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RE: Segment protection failure when recovery LSPs overlap
Nic,
RFC4873 by means of SRRO allows nodes to determine existence of
upstream/downstream recovery segments as carried in Path/Resv message.
Combined with RFC4426 and RFC4428 that refers to master/slave it results
that either C (or F) trigger a recovery action by means of disjoint
recovery segments.
Thanks,
-d.
> -----Original Message-----
> From: Nic Neate [mailto:Nic.Neate@dataconnection.com]
> Sent: Friday, February 20, 2009 4:05 PM
> To: ccamp@ops.ietf.org
> Cc: labn - Lou Berger; IBryskin@advaoptical.com;
> PAPADIMITRIOU Dimitri; Aria - Adrian Farrel Personal
> Subject: Segment protection failure when recovery LSPs overlap
>
> Hi CCAMP,
>
> I'd like to raise one more issue with RFC4873 segment
> recovery, which I believe will lead to data loss when
> overlapping segment recovery LSPs are used.
>
> RFC4873 allows topologies like this one:
>
> K-----------L
> / \
> A===B===C===D===E===F===G===H
> \ /
> I-----------J
>
> A working LSP A-B-C-D-E-F-G-H is protected by two overlapping
> segment recovery LSPs: B-K-L-F and C-I-J-G. The recovery
> scheme is 1:1 protection with extra traffic.
>
> Suppose the link D-E fails:
>
> K-----------L
> / \
> A===B===C===D x E===F===G===H
> \ /
> I-----------J
>
> My understanding is that the failure will be handled as follows.
>
> - D detects the link failure, and sends Notify to C (first
> Notify object
> in the received Path). C and G exchanged Notify messages
> to remove
> extra traffic from the C-I-J-G repair, and then send and receive
> traffic from the working LSP on C-I and G-J.
>
> - Meanwhile, E also detects the failure, and sends Notify to
> F (first
> Notify object in the received Resv). F likewise exchanges Notify
> messages with B to remove extra traffic from the B-K-L-F
> repair, and
> and then send and receive working LSP traffic on B-K and F-L.
>
> That results in the following data flow:
>
> K----->-----L
> / \
> A->-B <-C D E F-> G<--H
> \ /
> I-----<-----J
>
> Forward traffic reaches G on the link F-G. However, G has
> switched to send and receive on G-J, and so drops traffic
> received from F.
>
> Reverse traffic reaches B on C-B. However, B has switched to
> send and receive on B-K, and so drops traffic received from C.
>
> Thus traffic is lost in both directions.
>
> Can anyone point out an error in this analysis? Is this a
> topology that there is interest in supporting?
>
> Thanks,
>
> Nic
>