Subject: doubt about implecit NULL label

From: siva Reddy <[email-masked]>
Date: Wed, 18 Jul 2001 01:41:00 -0700 (PDT)

Dear all,
when we use implecit NULL (label valu 3) the
penultimate HOP has to pop the label insted of
replacing(swapping).afterwards it will be simply IP
packet only.can penultimate HOP forward the IP packet
to LSR.

please clarify me about this implecit NULL concept.
thanks in adv.
regards
lonka

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From: Ashish Kulshreshtha <[email-masked]>
Date: Wed, 18 Jul 2001 14:47:45 +0530

Penultimate hop doesn't need to do IP forwarding as it knows the
outgoing label for egress. So it will send using that label.
The advantage is that the egress doesn't need to perform a POP
operation.
From RFC 3031,
section 3.16......
 
From an architectural perspective, this is perfectly appropriate. The
purpose of the level m label is to get the packet to Rn. Once R[n-1] has
decided to send the packet to Rn, the label no longer has any function,
and need no longer be carried. 

There is also a practical advantage to doing penultimate hop popping. If
one does not do this, then when the LSP egress receives a packet, it
first looks up the top label, and determines as a result of that lookup
that it is indeed the LSP egress. Then it must pop the stack, and
examine what remains of the packet. If there is another label on the
stack, the egress will look this up and forward the packet based on this
lookup. (In this case, the egress for the packet's level m LSP is also
an intermediate node for its level m-1 LSP.) If there is no other label
on the stack, then the packet is forwarded according to its network
layer destination address. Note that this would require the egress to do
TWO lookups, either two label lookups or a label lookup followed by an
address lookup.From an architectural perspective, this is perfectly
appropriate. The purpose of the level m label is to get the packet to
Rn. Once R[n-1] has decided to send the packet to Rn, the label no
longer has any function, and need no longer be carried. There is also a
practical advantage to doing penultimate hop popping. If one does not do
this, then when the LSP egress receives a packet, it first looks up the
top label, and determines as a result of that lookup that it is indeed
the LSP egress. Then it must pop the stack, and examine what remains of
the packet. If there is another label on the stack, the egress will look
this up and forward the packet based on this lookup. (In this case, the
egress for the packet's level m LSP is also an intermediate node for its
level m-1 LSP.) If there is no other label on the stack, then the packet
is forwarded according to its network layer destination address. Note
that this would require the egress to do TWO lookups, either two label
lookups or a label lookup followed by an address lookup.

the text in bold tells that it uses the label of outgoing interface and
then POPS  the label.

----- Original Message ----- 
From: siva  <mailto:[email-masked]> Reddy 
To: [email-masked] <mailto:[email-masked]>  
Sent: Wednesday, July 18, 2001 2:11 PM
Subject: doubt about implecit NULL label

Dear all,

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From: siva Reddy <[email-masked]>
Date: Wed, 18 Jul 2001 03:03:48 -0700 (PDT)

thank you for javier and ashish for your reply ,
thanks once again
regards
lonka
--- Ashish Kulshreshtha <[email-masked]>
wrote:
> <http://personal.mail.yahoo.com/> 
> 
> 
>

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From: "Alexander Marhold" <[email-masked]>
Date: Wed, 18 Jul 2001 13:10:28 +0200

to clarify the implicit-Null tells to POP the outermost label only
it does not remove the complete label-stack if there are multiple levels of
labels
otherwise TE-tunnels AND MPLS/VPN could not work.
So the implicit-null tells to not use a label at the corresponding
stacklevel and not for all levels.

regards
Alexander

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-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of siva
Reddy
Sent: Wednesday, July 18, 2001 10:41 AM
To: [email-masked]
Subject: doubt about implecit NULL label

Dear all,