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From: Marek Lindner <lindner_marek@yahoo.de>
To: Daniel Halperin <dhalperi@cs.washington.edu>
Subject: Re: [PATCH] mac80211: minstrel_ht should not override user supplied rts setting
Date: Sun, 29 Jan 2012 03:58:34 +0800
Cc: Felix Fietkau <nbd@openwrt.org>, linux-wireless@vger.kernel.org
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On Sunday, January 29, 2012 03:17:32 Daniel Halperin wrote:
> Adaptive RTS/CTS is important to control for interference---RTS/CTS is
> there so that other Wi-Fi devices know not to interfere with your
> flow. It may well be the case that this is not a major problem in
> whatever your scenario you're working, Marek, but it makes a big
> difference in a many bad corner cases.

I am very well aware of what RTS/CTS is for and I am not against using it. 
However, having the rate algorithm overriding the user setting for a specific 
rate is extremely hard to grasp or debug.
To give you an understanding where I am coming from: In our networks we 
experienced extremely variable throughput ranging from 30Mbit/s (single 
stream) to 0.2 Mbit/s in the next second. As you can imagine we were trying to 
figure out what was going on. After spending some time and losing hair I 
noticed that the RTS/CTS implementation of our driver (ath9k) is buggy. Every 
once in a while the nodes would "battle" each other with RTS/CTS packets. 
Strange enough for us, sometimes no RTS packets were sent and sometimes they 
would battle. All attempts to disable RTS/CTS for the mere sake of testing(!) 
was impossible. First I thought ath9k did not properly implement the "iw rts" 
setting but that wasn't the case. After losing more hair I realized that 
minstrel_ht, a rate control algorithm(!), was overriding my rts settings but 
not always(!). The only way to make RTS shut up was to apply the posted patch.

We should make a difference between the idea (using RTS to increase the 
reliability) and its implementation. No matter from which angle I look, the 
current implementation gives me much pain.


> I'm with Felix: I could see adding an option that means "force never
> use RTS/CTS" but I think we're at the right operating point when
> interpreting the standard "RTS/CTS threshold" option.

Actually, this option exists already. It's just minstrel_ht overriding it:
iw phy <phyname> set rts <rts threshold|off>
Put special attention towards the "off" at the end.

If we let the rate algorithm override our rts settings it should at least be 
communicated. I found no such mention anywhere. 

I'd like to further stress the point that no other rate algorithm does this. 
We create incoherent and hard to debug behavior.

Regards,
Marek