Babel in Bird

[Ondrej Zajicek]

Hi

During testing of Babel we noticed a strange behavior and we are not sure
if it is a problem in the implementation or in the specification.

The sequence of events is like:

1) Router A is running, originated some routes with seqno old_seqno and
propagated them to a router B.

2) If router A (or Babel protocol) is restarted, the seqno counter used for
originating routes is also reset to 1.

3) Then routes are originated with seqno 1 and propagated to router B.

4) Router B already has these routes with old high seqno, so the update
is considered unfeasible and for selected entries, so it is ignored
(3.5.4).

5) The routes in the router B's table slowly expire.

6) When routes expire, router B sends seqno request (3.8.2.1) with
old_seqno+1.

7) Router A receives seqno request, increases seqno to 2 (3.8.1.2),
originates routes with seqno 2 and sends them to router B.

8) Router B still ignores them as 2 < old_seqno.

9) Subsequent updates are still ignored as router B holds old
timeouted entry as a selected entry with unreachable metric
(as there is no other one) until GC time.


I have two questions w.r.t. this sequence of events:

1) How is router restart and seqnos supposed to be handled without
waiting for route timeout?

2) If a route is selected, then becomes unreachable/retracted, and there
is no other route to be selected, is it still considered selected? I
would say that no as the selection process (3.6) forbids retracted routes
to be selected, but the BIRD implementation keeps the old selected route
(now unreachable) in this case.


-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
"To err is human -- to blame it on a computer is even more so."
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