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- Subject: Re: [chrony-users] asymmetric phy delay?
- From: Miroslav Lichvar <mlichvar@xxxxxxxxxx>
- Date: Mon, 10 Aug 2020 14:45:41 +0200
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On Mon, Aug 10, 2020 at 12:55:59PM +0100, Nuno Gonçalves wrote:
> SOC A to A shows a offset of about 50us+-25us
> SOC A to B shows a offset of about 0us+25us
> SOC B to B shows a offset of about 75us+-25us
> SOC B to A shows a offset of about 130us+-25us
> I understand that a eventual asymmetric tx/rx delay on the phy could
> cause this unbalance, but my understanding of the NTP protocol is that
> this would cancel each other on a query/response cycle of the same
> board type, which is not happening.
The asymmetry is likely in the timestamping. If you don't enable the
interleaved mode, only three of the four timestamps from the NTP
exchange will be kernel or hardware timestamps. Only when the
interleaved mode is enabled, the fourth timestamp (server TX) will be
symmetric to the client TX timestamp.
The client could be easily modified to ignore the more accurate
kernel/HW timestamp when not using interleaved mode, but the idea was
to prefer stability over symmetry between two identical machines.
There are many sources of asymmetry and this is just one of them.
--
Miroslav Lichvar
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