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
