Post-Stack MPLS Network Action (MNA) Header Specification
draft-ietf-mpls-mna-ps-hdr-08
| Document | Type | Active Internet-Draft (mpls WG) | |
|---|---|---|---|
| Authors | Jaganbabu Rajamanickam , Rakesh Gandhi , Royi Zigler , Jie Dong , Jisu Bhattacharya | ||
| Last updated | 2026-06-03 (Latest revision 2026-04-02) | ||
| Replaces | draft-jags-mpls-ps-mna-hdr | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews |
RTGDIR IETF Last Call Review due 2026-06-09
Incomplete
SECDIR IETF Last Call Review due 2026-06-09
Incomplete
OPSDIR IETF Last Call Review due 2026-06-09
Incomplete
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
|
||
| Document shepherd | Adrian Farrel | ||
| Shepherd write-up | Show Last changed 2026-04-14 | ||
| IESG | IESG state | In Last Call (ends 2026-06-09) | |
| Action Holder | |||
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Jim Guichard | ||
| Send notices to | adrian@olddog.co.uk | ||
| IANA | IANA review state | IANA OK - Actions Needed |
draft-ietf-mpls-mna-ps-hdr-08
MPLS Working Group J. Rajamanickam, Ed.
Internet-Draft R. Gandhi, Ed.
Intended status: Standards Track Cisco Systems, Inc.
Expires: 4 October 2026 R. Zigler
Broadcom
J. Dong
Huawei Technologies
J. Bhattacharya
Cisco Systems, Inc.
2 April 2026
Post-Stack MPLS Network Action (MNA) Header Specification
draft-ietf-mpls-mna-ps-hdr-08
Abstract
This document defines the Post-Stack MPLS Network Action (MNA) Header
Specification for carrying Network Actions encodings and Ancillary
Data after the MPLS label stack, based on the In-Stack MNA
Specification defined in "MPLS Network Action (MNA) Sub-Stack
Specification." MPLS Network Actions can be used to influence packet
forwarding decisions, carry additional Operations, Administration,
and Maintenance information in the MPLS packet, or perform user-
defined operations. This document follows the framework specified in
RFC 9789.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 4 October 2026.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Post-Stack MPLS Header Presence Bit Carried in In-Stack MNA
Sub-Stack . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Post-Stack MPLS Header Encoding . . . . . . . . . . . . . 6
3.2.1. Post-Stack MPLS Header Type . . . . . . . . . . . . . 6
3.2.2. Post-Stack Network Action Encoding . . . . . . . . . 7
3.2.3. Encoding of an MPLS Packet with a Post-Stack Header for
MNA . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. In-Stack Network Action Special Opcodes . . . . . . . . . . . 9
4.1. In-Stack Network Action Opcode for PSMH Start Offset for
MNA . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. In-Stack Network Action Opcode for Offset of End of
Post-Stack MPLS Header for MNA . . . . . . . . . . . . . 9
5. Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Processing Rules for P Bit . . . . . . . . . . . . . . . 10
5.2. Network Action Processing Order . . . . . . . . . . . . . 10
5.3. Node Capability Signaling . . . . . . . . . . . . . . . . 10
6. Processing the Network Action Sub-Stack and Post-Stack
Header . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Encapsulating Node Responsibilities . . . . . . . . . . . 11
6.2. Transit Node Responsibilities . . . . . . . . . . . . . . 11
6.3. Penultimate Node Responsibilities . . . . . . . . . . . . 12
6.4. Decapsulating Node Responsibilities . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8.1. First Nibble for Post-Stack MPLS Header . . . . . . . . . 12
8.2. Post-Stack MPLS Header Types Registry . . . . . . . . . . 12
8.3. Network Action Opcodes . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Appendix A: Examples . . . . . . . . . . . . . . . . 16
A.1. Examples of Post-Stack MPLS Header Encoding . . . . . . . 16
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A.1.1. NAS that only Indicates Post-Stack MPLS Header . . . 16
A.1.2. NAS that Indicates Post-Stack MPLS Header Start
Offset . . . . . . . . . . . . . . . . . . . . . . . 17
A.1.3. Post-Stack Network Actions with Two Opcodes . . . . . 17
A.1.4. Post-Stack Network Action with two Different
Scopes . . . . . . . . . . . . . . . . . . . . . . . 18
A.2. Examples of In-Stack and Post-Stack Network Actions . . . 19
A.2.1. NAS with In-Stack and Post-Stack NAs . . . . . . . . 19
A.2.2. NASes with Different In-Stack and Post-Stack
Scopes . . . . . . . . . . . . . . . . . . . . . . . 20
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 21
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
[RFC3032] defines the encoding of the MPLS label stack, the basic
structure used to define a forwarding path. There are applications
that require MPLS packets to perform special network actions and
carry optional Ancillary Data (AD) that can affect the packet
forwarding decision or trigger Operations, Administration, and
Maintenance (OAM) logging, for example as described in [RFC9791].
