[ntp:questions] high precision tracking: trying to understand sudden jumps

Unruh unruh-spam at physics.ubc.ca
Tue Apr 1 19:50:18 UTC 2008

hal-usenet at ip-64-139-1-69.sjc.megapath.net (Hal Murray) writes:

>>Yes, PTP can yield an accuracy better than 100 ns if both the NICs at the
>>clients and the server support hardware timestamping of sent/received PTP
>>On the other hand, also *every* network node between the PTP endpoints has
>>to be PTP-aware and compensate the packet delay it introduces, so you will
>>probably only get full PTP accuracy in your local network where you have
>>control over all the equipment.

>Suppose I have PTP network adapters but vanilla switches and my
>network is lightly loaded.

>Can I filter out the delays in the switches by sending 10 packets
>and throwing out the ones with long delays?  I'd expect the
>timings to be a cluster around the case where there was no delay
>in the switch and a tail for the ones that encountered some
>delay.  I think it would be easy to filter out that tail.

ntp already does. It throws away 85% of the packets it gets, keeping only
  the roughly 1/8 of them with the shortest round trip times (a real waste
of data I think but it certainly solves your problem). 

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