A stub-only implementation of Babel
===================================

Babel is a loop-avoiding distance vector protocol that is suitable for
both wired and wireless networks documented in RFC 6126.  The standard
implementation of Babel is a Unix daemon called babeld, that fits in a few
dozen kilobytes and implements a number of extensions to the protocol.
For more information about Babel and babeld, please see

    http://www.pps.univ-paris-diderot.fr/~jch/software/babel/

The sbabeld daemon is a tiny implementation of Babel that is only suitable
for stub routers (routers that only route between their connected networks
and the rest of the Internet).  It is designed to be small, easy to
understand, and easy to port.  If you can afford to, you should be running
the full babeld daemon, as it converges faster while remaining reasonably
small.

Sbabeld implements the following features of the Babel protocol:

  * bidirectional reachability detection (Hello/IHU);
  * learning the best IPv6 default route and timing it out;
  * announcing an IPv6 /64 prefix;
  * passive participation in the loop-avoidance protocol;
  * participation in the request/response protocol.

It does not implement the following features of the full babeld implementation:

  * IPv4;
  * learning a full routing table;
  * reannouncing learned routes;
  * keeping fallback routes for faster convergence after a failure;
  * automatic redistribution;
  * active participation in the loop-avoidance protocol;
  * link quality estimation for lossy and high-latency links;
  * detection of interface status;
  * history-sensitive route selection;
  * source-specific routing.

The code could be further simplified by removing route selection and
bidirectional reachability detection -- this would free us from having to
determine our link-local addresses, and would avoid the need for
a neighbours table.

Usage
-----

In order to just learn any default routes that are being announced:

    sbabeld eth0 wlan0

In order to additionally announce a /64 prefix:

    sbabeld -p 2001:db8:dead:beef:: eth0 wlan0

Porting
-------

Sbabeld was written for Linux.  It should be easy enough to port to other
systems that support IPv6 -- see the functions get_local_address and
install_default_route in util.c.

I have assumed that for link-local addresses, sin6_scope_id is the
interface index.  That is true on all systems known to me.

Author
------

Juliusz Chroboczek