1. doc.: IEEE 802.11-08/0636r0 Submission April 2008 L. Chu Etc.Slide 1 Symmetrical Airtime Link Metric Report and Path Loop Avoidance Date: 2008-04-28 Authors:

2. doc.: IEEE 802.11-08/0636r0 Submission April 2008 L. Chu Etc.Slide 2 Abstract The current 802.11s airtime link metric does not include a power saving factor which may give a power saving MP the same chance to become a forwarding node. This may increase end-to-end delay. The current airtime link report mechanism can not guarantee symmetrical airtime link metrics in peer link MPs which may create more unstable forwarding path and path loop. This presentation gives some solutions to these questions.

3. doc.: IEEE 802.11-08/0636r0 Submission April 2008 L. Chu Etc.Slide 3 Issue: Airtime Link Metric The power saving feature is not considered in calculating airtime link metric –A forwarding power saving MP has the same chance to be selected as a forwarding MP This causes a power consumption problem for power saving MPs. This also causes problems with maintaining end-to-end QOS. O: varies depending on PHYB t : 8192 bitse f : frame error rate Equation in Draft:

4. doc.: IEEE 802.11-08/0636r0 Submission April 2008 L. Chu Etc.Slide 4 Solution: Airtime Link Metric with Power Saving factor If a MP is a forwarding power saving MP, BI is the mesh beacon interval. If a MP is a normal MP, BI is zero. The BI term makes it less likely that a forwarding power saving MP to be selected as a forwarding node. Proposed Equation:

5. doc.: IEEE 802.11-08/0636r0 Submission April 2008 L. Chu Etc.Slide 5 Current Link Metric Report If a bi-directional link metric is required, a MP may report new link metric to its neighbor peer MPs. If a bi-directional link metric is required, a MP may request the link metric from its neighbor peer MPs. A MP may update its link metric information factoring in the link metric information received. All these “may” operations can create asymmetric link metrics between two peer neighbor MPs, which leads to an routing instability, path-loops, etc.

6. doc.: IEEE 802.11-08/0636r0 Submission April 2008 L. Chu Etc.Slide 6 Different Forward/Reverse Paths with Unequal Peer Link Metrics First source MP1 try to set up a path to destination MP5 (Suppose all MPs receives broadcast PREQ correctly): –MP1  MP5: MP1->MP2->MP4->MP5. –MP5  MP1: MP5->MP4->MP2->MP1. Then source MP6 try to set up a path to destination MP1 (Suppose all MPs receives broadcast PREQ correctly): –MP6  MP1: MP6->MP4->MP3->MP1. –MP1  MP6: MP1->MP3->MP4->MP6. Now the path from MP1 to MP5 and path from MP5 to MP1 become: –MP1  MP5: MP1->MP2->MP4->MP5. –MP5  MP1: MP5->MP4- >MP3->MP1. Each time MP1 or MP6 starts a route maintenance, there will be a path vibration! MP2 MP1MP4 MP3 Mesh Network MP5 MP6 1 11 1 2 2 1 1 2 22 2 x MPi MPj Link Metric from MPi to MPj

7. doc.: IEEE 802.11-08/0636r0 Submission April 2008 L. Chu Etc.Slide 7 Improved Link Metric Report If bi-directional link metric is required, a MP shall report a new link metric to its neighbor peer MPs each time a different link metric is calculated. If a bi-directional link metric is required, a MP may request a link metric from its neighbor peer MPs. A MP shall update its link metric information using the link metric information received. –A possible link metric updating between MP1 and MP2 is (MP1 metric + MP2 metric)/2.

8. doc.: IEEE 802.11-08/0636r0 Submission April 2008 L. Chu Etc.Slide 8 Path Loop with Asymmetrical Peer Link Metrics MP1 sends PREQ to MP6. Then MP5 sends PREQ (DO=0) to MP1. MP4 FIB DSTNHPDSN Metric MP1MP311 MP5 12 MP2 MP1MP4 MP3 MP5 2 21 1 1 1 2 2 1 1 1 1 MP2 MP1MP4 MP3 MP5 2 21 1 1 1 2 2 1 1 2 1 1 MP4 FIB DSTNHPDSN Metric MP1MP311 MP5 12 MP6 12 DST: Destination NHP: Next Hop DSN: DST Sequence Number FIB: Forward Information Table MP1 FIB DSTNHPDSN Metric MP6MP316 MP5’s PREQ PREP MP1’s PREQ MP6 2 2 2 2 MP3 FIB DSTNHPDSN Metric MP6MP414 MP1 11 MP3 FIB DSTNHPDSN Metric MP1 11

9. doc.: IEEE 802.11-08/0636r0 Submission April 2008 L. Chu Etc.Slide 9 Path Loop with Asymmetrical Peer Link Metrics (Cont’d) MP1 sends PREP to MP5 since it has an effective path to MP6. MP4 will accept the PREP because PREP.DSN==DSN in FIB. Path loop for MP6 occurs! –MP4  MP2  MP1  MP3  MP4 MP2 MP1MP4 MP3 MP5 2 2 1 1 1 1 2 1 1 1 1 MP4 FIB DSTNHPDSN Metric MP1MP3 11 MP5 12 MP6MP2 110 PREQ PREP DST: Destination NHP: Next Hop DSN: DST Sequence Number FIB: Forward Information Table MP2 MP1MP4 MP3 MP5 MP6 2 21 1 1 1 2 2 1 1 2 2 1 1 MP1 FIB DSTNHPDSN Metric MP6MP316 MP5MP213 MP2 FIB DSTNHPDSN Metric MP5MP412 MP6MP118 2 MP6 2 2

10. doc.: IEEE 802.11-08/0636r0 Submission April 2008 L. Chu Etc.Slide 10 Path Loop Avoidance Methods Solution 1: Disable PREQ with DO being set to 0. Solution 2: Create a new PREP based on FIB information when PREP[DST].DSN equals to FIB[DST].DSN and PREP[DST].metric is larger than FIB[DST].metric: –Sets PREP[DST].metric to FIB[DST].metric, –Sets PREP[DST].hop to FIB[DST].hop, –Sets PREP[DST].lifetime to FIB[DST].lifetime. Solution 3: Discard received PREP when PREP[DST].DSN equals to FIB[DST].DSN && PREP[DST].metric is larger than FIB[DST].metric.