netdev - Re: Extending socket timestamping API for NTP

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Date:   Tue, 07 Feb 2017 19:06:50 -0800
From:   Denny Page <dennypage@...com>
To:     sdncurious <sdncurious@...il.com>
Cc:     Miroslav Lichvar <mlichvar@...hat.com>, netdev@...r.kernel.org,
        Richard Cochran <richardcochran@...il.com>,
        Jiri Benc <jbenc@...hat.com>,
        "Keller, Jacob E" <jacob.e.keller@...el.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: Extending socket timestamping API for NTP

[Resend without rich text]

On Feb 07, 2017, at 12:17, sdncurious <sdncurious@...il.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