Segment protection failure when recovery LSPs overlap

[Nic Neate <Nic.Neate@dataconnection.com>]

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