[ntp:questions] frequency adjusting only

maxime louvel m.louvel at gmail.com
Tue Apr 22 12:30:08 UTC 2008


I am actually using 4 public NTP server, which whom my node-1 synchronises
Then node-1 broadcasts the time to all the other nodes in the subnet.

My goal is to achieve a synchronisation between the nodes (not with the
public NTP server) within 50 usec.
I don't care if the synchronisation to the public NTP is accurate around
half a second.
I think it's possible, because the nodes are close to each other and every
communication is using gigabit ethernet (cards + switch).

I achieve to get a small offset when nothing else than linux and ntpd is
running on the nodes
(offset arount 10 usec).

I have a program which sends data from one node to another.
The sending/reception are timestamped and the delay is then computed.

I would like the measured delay to be constrained in a 50usec wide range.

Do you think it should be possible ?

I have tried to run the test for 24h and didn't get the expected results.
The range of the measured delay is 2 milliseconds (between -1ms and +1ms)
with some spikes (ten or so for a milion measurments). The mean delay is
-0.066087 (ms) and the std dev is 0.22849
Do you think the spikes can influence ntp  ?

thanks very much


On Tue, Apr 22, 2008 at 1:15 PM, Danny Mayer <mayer at ntp.isc.org> wrote:

> maxime louvel wrote:
> > Thanks for your answer,
> >
> > I can't have a step on the clock because that would screw up my
> > applications.
> > However if I keep the load within a certain range I should fine, don't I
> > ?
> > I am synchronising one node to several public NTP servers, and the
> > others
> > nodes are synchronised to the first one.
> > There are 2 to 24 nodes in my sub net.
> > Do you think that should be feasible ?
> >
> > Maxime
> >
> You need at least 3 and preferably 4 public NTP servers in order for your
> local server to make decisions about which one is giving the most accurate
> and reliable time at any moment. 2 is not enough since there's no way to
> decide which is better. 3 allows 2 to gang up on the third.
> 2-24 nodes is nothing. Even bombarding the NTP server with queries will
> barely be noticed. Don't forget at a minimum a node won't send a query more
> frequently than once every 64 seconds unless you are using a badly
> implemented NTP client.
> Danny

Maxime Louvel
0044 7964 5555 80
43 Allen road
Whitemore reans
WV60AW Wolverhampton
United Kingdom

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