Ancillary Data can be used to carry additional information, for
network slice purpose, as an example [RFC9791].
In some cases, more AD may be required than can be carried in the
MPLS header, so these kinds of network actions and their AD are
encoded after the Bottom of Stack (BOS). This network action with AD
is called the Post-Stack (PS) MNA.
This document defines the Post-Stack MPLS Network Action (MNA) header
specification for carrying Network Actions encodings and Ancillary
Data after the MPLS label stack. The specification is based on the
In-Stack MNA Specification defined in [I-D.ietf-mpls-mna-hdr]. The
requirements for Post-Stack network action and Post-Stack data (PSD)
are described in [RFC9613]. This document follows the framework
specified in [RFC9789].
2. Conventions Used in This Document
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119] [RFC8174]
when, and only when, they appear in all capitals, as shown here.
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2.2. Abbreviations
The terminology defined in [RFC9789] and [RFC9613] are used in this
document.
+==============+===================+=========================+
| Abbreviation | Meaning | Reference |
+==============+===================+=========================+
| AD | Ancillary Data | [RFC9613] |
+--------------+-------------------+-------------------------+
| bSPL | Base Special | [RFC9017] |
| | Purpose Label | |
+--------------+-------------------+-------------------------+
| BOS | Bottom Of Stack | [RFC9789] |
+--------------+-------------------+-------------------------+
| HBH | Hop-By-Hop Scope | [RFC9789] |
+--------------+-------------------+-------------------------+
| I2E | Ingress-To-Egress | [RFC9789] |
| | Scope | |
+--------------+-------------------+-------------------------+
| IHS | I2E, HBH, or | [I-D.ietf-mpls-mna-hdr] |
| | Select Scope | |
+--------------+-------------------+-------------------------+
| ISD | In-Stack Data | [RFC9613] |
+--------------+-------------------+-------------------------+
| LSE | Label Stack Entry | [RFC9789] |
+--------------+-------------------+-------------------------+
| MNA | MPLS Network | [RFC9789] |
| | Actions | |
+--------------+-------------------+-------------------------+
| NAI | Network Action | [RFC9613] |
| | Indicator | |
+--------------+-------------------+-------------------------+
| NAL | Network Action | [I-D.ietf-mpls-mna-hdr] |
| | Length | |
+--------------+-------------------+-------------------------+
| NAS | Network Action | [RFC9789] |
| | Sub-Stack | |
+--------------+-------------------+-------------------------+
| NASL | Network Action | [I-D.ietf-mpls-mna-hdr] |
| | Sub-Stack Length | |
+--------------+-------------------+-------------------------+
| OAM | Operations, | [RFC6291] |
| | Administration, | |
| | and Maintenance | |
+--------------+-------------------+-------------------------+
| P bit | Post-Stack MPLS | This document |
| | Header Presence | |
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| | Bit | |
+--------------+-------------------+-------------------------+
| PS | Post-Stack | This document |
+--------------+-------------------+-------------------------+
| PSD | Post-Stack Data | [RFC9613] and [RFC9789] |
+--------------+-------------------+-------------------------+
| PSMH | Post-Stack MPLS | This document |
| | Header | |
+--------------+-------------------+-------------------------+
| TC | Traffic Class | [RFC5462] |
+--------------+-------------------+-------------------------+
| TTL | Time To Live | [RFC3032] |
+--------------+-------------------+-------------------------+
Table 1: Abbreviations
3. Overview
Two main parts are specified to support post-stack MNA:
* The Post-Stack MPLS header Presence Bit Carried in In-Stack MNA
Sub-Stack.
