A station can't reconnect after it wakes up

Igor Perminov igor.perminov at inbox.ru
Thu Jul 30 12:05:30 EDT 2009


Dear colleagues,

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.

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.

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 ===

Best regards,
Igor Perminov



More information about the HostAP mailing list