The MPLS WG Archive

["David Allan" <dallan@nortelnetworks.com>]

> > > No, if the packet arrives unlabeled then you necessarily had
> > > to have given out an explicit null label for that FEC.  
> > 
> > Seems to me there are a lot of potential reasons for an unlabelled 
> > packet turning up at the downstream LSR besides the downstream LSR 
> > offering implicit NULL.
> 
> Yes!  But the only valid one is that you gave out an explicit 
> null. So you check that you did in fact do that.  If not it's 
> an error.

OK, so you assume on a TTL exhaust, the LSP PING processing in the
downstream LSR sees the whole label stack of the packet and uniquely checks
that the label (or absence of label) is correct and if a label there, the
correct binding between label and FEC exists. aka there is a very tight
coupling between LSP and PING message processing.


> <snipped>
> 
> That's why appending an implicit null results in a PoP!
> 
Have to admit, still not parsing what the implicit null is appended to...?

cheers
Dave