[ntp:questions] Garmin 18 LVC: whether to fudge

Hans Jørgen Jakobsen hjj at wheel.dk
Sun Feb 15 11:08:36 UTC 2009

On Sun, 15 Feb 2009 04:07:33 GMT, Danny Mayer wrote:
> Richard B. Gilbert wrote:
>> Chris Adams wrote:
>>> Once upon a time, Richard B. Gilbert <rgilbert88 at comcast.net> said:
>>>> My bet would be that there is an asymmetry in your ADSL link!   If I'm 
>>>> not mistaken, the "A" in ADSL stands for asymmetric!
>>> The asymmetry in ADSL is in bandwidth, not path or latency.  More
>>> frequency space is used for downstream (ISP->end user) communication
>>> than for upstream, but both travel the same path.
>>> There may be asymmetric routing going on (very often the case when you
>>> talk to a host across the Internet, especially if either your ISP or the
>>> remote host's ISP are multihomed), but it is highly unlikely it is
>>> happening between the end user and his own ISP.
>> The difference in bandwidth would mean that a packet of N bytes would 
>> take more time in one direction than the other!  I don't know if that 
>> would be sufficient to account for the observed behavior but I can't 
>> think of another explanation.
> Since a regular ntp packet is just 48 bytes without extensions, the idea
> that ADSL would make a difference is rather unlikely. The asymmetry is
> related to the size of the packets but it affects TCP rather than UDP.

There ARE asymetric delays even for ntp packets.
For my 14M/1.5M VDSL line I see an offset of 7-800 microseconds:
 ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
-ns.tele.dk      .GPS.            1 u   48   64  377   22.450    0.891   0.096
-tix.ns.tele.dk  .GPS.            1 u    7   64  377   22.338    0.698   0.183
-puma.tele.dk    .GPS.            1 u   19   64  377   27.048    0.784   0.145
+GPS_HP(0)       .GPS.            0 l   10   16  377    0.000   -0.776   6.230
oPPS(0)          .1PPS.           0 l   10   16  377    0.000    0.012   0.015
+GPS_ONCORE(0)   .GPS.            0 l    8   16  377    0.000    0.035   0.006

20.6 ms of the roundtrip time are spend on the DSL line.
Some interleaving has been configured. It's better to have delay than dropped
The VDSL line runs at a distance from the CO where ADSL have better performance,
at least for download.

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