[PATCH] babel: Rework seqno request handling to track sender, not destination

[Juliusz Chroboczek]

> Right, so after going after this a couple of times, I think I see the
> discrepancy here: in Bird we're currently doing retransmission at every
> hop;

Yep, that's the intent of RFC 8966 Section 3.8.1 (I agree it's not
perfectly clear):

   A single request MUST NOT be duplicated: it MUST NOT be forwarded to
   a multicast address, and it MUST NOT be forwarded to multiple neighbours.

The goal is to have a bound on the number of requests in flight for
a given prefix: it's at most equal to the number of routers that are at
the border of the subnetwork for which the prefix became unfeasible.  If
you start resending requests within the network, you're multiplying a term
that's exponential in the diameter of the feasible part of the network.
And exponentials make me nervous.

-- Juliusz