QoS with EDCA Downgrading

1. QoS with EDCA Downgrading Date: 2009-11-19 Author: Dave Stephenson, Cisco

2. Abstract This presentation describes the behavior in IEEE 802.11-2007 dubbed “EDCA Downgrading” and its effect on managing QoS using admission control in an ESS Dave Stephenson, Cisco

3. 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

4. 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

5. 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

6. 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

7. 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