Subject: Question on LMP Fault Localization

From: "Baktha Muralidharan" <[email-masked]>
Date: Fri, 10 Dec 2004 12:44:38 -0500

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.

---

From: "Bradford, Richard" <[email-masked]>
Date: Fri, 10 Dec 2004 14:31:31 -0500

>configuration for the first example is as follows:
Yes, LMP needs to know this information.

>If yes, how will the LMP instance on Node 3 gather the LSP information?.

It's left as an implementation detail. Presumably the LMP module in each 
node exchanged information with the signalling module in each node. It is 
already assumed that the LMP module provides control channel, data-link and 
TE-Link information to the local signalling and routing modules. This is a 
case where the direction of that exchange is reversed.
-- Rich

---

From: "Baktha Muralidharan" <[email-masked]>
Date: Fri, 10 Dec 2004 16:50:31 -0500

Rich,
 
Thanks!.
 
So, if a Node (node A) sends a channel status to an upstream node (node B)
indicating that it  (node A) is experiencing LoL, for node B does the
following:
 
1. Figures out which LSPs are carried on the interface reported by node A
2. Each of the LSPs could be taking different route and so, could go out on
a different interface off node B. Determines the corresponding "upstream
side" interface for each of the LSPs.
3. On each of the "upstream side" interface, checks if it (node B) is
receiving light.
4. If even one of the "upstream side" interface is receiving light,  then
node B concludes that failure is clear on its' upstream side
 
Assuming a PXC (Lambda link switching) in this case, by the way.
 
Finally, it is interesting that a "lower level layer" (LMP) requires
knowledge of "higher level layers" to perform its function. Was it by
design?
 
Thanks,
/Baktha

  _____  

From: [email-masked] [mailto:[email-masked]] On Behalf
Of Bradford, Richard
Sent: Friday, December 10, 2004 2:32 PM
To: Baktha Muralidharan
Cc: [email-masked]
Subject: Re: Question on LMP Fault Localization

"

Thanks,
/Baktha
Cisco Systems, Inc.