[mesh-dev] We need to talk about batman

[Max B]

Not that I'm arguing in favor of layer 2 vs layer 3 forwarding (although 
we're already pretty deep in certain parts of layer 2 implementations), 
but why can't we just match the MTU of the open0 interface to the bat0 
interface?


On 10/17/14, 11:51 AM, Alexander Papazoglou wrote:
> Hello mesh-dev.
>
> I think we may finally have an explanation of the vexing issue of "I can't
> connect to the internet over peoplesopen.net <http://peoplesopen.net>."
>
> Marc and I spent some time staring at wireshark dumps and  thinking
> about why some clients are unable to consistently connect via the
> tunnel last night. I think Marc came up with a disappointing but correct
> answer: it is basically an mtu issue (mtu is not being discovered
> correctly), BUT there is no good fix because we are tunneling at layer 2.
>
> When a packet arrives at a node from a client with too large an mtu,
> what SHOULD happen in a normal forwarding situation (per RFC 1191)
> is that the node issue a ICMP "Destination Unreachable" packet with a 
> "Fragmentation required" code. The client then uses this information to
> reset its mtu.
>
> This doesn't happen because we aren't really forwarding (forwarding 
> happens
> at layer 3). Instead, our interfaces (open0 and bat0) are bridged. So 
> if a frame
> coming from open0 doesn't fit into bat0 it most likely gets silently 
> dropped.
>
> So bridging open0 with bat0 is a disaster. A quick fix might be to replace
> bridging with forwarding (at the IP level). I suspect this is not the 
> right thing
> to do. It might be better to abandon the idea of meshing at layer 2; there
> are numerous advantages to this.
>
> In any case; we should discuss options this Tuesday.
>
> Alex
>
>
>
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