[ntp:questions] Re: polling algorithm

Klas Bengtsson klasbengtsson99 at hotmail.com
Thu Sep 1 14:16:44 UTC 2005


Hi David,

and thanks for your answer. But I am still a bit confused over the behaviour 
I see here. I have peer printouts every minute and they look like this:

Tue Aug 16 08:43:53 JST 2005
     remote           refid      st t when poll reach   delay   offset  
jitter
==============================================================================
*148.17.101.4    148.17.100.2     2 u  984 1024  377    1.209   -0.669   
0.740
+148.17.101.5    148.17.100.2     2 u  990 1024  377    1.241   -0.764   
0.362

Tue Aug 16 08:44:53 JST 2005
     remote           refid      st t when poll reach   delay   offset  
jitter
==============================================================================
148.17.101.4    148.17.100.2     2 u   19 1024  377    0.962  1998.55 
1999.22
148.17.101.5    148.17.100.2     2 u   24 1024  377    1.025  1998.71 
1999.47

Here the synchronisation is lost, but the poll value remains at 1024. The 
poll value is 1024 until the next poll (17 minutes later), and then the poll 
value is decreased to 64 again, and the synchronisation is back.

.
.
.

Tue Aug 16 09:00:55 JST 2005
     remote           refid      st t when poll reach   delay   offset  
jitter
==============================================================================
148.17.101.4    148.17.100.2     2 u  981 1024  377    0.962  1998.55 
1999.22
148.17.101.5    148.17.100.2     2 u  986 1024  377    1.025  1998.71 
1999.47

Tue Aug 16 09:01:56 JST 2005
     remote           refid      st t when poll reach   delay   offset  
jitter
==============================================================================
*148.17.101.4    148.17.100.2     2 u   18   64  377    0.929  1990.20   
8.349
+148.17.101.5    148.17.100.2     2 u   22   64  377    0.966  1997.48   
1.230

Now to my question:
If we are just dealing with a lost packet here, why is the poll value 
decreased to 64 seconds when the synchronisation is back again after 17 
minutes?

Has it something to do with the large jitter and offset values we see? In 
that case I still would expect the poll value to decrease to 64 seconds when 
the synchronisation is lost, not when it comes back into synchronisation 
again.

BR,
Klas



>From: "David L. Mills" <mills at udel.edu>
>To: questions at lists.ntp.isc.org
>Subject: [ntp:questions] Re: polling algorithm
>Date: Thu, 25 Aug 2005 19:06:54 +0000
>
>Klas,
>
>Think about what you said. The only way the client knows the server is 
>reachable is to poll it. Even if the server does not answer a poll you 
>can't conclude it has become unreachable; a packet could have been lost.
>
>The poll interval increases in the first place because the feedback loop 
>has stabilized and doesn't need refresh very often. The design allows for 
>some packets to be lost and for the server quality metric (synchronization 
>distance) to accurately reflect the maximum error relative to that server. 
>If the server comes back online after a few polls or a couple of hours, the 
>clock discipline process just takes over as before and reducing the poll 
>interval is unnecessary and counterproductive.
>
>Dave
>
>Klas Bengtsson wrote:
>>Hi all!
>>
>>Anyone out there who can describe how the polling algorithm for NTP works?
>>
>>My problem is that when the poll interval is 1024 seconds and I suddenly 
>>lose synchronisation for some reason, I would expect the NTP server to 
>>decrease the polling interval back to 64 seconds again, but it does not.
>>That means that even if the reason for the synchronisation loss is fixed I 
>>have to wait up til 17 minutes before I get the synchronisation back 
>>again.
>>
>>Is this how it is supposed to work?
>>What makes the polling interval decrease again once it has reached 1024 
>>seconds?
>>Where can I find information about how the polling algorithm works?
>>
>>BR,
>>Klas
>>
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