[ntp:questions] Garmin 18 LVC: whether to fudge
Unruh
unruh-spam at physics.ubc.ca
Sun Feb 15 18:04:36 UTC 2009
Hans =?iso-8859-1?Q?J=F8rgen?= Jakobsen <hjj at wheel.dk> writes:
>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
Something is really weird here. .776ms from a GPS refclock is horrible.
That is about a factor of 300 worse than you would expect. In fact all of
your GPS refclocks show bad time What are you doing to your clocks?
>20.6 ms of the roundtrip time are spend on the DSL line.
How do you know it is exactly 20.6ms?
>Some interleaving has been configured. It's better to have delay than dropped
>packets.
>The VDSL line runs at a distance from the CO where ADSL have better performance,
>at least for download.
>/hjj
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