Supplicant Recovery on Roam Failures

Matt Causey matt.causey at gmail.com
Thu Dec 27 23:33:29 EST 2012


Thank you for your reply!

On Thu, Dec 27, 2012 at 4:43 PM, Jouni Malinen <j at w1.fi> wrote:

> On Thu, Dec 27, 2012 at 04:18:26PM -0500, Matt Causey wrote:
> > I have a question that hopefully you can help me with.  We run
> > wpa_supplicant 1.x and Linux 2.6.39 in a somewhat dense Cisco-based WLAN.
> > There are lots of clients and they spend a lot of time roaming between
> > access points.  Intermittently, the client gets into a state where
> > wpa_supplicant has received an RSSI_LOW event, and has selected a BSSID
> > that has better coverage.  The client gets to
> > ieee80211_send_probe_req<
> http://lxr.free-electrons.com/ident?i=ieee80211_send_probe_req>()
> > [1], and the BSSID that it has selected fails to respond to broadcast
> > probes.:
>
> Could you please test this with the current 2.0-devel snapshot of
> wpa_supplicant (i.e., snapshot of the master branch in
> git://w1.fi/srv/git/hostap.git)? There has been number of changes
> improving reaction to cases where authentication/association fails for
> whatever reason (very much including the cases where the AP network
> tries to use some proprietary load balancing mechanism that is likely
> the case here).
>

I will give that a shot and let you know what we find.



>
> > In the 802.11 capture data, we see clearly that the client is sending
> valid
> > Probe Requests, and the access point is failing to respond, though the
> same
> > access point is responding to other clients.  We have the vendor engaged
> to
> > answer the question of why the access point is not responding.
>
> I'd assume you have either load balancing or band steering enabled on
> the Cisco APs and that makes it not reply to some Probe Request frames.
> In this particular case the new AP seemed to be on the 5 GHz band, so
> I'd assume this was not caused by band steering, but load balancing is
> likely to have similar effects. I cannot say that I like the way these
> APs try to force load balancing, but well, that's what's out there.. It
> is especially harmful with the mac80211 mechanism of using Probe Request
> frames to probe the specific AP and it would have been nice if that
> mechanism would have been used only for the broadcast probe case, but I
> guess it could apply here, too.
>
>
We do have both "Band Steering" and "AP Load Balancing" disabled.  But I
agree, it does indeed look as though the AP is being silent in an attempt
to coerce the client into going to another AP.


>
> > What appears to happen after the MLME layer issues the times-out, is that
> > the supplicant then kicks off a scan and disconnects.  In our dense
> > environment, this scan takes seconds to complete, and breaks our clients.
> > In the case where these failures happen, the client already has a very
> full
> > scan list that contains a lot of very solid roam choices.  Is there some
> > way that I can cause the supplicant to attempt the next BSS on the list
> > rather than invoking a whole new scan?
>
> This is the part that has changed a lot in the latest wpa_supplicant and
> as such, I'd like to see a debug log from such a run. wpa_supplicant
> should use the temporary blacklist mechanism to allow other BSSes to be
> tried and there is now some cases where the "extra" scan can be avoided
> or at least limited to a subset of channels (though, in your particular
> case, even that may end up being rather large set of channels). There is
> now better framework in place for allowing old scan results to be used,
> so it should be much easier to extend these mechanisms based on the new
> debug log if needed.
>

Sounds great; thanks! Has the blacklist functionality improved in 2.x as
well?  In the 1.x code it seems like on small lab networks we routinely get
into a situation where we only have 2 APs online, one AP sends a Deauth for
some valid reason, becoming blacklisted.  Then the client has nowhere to
roam besides the AP he is on, so it repeatedly roams to itself at -80 RSSI
until I manually clear the blacklist or bounce the supplicant.

--
Matt
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