[ntp:questions] SNTP server + ntpd 4.2.4 client

Noob root at localhost.invalid
Mon Mar 17 13:07:02 UTC 2008


Hello Bill,

(Your news client often adds an extraneous =20 suffix to quotes.)

Bill Unruh wrote:

> 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.
> 
> 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 more
> than inbound packets for example) and your clock would be biased.

Point taken.

>>> I've been asked to evaluate the following time server, in order to reach
>>> a better accuracy than what the current setup provides.
> 
> You are not going to get better accuracy by changing ntp program

You have apparently misread my original post. I do not plan to change 
ntpd.

> 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)

This is precisely what I plan to do. Specifically, I plan to connect 
my box to the time server using a cross-over Ethernet cable. (My box 
has 4 gigabit Ethernet ports, I will devote one to NTP traffic.) The 
RTT is very stable at 80-85 µs.

>>> (AFAIU, this time server implements SNTP, not the full NTP.)
> 
> Then it is not a server.

I don't understand the point you are trying to make.
It is an SNTP server.

> You will never get your network ntp under a few ms.

Unless the stratum 1 server is on the same LAN...
(As you state later.)

>>> 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
> 
> It is NOT a stratum 1.
> A stratum 1 gets its time from an atomic clock.

I don't understand the point you are trying to make.

The time server has a GPS receiver, thus it is "attached" to a
stratum 0 device, thus it is a stratum 1 server.

> If you attach a GPS to one of your (Linux) machines, you will get 2 µs
> accuracy on that machine. On the machines attached on your LAN you will
> get 10s of µs accuracy, if they are unix type machines.

This is the setup I've been discussing, with the GPS receiver inside 
the "time server" I'm supposed to evaluate...

> 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.

I should have made clear that I am not using Windows.
The system to be synchronized runs Linux 2.6.22.1-rt9 and ntpd 4.2.4

Regards.




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