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List:       linux-wireless
Subject:    Re: [PATCH] mac80211: Send EAPOL frames at lowest rate
From:       John Lempkowski <johnl () memorylink ! com>
Date:       2015-02-27 19:27:32
Message-ID: loom.20150227T191942-361 () post ! gmane ! org
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Jouni Malinen <jouni@...> writes:

> In most cases, minstrel_ht is currently using the initial EAPOL-Key
> frames for probing higher rates and this results in only five link layer
> transmission attempts (one at high(ish) MCS and four at MCS0). While
> this works with most APs, it looks like there are some deployed APs that
> may have issues with the EAPOL frames using HT MCS immediately after
> association. Similarly, there may be issues in cases where the signal
> strength or radio environment is not good enough to be able to get
> frames through even at couple of MCS 0 tries.

This sounds similar to a problem I discovered while hacking on older
software. EAPOL frames were being dropped at a higher rate than were UDP
frames of any size, but only when there was other traffic. Eventually I
proved that most EAPOL frames in my test setup were actually received by
ath9k, but mac80211 did not always pass them up the stack. It turned out
that hostapd and/or wpa_supplicant were getting the EAPOL frames if and only
if they were not the 2nd or later subframe in an A-MPDU. If this has since
been discovered and fixed upstream, I apologize for the noise.

I spent a couple days trying to find a breakage mac80211's rx handlers which
were definitely getting the disappearing EAPOL packets. Eventually I gave up
and hacked the ath9k transmit routine so it breaks from the aggregation
logic and form a new A-MPDU whenever it encounters an EAPOL frame. This
worked around the auth+reauth issues that caused me to investigate.

The software exhibiting this behavior is a hacked OpenWrt checkout using the
following base versions:

Linux version 3.10.21
Compat-wireless 2013-04-16
Hostapd 2.1-devel
    PKG_VERSION:=20130807
    PKG_REV:=d2ba3d6bd9336ef8fd761a0cc7999824d4c4da41)

> It should also be noted that many drivers that do not use minstrel as
> the rate control algorithm are already doing similar workarounds by
> forcing the lowest TX rate to be used for EAPOL frames.

Perhaps this workaround avoids MPDU aggregation in all those drivers as well.

-John


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