[Babel-users] ETX metric for bandwidth limited links

Juliusz Chroboczek Juliusz.Chroboczek at pps.jussieu.fr
Thu May 20 16:25:08 UTC 2010

>>> Does it support asymmetric links?

>> Yes.

> Maybe I have not asked the question correctly: does it selects (by
> default) in both directions the best path (even if they are different)
> or does it selects only one path which is the best for both
> directions.

The Babel algorithm choses paths of smallest metric.  This can, in
general, lead to asymmetric routing.

The two metrics implemented in the current version of babeld (2-3 and
ETX) are symmetric -- i.e. the current implementation will give the same
costs to links in both directions.  This implies that in a stable
network, routing will be symmetric unless you manually tweak costs and
metrics (using e.g. filtering).

> Is there some reason why would be prefer in our networks paths which
> are the same for both directions?

Please read the ETX paper[1] to understand why ETX is defined the way it
is defined.

Babeld is fairly modular, and it's trivial to experiment with new
metrics in Babel, so if you have an ideas about how to improve on ETX,
I'm listening.  (But please read Sections 3.4.3, 3.5.2 and 3.6, as well
as Appendix A of the Babel draft[2] before you make any suggestions.)


[1] De Couto, D., Aguayo, D., Bicket, J., and R. Morris, "A
    high-throughput path metric for multi-hop wireless networks",
    Proc. MobiCom 2003, 2003.

[2] http://www.pps.jussieu.fr/~jch/software/babel/draft-chroboczek-babel-routing-protocol.txt

More information about the Babel-users mailing list