Re: [B.A.T.M.A.N.] batman goes looping...

Yang Su
2009-08-10T07:57:29Z

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|Marek Lindner <lindner_marek@yahoo.de> |
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| To: |
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|"The list for a Better Approach To Mobile Ad-hoc Networking" <b.a.t.m.a.n@lists.open-mesh.net> |
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| Date: |
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|10.08.2009 09:50 |
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|Re: [B.A.T.M.A.N.] batman goes looping... |
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| Sent by: |
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|b.a.t.m.a.n-bounces@lists.open-mesh.net |
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On Monday 10 August 2009 15:20:28 Yang Su wrote:
> For the echo cancellation example: the problem happens with the
interaction
> between A and B. Because of the asym link, A chooses B as its next
neighbor
> towards F. Because the asym path is short, B always receives F's OGM from
A
> first (before B receives the OGM with the same seqno from C). As a
result,
> from A's point of view, F's OGMs rebroadcasted by B are always echoes and
A
> will never update the TQ information towards F via B.
>
> When the quality of any links on the path B to F drops down, B might
choose
> A as its next neighbor towards F and a loop forms between A and B.

Ah, here we have the misunderstanding. Let me briefly outline how the echo
cancellation works (I refer to your PDF-example):

1. Node C emits a packet.
2. Node B receives it and writes the address of Node C in the previous
sender
field because it received that packet via C. Now, B rebroadcasts the OGM.

-- B has already rebroadcasted an earlier OGM with the same sequence number
(received from A). Will B rebroadcast the OGM with the same sequence number
(received from C) again?

3. Node C receives the packet it sent before and discards it as the packet
contains its own address in the previous sender field.
4. Node A happily receives the packet (and goes back to step 2).

Cheers,
Yang