Re: Extending socket timestamping API for NTP

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On Feb 07, 2017, at 12:17, sdncurious [off-list ref] wrote:

 If the NTP has access to the physical layer, then the timestamps are
   associated with the beginning of the symbol after the start of frame.
   Otherwise, implementations should attempt to associate the timestamp
   to the earliest accessible point in the frame.

The spec is unfortunately a bit ambiguous and probably should be clarified.

NTP is sensitive to transmission asymmetry. While using the SFD is appropriate for transmit timestamps, it is not appropriate for receive timestamps. A simple reason for this is port speed mismatch. Consider a 1Gb entity communicating with a 100Mb entity on a local switch: leaving aside internal switch delays, if SFD timestamping is used for both transmit and receive, then there is a baked in asymmetry of 6768ns between the forward and reverse paths; if SFD is used for transmit, and FCS end is used for receive, there is no asymmetry.

There is a good explanation of this written by David Mills (NTP's author) here: https://www.eecis.udel.edu/~mills/stamp.html#require

Denny