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Symmetrical Airtime Link Metric Report and Path Loop Avoidance

This presentation proposes solutions to improve the current 802.11s airtime link metric by including a power saving factor and ensuring symmetrical metrics in peer link MPs to avoid path loop and routing instability issues.

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Symmetrical Airtime Link Metric Report and Path Loop Avoidance

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  1. Symmetrical Airtime Link Metric Report and Path Loop Avoidance Date: 2008-07-14 Authors: L. Chu Etc.

  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. L. Chu Etc.

  3. 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. Issue: Airtime Link Metric Equation in Draft: O: varies depending on PHY Bt: 8192 bits ef: frame error rate L. Chu Etc.

  4. 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. Solution: Airtime Link Metric with Power Saving factor Proposed Equation: L. Chu Etc.

  5. 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. Current Link Metric Report L. Chu Etc.

  6. First source MP1 tries to set up a path to destination MP5 (Suppose all MPs receive broadcast PREQ correctly): MP1MP5: MP1->MP2->MP4->MP5. MP5MP1: MP5->MP4->MP2->MP1. Then source MP6 tries to set up a path to destination MP1 (Suppose all MPs receive broadcast PREQ correctly): MP6MP1:MP6->MP4->MP3->MP1. MP1  MP6: MP1->MP3->MP4->MP6. Now the path from MP1 to MP5 and the 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! Different Forward/Reverse Paths with Unequal Peer Link Metrics MP2 1 1 1 MP5 2 2 1 MP1 MP4 x x: Metric in MPj of the Link from MPi to MPj 2 2 MPj 2 MPi y: Metric in MPi of the Link from MPj to MPi 1 2 MP3 MP6 y 1 Mesh Network L. Chu Etc.

  7. If bi-directional link metric is required, a MP should 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 should request a link metric from its neighbor peer MPs. A MP should 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. Improved Link Metric Report L. Chu Etc.

  8. Path Loop with Asymmetrical Peer Link Metrics • MP1 sends PREQ to MP6. • Then MP5 sends PREQ (DO=0, RF=0) to MP6. x x: Metric in MPj of the Link from MPi to MPj MPj MPi y: Metric in MPi of the Link from MPj to MPi y MP5 MP2 MP5 MP1’s PREQ 2 2 1 MP2 2 2 1 MP5’s PREQ 1 1 1 1 PREP 1 1 1 MP1 MP4 1 1 MP1 MP4 1 2 2 1 DST: Destination NHP: Next Hop DSN: DST Sequence Number FIB: Forward Information Table 1 1 1 2 2 MP6 2 2 2 MP3 MP6 2 MP3 L. Chu Etc.

  9. Path Loop with Asymmetrical Peer Link Metrics • MP6 responds with PREP. x x: Metric in MPj of the Link from MPi to MPj MPj MPi y: Metric in MPi of the Link from MPj to MPi y MP5 MP5 MP1’s PREQ MP2 MP2 2 2 2 1 2 1 MP5’s PREQ 1 1 PREP 1 1 1 1 1 1 2 MP1 MP4 2 MP1 MP4 1 1 DST: Destination NHP: Next Hop DSN: DST Sequence Number FIB: Forward Information Table 1 1 1 1 2 2 2 2 MP6 MP6 2 2 2 2 MP3 MP3 L. Chu Etc.

  10. 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. x x: Metric in MPj of the Link from MPi to MPj MPj MPi y: Metric in MPi of the Link from MPj to MPi y MP1’s PREQ MP5’s PREQ MP2 MP5 MP2 MP5 2 1 2 2 2 1 PREP 1 1 1 1 1 1 DST: Destination NHP: Next Hop DSN: DST Sequence Number FIB: Forward Information Table 1 1 MP1 MP4 MP1 MP4 1 1 2 2 1 1 1 1 2 2 MP6 2 2 2 MP6 2 MP3 MP3 L. Chu Etc.

  11. Path Loop with Asymmetrical Peer Link Metrics (Cont’d) • Path loop for MP6 occurs! • MP4MP2MP1MP3MP4 MP2 MP5 MP1 MP4 MP6 MP3 L. Chu Etc.

  12. 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. L. Chu Etc.

  13. Straw Poll • Do you think all or part of Path Loop problem, Symmetric Link Metric problem, Power Saving Link Metric problem should be solved based on the methods or the updated methods proposed in 11-08/636? • Yes No Do not know L. Chu Etc.

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