Subject: Re: Queries regarding RFC 3032

From: "Chang, Chong Yie" <[email-masked]>
Date: Thu, 18 Apr 2002 03:15:16 +0800

I got some different view from NULL and implicit NULL label.
From my view, if a downstream LSR is not capable of handling
MPLS encoded packets but it needs the upstream LSR utilize the
function of label forwarding, it will distribute implicit NULL label
to its upstream.

If the downstream need the upstream forward packets according
to packet layer 3 address, it will distribute explicit NULL label (0).

Am I right or wrong?

    Best regards,

                                                    Chang, Chong Yie

David Charlap wrote:

George Thomas wrote:

If there is no LSP associated with the destination, (say for e.g.
unicast/muticast/broadcast packet), the packet can be sent as
native IP packet without NULL label encapsulation in a MPLS domain
or it can be discarded, based on the design of the router. This is
my understanding from your response.

Correct.

Then i would like to know when NULL label encapsulation is to be
used?

A router that isn't capable of popping a label stack may be required to
do so.  In this situation, it can swap to NULL.  When this happens, the
next-hop router is expected to pop off that NULL label prior to
processing the packet.

My initial understanding was: In a MPLS domain, for IP packets
which cannot be associated with a LSP, those packets should be
sent to the IP next hop with NULL label encapsulation in a LAN
media and it should not be sent as native IP packet.

NULL labels are a workaround to get around some kinds of hardware
restrictions.  I don't think they were ever meant to be the primary
means of forwarding unlabeled traffic.

-- David
.

---

From: David Charlap <[email-masked]>
Date: Wed, 17 Apr 2002 16:34:59 -0400

> function of label forwarding, it will distribute implicit NULL label

If the downstream LSR is incapable of forwarding labeled traffic, then
it has no business participating in MPLS signaling ;-)

If an egress router can not pop the label stack, then it may send an
implicit NULL upstream - requesting that the upstream router pop the
stack before sending the packet.  This may result in the egress router
receiving a labeled packet (if it's terminating the innermost LSP of a
hierarchicl LSP) or an unlabeled packet (if the next-hop is beyond the
edge of the MPLS cloud.)

> to packet layer 3 address, it will distribute explicit NULL label

From the standpoint of packet forwarding, implicit NULL is the same as
explicit NULL.  In both cases, the egress router must forward the packet
based on its contents after popping the label stack.  With implicit
NULL, the previous-hop router does the pop.  With explicit NULL, the
next-hop router does the pop.  There is no other significant difference.

-- David