RE: Resetting the sequence number in an authenticated BFD session

["Nitin Bahadur" <[e-mail]>]

Alexander,

 

   I agree that keeping the sequence number consistent between line
cards is not practical. We need a way for a system to indicate that it
wants to restart the sequence.

 

Nitin

 

________________________________

From: Alexander Vainshtein [mailto:[e-mail]] 
Sent: Thursday, January 10, 2008 12:42 PM
To: David Ward; Dave Katz
Cc: Ronen Sommer; BFD WG; Igor Danilovich
Subject: Resetting the sequence number in an authenticated BFD session
Importance: High

 

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