[Babel-users] Babel extension mechanism

[Juliusz Chroboczek]

> One of my thoughts for "flags" originally was to indicate a unicast ability

IHUs can be sent over unicast if desired.  Actually, the current
implementation will opportunistically send them over unicast (see
send_ihu in message.c).  It's only HELLOs that can only be sent over
multicast, since doing otherwise would confuse the HELLO seqno counters.

Note that it is not *that* clear that sending IHUs over unicast is
advantageous.  Every unicast packet requires an ND exchange unless
there already is unicast traffic flowing through that neighbour.
Additionally, sending IHUs over multicast allows aggregating multiple
IHUs into a single packet (txops and all of that).

Chaning babeld to send IHUs over unicast is a one-line change.  I'm
looking forward to hard data proving it's worthwile.

(But perhaps we shouldn't bother?  Babel is already an order of
magnitude more economic than all the other currently deployed mesh
routing protocols.)

-- Juliusz