, it only gives you an idea of the=20
>variability of the error. Theoretically, the error could be as much as=20
>15 to 25ms, plus the error from the stratum one to the stratum 2.
Agreed. The accuracy is bounded by the round trip time. The offset
fluctuations will give and estimate of those variations in round trip time.
But that time could be biased (ie outbound packets always take 10ms moere
than inbound packets for example) and your clock would be biased.
It is highly unlikely that packets take zero time to go out and 50ms to get
>> I've been asked to evaluate the following time server, in order to reac=
>> a better accuracy than what the current setup provides.
You are not going to get better accuracy by changing ntp program (well the
evidence is that chrony is a somewhat better client, but in your case the
difference will be negligible) . YOu are going to get a better time by
using a server that is closer to you and has more predictable round trip
times (ethernet, not ADSL)
going to get better accuracy
>> (AFAIU, this time server implements SNTP, not the full NTP.)
Then it is not a server.
>There are many network timeservers that implement full NTP, so I'm not=20
>sure what they are doing with just SNTP; maybe it is aimed at the=20
>Windows w32time market.
>Also, you can use a GPS timing receiver, with 1pps output, directly.
Yes. IF you want better accuracy, get yourself a Garmin 18LVC, wire it up
and use the PPM output. They you will have a few microsecond accuracy. YOu
will never get your network ntp under a few ms.
>> The idea would be to put this (stratum 1) time server in the same LAN a=
>> my box, and add it my ntp.conf. (The RTT on the LAN is on the order of =
It is NOT a stratum 1.
A stratum 1 gets its time from an atomic clock.
It you attach a GPS to one of your (Linux) machines, you will get 2usec
accuracy on that machine. On the machines attached on your lan yyou will
get 10s of usec accuracy, if they are unix type machines. As far as I know
windows does not impliment a proper clock control API so you will have be
happy with a few msec for those.
>> 100 =B5s.)
>> Will it work?
>It's a violation of NTP, so you the result will only be compliant as an=20
>SNTP client. It may or may not work, depending on whether or not early=20
>w32time implementations conformed to SNTP. Early versions of w32time=20
>didn't set enough of the response fields to sensible values to guarantee =
>that ntpd would work as a client.
>> Is it a problem that the time server only implements SNTP?
>> What kind of accuracy may I expect?
>If it works, you can probably expect most of the errors to be within=20
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