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Date: Fri, 29 Jul 2011 11:14:20 +0200
Message-ID: <CAGXE3d9gX0LQjHiYB6vpD8AvfdrwOr5DZs=sGxZzHYW5RSZHsQ@mail.gmail.com> (sfid-20110729_111434_023237_2252AE54)
Subject: mac80211 + hostapd: EAPOL frames rate selection
From: Helmut Schaa <helmut.schaa@googlemail.com>
To: linux-wireless <linux-wireless@vger.kernel.org>,
	hostap@lists.shmoo.com
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Sender: linux-wireless-owner@vger.kernel.org

Hi,

I just noticed that EAPOL frames generated by hostapd during the 4-way
handshake are sent out by mac80211 using a rate as selected by the rc
algorithm for data frames. In my case minstrel_ht selects a MCS rate for
11n clients which sometimes results in a 4-way handshake timeout under
low signal conditions.

I haven't found anything in 802.11-2007 if EAPOL frames have to be sent
at a low rate but I'd argue that it makes sense to send them at a basic
rate just like it's done for management frames.

We've got a nice little helper in mac80211 (rate_control_send_low) that
allows the rc algorithm to check if a frame should be sent at a low rate.
I thought I'd hook in there and just check skb->protocol to force EAPOL
frames to the lowest rate. However, this didn't work out because in AP
mode the EAPOL frames are injected through a monitor interface and as
such skb->protocol is never initialized (ieee80211_monitor_start_xmit).

The injected frames however already have an 802.11 header and therefore
figuring out the ethertype of the injected frame is not as straightforward as
I liked it to be :(

Can I always be sure that an injected data frame (!=nullfunc) has a rfc1042
header following after the 802.11 header?

Another option would be to let hostapd specify a fixed tx rate in the radiotap
header (and extend mac80211 to understand it). However, since some drivers
also make use of skb->protocol (to forbid aggregation for example) it sounds
more sane to initialize it also for injected frames.

Any other ideas?

Thanks,
Helmut