Re: IEEE80211 Acknowledgement

[Ivo van Doorn <ivdoorn@gmail.com>]

On Monday 04 May 2009, Johannes Berg wrote:
> On Mon, 2009-05-04 at 17:29 +0200, Ivo van Doorn wrote:
> > Hi,
> > 
> > Just as side information for other readers:
> > 
> > Alexandre is looking into the problem of rt2500usb/rt73usb and
> > possibly other USB drivers, that they cannot report the ACK status
> > of frames to mac80211 which is problematic for hostapd.
> 
> I realise that this probably doesn't help and is tangential to the
> question being asked, but I do not think processing ACK frames in
> software is possible for this, since the software does not know which
> frame was transmitted, due to four queues being used, and retries etc.

Well I believe the idea would be that (I'll see if I can dig up a reference
to the initial discussion about this feature on this list) the driver sets a flag
that mac80211 needs to keep a list of all frames send out to the driver and
listens for ACK's.

As soon as a ACK was passed from driver to mac80211 it could check if
the corresponding frame was in the list and remove the frame from the list
and generate a tx_status response indicating success.
Off course all frames must be checked for the timeout value as well to see
if the frame wasn't send (in which case the tx_status event is send as well).

There shouldn't be a problem with multi-queue with this if the list of frames
is put onto a single list instead of one list per queue.
As for the retries and other statistics, do we honestly care?
At the moment rt2500usb/rt73usb indicate the frame has not been acked
and set the retry value to 1. If we somehow can tell if the frame was send out
or not, isn't that a step forward already? The retry value is needed for the
rate algorithm, but it is receiving the same incorrect value now as well, so it wouldn't
be a step backward in that perspective. ;)

Ivo