* The Post-Stack MPLS header Encoding that includes Post-Stack MPLS
Header Type and Post-Stack Network Actions.
3.1. Post-Stack MPLS Header Presence Bit Carried in In-Stack MNA Sub-
Stack
Bit 20 in LSE Format B carried in the In-Stack Network Action Sub-
Stack (NAS) described in [I-D.ietf-mpls-mna-hdr] is defined as the P
bit in this document to indicate the presence of the Post-Stack MPLS
Header in the packet after the BOS.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode | 13-bit Data (Format B) |P|IHS|S| NASL |U| NAL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Post-Stack MPLS Header Presence Bit Carried in In-Stack
MNA Sub-Stack
The following flags are carried in an In-Stack NAS as defined in
[I-D.ietf-mpls-mna-hdr] and shown in Figure 1. These flags are also
applicable to Post-Stack MNA:
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* IHS (2 Bit): The In-Stack NAS for each scope with the P bit set
has a corresponding Post-Stack MPLS Header.
* U (1 Bit): Indicates the combined Unknown Action Handling of the
In-Stack and the Post-Stack Network Actions.
3.2. Post-Stack MPLS Header Encoding
The Post-Stack MPLS Header (PSMH) for MNA is encoded after the LSE
for which Bottom of the Stack (BOS) bit is set, either immediately
after the BOS (i.e., start offset of 0) or after any other Post-Stack
headers that follow the BOS (i.e., start offset of non-zero), as
described in Section 4.
The PSMH for MNA carries one or more Post-Stack Network Actions and
their Ancillary Data.
The PSMH for MNA consists of two main parts:
* Post-Stack MPLS Header Type
* Post-Stack Network Action Encoding
3.2.1. Post-Stack MPLS Header Type
The Post-Stack MPLS Header type is the top-header for all the Post-
Stack Network Actions that are encoded in the PSMH for each scope as
shown in Figure 2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PFN |Reserve| PSMH-Len | Type = MNA-Post-Stack-Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Post-Stack MPLS Header Type
* PFN (4 bits): The Post-Stack First Nibble (PFN) [RFC9790]
identifies the start of the PSMH. The PFN is set to 0x0 for the
Post-Stack MPLS header.
* Reserve (4 bits): Reserved bits.
* PSMH-Len (8 bits): The PSMH total length in 4-octet units that
includes Post-Stack Network Actions. The length excludes the
4-byte PSMH type header.
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* Type (16 bits): Type is set to 1 to indicate Post-Stack MPLS
Header Type for MNA. See Section 8.2. Note that other types of
Post-Stack headers (besides MNA) can be defined in the future and
is outside the scope of this document.
3.2.2. Post-Stack Network Action Encoding
The format shown in Figure 3, encodes a single Post-Stack Network
Action. By repeating this format, multiple Post-Stack Network
Actions and their corresponding Ancillary Data can be encoded.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA-PS-OP |R|R| PS-NAL | Post-Stack Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Continued Post-Stack Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Post-Stack Network Action Encoding
* MNA-PS-OP (7 bits): Post-Stack Network Action Opcode. See
Section 8.3.
* R (2 bits): Reserved bits.
* PS-NAL (7 bits): Post-Stack Network Action Length in 4-octet
units. This excludes the first 4-octets starting with MNA-PS-OP.
* Post-Stack Data (16 bits): Post-Stack Data associated with the
Post-Stack Network Action.
* Continued Post-Stack Data: Further Post-Stack Data associated with
the Post-Stack Network Action, as indicated by the value of PS-
NAL. The padding rules for Post-Stack Data that does not fill a
multiple of 32-bit units are described in the document that
defines the NA indicated by the MNA-PS-OP value.
