[ntp:questions] high precision tracking: trying to understand sudden jumps
martin.burnicki at meinberg.de
Tue Apr 1 09:23:34 UTC 2008
Hal Murray wrote:
>>>probably have make use of PTP (precision timing protocol).
>>No idea what that is. If you had wanted super precision you would have put
>>a GPS onto each machine, I hope.
>>From the Wikipedia entry on PTP it looks absolutely no different from ntp.
>>I have no idea what the idea is.
> The basic idea is to do the time stamping in hardware deep in
> the network adaper. That avoids lots and lots of jitter.
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.
Switches can very well insert a delay in the range of milliseconds. If there
are incoming packets at different ports at the same time which shall go out
on the same port then the packets have to be queued. Unless the network is
really heavily loaded this may happen only occasionally, but it may happen.
The switches included in our PTP starter kit
implement PTP boundary clocks for the ports in order to eliminate the
queuing delay. Without this special handling PTP would suffer from the same
latencies as NTP.
On the other hand, NTP yields quite good results without requiring special
hardware, even over WAN connections.
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