[ntp:questions] Compensating for asymmetric delay on a per-peer/server basis?

Charles Elliott elliott.ch at comcast.net
Thu Sep 11 20:47:00 UTC 2014


The offset may be a function of distance.
Try this experiment:

Set up your ntp.conf file to have three servers (all examples assume you
are located in Eastern USA):
1.  A relatively unused stratum 1 or 2 server as close to you as possible.
2.  A relatively unused stratum 1 or 2 server about 1,000 miles away (e.g., 
    ntp.melancthon.net)
3.  A relatively unused stratum 1 or 2 server more than 2,000 miles away
    (e.g., ntp1.tp.pl, ntp2.tp.pl, time.coi.pw.edu.pl, ntp.certum.pl).

On my computer, the offset is proportional to distance:

Remote	               Refid Stratum            Type	    When    Poll
Reach  Delay	  Offset	Jitter
BR-350P	                GPS	0	       Local clock	7	16
017	0.000	 -17.653	2.345
FreeNAS            time-c.nist.gov  2	       Unicast server	16	16
017	0.238	   0.008	0.037
nist1-pa.ustiming.org	  ACTS	1	       Unicast server	15	16
017	28.844   0.135	3.158
2a01:1102:0:b::2	        ATOM	1	       Unicast server	16
16	017	120.732 -5.145	2.151
2a01:1100:1::2	        ATOM	1	       Unicast server	15	16
017	128.756 -3.931	4.635
213.222.200.99	        PPS	      1	       Unicast server	13	16
017	110.727 -0.968	4.119
ntp.coi.pw.edu.pl	       OCX0	      1	       Unicast server	14
16	017	122.100 -4.253	0.584
serenity.melancthon.net	india.colorado.edu 2     Unicast server	35	32
003	53.520   2.019	3.556

Charles Elliott

> -----Original Message-----
> From: questions-bounces+elliott.ch=comcast.net at lists.ntp.org
> [mailto:questions-bounces+elliott.ch=comcast.net at lists.ntp.org] On
> Behalf Of mike cook
> Sent: Thursday, September 11, 2014 2:08 PM
> To: Questions List
> Subject: Re: [ntp:questions] Compensating for asymmetric delay on a
> per-peer/server basis?
> 
> 
> Le 11 sept. 2014 à 18:48, Rob a écrit :
> 
> > Paul <tik-tok at bodosom.net> wrote:
> >> As an aside has anyone tried shaping traffic to make the
> >> upstream/downstream latencies similar?  It would seem more efficient
> >> to apply network solutions to network problems if possible.
> >
> > That does not work.  The asymmetry is not caused by traffic but by
> > modem parameters.
> 
>   Did I miss something? The OP did not offer any evidence that there
> was path asymmetry, just that there was an unexplained offset between
> two GPS sync'd servers. It is often not possible to identify the origin
> of such an offset, but it would help if the suggested feature was
> implemented to take care of these corner cases. I saw that Dr Mills was
> in agreement back in 2005 but that the implementation is complex. If
> anyone wants a subject for an MSc project, this could be it.
> 
> >
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> > questions at lists.ntp.org
> > http://lists.ntp.org/listinfo/questions
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