On Thu, 2014-01-23 at 13:38 +0100, Michal Kazior wrote:
> On 23 January 2014 13:20, Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote:
> > On Thu, 2014-01-23 at 11:33 +0100, Michal Kazior wrote:
> >
> >> An extreme approach would to actually update AP IEs in mac80211. This
> >> way it would be possible to pull any interface into CSA implicitly if
> >> you're out of channels. This way single-channel multi-BSS AP could
> >> probably switch atomically without races and your multi-channel
> >> GO-possibly-follows-STA would still work.
> >
> > I don't really like updating IEs in mac80211 much. Asking userspace
> > shouldn't be *that* much slower.
>
> I'm not really fond the idea either. Though it's not about being
> slower but about the 'dragging other interfaces along' being possibly
> automatic when you're out of channels.
Yeah, I agree that updating the IEs in mac80211 is not a good idea.
That's why I would prefer to have the notifications to the user space
("your interface must switch channel") and wait for the userspace to
request a channel switch (with all the necessary information).
> >> > The CAC might take too long. If we have an AP1 and a STA and the STA
> >> > gets the CSA from its AP2 with a short count, AP1 may not have the time
> >> > to CAC. In this case, AP1 have two choices: trust that AP2 is doing the
> >> > right thing and moving to a usable DFS channel or shut itself down.
> >>
> >> That's the point. AP1 doesn't have time to perform CAC in this case,
> >> but it should still be possible for AP1 to _just_ beacon CSA as it's
> >> only a hint. AP1 could then be stopped to perform CAC, and once it's
> >> completed restart the AP1.
> >
> > I don't get this side discussion about CAC. There's no time to do CAC in
> > the middle of a CSA *anyway* since you can't be beaconing on the old
> > channel while doing CAC on the new channel, and if you stop beaconing
> > entirely for the time of the CAC then all clients will likely go away...
>
> You're right, but..
>
>
> > I'm not sure I understand how you can ever do a CSA to a radar channel
> > that would still need CAC?
>
> You're supposed to fulfil a uniform channel usage spread which
> includes "usable" DFS channels.
>
> With CSA you're at least able to tell stations to stop Tx and should
> wait for you (the AP) on a different channel.
I think that we should stop transmission in all interfaces that are on a
certain channel when that channel receives a CSA with "quiet" mode. I'm
not sure this would help your CAC case, but I think it's a wise thing to
do to prevent the other interfaces from transmitting.
--
Luca.
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