1. A Simplified Solution For Critical A-MPDU DoS Issues
July 2008
doc.: IEEE /1021r0
September 2008
A Simplified Solution For Critical A-MPDU DoS Issues
Date:
Authors:
Luke Qian etc.
Luke Qian etc, Cisco

2. July 2008
doc.: IEEE /1021r0
September 2008
Abstract
Current operation rules for A-MPDU and BAR facilitate a number of Denial of Service (DoS) attacks as presented in /0703r0. This submission proposes a simplified solution to mitigate the most damaging and easiest-to-launch ones.
Luke Qian etc.
Luke Qian etc, Cisco

3. Overview for the Issues
September 2008
Overview for the Issues
Per current 11n A-MPDU/BA rules, advanced SN in data frames or BAR can advance the left edge of the BA re-ordering buffer on the receiver.
However,
BAR is a control frame which is not encrypted, nor has any authentication information
SN in a data frame is not protected with encryption.
As a result,
a receiver running BA can be exposed to DoS attacks by rogue devices which move the receiver BA reordering buffer with falsely advanced SN, potentially causing subsequent valid frames to be discarded
Such identified DoS attacks include: (Ref /0703)
Forged packets with advanced Sequence Numbers (SN)
Captured and Replayed packets with modified SN.
Captured and Replayed packets with advanced SN without modification.
False Block ACK Request (BAR) with advanced SN.
False BA to prevent retransmission.
They can cause severe performance degradation, such as drop of voice calls, lost connection for TCP traffic etc.
Luke Qian etc.

4. Uniqueness of the DoS Issues
July 2008
doc.: IEEE /1021r0
September 2008
Uniqueness of the DoS Issues
Hit-and-run type of attack
as only one packet is needed to cause the DoS.
So an attacker does not need to be at the spot to launch attacks persistently, making it hard to identify or catch the attackers.
Significantly long period of DoS for a single attack
At the order of tens of seconds.
Can cause disassociations or dropped sessions, especially problematic for tcp sessions and voice connections
A regular DoS, CTS with excessive NAV setting for example, can only cause a DoS for a period of tens of ms, several order of magnitudes less than that of an A-MPDU DoS, and will have to repeatedly launch the attacks.
Luke Qian etc.
Luke Qian etc, Cisco

5. September 2008
The Proposed Approach
Focus on the two easiest-to-launch DoS for a better acceptance in TGn:
a) Forged packets with advanced Sequence Numbers (SN) .
b) False Block ACK Request (BAR) with advanced SN.
Note – they are the “fire and forget” attacks whereby an attacker need nothing but a single packet to launch a DoS.
Luke Qian etc.

6. A Simpler Solution September 2008
July 2008
doc.: IEEE /1021r0
September 2008
A Simpler Solution
Introduce a capability bit to signal the protection for backward compatibility
Transmitter rules:
Never sends BAR or data with a SN which would cause the receiver to advance the left edge over a “hole”
Sends an 11w type of encrypted management action frame, the protected ADDBA, to advance the left edge of the receiver window over a “hole” when needed.
Overload the existing ADDBA request frame
ADDBA request already contains all the required information
Just need to allow an ADDBA request to be used during an established BA session to move the left edge of receiver window
Receiver rules:
On receiving a BAR or data frame which advances the left edge of receiver window over a “hole”,
drop the BAR and flag a DoS attack (immediate detection of attack upon receipt of just one frame from attacker), and tear down BA session to minimize disruption
On receiving a protected ADDBA for an established BA session, adjust the left edge as requested.
Add a footnote in the 11n spec to allow an alternative ordering of BA Reordering after MPDU decryption on the receiver.
But preserve existing ordering option as well for backward compatibility.
Luke Qian etc.
Luke Qian etc, Cisco

7. A Capability Bit for Negotiation: RSN Element changes
September 2008
A Capability Bit for Negotiation: RSN Element changes
Pre-Auth No
Pairwise
PTKSA Replay
Counter
GTKSA Replay
Counter
Reserved
PeerKey
Enabled
Reserved
PBAC
Resv B0 B1 B2 B3 B4 B5 B6 B8 B9
B10
B11
B12
B13
B15
Modified RSN Capabilities subfield of the RSN Element
A bit for signaling the capability: PBAC – Protected BAR Capable
Indicates capability to perform modified BAR rules and decryption ordering
If both STA advertise PBAC=1, then PBAC SHALL be used
If at least one STA of a pair advertises PBAC=0, then PBA SHALL NOT be used
STA that supports PBAC must also indicate TGw (e.g. dot11RSNAProtectedManagementFramesEnabled)
Luke Qian etc.

8. Specification change for order of operations
September 2008
Specification change for order of operations
Allow alternative ordering of Block Ack Reordering AFTER A-MPDU decryption step, but preserve existing ordering option as well for legacy implementations.
Move MPDU Decryption and
Integrity Function to here
Luke Qian etc.