



MPLS Working Group                                  J. Rajamanickam, Ed.
Internet-Draft                                            R. Gandhi, Ed.
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: 4 April 2026                                          R. Zigler
                                                                Broadcom
                                                                   T. Li
                                                        Juniper Networks
                                                                 J. Dong
                                                     Huawei Technologies
                                                          1 October 2025


             Post-Stack MPLS Network Action (MNA) Solution
                     draft-ietf-mpls-mna-ps-hdr-03

Abstract

   This document defines the Post-Stack MPLS Network Action (MNA)
   solution for carrying Network Actions and Ancillary Data after the
   MPLS label stack, based on the In-Stack MNA solution defined in "MPLS
   Network Action (MNA) Sub-Stack Solution."  MPLS Network Actions can
   be used to influence packet forwarding decisions, carry additional
   Operations, Administration, and Maintenance (OAM) information in the
   MPLS packet, or perform user-defined operations.  This solution
   document addresses the Post-Stack network action and Post-Stack data
   specific requirements found in "RFC 9613".  This document follows the
   architectural framework for the MPLS Network Actions (MNA)
   technologies 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 April 2026.






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Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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 Header Presence Bit Carried in In-Stack MNA
           Sub-Stack . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Post-Stack Header Encoding  . . . . . . . . . . . . . . .   6
       3.2.1.  Post-Stack Header Type  . . . . . . . . . . . . . . .   6
       3.2.2.  Post-Stack Network Action Encoding  . . . . . . . . .   6
   4.  In-Stack Network Action Special Opcodes . . . . . . . . . . .   7
     4.1.  In-Stack Network Action Opcode for PSH Start Offset . . .   7
     4.2.  In-Stack Network Action Opcode for Offset of End of
           Post-Stack Header . . . . . . . . . . . . . . . . . . . .   8
   5.  Procedure . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Processing Rules for P Bit  . . . . . . . . . . . . . . .   8
     5.2.  Network Action Processing Order . . . . . . . . . . . . .   9
     5.3.  Node Capability Signaling . . . . . . . . . . . . . . . .   9
   6.  Node Responsibilities . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Encapsulating Node Responsibilities . . . . . . . . . . .   9
     6.2.  Transit Node Responsibilities . . . . . . . . . . . . . .   9
     6.3.  Penultimate Node Responsibilities . . . . . . . . . . . .  10
     6.4.  Decapsulating Node Responsibilities . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  First Nibble for Post-Stack Headers . . . . . . . . . . .  10
     8.2.  Post-Stack Header Types Registry  . . . . . . . . . . . .  11
     8.3.  Network Action Opcodes  . . . . . . . . . . . . . . . . .  11
   9.  Appendix A: Examples  . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Examples of Post-Stack Header Encoding  . . . . . . . . .  12
       9.1.1.  NAS that only Indicates Post-Stack Header . . . . . .  12
       9.1.2.  NAS that Indicates Post-Stack Header Start Offset . .  13



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       9.1.3.  Post-Stack Network Actions with Two Opcodes . . . . .  14
       9.1.4.  Post-Stack Network Action with two Different
               Scopes  . . . . . . . . . . . . . . . . . . . . . . .  15
     9.2.  In-Stack and Post-Stack Network Actions . . . . . . . . .  16
       9.2.1.  NAS with In-Stack and Post-Stack NAs  . . . . . . . .  16
       9.2.2.  NASes with Different In-Stack and Post-Stack
               Scopes  . . . . . . . . . . . . . . . . . . . . . . .  17
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     10.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   [RFC3032] defines the encoding of the MPLS label stack, the basic
   structure used to define a forwarding path.  Forthcoming applications
   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.  AD can be used to carry additional
   information, such as for In Situ OAM (IOAM) as described in
   [RFC9791].  User-defined network actions allow local actions to be
   defined.  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 Post-Stack MNA.