3.2.3. Encoding of an MPLS Packet with a Post-Stack Header for MNA
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
M | MNA Label | TC |0| TTL | N
P +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A
L |Opcode=2=NoOp| 0 |1|IHS|0| NASL=0|U|NAL=0| S
S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
H ~ |0| ~
D +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R | |1| |
- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
P | 0x0 |Reserve| PSMH-Len | Type = MNA-Post-Stack-Header |PSMHT
S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
M | MNA-PS-OP |R|R| PS-NAL | Post-Stack Data | |
H +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+PSNA
| ~ Continued Post-Stack Data ~ |
- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
~ ~
~ Payload ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Encoding of an MPLS Packet with a Post-Stack MPLS
Header for MNA
An example of an MPLS packet encoding that carries an MPLS header and
a Post-Stack MPLS header for the MNA Post-Stack Header type is shown
in Figure 4. The encoding contains the following fields.
* MPLS HDR: MPLS Header containing an MPLS label stack and MNA Sub-
Stacks (NAS).
* NAS: Network Action Sub-Stack. It has the P bit set to 1 to
indicate the presence of a Post-Stack MPLS header for the MNA
Post-Stack Header type in the packet. An MPLS packet may carry
more than one NAS, not shown in Figure 4.
* PSMH: Post-Stack MPLS Header. This includes the Post-Stack MPLS
header type (PSMHT) and Post-Stack network actions (PSNA).
* PSMHT: Post-Stack MPLS Header Type. This includes the type of
Post-Stack MPLS header, version, and Post-Stack MPLS Header
length. In this example, the type is set to the MNA Post-Stack
Header.
* PSNA: Post-Stack Network Actions. Applicable only when the type
is set to the MNA Post-Stack Header. An MPLS packet may have more
than one Post-Stack Network Action, not shown in Figure 4.
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4. In-Stack Network Action Special Opcodes
4.1. In-Stack Network Action Opcode for PSMH Start Offset for MNA
Opcode: TBA2
Purpose: This opcode carries the start offset of the PSMH for MNA
from the BOS.
LSE Format: B or C (defined in [I-D.ietf-mpls-mna-hdr])
Data: The data value of the LSE contains the offset from the MPLS BOS
in 4-octet units.
Scope: This opcode can be used with any scope.
This opcode allows the existing Post-Stack Headers (PSHs) [RFC9790],
e.g., the Generic Control Word (0000b) [RFC4385] and G-ACh (0001b)
[RFC5586], and any other PSH defined in the future to be placed
immediately after the BOS.
If in the given NAS, the P bit is set and the PSMH Start Offset
opcode is absent, the PSMH is encoded immediately after the MPLS BOS.
A data value of 1 indicates that the PSMH starts at 4-octets after
the BOS.
4.2. In-Stack Network Action Opcode for Offset of End of Post-Stack
MPLS Header for MNA
Opcode: TBA3
Purpose: This opcode carries the offset of the end of the Post-Stack
MPLS Header for MNA from the BOS.
LSE Format: B or C (defined in [I-D.ietf-mpls-mna-hdr]).
Data: The data value of the LSE contains the offset from the MPLS BOS
in 4-octet units. A data value of 5 indicates that the Post-Stack
MPLS Header ends at 20-octets after the BOS.
Scope: This opcode can be used with any scope.
The offset of the end of the PSMH for MNA and the offset of the start
of the PSMH for MNA allow the NAS parser implementation to know the
size of the PSMH without having to parse the PSMH (e.g., for Readable
Label Depth with Post-Stack MPLS Header checks).
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5. Procedure
5.1. Processing Rules for P Bit
The P bit MUST be set to 1 in an In-Stack Network Action Sub-Stack
when the corresponding Post-Stack MPLS Header is added in the packet.
By default, the PSMH starts immediately after the BOS. The offset of
the PSMH that does not start immediately after the BOS is indicated
using the PSMH Start Offset Opcode TBA2.
