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Symmetrical Airtime Link Metric Report and Path Loop Avoidance
Feb, 2008 doc.: IEEE /xxxx July 2008 Symmetrical Airtime Link Metric Report and Path Loop Avoidance Date: Authors: L. Chu Etc. Liwen Chu, STMicroelectronics
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Feb, 2008 doc.: IEEE /xxxx July 2008 Abstract The current s 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. Liwen Chu, STMicroelectronics
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Issue: Airtime Link Metric
July 2008 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. Equation in Draft: O: varies depending on PHY Bt: 8192 bits ef: frame error rate L. Chu Etc.
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Solution: Airtime Link Metric with Power Saving factor
July 2008 Solution: Airtime Link Metric with Power Saving factor BI is the default mesh beacon interval. If a MP is a forwarding power saving MP, K is set to 1. If a MP is a normal MP, BI is set to zero. The new airtime link metric makes it less likely that a forwarding power saving MP to be selected as a forwarding node. Proposed Equation: L. Chu Etc.
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Current Link Metric Report
July 2008 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. L. Chu Etc.
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Different Forward/Reverse Paths with Unequal Peer Link Metrics
July 2008 Different Forward/Reverse Paths with Unequal Peer Link Metrics First source MP1 tries to set up a path to destination MP5 (Suppose all MPs receive broadcast PREQ correctly): MP1MP5: MP1->MP2->MP4->MP5. MP5MP1: MP5->MP4->MP2->MP1. Then source MP6 tries to set up a path to destination MP1 (Suppose all MPs receive broadcast PREQ correctly): MP6MP1: 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: MP5MP1: MP5->MP4- >MP3->MP1. 1 MP2 1 1 MP5 2 2 MP1 MP4 1 x x: Metric in MPj of the Link from MPi to MPj 2 2 2 MPi MPj y: Metric in MPi of the Link from MPj to MPi 1 MP3 2 1 MP6 y Mesh Network L. Chu Etc.
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July 2008 Different Forward/Reverse Paths with Unequal Peer Link Metrics (Cont’d) Each time MP1 or MP6 starts a route maintenance, there will be a path vibration! Path to MP1 Path to MP5 Path to MP6 MP2 MP5 MP1 sent up the path to MP5 MP1 MP4 MP6 tries to sent up path to MP1 MP1 maintains the path to MP5 MP3 MP6 L. Chu Etc.
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Improved Link Metric Report
July 2008 Improved Link Metric Report 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. L. Chu Etc.
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Path Loop with Asymmetrical Peer Link Metrics
July 2008 Path Loop with Asymmetrical Peer Link Metrics MP1 sends PREQ to MP6. Then MP5 sends PREQ (DO=0, RF=0) to MP6. x MPi MPj x: Metric in MPj of the Link from MPi to MPj y: Metric in MPi of the Link from MPj to MPi y MP5 MP5 MP2 2 MP2 2 1 MP1’s PREQ 2 2 1 1 MP5’s PREQ 1 1 1 1 1 MP1 1 1 MP4 PREP MP1 1 1 MP4 2 2 1 1 DST: Destination NHP: Next Hop DSN: DST Sequence Number FIB: Forward Information Table 1 1 2 2 2 2 MP6 2 MP6 2 MP3 MP3 MP4 FIB DST NHP DSN Metric MP1 MP3 1 2 MP5 MP6 FIB DST NHP DSN Metric MP1 MP4 1 4 MP3 FIB DST NHP DSN Metric MP1 1 MP4 FIB DST NHP DSN Metric MP1 MP3 1 2 L. Chu Etc.
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Path Loop with Asymmetrical Peer Link Metrics
July 2008 Path Loop with Asymmetrical Peer Link Metrics MP6 responds with PREP. x MPi MPj x: Metric in MPj of the Link from MPi to MPj y: Metric in MPi of the Link from MPj to MPi y MP5 MP5 MP1’s PREQ 2 MP2 2 MP2 2 1 2 1 MP5’s PREQ 1 1 1 1 1 1 PREP MP1 1 1 MP4 2 MP1 1 1 MP4 2 DST: Destination NHP: Next Hop DSN: DST Sequence Number FIB: Forward Information Table 1 1 1 1 2 2 2 2 2 MP6 MP6 MP3 2 2 MP3 2 MP2 FIB DST NHP DSN Metric MP5 MP4 1 2 MP4 FIB DST NHP DSN Metric MP1 MP3 1 2 MP5 MP6 MP1 FIB DST NHP DSN Metric MP6 MP3 1 6 MP3 FIB DST NHP DSN Metric MP6 MP4 1 2 MP1 L. Chu Etc.
