Re: [B.A.T.M.A.N.] link alternation when radios are not on batman-adv router?

[gtolon@inti.gob.ar]

Hi Simon, thanks for your reply!


El 15/06/2012 06:55 a.m., Simon Wunderlich escribió:
> On Thu, Jun 14, 2012 at 04:51:01PM -0300, gtolon@inti.gob.ar wrote:
>> Hi,
>>
>>      we are interested too in interface alternating, so we made some
>> tests to understand how it works. As you can see on the attached
>> sketch.png, we connected two pair of routers using their ethernet
>> interfaces, E6 with E7, and E8 with E9. All of them have eth0, and
>> an ad hoc interface, wlan0-1, managed by batman. E6 and E8 are in
>> channel 11, whereas E7 and E9 are in channel 1. Besides we used two
>> other routers, E12 and E13, both in channel 11, with their tx power
>> set to just 0 dbm, to avoid a direct sight between them.
>>
>>      Then we sent traffic from E12 to E13. We expected that packets
>> travelled from E12 to E6, and that E6 forwarded them to his eth0 to
>> use the interface alternating feature, making traffic flow to E7,
>> then E9, E8 and finally E13. But instead, we observed that the
>> actual path was E12--E6--E8--E13. The resulting routes for each
>> router are attached in a text file, and also the graph from the
>> batctl vd dot command.
>>
>>      After this result, we read again the thread mentioned by Guido,
>> specially in this part:
>>
>> https://lists.open-mesh.org/pipermail/b.a.t.m.a.n/2012-March/006344.html
>>
>>     And if we understand correctly, the alternation feature works
>> after the batman path has been selected. So in our case, E12 looks
>> at his table to know where to send a packet to E13, and finds E6.
>> Then E6 receives the packet and looks in his own table, finding that
>> the best path to reach E13 is E8. At this point, the alternating
>> should work, but there's only one interface directly connected to
>> E8, so the packet goes there, and so on. We think that if E6 and E7
>> were not two different routers running batman-adv but they were two
>> radios of the same batman-adv router, and the same for E8 and E9,
>> the alternating would work, because the unique router would choose
>> the best path, and then would find two possible interfaces to the
>> same next-hop, changing the interface.
> This is entirely correct - batman-adv has only one link to choose from
> (E6 ->  E8) to reach its best nexthop E8, so there is no way to
> "alternate" the interfaces.
>
>>      We'd like to know if this interpretation is correct, and in that
>> case, if it were possible to use interface alternating in a case
>> like this, with two routers connected to work together. Thanks!
> Mhm, with the current implementation - no, unfortunately not. We would
> need some kind of multipath routing to select between routes, this is
> much more complex.

Ok, i understand.

>
> An alternative might be to use the routers E7/E9 as secondary routers
> without batman, but only forwarding traffic between Ethernet and
> WiFi. Then the "primary" routers (E6 ->  E8) would think they have
> an alternative route via Ethernet (because they don't see the
> intermediate hops E7/E9). This comes with some caveats however, e.g.
> 4-addr mode in Ad-Hoc, you need some very simple ethernet forwarder,
> and most probably other things I forgot.

We had tried with something like that using ap and sta modes in E7 and 
E9, and it hadn't worked. Thanks to your suggestion we noticed the 
necessity of the 4-address mode, so we are now trying with wds:

http://wiki.openwrt.org/doc/recipes/atheroswds

Unfortunately, we haven't found yet a way to use 4-address mode in ad 
hoc. Apparentrly, it's not possible:

http://linuxwireless.org/en/users/Documentation/iw#Using_4-address_for_AP_and_client_mode

Best Regards

Gabriel