Message-ID: <20150223111837.11955irpcjwu7dkt@horde.inescporto.pt> (sfid-20150223_111851_198179_4AA5EFBC)
Date: Mon, 23 Feb 2015 11:18:37 +0100
From: =?iso-8859-1?b?TeFyaW8=?= Lopes <mjlopes@inescporto.pt>
To: wim torfs <wtorfs@gmail.com>
Cc: linux-wireless@vger.kernel.org
Subject: Re: Issue with frame injection on monitor interface (ath9k)
References: <20150219155345.54713ultwqjbtrsp@horde.inescporto.pt>
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	<20150220111149.11345nax1t6qevdx@horde.inescporto.pt>
	<54E83020.601@gmail.com>
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Hi.

When doing "iw dev wlan0 set noack_map 0x0009", frames are sent with  
"Ack Policy" set to "No Ack" (at QoS header), receiver don't ack those  
frames and I guess that "minstrel_ht" falls to lowest bitrate possible  
or it is disabled when iw command is executed. At this point, manually  
setting a bitrate with "iw dev wlan0 set bitrates ht-mcs-5 8" doesn't  
throw a error and also doesn't change bitrate to the desired one, so,  
it doesn't work.
I partially agree with you, setting for example MCS 15 after  
noack_map, due to lack of acks and block acks, bitrate could be higher  
than expected because when transmitter is transmitting, with or  
without frame aggregation, it doesn't have to pass to receiver mode  
(hardware level), wait and process each ack/back in order to transmit  
the next frame or next A-MPDU/A-MSDU. Also, it is expected that  
modulation type change to 64-QAM with 2 streams and in practice that  
doesn't happen, it continues at BPSK with 1 spatial stream, probably  
at 802.11a because iwinfo reports 6.0 Mbit/s PHY rate.
Finally, thanks for your suggestion about flags on code.

BR,
M?rio Lopes.


Quoting wim torfs <wtorfs@gmail.com>:

>
>
> On 02/20/2015 11:11 AM, M?rio Lopes wrote:
>
>>
>> Quoting wim torfs <wtorfs@gmail.com>:
>>
>>>
>>>
>>> On 02/19/2015 03:53 PM, M?rio Lopes wrote:
>>>> Hi everyone.
>>>>
>>>> When using frame injection over monitor interface, with handmade packet
>>>> with Radiotap header + QoS + Data, at sender I capture the packet with
>>>> tcpdump and it is equal to the one I sent.
>>>> Although, at receiver station, the packet is diferent, FCS was
>>>> recalculated or forced to active and calculated, MCS is not the one I
>>>> supplied, sequence number (QoS field) is not the same, amongst other
>>>> things.
>>>> Also, QoS Ack policy was "No Ack" at sender (QoS Control = 0x0020), a
>>>> Ack frame was transmitted to sender and the received packet arrived with
>>>> QoS Control = 0x0000.
>>>>
>>>
>>> Mario,
>>>
>>> If I'm not mistaken, the mac80211 parses your injected packet,
>>> retrieves relevant information, such as flags from the radiotap
>>> header, removes the radiotap header from your packet and then proceeds
>>> with sending the packet as any normal packet towards the radio interface.
>>>
>>> This, however, means that either the mac80211 rate control algorithm
>>> is calls or the ath9k rate control algorithm, depending on your kernel
>>> configuration. The ath9k driver uses this rate control information to
>>> compose the tx descriptor, after which the packet is handed over to
>>> the hardware.
>>> I believe it is also the hardware (transmitter side or receiver side,
>>> that I don't know), which fills in the actual rate used for the
>>> transmission in your packet, since the hardware is capable of
>>> retransmitting your packet with a lower rate when transmissions fail
>>> too much at the highest rate from the rate selection algorithm.
>>>
>>> You mentioned that you monitor interface captures the packet as you
>>> have sent it. If I remember correctly, the mac80211 forwards this
>>> packet towards the monitor interface when the tx status of ath9k has
>>> been received, so basically, you receive transmission status
>>> information, along with your originally sent packet. Sequence numbers
>>> should have been adjusted in this packet I think, however, the
>>> transmission rate will be the same as you have specified.
>>>
>>> If you would like to send at a specific rate, you would need to use
>>> the relevant iw commands to fix the transmission rate (or hack ath9k
>>> to transmit at a fixed rate).
>>>
>> Before trying Radiotap header approach, I did disable Ack's with iw but
>> the transmition rate remained at basic rate (6 Mbit/s) despite forcing
>> higher MCS with iw, so it was useless.
>> Could be this a bug in the driver, unicast traffic sent with NoAck flag
>> treated as a broadcast, despite higher bitrate forcing?
>> Returning to Radiotap, what's the point of injecting a frame with
>> radiotap header if it isn't going to be used? I didn't try all the
>> possible combinations, but the ones I tryied, they where changed before
>> arriving at receiver.
>>
>> Thanks.
>>
>> BR,
>> M?rio Lopes.
>>
>
> Mario,
>
> I don't see the relation between the disabling of acks and the rate  
> selection. I don't have a working setup currently, so I can not test  
> it, but fixing the rate with iw should work. See whether you did  
> everything correctly.
>
> I don't have experience with the NoAck flag, so I cannot comment on  
> that, however, when reflecting the standard and previous work, I  
> don't see any reason why the driver would stick to basic rates.  
> Possibly MPDU aggregation is disabled if the NoAck means also no  
> block acks, but this does not prevent you to use MCS rates. Look,  
> this is me just thinking out loud, so I might be wrong here.
>
> As regards radiotap, it does have its use. For instance, if you look  
> into net/mac80211/tx.c and search for ieee80211_parse_tx_radiotap,  
> you will notice that it ensures whether or not the  
> IEEE80211_TX_INTFL_DONT_ENCRYPT and IEEE80211_TX_CTL_DONTFRAG flags  
> are set. Also, it allows you to set the IEEE80211_TX_CTL_NO_ACK flag  
> on a per packet base.
>
> Best regards,
> Wim.
>


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