Subject: RE: Last call on LSP Ping

From: [email-masked]
Date: Mon, 9 Dec 2002 23:30:47 -0000

Curtis,

Please see below.  regards, Neil 

Curtis Villamizar wrote 09 December 2002 22:43

<snipped>
> > This is what RFC1812 says:
> > 
> >    A router MUST NOT discard a datagram just because it was received
> >    with TTL equal to zero or one; if it is to the router 
> and otherwise
> >    valid, the router MUST attempt to receive it.
> > 
> > I agree that if LSP-pings ends up in a router that it was 
> not intended
> > it will get discarded. However, I am wondering what is the 
> required behavior
> > in LSP-ping draft? Is it to get a reply even from an egress 
> LSR that is not
> > the correct egress? If so then how can that egress use the 
> RSVP return path
> > to respond?
> 
> That is the case 1 I had listed as a deficiency to your suggestion.
> So you agree case 1 is valid.
> 
> If packets were misdirected, then there has to be an RSVP return path
> for a TE tunnel and LDP requires that their be IP routes.
NH=> I refer to the RSVP bit.  If a pkt gets misdirected and is requesting a
return by the RSVP control-plane, then of course as you rightly observe
there has to be such a return control-plane path available.  But how is it
possible that a pkt which is misdirected to some arbitrary egress point will
have a RSVP return path back to some fixed source point?  This appears not
to be a sensible thing to assume.  So this option (ie using RSVP) does not
appear to cater for the needs of identifying where misdirected traffic ends
up.
<snipped to end>

---

From: Curtis Villamizar <[email-masked]>
Date: Mon, 09 Dec 2002 22:32:44 -0500

In message <[email-masked]

> appear to cater for the needs of identifying where misdirected traffic ends

I looked at this again and it appears I had this wrong.  If the
"Downstream Mapping" was used to form a RESV, then this would be fine,
but it is not the case.

In order to have *any* MPLS/TE LSP going to an adjacent router, there
must be an RSVP adjacency.  The route up to the point where the path
went wrong is in the MPLS-ping packet payload if the "Downstream
Mapping" is present so the router could in principle send the packet
back hop by hop in RSVP.  I was assuming that the "Downstream Mapping"
could be inverted but I see that the intent is to simply add the
LSP_ECHO in the normal RESV for the LSP.  As is it is just a
continuity test.

It seems that error indication on misdirected traffic is only possible
if IP routing is used to deliver the reply.

Even so, if the packet is misdirected, at some hop the packet arrives
at the wrong router.  In traceroute mode you can determine when this
occurs and which router the traffic is being misdirected towards (if
IP can be used to return the packet).  Use of 127/8 directs the
request to "whoever gets this".  If the expected router-id is used as
the address then the request times out.  A timeout cannot be
distinguished from a MPLS-ping incapable router.  For LDP there is no
way to determine the router-id of the router one hop beyond an
MPLS-ping incapable router, so 127/8 must be used and TTL incremented.

Curtis