netdev - Re: Extending socket timestamping API for NTP

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Date:   Tue, 07 Feb 2017 17:18:36 -0800
From:   Denny Page <dennypage@...com>
To:     Miroslav Lichvar <mlichvar@...hat.com>
Cc:     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


> On Feb 07, 2017, at 06:01, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> 
> 1) new rx_filter for NTP
> 
>   Some NICs can't timestamp all received packets and are currently
>   unusable for NTP with HW timestamping. The new filter would allow
>   NTP support in new NICs and adding support to existing NICs with
>   firmware/driver updates. The filter would apply to IPv4 and IPv6
>   UDP packets received from or sent to the port number 123.
> 

I think this is a good idea. Even if the hardware doesn’t support it, the filtering could be done in the kernel. Save a huge number of context switches.



> 4) allow sockets to use both SW and HW TX timestamping at the same time
> 
>   When using a socket which is not bound to a specific interface, it
>   would be nice to get transmit SW timestamps when HW timestamps are
>   missing. I suspect it's difficult to predict if a HW timestamp will
>   be available. Maybe it would be acceptable to get from the error
>   queue two messages per transmission if the interface supports both
>   SW and HW timestamping?
> 

Highly agreed. The current interface pretty much forces a socket per physical interface, which should not be necessary.


> 5) new SO_TIMESTAMPING options to get transposed RX timestamps
> 
>   PTP uses preamble RX timestamps, but NTP works with trailer RX
>   timestamps. This means NTP implementations currently need to
>   transpose HW RX timestamps. The calculation requires the link speed
>   and the length of the packet at layer 2. It seems this can be
>   reliably done only using raw sockets. It would be very nice if the
>   kernel could tranpose the timestamps automatically.
> 
>   The existing SOF_TIMESTAMPING_RX_HARDWARE flag could be aliased to
>   SOF_TIMESTAMPING_RX_HARDWARE_PREAMBLE and the new flag could be
>   SOF_TIMESTAMPING_RX_HARDWARE_TRAILER.
> 
>   PTP has a similar problem with SW RX timestamps, which are closer
>   to the trailer timestamps rather than preamble timestamps. A new
>   SOF_TIMESTAMPING_RX_SOFTWARE_PREAMBLE flag could be added for PTP
>   implementations to get transposed timestamps in order to improve
>   accuracy.
> 

Also highly agreed.

Denny