The P bit MUST be set to 1 when the network action with opcode TBA2
is added to the In-Stack Network Action Sub-Stack. The node that
recognizes the network action with Opcode TBA2 MUST process the
packet according to the U flag if the P bit is not set.
The P bit MUST be set to 1 when the network action with opcode TBA3
is added to the In-Stack Network Action Sub-Stack. The node that
recognizes the network action with Opcode TBA3 MUST process the
packet according to the U flag if the P bit is not set.
The node that supports the P bit, processes the Post-Stack Network
Actions in the Post-Stack MPLS Header as defined in this document.
Conversely, the node that does not support the P bit skips processing
the Post-Stack MPLS Header altogether.
5.2. Network Action Processing Order
The Post-Stack Network Actions are processed in the same order they
are encoded after the BOS. By default, they are processed after the
In-Stack Network Actions in the Network Action Sub-Stack. However,
Post-Stack Network Action Opcodes for applications (such as IOAM as
described in [RFC9791]) can be added to the In-Stack Network Action
Sub-Stack to process Post-Stack Network Actions in a certain order
with respect to the Opcodes in the In-Stack Network Action Sub-Stack.
5.3. Node Capability Signaling
The ingress node that is adding a Post-Stack MPLS Header MUST ensure
that the egress node is capable of Post-Stack MNA processing and can
remove the PSMH from the packet.
* Each participating node MUST signal the Post-Stack network actions
that it supports to the encapsulating node.
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* Each participating node MUST signal its "Readable Label Depth
including Post-Stack MPLS Header" that can be encoded by the
encapsulating node. Note that the "Readable Label Depth" defined
in [I-D.ietf-mpls-mna-hdr] is extended in this document to include
the length of the Post-Stack MPLS Header.
The above capability signaling will be added in the appropriate
protocols. Signaling details are outside the scope of this document.
6. Processing the Network Action Sub-Stack and Post-Stack Header
The processing of the Network Action Sub-Stack described in
[I-D.ietf-mpls-mna-hdr] are also applicable to the procedures defined
in this document. This section defines the specific responsibilities
for nodes along an MPLS path for processing a Post-Stack MPLS Header.
6.1. Encapsulating Node Responsibilities
The encapsulating node MAY add a Post-Stack MPLS Header to the
packet. The location of the header MAY be in accordance with local
policy. The location may also be subject to any placement
restrictions inherent in the implementation.
The encapsulating node MUST NOT add a Post-Stack MPLS Header to the
packet if the decapsulating node does not support Post-Stack MPLS
Header.
The encapsulating node MUST add the Post-Stack MPLS Header in the
same order as the corresponding In-Stack NAS in the MPLS header.
6.2. Transit Node Responsibilities
A transit node MAY modify the Ancillary Data in the Post-Stack MPLS
Header.
A transit node MUST respect the Unknown Action Handling flag encoded
in the corresponding NAS when processing the PSMH.
A transit node that removes an NAS with the Select scope, MUST also
remove the associated PSMH.
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6.3. Penultimate Node Responsibilities
In addition to the transit node responsibilities above, the
penultimate node MUST NOT remove an HBH or I2E NAS
[I-D.ietf-mpls-mna-hdr] and the associated PSMH when the NAS is
exposed after removing the forwarding (transport) label. This allows
the egress node to receive and process the NAS and the associated
PSMH.
6.4. Decapsulating Node Responsibilities
The decapsulating node MUST remove the Post-Stack MPLS Headers from
the packet when it removes the NASes.
7. Security Considerations
The security considerations in [RFC3032], [RFC9789], and
[I-D.ietf-mpls-mna-hdr] also apply to this document.
System designers must be aware that information included in Post-
Stack Ancillary Data may be transmitted "in the clear." Network
actions that require the exchange of sensitive data, must be defined
in such a way that the data is encrypted in transit.
8. IANA Considerations
8.1. First Nibble for Post-Stack MPLS Header
This document requests that IANA update the "Post-Stack First Nibble"
registry created by [RFC9790] to include the PFN value 0x0 for the
Post-Stack MPLS Header. This should be added as a new entry in the
registry.
