The MPLS WG Archive

[Curtis Villamizar <curtis@fictitious.org>]

I'm replying to myself because I gave Shahram some overly short
answers.  Sorry.  I wanted to get out the part 2 message.

> > 4) What happens if in an FEC stack ping, some of the FECs are confirmed
> > and some are not? Do we need more error codes? How many more?
> 
> Good question.  Let me answer that one separately in another message.
> It is an ommision in the document.

There are a couple of legitimate situations where this will happen.
With varying degrees of "legitimate".  Some have to do with TTL
processing with hierarchy.  Another is situation is an LSR that
doesn't decrement TTL (hopefully not on an LDP LSP that loops) or one
that doesn't understand lsp-ping.

What you'd see at the ingress is an error.  If you resent the Echo
Request without any of the prior mapping TLVs, then you'd see an Echo
Reply that confirmed the FEC and gave a downstream mapping TLV.  You
could then continue the traceroute.  Having reached the egress you
have a portion of the trace for which you aren't certain how the
packets go.  If the pings work and the area of uncertainty is outside
your administrative domain, you have to accept that delivery happens
by some undisclosed magic until delivery fails to happen.  If it is
within your administrative domain, you can MIB query until such time
as you can resolve the problem to get complete traces.

> > 5) How are the ATM/FR labels supported in Label-mapping TLV?
<snip>

Lets try again on this one.

AFAIK, an ATM or FR LSR doesn't decrement TTL.  Therefore this would
appear to to be a transparent couple of hops just like FSC, LSC, TDM.

This might be one of those situations in point 4 above where there is
a discontinuity in the sanity of the traceroute.  The downstream
mapping gives the next ATM LSR but incrementing TTL does to the end of
the VC if TTL isn't decremented.

Curtis

ps - IMHO - PHP is a thing of technical elegance and beauty compared
to MPLS over ATM (or anything over ATM).  :-)