RE: Resetting the sequence number in an authenticated BFD session

Alexander Vainshtein <Alexander.Vainshtein@ecitele.com> Fri, 11 January 2008 10:11 UTC

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Thread-Topic: Resetting the sequence number in an authenticated BFD session
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From: Alexander Vainshtein <Alexander.Vainshtein@ecitele.com>
To: David Ward <dward@cisco.com>, Nitin Bahadur <nitinb@juniper.net>
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Subject: RE: Resetting the sequence number in an authenticated BFD session
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David, Nitin and all,
Please see inline below (blue italics)..Unfortunately it seems that none of the options explicitly proposed by David resolve the issue.
 
Regards,
                 Sasha

________________________________

From: David Ward [mailto:dward@cisco.com]
Sent: Thu 1/10/2008 11:51 PM
To: Nitin Bahadur
Cc: David Ward; Alexander Vainshtein; Dave Katz; Ronen Sommer; BFD WG; Igor Danilovich
Subject: Re: Resetting the sequence number in an authenticated BFD session


Solutions include (and are alluded to in the drafts): 

run BFD on bundled interfaces (any flavor) centrally [Sasha] This rasies the issue of the central component failover (as mentioned in my oriinal message).
 
run BFD on all component links independently [Sasha] IMHO this is not a viable option. E.g., consider a L3 1-hop situation where the bundled interfaces run between one of the routers and a L2 switch, while the L3 adjacency is unaware of bundling. This is easily achieved with LAG. 
 
run BFD on a master component link [Sasha] The failure of the LC that carries the "master componet link" is the original scenario described in my original email, and the issue remains unsloved IMO.

There are other variants as well.

-DWard

On Jan 10, 2008, at 3:41 PM, Nitin Bahadur wrote:


	
	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:Alexander.Vainshtein@ecitele.com] 
	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