Re: [chrony-users] Dealing with SNTP sources

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On Wed, 4 May 2011, Harald Krammer wrote:

Hello,
I have a question about dealing with SNTP sources.

when I interact with two SNTP servers (also named as NTP server),

An sntp server must, AFAIK be a stratum 0 server.

then it's hard to find a solution to prevent the case of 'no majority'.
The most (S)NTP devices solved this problem with selecting of one
primary time source and one backup time source . More cannot be
adjusted.

I did it with following setting:
server <server-1-ip> iburst minpoll 6 maxpoll 8 prefered
server <server-2-ip> iburst minpoll 6 maxpoll 8 minstratum 8

The option 'prefered' didn't help, but lowering of stratum was
a solution to prevent the case of 'no majority'.

With following settings:
server <server-1-ip> iburst minpoll 6 maxpoll 8 prefered
server <server-2-ip> iburst minpoll 6 maxpoll 8

I got 'Can't synchronise: no majority' and here is my traces:

Recall that 2 servers is a worst case situation. If they disagree, noone can
say which is correct. You need three at least.



sources.c:324:(SRC_SetSelectable)[04-14:40:35] 10.126.9.246
sources.c:302:(SRC_AccumulateSample)[04-14:40:35] ip=[10.126.9.246] t=Wed
05/04/11 14:40:35.145764 ofs=-0.003653 del=0.000741 disp=0.000629 str=2
sourcestats.c:574:(SST_GetSelectionData)[04-14:40:35] n=4 off=0.000498
del=0.000897 dis=0.001328 var=0.000000 pdist=0.001318 avoff=-0.000005
avok=1 selok=1
sourcestats.c:574:(SST_GetSelectionData)[04-14:40:35] n=4 off=-0.003653
del=0.000741 dis=0.000632 var=0.000000 pdist=0.001000 avoff=-0.004032
avok=1 selok=1
sources.c:624:(SRC_SelectSource)[04-14:40:35] Can't synchronise: no
majority

210 Number of sources = 2
MS Name/IP address   Stratum Poll LastRx Last sample
=================================================================
^x 10.126.9.245       2    6     33    +20ns[-1669us] +/-  398us
^x 10.126.9.246       2    6     33  -4332us[-4332us] +/-  857us

The intervals do not overlap. It cannot figure out which is best. Trying to
force something is silly, since you cannot figure out what the right one to
force is. Use three servers.



After a while is again synchronized
210 Number of sources = 2
MS Name/IP address Stratum Poll LastRx Last sample
===============================================================
^* 10.126.9.245      2    6     50   -111us[ -278us] +/-  378us
^+ 10.126.9.246      2    6     49   +737us[ +737us] +/-  521us

Now they overlap (just barely) .


After 2..3 min again: Can't synchronise: no majority
210 Number of sources = 2
MS Name/IP address  Stratum Poll LastRx Last sample
================================================================
^x 10.126.9.245       2    6      4   -403us[ -403us] +/-  529us
^x 10.126.9.246       2    6      2  +2207us[+2207us] +/-  353us

And now they do not. Get a third source so you and it can figure out which is the bad source.



and so on

=======================================================================
  Date (UTC) Time     IP Address   St   Freq ppm   Skew ppm     Offset
=======================================================================
2011-05-04 14:40:28 0.0.0.0          0     19.521     42.166  0.000e+00
2011-05-04 14:40:33 10.126.9.246     3     19.521     42.166 -2.127e-03
2011-05-04 14:40:34 10.126.9.245     3     19.521     42.166  4.087e-03
2011-05-04 14:40:35 0.0.0.0          0     19.521     42.166  0.000e+00
2011-05-04 14:41:39 10.126.9.245     3     -0.505     50.104 -1.669e-03
2011-05-04 14:41:40 0.0.0.0          0     -0.505     50.104  0.000e+00
2011-05-04 14:42:43 10.126.9.245     3     -9.630     15.559 -8.059e-04
2011-05-04 14:42:45 0.0.0.0          0     -9.630     15.559  0.000e+00
2011-05-04 14:43:49 10.126.9.245     3    -11.892      9.416 -3.936e-04
2011-05-04 14:44:53 10.126.9.245     3    -12.749      5.457 -1.671e-04
2011-05-04 14:45:57 0.0.0.0          0    -12.749      5.457  0.000e+00
2011-05-04 14:49:11 10.126.9.246     3    -12.434      8.937  5.796e-03
2011-05-04 14:50:16 10.126.9.246     3    -11.686     15.050  6.131e-04
2011-05-04 14:51:19 10.126.9.245     3     -1.017     27.162  1.354e-04
2011-05-04 14:52:20 0.0.0.0          0     -1.017     27.162  0.000e+00
2011-05-04 14:52:25 10.126.9.245     3     -1.017     27.162  2.711e-04


The solution 'minstratum 8' is fine for me. The behavior
without setting of minstratum was unclear for me.
Is it explainable?


Nice greetings,
Harald


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