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

Yang Su
2009-08-10T07:20:28Z

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.

Regards,
Yang

On Friday 07 August 2009 23:13:46 Yang Su wrote:
> - Some updates: I did more thorough testing. It turns out that this patch
> does really solve the problem. This method seems not able to completely
> eliminate all possible forms of looping which happen in the test cases.
> This result in an special pattern: the rerouting is fast (~15 seconds),
> then (after 3~ 5 seconds) the network enters the looping state and it
> normally takes long time to recover. Last time when I did tests, I stop
> the tests right after I observed the successful rerouting. I didn't look
> into the cause of this problem yet. Any inputs are welcome.

Can you provide logs from the nodes involved ? You also can send them
off-list
to avoid sending too big attachments.

> - In the following example (also appended in the attachment :) ), if I
> understand the current echo cancellation implementation correctly, batman
> will enter permanent looping between A and B. In this example, A send to
F,
> all the links are perfect and have the same delay. Only exception is link
> A-E. It is an asymmetric link.

Why do you think it would loop permanently and why do you think it is the
fault of the echo cancellation ?
Actually, this example should[tm] be pretty easy. Every time the OGM from E
or
F arrive at A via the asymetric link A will apply a severe asymetric link
penalty. All nodes will route via the longer path. Did you try to run this
scenario in your testbed ?

Regards,
Marek

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