[Babel-users] MTU based routing for tunnel based babel networks?

[Johannes Kimmel]

Hi,

I also wish there was some form of ensuring a path with a minimum MTU. 
My use case is providing a minimum MTU for VXLAN overlay networks in 
very heterogeneous networks consisting of different tunnel mechanisms 
(gre, wireguard, via v4 and v6), direct ethernet links and ptp connections.

To allow proper L2 connections with an MTU of 1500, links must at least 
have an MTU of 1570 to have room for unfragmented VXLAN packets. This is 
important since VXLAN VTEPs must not fragment packets [0], which is very 
annoying in this case.

Having some sort of mechanism within babel that propagates routes 
between (not necessarily directly connected) VTEPs, that ensures a 
minimum MTU along a path would be very welcome. Otherwise packets might 
choose a path with lower metric and insufficient MTU, which will cause 
in dropped packets.

Cheers

[0]: https://datatracker.ietf.org/doc/html/rfc7348#section-4.3

On 16.07.23 20:51, Daniel Gröber wrote:
> Hi babelers,
> 
> I've been running babel on top of my wireguard IPv6 network for a while now
> and I have a problem that keeps biting me and I can't find a good solution
> for: babel is oblivious to a link's MTU and picks paths that involve
> wireguard-in-wireguard tunnels even though paths without this stacking are
> available.
> 
> The stacking (and subsequent path MTU reduction) is I belive not even
> bounded, so there is no static MTU I could configure on all my hosts to
> take care of this like one would do with a plain wireguard setup.
> 
> I was able to fix this on my routers by configuring the firewall to drop
> UDP tunnel packets that are going to traverse interfaces with
> MTU<=1440. This works alright but I also have babel running on workstations
> that are behind these routers and there is no good way to classify which
> UDP packets are part of my network's wireguard tunnels and which aren't.
> 
> So this got me thinking (for the hundreth time) perhaps this should be
> something the routing protocol takes care of? Babeld would essentially have
> to pad it's hello packets to a (configurable) size to detect if
> fragmentation is required (or they are being blackholed outright).
> 
> My use-case would be well served if I could just specify a minimum MTU all
> paths must satisfy though more elaborate things could be done I suppose
> (metric based on MTU?).
> 
> Opinions? Anybody have any better ideas on how to prevent this sort of
> tunnel stacking?
> 
> Thanks,
> --Daniel
> 
> PS: Just to clarify why the tunnel stacking happens in my setup: my network
> tunnels IPv6 over IPv4 (most of the time), but I want to support IPv6-only
> underlay networks so I have wireguard tunnels with IPv6 endpoints which can
> in turn get routed over V6-over-V4 wg tunnels (when the ether is flowing
> just right).
> 
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