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Path Loop with Asymmetrical Peer Link Metrics (Cont’d)
July 2008 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 MPj x: Metric in MPj of the Link from MPi to MPj MPi y: Metric in MPi of the Link from MPj to MPi y MP1’s PREQ MP2 MP5 MP2 MP5’s PREQ 2 2 1 2 2 MP5 1 PREP 1 1 1 1 1 1 1 DST: Destination NHP: Next Hop DSN: DST Sequence Number FIB: Forward Information Table MP1 1 MP4 MP1 1 1 MP4 1 2 1 1 1 2 2 2 MP6 2 2 MP3 MP6 2 2 MP3 MP4 FIB DST NHP DSN Metric MP1 MP3 1 MP5 2 MP6 MP2 8 MP1 FIB DST NHP DSN Metric MP6 MP3 1 4 MP5 MP2 3 MP2 FIB DST NHP DSN Metric MP5 MP4 1 2 MP6 MP1 6 L. Chu Etc.
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Path Loop with Asymmetrical Peer Link Metrics (Cont’d)
July 2008 Path Loop with Asymmetrical Peer Link Metrics (Cont’d) Path loop for MP6 occurs! MP4MP2MP1MP3MP4 MP4 FIB DST NHP DSN Metric MP1 MP3 1 MP5 2 MP6 MP2 8 MP2 FIB DST NHP DSN Metric MP5 MP4 1 2 MP6 MP1 6 MP2 MP5 MP1 MP4 MP3 MP6 MP1 FIB DST NHP DSN Metric MP6 MP3 1 4 MP5 MP2 3 MP3 FIB DST NHP DSN Metric MP6 MP4 1 2 MP1 L. Chu Etc.
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Possible Path Loop Avoidance Methods
July 2008 Possible Path Loop Avoidance Methods Solution 1: a MP disables PREQ with DO being set to 0. This create to much restriction to the routing protocol and the path creation delay may become longer Solution 2: A MP shall create a new PREP based on FIB information when PREP[D].DSN equals to FIB[D].DSN and PREP[D].metric is larger than FIB[D].metric: Sets PREP[D].metric to FIB[D].metric, Sets PREP[D].hop to FIB[D].hop, Sets PREP[D].lifetime to FIB[D].lifetime. Solution 3: A MP shall discard received PREP when PREP[D].DSN equals to FIB[D].DSN && PREP[D].metric is larger than FIB[D].metric. Since receiving a PREP with PREP[D].DSN being equals to FIB[D].DSN means that PREP creates a path loop, accepting PREP with PREP[D].metric being smaller than FIB[D].metric may also create a path loop. L. Chu Etc.
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Path Loop With Possible Solution 3
July 2008 Path Loop With Possible Solution 3 MP8 sends PREQ to MP1. MPi x MPj x: Metric between MPi and MPj MP8 5 15 MP9FIB DST NHP DSN Metric MP8 1 5 MP7 FIB DST NHP DSN Metric MP8 1 15 MP9 MP7 1 MP5 MP2 1 1 1 1 MP4FIB DST NHP DSN Metric MP8 MP9 1 6 MP2 FIB DST NHP DSN Metric MP8 MP4 1 7 MP1 MP4 MP10 1 1 1 MP3 1 MP6 MP1FIB DST NHP DSN Metric MP8 MP2 1 8 PREQ PREP L. Chu Etc.
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Path Loop With Solution 3 (Cont’d)
July 2008 Path Loop With Solution 3 (Cont’d) MP1 replies with PREP to MP8. MP5 sends PREQ (DO=0, RF=0) to MP8. MP7 replies PREP. x: Metric between MPi and MPj MPi x MPj MP8 5 15 MP8 FIB DST NHP DSN Metric MP1 MP9 1 8 MP9 FIB DST NHP DSN Metric MP8 1 5 MP1 MP4 3 MP9 MP7 MP5 1 MP2 1 1 1 1 MP4 FIB DST NHP DSN Metric MP8 MP9 1 6 MP1 MP2 2 MP7 FIB DST NHP DSN Metric MP8 1 15 MP5 MP1 MP4 MP10 1 PREQ 1 1 MP3 1 MP6 PREP MP2 FIB DST NHP DSN Metric MP8 MP4 1 7 MP1 MP5 FIB DST NHP DSN Metric MP8 MP7 1 16 L. Chu Etc.
