Re: A station can't reconnect after it wakes up

[Igor Perminov <igor.perminov@inbox.ru>]

On Fri, 31/07/2009 at 18:16 +0200, Artur Skawina wrote:
> [linux-wireless added to cc list]
> 
> Igor Perminov wrote:
> > I have an issue related to handling power-saving stations by hostapd
> > and/or mac80211 stack. A station can't reconnect after it wakes up.
> > 
> > The problems looks similar to another one having been reported to this
> > list earlier (STA can connect, but fails to reconnect within
> > ap_max_inactivity):
> > http://lists.shmoo.com/pipermail/hostap/2009-February/019192.html
> > 
> > AP: Linux box with D-Link DWA-110 USB Wi-Fi stick (rt73usb kernel
> > driver), kernel 2.6.30 with some patches, hostapd 0.6.9.
> > Station: Toshiba G900 PDA under Windows Mobile 6.0.
> > 
> > The environment is described in details here:
> > http://rt2x00.serialmonkey.com/phpBB/viewtopic.php?f=5&t=5486&start=10
> > 
> > Consider the following step-by-step:
> > 1. A station authenticates and associates with the AP and exchanges
> > traffic.
> > 2. The station indicates to the AP that it is going to sleep.
> > 3. The station device goes to the stand-by mode (not only its wi-fi
> > card, but the device itself).
> > 4. The station device wakes up and begins authentication with an
> > Authentication management frame.
> > 
> > This is the behavior of my PDA.
> > 
> > The problem is the mac80211 stack at the point 4 "remembers" that the
> > station has gone to sleep. So, the response frames from hostapd are
> > buffered by mac80211.
> > The station indicates in the Authentication frame that it isn't sleeping
> > anymore. But the mac80211 stack analyzes sleep/wake transitions in
> > _data_ frames only, but not in management ones. See
> > ieee80211_rx_h_sta_process in net/mac80211/rx.c. A comment there notes:
> > "Ignore doze->wake transitions that are indicated by non-data frames,
> > the standard is unclear here".
> > As the result, the station never receives the authentication response
> > from the AP.
> > 
> > One solution against this problem could be implemented in hostapd: to
> > force the mac80211 stack to "forget" the station just after receiving an
> > authentication frame (the patch is below). After this change the station
> > can reconnect successfully.
> 
> i just did a quick test and your hostapd patch does indeed fix my problem
> too (p54+hostapd with a winmobile device that couldn't reconnect after
> turning the wifi module off and on). 
> 
> > Another solution (in theory) would be to improve the mac80211
> > implementation: to analyze not only data frames, but also
> > management ones (or may be just some kinds of them) in
> > ieee80211_rx_h_sta_process.
> 
> would seem to make sense, but having not read the spec i have no idea
> if that's the right answer; hence the linux-wireless cc...
> 
> > I've asked this question to the linux-wireless mailing list few days
> > ago, but nobody has answered still:
> > http://marc.info/?l=linux-wireless&m=124879549212741&w=2
> > 
> > And what is your opinion, what is a better way: should this problem be
> > fixed in hostapd or in mac80211?
> > 
> > === Begin diff ===
> > --- a/hostapd/ieee802_11.c	2009-06-29 14:21:59.000000000 +0400
> > +++ b/hostapd/ieee802_11.c	2009-07-21 16:28:17.000000000 +0400
> > @@ -583,6 +583,13 @@
> >  		goto fail;
> >  	}
> >  
> > +	res = hostapd_sta_remove(hapd, mgmt->sa);
> > +	if (res) {
> > +		wpa_printf(MSG_DEBUG, "authentication: STA=" MACSTR
> > +				", hostapd_sta_remove returned %d\n",
> > +				MAC2STR(mgmt->sa), res);
> > +	}
> > +
> >  	if (vlan_id > 0) {
> >  		if (hostapd_get_vlan_id_ifname(hapd->conf->vlan,
> >  					       sta->vlan_id) == NULL) {
> > === End diff ===
> 
> Thanks for the cc, added this to my local hostapd patch set, until the
> issue gets resolved one way or another.
> 
> artur

I've looked through the 802-11.2007 document and now I consider the
source of the problem is misunderstanding by an AP and a STA states of
each other. When the station tries to reconnect it considers itself
non-authenticated. Whereas the AP considers the station both
authenticated and associated.

I think it would be better to fix the problem in hostapd, because it's a
hostapd's responsibility - to manage the state of a station being known
to the AP. So, when hostapd receives an Auth frame from a station it
should consider the station being non-authenticated and force removing
the station from the underlying stack.

I'll submit a patch for hostapd in a nearest future.

Best regards,
Igor