+==========+=======+========================+===============+
| Protocol | Value | Description | Reference |
+==========+=======+========================+===============+
| MPLS | 0x0 | Post-Stack MPLS Header | This document |
+----------+-------+------------------------+---------------+
Table 2: Post-Stack First Nibble Registry
8.2. Post-Stack MPLS Header Types Registry
This document requests that IANA create a new registry with the name
"Post-Stack MPLS Header Types" as follows. The registration
procedure for this registry is "IETF Review", "Experimental Use" and
"Private Use". The fields are "Type" (integer), "Description"
(string), and "Reference" (string).
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The Registration Procedure(s) for this registry are:
+=============+==================+===============+
| Type | Description | Reference |
+=============+==================+===============+
| 0 | Reserved | This document |
+-------------+------------------+---------------+
| 1-65520 | IETF Review | This document |
+-------------+------------------+---------------+
| 65521-65524 | Experimental Use | This document |
+-------------+------------------+---------------+
| 65525-65535 | Private Use | This document |
+-------------+------------------+---------------+
Table 3: Post-Stack MPLS Header Types Registry
The initial assignments for this registry are:
+======+=====================================+===============+
| Type | Description | Reference |
+======+=====================================+===============+
| 0 | Reserved | This document |
+------+-------------------------------------+---------------+
| 1 | Post-Stack MPLS Header Type For MNA | This document |
+------+-------------------------------------+---------------+
Table 4: Post-Stack MPLS Header Types
8.3. Network Action Opcodes
IANA maintains the "Network Action Opcodes" registry that will be
created by [I-D.ietf-mpls-mna-hdr]. IANA is requested to perform two
actions for this registry:
1. Assign two code points for network actions defined in this
document.
2. Add a new column to indicate whether network action opcodes may
be used In-Stack or Post-Stack.
The resulting entries in the registry are as follows:
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+========+=============+================+=========================+
| Opcode | Description | In-Stack Only, | Reference |
| | | Post-Stack | |
| | | Only, In-Stack | |
| | | and Post-Stack | |
+========+=============+================+=========================+
| 0 | Reserved | Not Applicable | [I-D.ietf-mpls-mna-hdr] |
+--------+-------------+----------------+-------------------------+
| 1 | Flag-Based | In-Stack Only | [I-D.ietf-mpls-mna-hdr] |
| | Network | | |
| | Action | | |
| | Indicators | | |
| | without AD | | |
+--------+-------------+----------------+-------------------------+
| 2 | No | In-Stack and | [I-D.ietf-mpls-mna-hdr] |
| | operation | Post-Stack | |
| | Opcode | | |
+--------+-------------+----------------+-------------------------+
| TBA2 | Offset of | In-Stack Only | This document |
| | Start of | | |
| | Post-Stack | | |
| | MPLS Header | | |
+--------+-------------+----------------+-------------------------+
| TBA3 | Offset of | In-Stack Only | This document |
| | End of | | |
| | Post-Stack | | |
| | MPLS Header | | |
+--------+-------------+----------------+-------------------------+
| 127 | Opcode | In-Stack and | [I-D.ietf-mpls-mna-hdr] |
| | Range | Post-Stack | |
| | Extension | | |
| | Beyond 127 | | |
+--------+-------------+----------------+-------------------------+
Table 5: Network Action Opcodes
9. References
9.1. Normative References
[I-D.ietf-mpls-mna-hdr]
Rajamanickam, J., Gandhi, R., Zigler, R., Song, H., and K.
Kompella, "MPLS Network Action (MNA) Sub-Stack
Specification including In-Stack Network Actions and
Data", Work in Progress, Internet-Draft, draft-ietf-mpls-
mna-hdr-21, 24 February 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
mna-hdr-21>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
<https://www.rfc-editor.org/info/rfc3032>.
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385,
February 2006, <https://www.rfc-editor.org/info/rfc4385>.
[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
2009, <https://www.rfc-editor.org/info/rfc5462>.
[RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
"MPLS Generic Associated Channel", RFC 5586,
DOI 10.17487/RFC5586, June 2009,
<https://www.rfc-editor.org/info/rfc5586>.
[RFC6291] Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
D., and S. Mansfield, "Guidelines for the Use of the "OAM"
Acronym in the IETF", BCP 161, RFC 6291,
DOI 10.17487/RFC6291, June 2011,
<https://www.rfc-editor.org/info/rfc6291>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9017] Andersson, L., Kompella, K., and A. Farrel, "Special-
Purpose Label Terminology", RFC 9017,
DOI 10.17487/RFC9017, April 2021,
<https://www.rfc-editor.org/info/rfc9017>.
[RFC9789] Andersson, L., Bryant, S., Bocci, M., and T. Li, "MPLS
Network Actions (MNAs) Framework", RFC 9789,
DOI 10.17487/RFC9789, July 2025,
<https://www.rfc-editor.org/info/rfc9789>.
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[RFC9790] Kompella, K., Bryant, S., Bocci, M., Mirsky, G., Ed.,
Andersson, L., and J. Dong, "IANA Registry and Processing
Recommendations for the First Nibble Following a Label
Stack", RFC 9790, DOI 10.17487/RFC9790, July 2025,
<https://www.rfc-editor.org/info/rfc9790>.
9.2. Informative References
[RFC9613] Bocci, M., Ed., Bryant, S., and J. Drake, "Requirements
for Solutions that Support MPLS Network Actions (MNAs)",
RFC 9613, DOI 10.17487/RFC9613, August 2024,
<https://www.rfc-editor.org/info/rfc9613>.
[RFC9791] Saad, T., Makhijani, K., Song, H., and G. Mirsky, "Use
Cases for MPLS Network Action Indicators and Ancillary
Data", RFC 9791, DOI 10.17487/RFC9791, July 2025,
<https://www.rfc-editor.org/info/rfc9791>.
Appendix A. Appendix A: Examples
A.1. Examples of Post-Stack MPLS Header Encoding
A.1.1. NAS that only Indicates Post-Stack MPLS Header
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA Label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Opcode=2=NoOp| 0 |1|IHS|0| NASL=0|U|NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ [0| ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |1| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0 |Reserve| PSMH-Len=1 | Type = MNA-Post-Stack-Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA-PS-OP |R|R| PS-NAL=0 | Post-Stack Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ Payload ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: NAS that only indicates Post-Stack MPLS Header
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In some cases, the NAS may encode only the presence of Post-Stack
MPLS Header. The Post-Stack MPLS Header starts immediately after the
BOS.
A.1.2. NAS that Indicates Post-Stack MPLS Header Start Offset
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA Label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode=TBA2 | Post-Stack Offset = 2 |1|IHS|0| NASL=0|U|NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ [0| ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |1| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0 |Reserve| PSMH-Len=1 | Type = MNA-Post-Stack-Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA-PS-OP |R|R| PS-NAL=0 | Post-Stack Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ Payload ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: NAS Indicates Post-Stack MPLS Header Start Offset
The NAS may encode the start offset of the Post-Stack MPLS Header
with a non-zero value, for example, when it is after another header
such as G-ACh or Control Word header. In this example, the PSMH
starts at an offset of 8 bytes after the BoS.
A.1.3. Post-Stack Network Actions with Two Opcodes
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA Label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Opcode=2=NoOp| 0 |1|IHS|1| NASL=0|U|NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0 |Reserve| PSMH-Len=3 | Type = MNA-Post-Stack-Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA-PS-OP=2|R|R| PS-NAL=0 | Post-Stack Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA-PS-OP=3|R|R| PS-NAL=1 | Post-Stack Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Post-Stack Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Optional Payload + Padding |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Post-Stack NA with Two Opcodes
This is an example where the Post-Stack MPLS Header encodes two
different Post-Stack Network Actions.
Details:
PSMH-Len=3: This is the total length of Post-Stack MPLS Header.
MNA-PS-OP=2: Post-Stack NA Opcode 2.
