Re: [B.A.T.M.A.N.] MTU/connection problem

[Clemens John <clemens-john@gmx.de>]

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On Thursday 17 June 2010 14:07:09 Linus Lüssing wrote:
> Yep, tap mode in openvpn is adding an extra header, it
> encapsulates not only the ip-packet but also the ethernet frame
> into udp or tcp. (By the way, tun-mode also adds an
> extra header, but the packets are smaller - only IP packets
> encapsulated in UDP or TCP.)
> 
> May I ask whether you are bridging tap0 or if you are routing the
> packets (so having an ip address on tap0 and having according
> entries in your routing table)? If it's the latter, then you could
> just decrease the MTU on the tap0 interfaces to a fitting size and
> let the VPN routers do the PMTU discovery stuff automatically. But
> of course, then you probably wouldn't need tap-mode in this
> scenario, as it just adds additional overhead with the unnecessary
> ethernet frame in between.
> 
> I know that tinc has two little, a bit hacky features in case of
> bridging tap0 with tap-mode (they call it switch-mode) to inform
> the other machines of the lower MTU in between. But I haven't
> heard of OpenVPN having similar features.
> 
> So I guess the easiest step would be the first suggestion, to do
> routing in between and lowering the MTU on the tap interfaces for
> a start before starting to experiment with (experimental) features
> and/or more complicated setups :).
> 
> Cheers, Linus

We tried tinc now, but the problem does still exist.
Our Tinc config looks like this:

----------
root@OpenWrt:~# cat /etc/tinc/batvpn/tinc.conf 
Hostnames=yes
Mode=Switch
name=floh1111
ConnectTo=batgw
#ConnectTo=harlingen
-----------

Normal ping over vpn till package size 1453 works.
If I increase the package size to 1454 ping fails.

All devices have an mtu of 1530 except bat0 and br-mesh which have an mtu of 
1500.
Batman is running on the tinc device tap0 and our bridge looks like this:

---------
config 'interface' 'mesh'
        option 'type' 'bridge'
        option 'ifname' 'ath0 bat0'
        option 'proto' 'static'
        option 'ipaddr' '10.18.1.101'
        option 'netmask' '255.255.128.0'
        option 'mtu' '1500'
        option 'dns'   '10.18.0.1 10.18.0.254'
--------------

If I decrease the mtu on the client that has no batman advanced to 1481, 
everything works fine.

Think we need help again^^
Clemens

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