[mesh-dev] We need to talk about batman

[Alexander Papazoglou]

Max,

We can't do what you're suggesting because the open0 interface is not
operating as a layer 3 interface. It is
bridged to br-openmesh along with bat0. This means that an over-sized
packet (e.g. one that doesn't fit in the
L2TP envelope of size ~1400bytes) arriving at open0 and headed toward bat0,
wouldn't trigger an ICMP
response, and would be unceremoniously dropped.

The ICMP response is triggered by the IP protocol layer (layer 3). That
response is also the only way
a Windows (and I think OS X) client would know that the mtu is smaller than
it thinks.

Assuming this is correct, which is still up for debate, we have two options:

(1) find a way to make the Win/OSX (Android/iOS?) client understand that it
must use a lower mtu
     (DHCP is not an option).
(2) remove the bridge and forward at layer 3 (so that ICMP responses would
be triggered, and the
     client can discover its mtu).

Alex


2014-10-17 12:06 GMT-07:00 Max B <maxb.personal at gmail.com>:

>  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."
>
> 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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