   This document defines the Post-Stack MPLS Network Action (MNA)
   solution for carrying Network Actions and Ancillary Data after the
   MPLS label stack.  It is based on the In-Stack MNA solution defined
   in [I-D.ietf-mpls-mna-hdr].  This solution document addresses the
   Post-Stack network action and Post-Stack data specific requirements
   found in [RFC9613].  This document follows the architectural
   framework for the MPLS Network Actions (MNA) technologies 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   | [RFC3032]               |
      +--------------+-------------------+-------------------------+
      | 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 | [RFC3032]               |
      +--------------+-------------------+-------------------------+
      | 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 Header | This document           |
      |              | Presence Bit      |                         |



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      +--------------+-------------------+-------------------------+
      | PSD          | Post-Stack Data   | [RFC9613] and [RFC9789] |
      +--------------+-------------------+-------------------------+
      | PSH          | Post-Stack Header | This document           |
      +--------------+-------------------+-------------------------+
      | TC           | Traffic Class     | [RFC5462]               |
      +--------------+-------------------+-------------------------+
      | TTL          | Time To Live      | [RFC3032]               |
      +--------------+-------------------+-------------------------+

                          Table 1: Abbreviations

3.  Overview

   The Post-Stack MNA solution contains two main parts:

   *  Post-Stack Header Presence Bit Carried in In-Stack MNA Sub-Stack

   *  Post-Stack Header Encoding that includes Post-Stack Header Type
      and Post-Stack Network Actions

3.1.  Post-Stack Header Presence Bit Carried in In-Stack MNA Sub-Stack

   The 21st bit from the left 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 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    |        Data             |P|IHS|S|U|  NASL | NAL |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 1: Post-Stack Header Presence Bit Carried in In-Stack MNA
                                 Sub-Stack

   The following flags are carried in In-Stack NAS as defined in
   [I-D.ietf-mpls-mna-hdr].  These flags are also applicable to Post-
   Stack MNA:

   *  IHS (2 Bit): Indicates the combined scope of the In-Stack and the
      Post-Stack Network Actions.  In-Stack NAS for each scope with P
      bit set will have its corresponding Post-Stack Header.

   *  U (1 Bit): Indicates the combined Unknown Action Handling of the
      In-Stack and the Post-Stack Network Actions.




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3.2.  Post-Stack Header Encoding

   The Post-Stack Header is encoded after the Bottom of the MPLS Label
   Stack (BOS), 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 Post-Stack Header carries one or more Post-Stack Network Actions
   and their Ancillary Data.

   The PSH consist of two main parts:

   *  Post-Stack Header (PSH) Type

   *  Post-Stack Network Action Encoding

3.2.1.  Post-Stack Header Type

   The Post-Stack Header type is the top-header for all the Post-Stack
   Network Actions that are encoded in the PSH for each scope.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | TBA1  |Version| PS-HDR-LEN    | TYPE = MNA-POST-STACK-HDR = 1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2: Post-Stack Header Type

   *  TBA1 (4 bits): This first nibble [RFC9790] identifies the start of
      the PSH.  A new value (value TBA1) is to be assigned by IANA for
      Post-Stack Headers.

   *  Version (4 bits): This is PSH version.  The initial version is set
      to 0.

   *  PS-HDR-LEN (8 bits): PSH Total Length in 4-octet units that
      includes Post-Stack Network Actions.  This excludes the PSH type
      header.

   *  TYPE (16 bits): Type is set to 1 to indicate Post-Stack Header
      Type For MNA.  See Section 8.2.

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.



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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-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.  Opcodes
      will be assigned by IANA accordingly.

   *  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 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 is described in the document that defines
      the NA indicated by the MNA-PS-OP value.

4.  In-Stack Network Action Special Opcodes

4.1.  In-Stack Network Action Opcode for PSH Start Offset

   Opcode: TBA2

   Purpose: This opcode carries the start offset of the PSH from 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 units of 4 octets.  This allows the Generic Control Word (0000b)
   [RFC4385] and G-ACh (0001b) [RFC5586] fields to be placed immediately
   after the BOS.  In the absence of this opcode, the PSH is encoded
   immediately after the MPLS BOS.  A data value of 1 indicates that the
   PSH starts 4 octets after the BOS.

   Scope: This opcode can be used with any scope.






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4.2.  In-Stack Network Action Opcode for Offset of End of Post-Stack
      Header

   Opcode: TBA3

   Purpose: This opcode carries the offset of end of the Post-Stack
   Header from 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 units of 4 octets.  A data value of 5 indicates that the Post-
   Stack Header ends 20 octets after the BOS.

   Scope: This opcode can be used with any scope.

5.  Procedure

5.1.  Processing Rules for P Bit

   The P bit MUST be set to 1 in In-Stack Network Action Sub-Stack when
   corresponding Post-Stack Header is added in the packet.

