[manet] RFC7181 (OLSRv2) trouble with ANSN and router restart

["Rogge, Henning" <[e-mail]>]

Hi,

I think I have potentially identified an issue with OLSRv2 that can lead to a stable desynchronization of the OLSRv2 TC database after a router restart.

The trouble happens because the ANSN (Advertised Neighbor Set Number) can (and should) become stable when the locally reachable neighbors and their metrics don't restart anymore.

The sequence that leads to the issue is:

1) router A restarts
2) router A (randomly?) selects a new Message Sequence Number which is HIGHER (in terms of cyclical comparison) than the last one it used
3) router A selects a new ANSN which is LOWER (or the same) than the last one it used
4) router B sees the new message sequence number/ANSN in TCs from router A
   => router B does not allow the old TC data to timeout (message sequence number is higher!)
   => router B does NOT overwrite the old TC data (ANSN is lower)

this situation will continue as long as the ANSN or router A (which can be stable for an arbitrary time) stays below the ANSN used by the router before its restart

There are two parts to this problem, one of them easy to fix.

a) the ANSN after the restart is lower than the ANSN before. We could just demand that a router does NOT increase the validity time of the TC entry in this case... or that it overwrites the TC entry (the combination of "new message seqno" and "old ANSN" should only happen after a restart)
b) the ANSN after the restart is the SAME as before... this is tricky, I have no idea how to resolve this at the receiver without comparing the TC data with the database, which is not reliable when we deal with incomplete TCs.

What do you think?

Henning Rogge