1. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 1 OBSS HCCA Race Condition Date: 2010-01-17 Authors:

2. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 2 Abstract When implementing the OBSS solution from 802.11aa draft 0.03, a race condition has been discovered in the HCCA TXOP advertisement. The presentation attempts to describe the issue and provides the outline of a potential solution.

3. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 3 HCCA TXOP Advertisement element The HCCA TXOP Advertisement element is used by an AP to advertise its TXOP state to its overlapping APs. Figure 7-aa19 –HCCA TXOP Advertisement element Figure 7-aa20 – TXOP Reservation field

4. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 4 Example (1/4) Two homes, with an AP in each home Both APs from same vendor (i.e. same scheduling alg) Each AP is streaming one 25fps 16Mbps video stream AP 1 AP 2 BEACON HCCA_TXOP[start=2870, dur=10ms interval=40ms] BEACON HCCA_TXOP[start=2885, dur=8ms, interval=40ms]

5. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 5 Example (2/4) Once both APs have received a beacon from the other AP with the HCCA TXOP Advertisement element, they know each other’s current HCCA schedule. However, what happens when both APs receive TSPEC requests in-between beacons?

6. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 6 Example (3/4) time 2870288028852893 Current steady state situation AP A receives a request for a 8Mbps video stream time 2870288028852893 New schedule 29032898 At the next beacon, AP B learns of the new schedule but what if AP B receives a TSPEC request before then?

7. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 7 Example (4/4) time 2870288028852893 Current steady state situation AP A receives a request for a 8Mbps video stream time 2870288028852893 AP A schedule 29032898 AP B also receives a request for a 8Mbps video stream time 2870288028852893 AP B schedule 29032898

8. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 8 Race Condition In-between successfully received beacons, an AP is unaware of new HCCA allocations in neighbouring QAPs But how likely is it that multiple APs will grant TSPECs in the same beacon period? –Murphy’s law – “If it can go wrong, it will” –After a power cut – many devices activating at the same time? –When trying to simulate the OBSS solution

9. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 9 First attempt to fix the problem Randomly choose to allocate at the start or the end of the service interval –Reduces chances of colliding allocations –Fairly easy to implement However, didn’t remove the problem –Still got clashes –Birthday Paradox? –Murphy’s Law? –Bad random number generator?

10. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 10 Second Attempt When making an allocation, send a frame to the other APs telling them of the allocation –Stops other AP allocating the same time slot –No need to wait a beacon period Mostly worked for the case of two APs –Still suspect race condition with >2 APs If both APs got their requests virtually simultaneously, they still made clashing allocations, and then sent a frame to the other AP to tell them about it

11. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 11 Third Attempt When making an allocation, send a request to the other APs telling them of the allocation –If other AP has made a clashing allocation, it reserves a new slot of the same duration as the request and replies with a frame “please don’t use that schedule, here’s a new proposed schedule” –The temporary allocation is held for 3 beacon frames If a request is received from another AP while waiting for a reply to its own TSPEC request, combine the allocation from the received request with own request and issue a “new schedule proposal” –Sort these allocation requests by BSSID

12. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 12 Key for following diagrams Allocated Traffic for AP 1 Allocated Traffic for AP 2 Desired Allocation for AP 1 Temporary Allocation for AP 2 Desired Allocation for AP 2 Temporary Allocation for AP 1 Existing scheduled traffic A desired time slot for a new TSPEC request A temporary allocation, used to offer an alternative to an allocation request from another AP

13. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 13 “Simple” Case Both APs get TSPEC requests at same time, one AP sends notification AP 1AP 2 121212 Allocation notification Alternate schedule 1221 Allocation notification 121 1221 OK 2 1212 Allocation notification 1221 OK 1212 1221 1 21 12 2

14. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 14 “Worst” Case Both APs send their notification frames at the “same” time AP 1AP 2 121212 Allocation notification 1 2 12211221 Alternate schedule Alternate Schedule 2121 1221 1221 Allocation notification OK 12 1221 1221 21 Allocation notification 1221 1221

15. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 15 Conclusions Simulation results suggest there is a race condition –But how often does it occur in the real world? A potential solution is proposed –Requires a new frame exchange sequence

16. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 16 Straw Poll 1 Is this race condition something TGaa needs to solve? –Yes –No

17. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 17 Straw Poll 2 Does the proposed solution outlined in this presentation seem plausible enough to want to see normative text? –Yes –No

18. doc.: IEEE 802.11-10/0062r0 Submission Jan 2010 Alex Ashley, NDS LtdSlide 18 References