[PATCH RFC] mac80211 default tx_last_beacon false (congestion)

[PATCH RFC] mac80211 default tx_last_beacon false (congestion)

[Tim Harvey]

The 802.11 spec states that the STA that generated the last Beacon frame shall
be the STA that response to a probe request.  This is important for congestion
reduction when a probe request is received - only 1 node in an adhoc BSS
will transmit a response.  While mac80211 drivers should provide the
tx_last_beacon function to report if they transmitted the last beacon many
do not.  As an attempt to reduce probe response congestion default this
to 0 such that a node not implementing this capability does not contribute
to unnecessary congestion.

In a modern medium sized office environment I see upwards of 100 probe
requests per second received at a given node from various hardware/OS/drivers
doing zeroconf 'active probing' as opposed to passively listening for beacons.
With a modest 10-node adhoc network consisting of drivers that do not implement
this tx_last_beacon feature, I have seen this result in the simultaneous xmit
of probe responses accumulating to 500 probe responses per second because of
collisions which brings the adhoc network to its knees as well as causes
needless congestion.
---
 net/mac80211/driver-ops.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/net/mac80211/driver-ops.h b/net/mac80211/driver-ops.h
index 4244554..af0c439 100644
--- a/net/mac80211/driver-ops.h
+++ b/net/mac80211/driver-ops.h
@@ -367,7 +367,7 @@ static inline void drv_reset_tsf(struct ieee80211_local *local)
 
 static inline int drv_tx_last_beacon(struct ieee80211_local *local)
 {
-	int ret = 1;
+	int ret = 0; /* default unsuported op for less congestion */
 
 	might_sleep();
 
-- 
1.7.0.4

Re: [PATCH RFC] mac80211 default tx_last_beacon false (congestion)

[Johannes Berg]

On Thu, 2010-12-09 at 13:15 -0800, Tim Harvey wrote:
> The 802.11 spec states that the STA that generated the last Beacon frame shall
> be the STA that response to a probe request.  This is important for congestion
> reduction when a probe request is received - only 1 node in an adhoc BSS
> will transmit a response.  While mac80211 drivers should provide the
> tx_last_beacon function to report if they transmitted the last beacon many
> do not.  As an attempt to reduce probe response congestion default this
> to 0 such that a node not implementing this capability does not contribute
> to unnecessary congestion.
> 
> In a modern medium sized office environment I see upwards of 100 probe
> requests per second received at a given node from various hardware/OS/drivers
> doing zeroconf 'active probing' as opposed to passively listening for beacons.
> With a modest 10-node adhoc network consisting of drivers that do not implement
> this tx_last_beacon feature, I have seen this result in the simultaneous xmit
> of probe responses accumulating to 500 probe responses per second because of
> collisions which brings the adhoc network to its knees as well as causes
> needless congestion.

FWIW, I'm OK with this. It shouldn't really have much adverse effect.

johannes