The MPLS WG Archive

[Jack Brennen <John.Brennen@marconi.com>]

Eric Gray wrote:
> No.  Router B has done a BAD THING.

I won't argue this point, merely because it's not fundamental to my finding
of a possible protocol instability.  The fundamental problem is that an "update"
Label Mapping message can "cross on the wire" with a Label Release message for
the same FEC/Label combination.  It is possible, in several ways, for Router B
to accept the "update" Label Mapping message as if it were a normal unsolicited
Label Mapping, even though it just recently released a Label Mapping for the
same FEC.  Reasons why Router B might do this:

   (1)  The original Label Mapping caused a "loop detection" failure,
        but the "update" Label Mapping did not.
   (2)  Router A was not the next hop when the original Label Mapping
        was processed, but is the next hop when the "update" Label Mapping
        is processed.
   (3)  Router B had no available memory to keep the original Label Mapping,
        but memory became available by the time the "update" Label Mapping
        was processed.

If "update" Label Mapping messages were syntactically different than normal
unsolicited Label Mapping messages, the whole problem could be avoided.  That
is how I would fix the specification if it were my decision.

> There is always a 'robustness' behavior that is well understood, if not exactly
> specified.  If Router A is getting labels it does not know how to handle (it
> has no corresponding ILM for these labels), it is a simple matter of software
> to realize that it should send a Label Release for those labels to the LSR that
> is sending them.

In my examples, Router A thinks that the label has been released by Router B,
so it is free to reassign it to a different FEC -- there may be a corresponding
ILM for the label (on Router A) when Router B actually puts traffic onto it,
in which case Router A won't see anything amiss.

Also, I think you mean that Router A should send a Label Withdraw.  I don't
necessarily agree, but I wouldn't argue too hard against it.

    Jack