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

|------------> | From: | |------------> |Marek Lindner <lindner_marek(a)yahoo.de> | |------------> | To: | |------------> |"The list for a Better Approach To Mobile Ad-hoc Networking" <b.a.t.m.a.n(a)lists.open-mesh.net> | |------------> | Date: | |------------> |03.08.2009 12:23 | |------------> | Subject: | |------------> |Re: [B.A.T.M.A.N.] batman goes looping... | |------------> | Sent by: | |------------> |b.a.t.m.a.n-bounces(a)lists.open-mesh.net | On Thursday 30 July 2009 22:32:04 Yang Su wrote: a It Changing the route based on the (fastest) sequence number has some drawbacks which we experienced before (BATMAN III). It tends to discriminate longer but better routes and favors short paths. In some asymetric link (worst case) scenarios it would route against all odds, simply because receiving a fast packet (newest seqno) does not mean we can send the same way back. If possible I'd like to avoid this strict seqno check. - You are right. Under certain circumstance, this approach may lead to less optimal route selection. Just to understand batman algorithm better, I try to make an example (please correct me if something's wrong here): There are two direct neighbors via which a sender communicates with a receiver. Via the good neighbor, their is a single good route with long delay. Between the bad neighbor and the receiver there are many parallel bad routes. Each bad route has high packet loss rate but very short delay. The receiver's OGMs arrive at the sender first via the bad neighbor. Although there is substantial OGM loss on each individual bad routes, by aggregation over those parallel bad routes, the bad neighbor is still able to relay most of sender's OGM to the receiver. As a result, if the sender already chooses the bad neighbor as the next hop, it will stick to it for quite long time and does not switch to the good neighbor. I attached another patch which will conduct a route switch only if the TQ of the sending neighbor is better than our current best route (no negative switching anymore). I tested the patch here and it works so far but my environment is less controlled than yours. Could you perform the same test on your setup ? - This approach achieves the similar goal as the seqno-based one: switch to a neighbor only when this neighbor is better and has more up-to-date information than the current best route. But this approach is less constrained and seems not to suffer from the above problem for the seqno-based approach. I have tested the patch on our test environment. It works pretty well on the test cases. observation: contained echoed towards happens use Right, every node will emit his currently best TQ value but I did not understand how we can use that. If we send him a better TQ he will send back that number. If we send a bad TQ he will send his good number. Furthermore, each hop will apply some asymetric / hop / wifi penalty that we pull into our routing database ? - The problem might happen when the neighbor thinks that I am his best neighbor (his good number is actually from me). In this case, my estimation of that neighbor's TQ is actually decided by my own TQ (my TQ minus one hop penalty). When my TQ drops down, the correct behavior should be that my estimation of that neighbor's TQ also drops down accordingly. However, when echo cancellation steps in, my estimation of that neighbor's TQ will not be updated and remains to be the stale value. This might cause problems (e.g. looping) in corner cases even with the new patch. Does this analysis make sense or I miss some details of the echo cancellation? Regards, Marek [attachment "switch-route-with-better-tq.patch" deleted by Yang Su/NSUYAN/CH/Ascom] _______________________________________________ B.A.T.M.A.N mailing list B.A.T.M.A.N(a)lists.open-mesh.net https://lists.open-mesh.net/mm/listinfo/b.a.t.m.a.n