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- To: chrony-dev@xxxxxxxxxxxxxxxxxxxx
- Subject: Re: [chrony-dev] SW/HW timestamping on Linux
- From: Denny Page <dennypage@xxxxxx>
- Date: Mon, 21 Nov 2016 17:26:46 -0800
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I checked this. It appears to be completely symmetrical at the switch level. For both the 1Gb and 100Mb ports, the delta I see for NTP packets is a very consistent 912ns on a 1Gb mirror port. This is exactly as expected:
preamble: 56 bits
SFD: 8 bits
MAC dest: 48 bits
MAC src: 48 bits
Length: 16 bits
IP/UDP/NTP: 608 bits
FCS: 32 bits
IPG: 96 bits
Total of 912 bits, which equates to 912ns on a 1Gb connection.
NB: The packet cannot be forwarded until it is received, so the preamble and IPG count when looking at packet to packet deltas when mirroring.
I’ll have to look at the driver/kernel next. It may just be an inherent offset due to software timestamping. I may hard code a correction for the hardware timestamps in chrony for testing.
Denny
> On Nov 21, 2016, at 10:50, Denny Page <dennypage@xxxxxx> wrote:
>
> I am going to set up a span port to capture the packets on both interfaces which may give an indication.
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