PS-NAL=0: Post-Stack Network Action does not contain any
additional data.
MNA-PS-OP=3: Post-Stack NA Opcode 3.
PS-NAL=1: Post-Stack Network Action contains 1 additional 4-octet
Ancillary Data.
A.1.4. Post-Stack Network Action with two Different Scopes
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA Label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Opcode=2=NoOp| 0 |1|HBH|0| NASL=0|U|NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA Label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode=TBA2 | Post-Stack Offset = 2 |1|I2E|1| NASL=0|U|NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0 |Reserve| PSMH-Len=1 | Type = MNA-Post-Stack-Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA-PS-OP=2|R|R| PS-NAL=0 | Post-Stack Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0 |Reserve| PSMH-Len=2 | Type = MNA-Post-Stack-Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA-PS-OP=3|R|R| PS-NAL=1 | Post-Stack Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Post-Stack Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Optional Payload + Padding |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Post-Stack NA with two Different Scopes
This is an example of Post-Stack MPLS Header encoding, that encodes
two different different scoped Post-Stack Network Actions. The first
scope is Hop-By-Hop and the second scope is Ingress-To-Egress.
Details:
The offset of the Hop-By-Hop scoped Post-Stack Network Action is
0.
Opcode TBA2 carries the offset of the Ingress-To-Egress scoped
Post-Stack Network Action. The data is 2, i.e., the Post-Stack
MPLS Header starts 8 bytes after the MPLS Bottom of Stack.
A.2. Examples of In-Stack and Post-Stack Network Actions
A.2.1. NAS with In-Stack and Post-Stack NAs
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA Label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode=1 | Flag-Based NAIs |1|IHS|0| NASL=0|U|NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ [0| ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |1| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0 |Reserve| PSMH-Len=1 | Type = MNA-Post-Stack-Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA-PS-OP |R|R| PS-NAL=0 | Post-Stack Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ Payload ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: NAS with In-Stack and Post-Stack NAs
In some cases, the NAS may encode In-Stack NAs and indicate the
presence of a Post-Stack MPLS Header. The IHS field indicates the
scope of both the In-Stack and Post-Stack NAs.
A.2.2. NASes with Different In-Stack and Post-Stack Scopes
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA Label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode=1 | Flag-Based NAIs |0|HBH|0| NASL=0|U|NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA Label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Opcode=2=NoOp| 0 |1|I2E|1| NASL=0|U|NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0 |Reserve| PSMH-Len=1 | Type = MNA-Post-Stack-Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNA-PS-OP |R|R| PS-NAL=0 | Post-Stack Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ Payload ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 10: NASes with Different In-Stack and Post-Stack Scopes
In some cases, the label stack may need to carry In-Stack NAs with
Hop-By-Hop scope and Post-Stack NAs with I2E scope. In this case,
there will be two NASes in the label stack. In this case, the first
NAS will encode the In-Stack NA with the Hop-By-Hop scope and the
second NAS will encode the presence of I2E-scoped Post-Stack NAs.
Acknowledgments
The authors would like to thank the authors and contributors of the
draft-ietf-mpls-mna-hdr as this document borrows some text from the
earlier version of that document. The authors would like to thank
Greg Mirsky, Loa Andersson, Haoyu Song, Adrian Farrel, Yao Liu, and
Joel Halpern for reviewing this document and providing many useful
comments.
Contributors
The following people have substantially contributed to this document:
John Drake
Juniper Networks
United States
Email: jdrake@juniper.net
Authors' Addresses
Jaganbabu Rajamanickam (editor)
Cisco Systems, Inc.
Canada
Email: jrajaman@cisco.com
Rakesh Gandhi (editor)
Cisco Systems, Inc.
Canada
Email: rgandhi@cisco.com
Royi Zigler
Broadcom
Email: royi.zigler@broadcom.com
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Jie Dong
Huawei Technologies
Beijing
China
Email: jie.dong@huawei.com
Jisu Bhattacharya
Cisco Systems, Inc.
Canada
Email: jisu@cisco.com
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