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Path Loop Path Loop With Solution 3 (Cont’d)
July 2008 Path Loop Path Loop With Solution 3 (Cont’d) The path to MP8 in MP4, MP9 become invalid and then are removed. Both AODV and HWMP do not require an implementation to increase DSN by one for the invalid path. HWMP does not define the time after which an MP can delete the invalid path. MP2 MP1 MP4 MP3 MP5 1 15 5 MP6 MP9 MP7 MP8 MP10 MPi x MPj x: Metric between MPi and MPj MP9 FIB DST NHP DSN Metric MP8 1 5 MP1 MP4 3 MP4 FIB DST NHP DSN Metric MP8 MP9 1 6 MP1 MP2 2 MP9 FIB DST NHP DSN Metric MP1 MP4 1 3 MP4 FIB DST NHP DSN Metric MP1 MP2 1 2 L. Chu Etc.
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Path Loop Path Loop With Solution 3 (Cont’d)
July 2008 Path Loop Path Loop With Solution 3 (Cont’d) MP10 sends PREQ (DO=0, RF=0) to MP8. MP6 sends PREQ (DO=0, RF=0) to MP8. MPi x MPj x: Metric between MPi and MPj MP8 5 15 MP9 MP7 MP3 FIB DST NHP DSN Metric MP6 MP4 1 2 MP1 FIB DST NHP DSN Metric MP8 MP2 1 8 MP6 MP3 3 1 MP5 MP2 1 1 1 1 MP1 MP4 MP10 MP4 FIB DST NHP DSN Metric MP1 MP2 1 2 MP10 MP6 MP7 FIB DST NHP DSN Metric MP8 1 15 MP5 MP10 MP4 2 1 1 MP10’s PREQ 1 MP3 1 MP6 MP6’s PREQ L. Chu Etc.
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Path Loop With Solution 3 (Cont’d)
July 2008 Path Loop With Solution 3 (Cont’d) MP7 replies PREP. MP1 replies PREP. x: Metric between MPi and MPj MPi x MPj MP8 MP10 FIB DST NHP DSN Metric MP8 MP4 1 17 MP3 FIB DST NHP DSN Metric MP6 MP4 1 2 MP8 MP1 9 5 15 MP9 MP7 1 MP5 MP2 1 1 1 MP4 FIB DST NHP DSN Metric MP1 MP2 1 2 MP10 MP6 MP8 MP3 10 1 MP4 FIB DST NHP DSN Metric MP1 MP2 1 2 MP10 MP6 MP8 MP7 16 MP1 MP4 MP10 1 1 1 MP3 1 MP6 MP6 FIB DST NHP DSN Metric MP8 MP4 1 11 MP7’s PREP MP4 accepts PREP being forwarded from MP3 since PREP[D].DSN equals to FIB[D].DSN && PREP[D].metric is smaller than FIB[D].metric! MP1’s PREP L. Chu Etc.
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Path Loop With Solution 3 (Cont’d)
July 2008 Path Loop With Solution 3 (Cont’d) Path loop for MP8 occurs! MP4MP3MP1MP2MP4 MP4 FIB DST NHP DSN Metric MP1 MP2 1 2 MP10 MP6 MP8 MP3 10 MP2 FIB DST NHP DSN Metric MP8 MP4 1 7 MP1 MP2 MP1 MP4 MP3 MP5 MP6 MP9 MP7 MP8 MP10 MP1 FIB DST NHP DSN Metric MP8 MP2 1 8 MP6 MP3 3 MP3 FIB DST NHP DSN Metric MP6 MP4 1 2 MP8 MP1 9 L. Chu Etc.
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Path Loop Avoidance Methods
July 2008 Path Loop Avoidance Methods Solution 1: A MP shall create a new PREP based on FIB information when PREP[D].DSN equals to FIB[D].DSN and PREP[D].metric is larger than FIB[D].metric: Sets PREP[D].metric to FIB[D].metric, Sets PREP[D].hop to FIB[D].hop, Sets PREP[D].lifetime to FIB[D].lifetime. A MP shall increase DSN by 1 when an active path becomes invalid. A MP shall keep the invalid path for K*dot11MeshHWMPactivePathTimeout. Solution 2: A MP shall discard received PREP when PREP[D].DSN equals to FIB[D].DSN && PREP[D].metric is larger than FIB[D].metric. L. Chu Etc.
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July 2008 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 updated methods proposed in 11-08/636? Yes No Do not know L. Chu Etc.
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