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

Heiko Gerstung heiko_removeme_.gerstung at meinberg.de
Wed Apr 2 14:51:24 UTC 2008

Hal Murray schrieb:
>> 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
>> packets.
>> 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.

Yes, that is possible. The main problem with vanilla switches is the asymmetric 
delays you get when the (store-n-forward) switch starts to queue packets due to 
higher network load. This is a rare occurence in lightly loaded networks.

The PTP standard (IEEE1588) describes how a client (called slave in PTP 
terminology) finds out the offset between his own clock and the servers 
("master") clock. It does not specify what you do with this offset, i.e. you can 
step your clock or apply small corrections in order to keep things running 
smoothly. There is nothing that prevents you from applying filters and 
statistics on the offset values before adjusting your clock.

If you want to play around with PTP, you can get a free (software-only) version 
at Sourceforge: ptpd.sf.net

The developers of this implementation state that "PTPd should be able to 
coordinate the clocks of your computers within tens of microseconds", which is 
around the performance of ntpd. As Martin already said, you can get NTP to be as 
accurate as PTP if you add hardware timestamping and you can get PTP to be as 
accurate as NTP by taking the hardware timestamping away from it.

Best Regards,

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