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 sometests 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 packetstravelled 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 thatcase, 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.
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.
Cheers, Simon