• Jouni Malinen's avatar
    mac80211: Send EAPOL frames at lowest rate · 9c1c98a3
    Jouni Malinen authored
    The current minstrel_ht rate control behavior is somewhat optimistic in
    trying to find optimum TX rate. While this is usually fine for normal
    Data frames, there are cases where a more conservative set of retry
    parameters would be beneficial to make the connection more robust.
    
    EAPOL frames are critical to the authentication and especially the
    EAPOL-Key message 4/4 (the last message in the 4-way handshake) is
    important to get through to the AP. If that message is lost, the only
    recovery mechanism in many cases is to reassociate with the AP and start
    from scratch. This can often be avoided by trying to send the frame with
    more conservative rate and/or with more link layer retries.
    
    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.
    
    The best approach for this would likely to be to reduce the TX rate for
    the last rate (3rd rate parameter in the set) to a low basic rate (say,
    6 Mbps on 5 GHz and 2 or 5.5 Mbps on 2.4 GHz), but doing that cleanly
    requires some more effort. For now, we can start with a simple one-liner
    that forces the minimum rate to be used for EAPOL frames similarly how
    the TX rate is selected for the IEEE 802.11 Management frames. This does
    result in a small extra latency added to the cases where the AP would be
    able to receive the higher rate, but taken into account how small number
    of EAPOL frames are used, this is likely to be insignificant. A future
    optimization in the minstrel_ht design can also allow this patch to be
    reverted to get back to the more optimized initial TX rate.
    
    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.
    
    Cc: stable@vger.kernel.org
    Reported-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Tested-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarJouni Malinen <jouni@qca.qualcomm.com>
    Signed-off-by: default avatarJohannes Berg <johannes.berg@intel.com>
    9c1c98a3
tx.c 87.9 KB