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

Danny Mayer mayer at ntp.isc.org
Sun Feb 15 04:07:33 UTC 2009


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.

Danny



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