1. July 2008
doc.: IEEE /0833r0
July 2008
A Proposed Scale-down Solution to A-MPDU DoS Related Comments in LB 129
Date:
Authors:
Luke Qian etc, Cisco
Luke Qian etc, Cisco

2. July 2008
doc.: IEEE /0833r0
July 2008
Overview
A number of new types of Deny of Service (DoS) associated with the n A-MPDU BA operations have been identified, commented and acknowledged since LB 115 for n. Resolutions for the relating comments in the recent LB 124 called for solutions less complicated and lower implementation cost than those in /0665r0, the jointly developed solutions. Following the thinking outlined in /0755r1, we present here a scaled-down version of 08/0665r0 which focuses on the DoS types with the most significant damages. Also see LB129 CID 8075, 8076.
Luke Qian etc, Cisco
Luke Qian etc, Cisco

3. Block Ack Security problems
July 2008
Block Ack Security problems
The following security problems exist: ( /0665r0)
The SN values of data packets are not protected – yet, SN values of data packets can be used to adjust the RX Buffer LE value. A single forged SN value can cause the recipient to move the LE value too far forward, thereby causing the recipient to discard frames below the new LE that should not have been discarded. Data is lost at the recipient.
A single forged SN value in a data packet can also cause the recipient to place the received frames in an incorrect order, which can cause problems both when the security layer examines the sequence of PN values in the MAC SN-ordered frames and when the frames are passed to the next layer for processing.
A single forged SN value in a data packet can cause RX scorecard information to be updated, and a subsequent transmission of a BA frame in response to a legitimate AMPDU can include this bogus scorecard information.
A captured and replayed packet cannot be detected except by replay detection in the security layer. If the RX buffer reordering is performed before this check, then the SN in that replayed packet can cause incorrect RX Buffer LE movement.
The BAR frame is not protected – yet the BAR frame SSN value is used to adjust the RX Buffer LE value. A single forged SN value can cause the recipient to move the LE value too far forward, thereby causing the recipient to discard frames below the new LE that should not have been discarded. Data is lost at the recipient.
The BA frame is not protected – yet the BA frame SSN value is used to adjust the originator’s TX scorecard LE value. Forged BA frames can cause false adjustments to the LE value that result in some data packets not being transmitted to the recipient, since they now have SN values below the new LE value. Data is lost.
Forged BA frames can suppress retransmission of frames that were not successfully received (even without moving LE at TX)‏
Luke Qian etc, Cisco

4. Prioritizing the A-MPDU DoS Attacks
July 2008
doc.: IEEE /0833r0
July 2008
Prioritizing the A-MPDU DoS Attacks
Sort the A-MPDU DoS Types on their ease of launching:
(see /0755r1)
1) Forged packets with advanced Sequence Numbers (SN)
easy to launch, can be addressed, e.g., by reversing the order of BA reordering and decryption.
4) False Block ACK Request (BAR) with advanced SN.
easy to launch, can be addressed, e.g., by protecting the BAR by wrapping it in an encrypted management frame, an 11w mechanism.
2) Captured and Replayed packets with modified SN.
more difficult, can be addressed by encrypting the SN, ( drop this one ?)
3) Captured and Replayed packets with advanced SN without modification.
more difficult, less likely to be successful, can be addressed by, e.g., a replay check before BA reordering, ( drop this one?)
5) False BA to prevent retransmission.
less likely be successful, not unique since regular ACK can cause similar DoS., (drop this one?)
The following proposed solution will focus on the most significant and easy-to-launch ones: 1), and 4) .
Luke Qian etc, Cisco
Luke Qian etc, Cisco

5. A Scaled-down Solution
July 2008
doc.: IEEE /0833r0
July 2008
A Scaled-down Solution
A scaled-down solution addressing the most significant few of the problems is:
Use a new protected form of the BAR frame to convey BAR information, and allow this protected BAR frame to cause RX Buffer LE movement while forbidding unprotected BAR frames from making RX Buffer LE changes
Allow alternative architectural ordering of Block Ack Reordering AFTER MPDU decryption, and include a new Block Ack Replay Detection function just before the Block Ack Reordering but preserve existing ordering option as well for legacy implementation
Luke Qian etc, Cisco
Luke Qian etc, Cisco

6. New Rules for the Solution
July 2008
New Rules for the Solution
Unencrypted BAR is not used to shift recipient RX BUFFER LE
Encrypted BAR can shift recipient RX BUFFER LE
STA with hybrid support for secure PN but no support for encrypted BAR can still use unencrypted BAR to shift recipient LE
Only the new protected MGMT frame can be used to perform BAR-style RX BUFFER pointer moves
Luke Qian etc, Cisco

7. Encrypted BAR frame New Action frame Encrypted according to TGw
July 2008
Encrypted BAR frame
New Action frame
Category = Block Ack
Action = BAR
Body = BAR Control, BAR Information (see TGn draft)
Multi-TID version allowed
Uncompressed?
Encrypted according to TGw
Luke Qian etc, Cisco

8. Encrypted BA frame New Action frame Encrypted according to TGw
July 2008
Encrypted BA frame
New Action frame
Category = Block Ack
Action = BA
Body = BAR Control, BAR Information (see TGn draft)
Multi-TID version allowed
Uncompressed?
Optionally includes recipient RX Buffer LE value
To allow originator to synch its TX Buffer with RX Buffer
Encrypted according to TGw
Luke Qian etc, Cisco

9. Specification change for order of operations
July 2008
Specification change for order of operations
Allow alternative ordering of Block Ack Reordering AFTER A new Block Ack Replay Detection function that includes a preceding MPDU decryption step, but preserve existing ordering option as well for legacy implementations.
Add a BlockAck Replay
Detection function here
Move MPDU Decryption and
Integrity Function to here
Luke Qian etc, Cisco