1. Revisiting Path Switch
July 2007
doc.: IEEE yy/xxxxr0
Revisiting Path Switch
Date: 18 November 2009
Authors:
Menzo Wentink, Conexant

2. Issue
Switching between AP path to direct path and vice-versa without causing any layer-2 or layer-3 reordering of MSDUs. The objective is to define a deterministic mechanism to effect a clean switch
It’s ok to assume that only one path be used at anytime, i.e. it is not suggested that the 2 paths may be used at the same time. Additionally, not concerned with which path is used.
One may choose path that is considered to be viable and keep only one path active
Still we have path transition, which may cause MSDU reordering, and that needs to be addressed.
The main problem is only in the case of switching from AP path to direct path, and this is arising because AP can buffer traffic. As this may, potentially, cause the peer STA to get packets from both paths
Clean switch from direct path to AP path can be done and is transmitter STA’s responsibility, i.e. the Tx STA’s implementation should be able to handle it itself

3. Solutions
During transitioning from AP path to direct path, make sure that there is no traffic queued for TDLS peer at the AP from TDLS originator or
During TDLS setup, ensure there is no traffic queued for TDLS peer at the AP from TDLS originator
Ensuring AP has no traffic
Wait for TBD time – not deterministic and inefficient
Exchange information about last sequence number received at peer STA
Not possible as originator STA may not know AP’s sequence numbers
Insert a Path Switch Request frame (or similar broom frame) and the peer STA can indicate the reception of such a frame

4. Solution #1: Path Switch
Already discussed and present in Draft 2.0 of 11z
Path Switch Request frame should be queued for all ACs in use by originator STA
Is it ok to just queue for lowest priority? (in most cases AC_BK)
Path Switch Response frame should be sent only on direct link
Concern is only with switching between AP path to direct path
Benefit
Path switch can be done without destroying TDLS link

5. Solution #2: Using existing messages
TDLS initiator stops traffic through AP (to peer STA) and queues TDLS setup request frame
TDLS responder stops its traffic through AP (to peer STA) after sending TDLS setup response
Make TDLS setup and response messages to go through the lowest priority active queue to the peer STA through AP (like AC_BK instead of AC_BE)
TDLS initiator may resume sending data to peer STA over direct path after sending TDLS confirm to peer
TDLS responder may resume sending data to peer STA over direct path after receiving TDLS confirm
Only point (3) is the change required in the draft, and (1, 2, 4, and 5) can be recommendations