   By default, the PSH starts immediately after the BOS.  The offset of
   the PSH that does not start immediately after the BOS is indicated
   using the PSH 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 Header as defined in this document.
   Conversely, the node that does not support the P bit, will skip
   processing the Post-Stack Header altogether.










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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 in 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 Header MUST ensure that
   the egress node is capable of Post-Stack MNA and can remove the PSH
   from the packet.

   *  Each participating node MUST signal the network actions that it
      supports.

   *  Each participating node MUST signal its "Readable Label Depth
      including Post-Stack Header" that can be encoded.

   The above capability signaling will be added in appropriate
   protocols.  Signaling details are outside the scope of this document.

6.  Node Responsibilities

   This section defines the specific responsibilities for nodes along an
   MPLS path for processing a Post-Stack Header.

6.1.  Encapsulating Node Responsibilities

   The encapsulating node MAY add Post-Stack Header to the packet in
   accordance with its policies, the placement restrictions, and the
   limitations.

   The encapsulating node MUST NOT add a Post-Stack Header to the packet
   if the decapsulation node does not support Post-Stack Header.

   If the encapsulating node is also a transit node, then it MUST also
   respect transit node responsibilities.

6.2.  Transit Node Responsibilities

   A transit node MAY modify the Ancillary Data in the Post-Stack
   Header.





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   A transit node MUST respect the Unknown Action Handling flag encoded
   in the corresponding NAS when processing the PSH.

   A node that removes the last copy of a NAS that has the P bit set
   MUST remove all Post-Stack Headers.

6.3.  Penultimate Node Responsibilities

   In addition to the transit node responsibilities above, the
   penultimate node MUST NOT remove the last copy of an HBH or I2E NAS
   and associated PSH when the NAS is exposed after removing the
   forwarding (transport) label.  This allows the egress node to process
   the NAS and associated PSH.

6.4.  Decapsulating Node Responsibilities

   The decapsulating node MUST remove any Post-Stack Header it receives.

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 Headers

   This document requests that IANA allocate a value (TBA1) for the
   Post-Stack Headers from the "Post-Stack First Nibble" registry
   created by [RFC9790] to indicate the start of the PSH after BOS.

         +==========+=======+====================+===============+
         | Protocol | Value | Description        | Reference     |
         +==========+=======+====================+===============+
         | PSH      | TBA1  | Post-Stack Headers | This document |
         +----------+-------+--------------------+---------------+

                 Table 2: Post-Stack First Nibble Registry








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8.2.  Post-Stack Header Types Registry

   This document requests that IANA create a new registry with the name
   "Post-Stack 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).

   The assignments for this registry are:

      +=============+==============================+===============+
      | Type        |         Description          | Reference     |
      +=============+==============================+===============+
      | 0           | Reserved, not to be assigned | This document |
      +-------------+------------------------------+---------------+
      | 1-65520     | IETF Review                  | This document |
      +-------------+------------------------------+---------------+
      | 65521-65524 | Experimental Use             | This document |
      +-------------+------------------------------+---------------+
      | 65525-65535 | Private Use                  | This document |
      +-------------+------------------------------+---------------+

                Table 3: Post-Stack Header Types Registry

   The initial assignment for this registry is:

         +======+================================+===============+
         | Type |          Description           | Reference     |
         +======+================================+===============+
         | 1    | Post-Stack Header Type For MNA | This document |
         +------+--------------------------------+---------------+

                      Table 4: Post-Stack Header Types

8.3.  Network Action Opcodes

   IANA maintains the "Network Action Opcodes" registry to be created by
   [I-D.ietf-mpls-mna-hdr].  IANA is requested to perform two actions on
   this registry:

   1.  Assign two code points for network actions defined in this
   document.

   2.  Add a new column to say 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  |                |                         |
    |        | Header      |                |                         |
    +--------+-------------+----------------+-------------------------+
    | TBA3   | Offset of   | In-Stack Only  | This document           |
    |        | End of      |                |                         |
    |        | Post-Stack  |                |                         |
    |        | Header      |                |                         |
    +--------+-------------+----------------+-------------------------+
    | 127    | Opcode      | In-Stack and   | [I-D.ietf-mpls-mna-hdr] |
    |        | Range       | Post-Stack     |                         |
    |        | Extension   |                |                         |
    |        | Beyond 127  |                |                         |
    +--------+-------------+----------------+-------------------------+

                      Table 5: Network Action Opcodes

9.  Appendix A: Examples

9.1.  Examples of Post-Stack Header Encoding

9.1.1.  NAS that only Indicates Post-Stack Header










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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|0|U| NASL=0|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                             [0|               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                             |1|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | TBA1  |Version| PS-HDR-LEN=1  | TYPE = MNA-POST-STACK-HDR = 1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  MNA-PS-OP  |R|R|  PS-NAL=0   |       POST-STACK DATA         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   ~                           Payload                             ~
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 4: NAS that only indicates Post-Stack Header

   In some cases, the NAS may encode only the presence of Post-Stack
   Header.  The Post-Stack Header starts immediately after the BOS.

