[Babel-users] Neighbours vs. Router-IDs [was: wanted to map the babel network]

[Juliusz Chroboczek]

> BTW, it would be much nicer if one could get the router IDs of all 
> neighbours instead of only their link-local IP addresses. But I am not 
> sure if those are even exposed in the protocol messages? The problem is 
> that without the router IDs you need to keep track of all the link-local 
> IPs of all the nodes in order to reconstruct the full topology.

As you guessed, it is not easy, and not even possible in general, to 
associate a given link-local IP with a Router-ID.  Since babeld is 
a routing daemon, and since this information is not needed for routing, 
babeld doesn't even attempt to guess -- it leaves it to whoever is 
listening on the monitoring interface.

Let me explain.  A Router-ID is used for identifying the originator of 
a route; if a route goes through 42 hops, the associated Router-ID 
identifies the router that is 42 hops away.  On the other hand, neighbours 
are identified by pairs of the form (interface, link-local address).  The 
two notions are distinct for a very good reason: if a router has multiple 
radios, it counts as multiple neighbours, but it still has just a single 
Router-ID -- otherwise the loop-avoidance mechanism wouldn't reliably 
eliminate loops that go through different interfaces.

Please squint at the following tpology:

   (A1) (B1) (B2)

    *    *     *
    |     \   /
    |      \ /
    A       B

Here, A is a router with a single radio A1, B has two radios B1 and B2 
tuned at the same frequency (for example an omni and a directional 
antenna, or two sector antennas).  If all radios hear each other, A's 
neighbour table looks as so:

   B1 through A1, cost=256
   B2 through A1, cost=321

which is why the link-quality estimator is able to distinguish between B1 
and B2 and choose the better of the two.  Interestingly enough, B's 
neighbour table will be as follows:

   A1 through B1
   A1 through B2
   B1 through B2
   B2 through B1

but the latter two will be discarded by the loop-avoidance mechanism -- 
which only looks at Router-IDs, and hence can detect that B1-B2 is a loop.  
(Yes, Virginia, there is such a thing as a one-hop loop.)

So how do you guess the Router-ID associated with a link-local address? 
Any route announced with a refmetric of 0 will carry the Router-ID you're 
looking for.  If there are no such routes, you're out of luck -- if the 
router is not originating any routes, it's not announcing its Router-ID, 
and the information is simply not available.

-- Juliusz