[Resend without rich text] On Feb 07, 2017, at 12:17, sdncurious <sdncuri...@gmail.com> 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
