Re: mac80211 + hostapd: EAPOL frames rate selection

[Felix Fietkau <nbd@openwrt.org>]

On 2011-07-29 7:55 PM, Helmut Schaa wrote:
> On Fri, Jul 29, 2011 at 7:37 PM, Jouni Malinen<j@w1.fi>  wrote:
>>  On Fri, Jul 29, 2011 at 11:14:20AM +0200, Helmut Schaa wrote:
>>>  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.
>>
>>  That sounds like an issue that should be fixed in the rate control
>>  algorithm if it is indeed using unsuitable rate immediately after
>>  association. Dropping data frames completely is not really a good thing
>>  regardless of whether they are EAPOL packets or not..
>
> True. Nevertheless other drivers like madwifi or the ralink legacy drivers also
> force EAPOL frames to a low rate. That's why I had the idea in the first place.
I think this mainly occurs on devices/drivers that use minstrel_ht, but 
lack proper multi-rate retry. minstrel_ht may pick a high rate for 
probing and on devices with a simple fallback table (e.g. rt2x00), it 
may give up on the frame before having tried a low enough rate.

On ath9k this shouldn't be an issue with minstrel_ht, because it always 
keeps the max_prob_rate in a retry slot.

- Felix