Subject: Re: Queries regarding RFC 3032

From: "George Thomas" <[email-masked]>
Date: 16 Apr 2002 11:18:40 -0000

hi,

Thanks for your response.

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

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.

Please clarify.

Regards
George.

George Thomas wrote:
>
>1. If there is no LSP associated with a destination (Unicast/
>    Multicast), whether it is mandatory to use NULL Label
>Encapsulation, and encoding the ethertype value to 
>0x8847/0x8848.
>or Is it valid to send as Native IP packet with Ethertype
>value as 0x800 in the MPLS network.

If an LER receives an unlabeled packet, and there is no forwarding 
table
information descrbing what LSP to send the packet on, then the 
behavior
is beyond the scope of MPLS.

It would be legal to forward the packet hop-by-hop, as a non-MPLS 
router
would.  It would also be legal to drop the packet and generate an 
ICMP
error (treating it as a non-MPLS router would if there is no route 
to
the destination.)

I would not expect a NULL label to be used in this situation.

>2. If NULL label is Mandatory, for a broadcast packet, what
>ethertype value should be used? Whether the Mulitcast ethertype
>value can be used?
>
>If NULL label is not mandatory, it implies that any native IP
>packet can be sent with ethertype value as 0x800 in a MPLS 
>domain
>without Label encapsulation. Am i right?

MPLS routers have to be able to send and receive non-labeled 
packets.
That's how the routing and signaling protocols all work.  Whether 
they
also have the capability of forwarding unlabeled packets is up to 
the
design of the router, of course.  I think such capability is 
required
for some protocol features (like extended-discovery LDP 
peering),
however.

Of course, Ethertypes are only valid on those interfaces that 
use
Ethertypes.  Other interfaces would use whatever layer-2 types 
are
appropriate for the IP packet.

-- David

---

From: David Charlap <[email-masked]>
Date: Tue, 16 Apr 2002 10:03:17 -0400

> my understanding from your response.

Correct.

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

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.

> 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