'A' bit problem statement

During today's MPLS session it was clear that more discussion/clarification was required on the problem space addressed by the I-D.

There is two aspects to the problem discussed in the draft.

1) Mixing e2e path testing with adjacency fault detection has coorindation problems when failures occur. If I have an LSP hierarchy that I am pinging at some low level, and a failure occurs, some number of the pings will be affected, and a response/alarm management will be hard to synchronize as ping sources are not local to the fault (and the fault may also be detected locally, e.g. link failure). IMHO this is an artifact of the delta between using ping to reactively to check routing policy vs. using ping proactively to detect data plane failures.

2) Reserved labels define functions instead of forwarding. Fate sharing between functions and LSPs is useful. Currently ECMP breaks fate sharing between LSPs and LSP functions defined by reserved labels.

The answer to #1 is to try to localize detection of data plane problems, the ability to do the equivalent of the router alert label that fate shares with the LSP is one potential mechanism that could be used to increase locality in detecting data plane problems. Specific data plane flows could be inspected for consistency by intermediate LSRs as they were forwarded down the path.

The answer to #2 is to provide an alternative to reserved labels for LSP functions, the MPLS/PW PID is a candidate for doing this. The 'A' bit was one example of providing such functionality by defining a replacement for router alert. A side note is that there is a much higher liklihood of commonality of forwarding of a solution that had the label of interest as the top label vs. prepending with  the router alert label.

Comments?

cheers
Dave