[Babel-users] Questions

[Mitar]

Hi!

I admit I have still not read the whole Babel's draft but I am
skipping from sections to sections as I find them interesting.

Does Babel require bidirectional reachability? In draft I read that
Babel tries to determine bidirectional reachability, but I was not
sure if this means only such links are taken into consideration for
routing? In "Cost Computation" section it is written that "if the
txcost is infinite, then the cost is infinite" so probably this means
such links are seen as non-existent (worst cost).

This is of course reasonable, as all packets require ACKs on
link-level so at least something should be coming back. But does
bidirectional reachability than imply also symmetric routing? Or can
Babel decide on an asymmetric route? As I understand it can. But then
why this "artificial" (?) requirement of bidirectional reachability?
Shouldn't this be just left out and if packets get to the other side
(probably with ACKs on link-level) then this is it, this is all we
want to know for routing?

There is probably a spelling error here: "How a the txcost and rxcost
are combined"

I must say that I have problems understanding the symbols in
explanation of the Bellman-Ford algorithm, but probably this is
because I am expecting something else. If I understand now the
description correctly (and my test implementation of it in Haskell
works correctly), then all nodes are computing shortest paths how they
are reachable from other nodes. Not what can they reach? How/when do
then nodes exchange this information?

(I am not reimplementing Babel, just trying to port shortest path
algorithm into my framework so that I can maybe play with other
approaches than shortest path later on.)

So what I am doing is that each node (name one B) sends packets to all
its neighboring nodes about all its known shortest paths to know
nodes. After each node (name one A) receives such packet it checks if
the path over this node (B) is better than currently known path
(taking into the account the cost of the edge from B to A). So what I
am getting at the end is that each node (like A) has information about
how is best for other nodes to send it packets. But this is not really
useful. I would need information in other direction.


Mitar