Re: mac80211 + hostapd: EAPOL frames rate selection

On Fri, Jul 29, 2011 at 2:44 PM, Helmut Schaa
[off-list ref] wrote:

Hi,

Hi Helmut,

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 am occasionally seeing this issue in ath9k Station under  heavy
traffic conditions
and low signal(only when I the distance between STA and AP is very
much significant),
some times I could no recreate the issue. I use ath9k-rate control and
I still found that the
EAPOL frames are being sent at lower rate.
with the sniffer capture there are lot of retries for 2nd message.
the timeout comes after 2nd message

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.

in ath9k Raj pointed out that we are not doing aggregation if the
frame is of EAPOL, I can
check if I am missing something.

Any other ideas?

Thanks,
Helmut
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shafi