9.1.2.  NAS that Indicates Post-Stack Header Start Offset

























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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=TBA2 |  Post-Stack Offset = 2  |1|IHS|0|U| NASL=0|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                             [0|               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                             |1|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | TBA1  |Version| PS-HDR-LEN=1  | TYPE = MNA-POST-STACK-HDR = 1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  MNA-PS-OP  |R|R|  PS-NAL=0   |       POST-STACK DATA         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   ~                           Payload                             ~
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 5: NAS Indicates Post-Stack Header Start Offset

   The NAS may encode the start offset of the Post-Stack Header with a
   non-zero value, for example, when it is after other header such as
   GACH or CW header.  In this example, the PSH starts at offset of 8
   bytes after the BoS.

9.1.3.  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|U| NASL=0|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | TBA1  |Version| PS-HDR-LEN=3  | TYPE = MNA-POST-STACK-HDR = 1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  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 6: Post-Stack NA with Two Opcodes

   This is an example, Post-Stack Header encodes two different Post-
   Stack Network Actions.

   Details:

      PS-HDR-LEN=3: This is the Total Length of Post-Stack 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.

9.1.4.  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|U| NASL=0|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          MNA Label                    | TC  |0|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=TBA2 |  Post-Stack Offset = 2  |1|I2E|1|U| NASL=0|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | TBA1  |Version| PS-HDR-LEN=1  | TYPE = MNA-POST-STACK-HDR = 1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  MNA-PS-OP=2|R|R|  PS-NAL=0   |       POST-STACK DATA         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | TBA1  |Version| PS-HDR-LEN=2  | TYPE = MNA-POST-STACK-HDR = 1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  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 Different Scopes

   This is an example of Post-Stack Header encoding, that encode two
   different different scoped Post-Stack Network Actions.  The first
   scope is Hop-By-Hop and the second scope is Ingress-To-Egress scoped
   Post-Stack Network Action.

   Details:

      The offset of the Hop-By-Hop scoped Post-Stack Network Action 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
      Header starts 8 bytes after the MPLS Bottom of Stack.

9.2.  In-Stack and Post-Stack Network Actions

9.2.1.  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|U| NASL=0|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                             [0|               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                             |1|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | TBA1  |Version| PS-HDR-LEN=1  | TYPE = MNA-POST-STACK-HDR = 1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  MNA-PS-OP  |R|R|  PS-NAL=0   |       POST-STACK DATA         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   ~                           Payload                             ~
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 8: NAS with In-Stack and Post-Stack NAs

   In some cases, the NAS may encode In-Stack NAs and indicate the
   presence of Post-Stack Header.  The IHS field indicates the scope of
   both the In-Stack and Post-Stack NAs.

9.2.2.  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|U| NASL=0|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          MNA Label                    | TC  |0|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Opcode=2=NOOP|    0                    |1|I2E|1|U| NASL=0|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | TBA1  |Version| PS-HDR-LEN=1  | TYPE = MNA-POST-STACK-HDR = 1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  MNA-PS-OP  |R|R|  PS-NAL=0   |       POST-STACK DATA         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   ~                           Payload                             ~
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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       Figure 9: 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.

10.  References

10.1.  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 Solution",
              Work in Progress, Internet-Draft, draft-ietf-mpls-mna-hdr-
              15, 5 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
              mna-hdr-15>.

   [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>.







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   [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>.

   [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>.

   [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>.

   [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>.

10.2.  Informative References

   [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>.

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, and Joel
   Halpern for reviewing this document and providing many useful
   comments.





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Contributors

   The following people have substantially contributed to this document:

   Jisu Bhattacharya
   Cisco Systems, Inc.
   Email: jisu@cisco.com


   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


   Tony Li
   Juniper Networks
   Email: tony.li@tony.li


   Jie Dong
   Huawei Technologies
   Beijing
   China
   Email: jie.dong@huawei.com








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