1. QoS with EDCA Downgrading
November 2009
doc.: IEEE yy/xxxxr0
November 2009
QoS with EDCA Downgrading
Date:
Author:
Dave Stephenson, Cisco
John Doe, Some Company

2. November 2009
doc.: IEEE yy/xxxxr0
November 2009
Abstract
This presentation describes the behavior in IEEE dubbed “EDCA Downgrading” and its effect on managing QoS using admission control in an ESS
Dave Stephenson, Cisco
John Doe, Some Company

3. What is “EDCA Downgrading”
November 2009
doc.: IEEE yy/xxxxr0
November 2009
What is “EDCA Downgrading”
EDCA downgrading is the behavior in which a STA transmits un-admitted traffic from a queue requiring admission control whose EDCAF uses the EDCA parameters from a lower queue not requiring admission control
Definitions:
un-admitted traffic: traffic which has not been admitted via an ADDTS request that’s transmitted on an AC configured for mandatory admission control
EDCA uses a fixed UP to AC mapping (Table 9-1)
Example:
QoS AP designates AC_VO as requiring admission control (ACM=1)
non-AP QoS STA does not have admitted TSPEC for UP=6, but frame is transmitted from AC_VO queue having TID in qos_control field set to 6
AC_VO’s EDCAF uses AC_BE’s EDCA parameters (AIFSN, CWmin, CWmax)
Dave Stephenson, Cisco
John Doe, Some Company

4. Managing QoS via Admission Control
November 2009
doc.: IEEE yy/xxxxr0
November 2009
Managing QoS via Admission Control
Admission control effectively manages QoS by enforcing an upper limit on the offered load of prioritized traffic transmitted on a channel
Each STA is permitted to use a metered portion of the available medium time on a channel
An AP measures and maintains a record of channel medium time usage per access category
Frames transmitted using AC_VO, AC_VI and AC_BE EDCA parameters
Frames transmitted by non-AP QoS STAs associated to the QoS AP
Frames transmitted by co-channel QoS APs and STAs
An AP admits/denies each ADDTS Request by comparing channel usage to its internal threshold for that AC
An AP (in promiscuous mode) accounts for traffic according to the TID in the received frame
The EDCA parameters used by transmitter are inferred from the TID—EDCA downgrading breaks the TID:EDCA parameter binding
Dave Stephenson, Cisco
John Doe, Some Company

5. A QoS AP Shouldn’t use EDCA Downgrading
November 2009
doc.: IEEE yy/xxxxr0
November 2009
A QoS AP Shouldn’t use EDCA Downgrading
EDCA downgrading causes head-of-line blocking
Example: frame belonging to un-admitted TS reaches the front of AP’s AC_VO queue and is transmitted using AC_BE’s EDCA parameters
Frames belonging to admitted TSs also enqueued in AC_VO and destined to other non-AP QoS STAs must suffer access delay to medium imposed by AC_BE queuing parameters
Possible QoS DoS attack if non-AP QoS STA sets up un-admitted high datarate downstream flow—repetitive head-of-line blocking
QoS DoS attack also possible with U-APSD operation
Dave Stephenson, Cisco
John Doe, Some Company

6. November 2009
doc.: IEEE yy/xxxxr0
November 2009
Breakage of U-APSD
non-AP STA may have expectation that a bi-directional TS will have the same UP used for downstream frames and upstream frames
If both QoS AP and non-AP QoS STA use EDCA downgrading, then U-APSD still works properly
With same value of UP in both downstream and upstream direction, non-AP QoS STA can properly configure U-APSD trigger and delivery enabled queuing
If (layer above MAC) in QoS AP re-marks UP to avoid head-of-line blocking, then U-APSD breaks
Downstream frame won’t be enqueued in “expected” AC
Dave Stephenson, Cisco
John Doe, Some Company

7. November 2009
doc.: IEEE yy/xxxxr0
November 2009
Recommendation
Add MIB variable dot11RejectUnadmittedTraffic which causes MA-UNITDATA primitive to reject unadmitted traffic and return a status code in MA-UNITDATA.confirm
Layer above MAC can change priority in subsequent MA-UNITDATA.request
Dave Stephenson, Cisco
John Doe, Some Company