Resetting the sequence number in an authenticated BFD session

[Alexander Vainshtein <[e-mail]>]

Hi all,
I have a question related to the expected behavior of sequence numbers in an aythenticated (MD5 or SHA1) BFD session.
 
The corresdponding sections of draft-ietf-bfd-base-06 state that, once the packet has been authenticated by the receiver, its sequence number MUST be checked; if its value is out of range defined by the last received sequence number and the Detect Multiplexor, the packet MUST be discarded.
 
This may result in the a BFD session going down in the situation when the transceiver "loses" the information about its last transmitted sequence number. A suitable use case is a multilink interface (LAG, ML-PPP, etc.) with the links residing in different line cards, and e BFD implemented in one of these cards: if this card fails, the BFD would could be re-started in one of the remaining cards. Such a restart would not affect the local session because the BFD machine would be restarted with bfd.AuthSeqKnown = 0, but keeping bfd.XmitAuthSeq consistent between different line cards seems problematic. (Implemeting BFD in some common card would resolve the situation with the multilink interfaces but would raise similar issues when the common card fails).
 
Note that this problem would not occur for a non-authenticated BFD session.
 
IMHO this problem is real, and I do not see a simple solution for it. 
I would highly appreciate any feedback from the draft authors and/or from the WG.
 
Regards,
                  Sasha