Question on LMP Fault Localization

["Baktha Muralidharan" <muralidb@cisco.com>]

I have question about LMP fault localization process. It appears that fault
localization process requires that LMP be "LSP aware". All other LMP
procedures, in contrast, are "LSP agnostic" and work only with interfaces.

The LMP draft illustrates LMP Fault Localization with two examples. The
configuration for the first example is as follows:

       +-------+        +-------+        +-------+        +-------+
       + Node1 +        + Node2 +        + Node3 +        + Node4 +
       +       +-- c ---+       +-- c ---+       +-- c ---+       +
   ----+---\   +        +       +        +       +        +       +
   <---+---\\--+--------+-------+---\    +       +        +    /--+--->
       +    \--+--------+-------+---\\---+-------+---##---+---//--+----
       +       +        +       +    \---+-------+--------+---/   +
       +       +        +       +        +       +        +       +
       +-------+        +-------+        +-------+        +-------+


"
   In the first example [see Fig. 2(a)], there is a failure on one
   direction of the bi-directional LSP. Node 4 will detect the failure
   and will send a ChannelStatus message to Node 3 indicating the
   failure (e.g., LOL) to the corresponding upstream node. When Node 3
   receives the ChannelStatus message from Node 4, it returns a
   ChannelStatusAck message back to Node 4 and correlates the failure
   locally. When Node 3 correlates the failure and verifies that the
   failure is clear, it has localized the failure to the data link
   between Node 3 and Node 4. At that time, Node 3 should send a
   ChannelStatus message to Node 4 indicating that the failure has been
   localized.
"

In the illustration above, Node 3 verifies that the "failure is clear",
presumably by checking if the interface on its upstream side (i.e. facing
Node 2) is receiving light. However, in this case, there is only one other
interface (besides the one Node 4 reported on) emenating from Node 3 and so,
seems simple enough to check if that interface is receiving light.

Consider the following, slightly modified configuration, in which Node 3 has
multiple interfaces:

       +-------+        +-------+        +-------+        +-------+
       + Node1 +        + Node2 +        + Node3 +        + Node4 +
       +       +-- c ---+       +-- c ---+       +-- c ---+       +
   ----+---\   +        +       +        +       +        +       +
   <---+---\\--+--------+-------+---\    +       +        +    /--+--->
       +    \--+--------+-------+---\\---+-------+---##---+---//--+----
       +       +        +       +    \---+-------+--------+---/   +
       +       +        +       +        +       +        +       +
       +-------+        +-------+        +-------+        +-------+
                                          | | | |
                                          | | | |
                                          | | | |

How does Node 3 know which interface to check to see if the failure is
further "upstream"? It looks like Node 3 needs LSP route knowledge to locate
the "upstream" interface (corresponding to the interface Node 4 reports on)?
If yes, how will the LMP instance on Node 3 gather the LSP information?.
Channel Status only provides interface ID.

Thanks,
/Baktha
Cisco Systems, Inc.