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

mike cook michael.cook at sfr.fr
Sat Sep 13 07:04:55 UTC 2014


Le 13 sept. 2014 à 07:46, Rich Wales a écrit :

> Replying to Charles Elliott:
> 
>> The offset may be a function of distance.  Try this experiment:
>> Set up your ntp.conf file to have three servers . . . :
>> 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
>> 3.  A relatively unused stratum 1 or 2 server more than 2,000 miles away
> 
> OK, here is information taken from two local servers under my control.
> 
> ==============================================================================
> 
> This first machine (171.67.203.16) is on the Stanford University campus.  The
> first two peers listed below are located on the Stanford campus; the third
> peer is also run by Stanford but is about 50 miles east of the campus.
> 
> +171.64.7.105    .PPS.            1 u 1012 1024  377    0.465   -0.045   0.076
> +171.64.7.67     .PPS.            1 u  217 1024  377    0.584   -0.043   0.081

  delay is low so you are nearby in a network sense, maybe on the same net (what is your netmask).

> *204.63.224.70   .PPS.            1 u  737 1024  377    1.803   -0.031   0.252
> 
> This next peer is my home machine (the one I described earlier as being
> connected to the Internet via a cable modem), with its own local GPS clock.
> 
> -68.65.164.12    .PPS.            1 u   13   16  372    8.168   -2.126   4.585

  I thought that you were connecting over a VPN?  The delay is high but not exceptionally and you can get  good stability even so. However,  the jitter is huge and your link is not stable as your reach is not 377. Here you missing polls , possibly due to a timeout or dropped packets.  These are not symptoms of link asymmetry, but might be due to unbalanced packet priority or something. Do you have HD TV on at the same time. 

> 
> Finally, these two servers are in Utah (Xmission) and Poland.
> 
> -198.60.22.240   .GPS.            1 u  721 1024  377   18.931    0.239   0.045
> -213.222.200.99  .PPS.            1 u  833 1024  377  172.099   -4.083   1.167
> 
> ==============================================================================
> 
> Now, here is the info from my home machine (68.65.164.12).  First, my local
> GPS reference clock:
> 
> *127.127.28.1    .PPS.            0 l   14   16  377    0.000   -0.003   0.025
> x127.127.28.0    .GPS.            8 l    7   16  377    0.000  -37.762  13.399
> 
> Next, my campus machine (see above for details):
> 
> -171.67.203.16   204.63.224.70    2 u    2   16  333    9.689    2.146   3.930

   again, reach is showing a 'broken' link.

> 
> And finally, the same two remote servers (in Utah and Poland) that I used on
> my campus machine:
> 
> +198.60.22.240   .GPS.            1 u   45   64  377   35.222    6.542   2.864
> +213.222.200.99  .PPS.            1 u   18   64  377  191.192    4.890   8.968

  These are not showing bad reach, at least not on these samples. Do they ever show a non 377 value?

> 
> ==============================================================================
> 
> Any thoughts?

  You are up late. 

> 
> Rich Wales
> richw at richw.org
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