[PATCH] wpa_supplicant: MLD STA: find partner links by BSSID and SSID

[Benjamin Berg]

Hi Michael,

On Mon, 2024-04-22 at 02:35 +0000, Michael-cy Lee (李峻宇) wrote:
> On Wed, 2024-04-17 at 09:06 +0200, Benjamin Berg wrote:
> > On Tue, 2024-04-09 at 14:50 +0800, Michael-CY Lee wrote:
> > > MLD STA finds MLD AP's partner links by BSSID from the scan results.
> > > However, if the scan results contain BSSs with same BSSID but different
> > > BSS information, the MLD STA might assign a wrong BSS to one of the MLD
> > > AP's partner links.
> > 
> > As a clarification, why/how do we get into the situation that the
> > same BSSIDs exists with the wrong SSID in the supplicant?
> > 
> > Did the AP change its SSID and we still have the old one in the
> > cache?
> 
> Yes, that's exactly the situation we've faced. 
> To be more precise, after we changed the SSID and restarted the AP MLD,
> we triggered the STA scan and saw the STA's scan results containing two
> BSSs with the same BSSID but different SSIDs, then the STA incorrectly
> matched one of the AP MLD's links to the wrong BSS (old SSID). 
> 
> > 
> > If that is the case, then maybe what we should be doing is to verify
> > that the BSS parameter change count matches. If not, it makes sense to
> > reject the BSS for now, which should trigger an ML probe request to
> > update the information and the next attempt will work fine.
> 
> We think the BSS parameter change count (BPCC) might not be helpful
> here since a change in SSID will not trigger a critical update.
> Additionally, An SSID change is usually followed by an AP restart,
> which will reset the BPCC value to 0.

Right, I guess that is fair then.

I guess it feels a bit weird to me overall, because I would expect that
we only ever have one BSS entry for a BSSID (except as a workaround for
some broken legacy APs). And it seems to me like one could increase the
BPCC across an AP restart.

So, I don't have a good alternative to your solution, I think.

That said, maybe we should still add an additional check for the BPCC?

Benjamin