Re: pull-request: wireless-next-2023-03-30

[Richard Cochran <richardcochran@gmail.com>]

On Tue, Apr 25, 2023 at 07:03:50AM +0000, Stern, Avraham wrote:

> Having the timestamps of the frames seemed like a basic capability that userspace will need to implement ptp over wifi, regardless of the selected approach.

Having time stamps on unicast PTP frames would be a great solution.
But I'm guessing that you aren't talking about that?

> Apparently you had other ways in mind, so I would love to have that discussion and hear about it.  

Let's back up a bit.  Since you would like to implement PTP over Wifi
in Linux, may I suggest that the first step is to write up and
publish your design idea so that everyone gets on the same page?

Your design might touch upon a number of points...

- Background
  - Difficulty of multicast protocols (like PTP) over WiFi.
  - What do the networking standards say?
    - IEEE Std 802.11-2016
      - Timing Measurement (TM)
      - Fine Timing Measurement (FTM)
    - IEEE 1588
      - Media-Dependent, Media-Independent MDMI
      - Special Ports
    - 802.1AS
      - Fine Timing Measurement Burst
  - Which of the above can be used for a practical solution?
    - What are the advantages/disadvantages of TM versus FTM?
    - What alternatives might we pursue?
      - unicast PTP without FTM
      - AP as transparent clock
- Existing Linux interfaces for time synchronization
  - What can be used as is?
  - What new interaces or extensions are needed, and why?
- Vendor support
  - How will we encourage broad acceptance/coverage?

IMO, the simplest way that will unlock many use cases is to provide
time stamps for single unicast frames, like in
ieee80211_rx_status.device_timestamp and expose an adjustable PHC
using timecounter/cyclecounter over the free running usec clock.  Then
you could synchronize client/AP over unicast IPv4 PTP (for example)
with no user space changes needed, AND it would work on all radios,
even those that don't implement FTM.

Thanks,
Richard