[Babel-users] IPv4+6 routing in Babel

[Juliusz Chroboczek]

> Think about a chain of nodes A-B-C. Each node can see each other,
> except A and C.
>
> With IPv6 DAD it can happen that A and C choose the same link-local IP
> for their interface, because they cannot see each other. This would
> make it "difficult" for B to address A and C.

As Gabriel mentioned, there are 62 bits available for choosing the
link-local address, so a collision is rather unlikely.

A collision may happen because of broken hardware, or a broken random
number generator.  In your particular case, packets from B will be sent
to whichever among A and C replies first to a neighbour solicitation,
which might create a black hole.

Note that since we are using a distance vector protocol, only the
traffic that goes through B and one of A and C will be blackholed; no
global pathologies will appear, and the rest of the network will
continue to function normally.

Note further that the pathology requires that A and C be ``near'' each
other.  In the following topology:

  A - B - B' - C

nothing bad will happen if A and C have the same link-local address.
This means that the somewhat dubious practice of ``borrowing'' a wifi
interface's MAC address for constructing the link-local address of
a tunnel interface doesn't break Babel.

                                        Juliusz