



MPLS Working Group                                  J. Rajamanickam, Ed.
Internet-Draft                                            R. Gandhi, Ed.
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: 21 January 2026                                       R. Zigler
                                                                Broadcom
                                                                 H. Song
                                                  Futurewei Technologies
                                                             K. Kompella
                                                        Juniper Networks
                                                            20 July 2025


              MPLS Network Action (MNA) Sub-Stack Solution
                       draft-ietf-mpls-mna-hdr-13

Abstract

   This document defines the MPLS Network Actions (MNA) sub-stack
   solution for carrying Network Actions and Ancillary Data in the label
   stack.  MNA 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 solution document specifies In-stack network action and In-stack
   data specific requirements found in "Requirements for MPLS Network
   Actions".  This document follows the architectural framework for the
   MNA technologies specified in "MPLS Network Actions (MNA) Framework".

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 21 January 2026.

Copyright Notice

   Copyright (c) 2025 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  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Label Stack Entry Formats . . . . . . . . . . . . . . . . . .   6
     4.1.  LSE Format A: The MNA Sub-Stack Indicator . . . . . . . .   6
     4.2.  LSE Format B: The initial opcode  . . . . . . . . . . . .   6
     4.3.  LSE Format C: Subsequent opcodes  . . . . . . . . . . . .   7
     4.4.  LSE Format D: Additional Data . . . . . . . . . . . . . .   8
   5.  The MNA Sub-Stack . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Opcodes . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Data  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.4.  Unknown Network Action Handling . . . . . . . . . . . . .  11
     5.5.  Ordering  . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Special Opcodes . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  bSPL Protection . . . . . . . . . . . . . . . . . . . . .  12
     6.2.  Flag-Based NAIs without AD  . . . . . . . . . . . . . . .  12
     6.3.  No-Operation Opcode . . . . . . . . . . . . . . . . . . .  13
     6.4.  Extension Opcode  . . . . . . . . . . . . . . . . . . . .  13
   7.  NAS placement in the Label Stack  . . . . . . . . . . . . . .  13
     7.1.  Actions when Pushing Labels . . . . . . . . . . . . . . .  14
   8.  Node Capability Signaling . . . . . . . . . . . . . . . . . .  15
   9.  Processing the Network Action Sub-Stack . . . . . . . . . . .  15
     9.1.  Encapsulating Node Responsibilities . . . . . . . . . . .  15
     9.2.  Transit Node Responsibilities . . . . . . . . . . . . . .  16
     9.3.  Penultimate Node Responsibilities . . . . . . . . . . . .  16
     9.4.  Egress Node Responsibilities  . . . . . . . . . . . . . .  16
   10. Network Action Indicator Opcode Definition  . . . . . . . . .  16
   11. Backward Compatibility  . . . . . . . . . . . . . . . . . . .  17
   12. Implementation Status . . . . . . . . . . . . . . . . . . . .  18
     12.1.  University of Tuebingen Implementation . . . . . . . . .  18
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  18
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     14.1.  MNA bSPL Label . . . . . . . . . . . . . . . . . . . . .  19
     14.2.  MPLS Network Actions Parameters  . . . . . . . . . . . .  19



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     14.3.  Network Action Flags Without Ancillary Data  . . . . . .  19
     14.4.  Network Action Opcodes . . . . . . . . . . . . . . . . .  20
   15. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  21
     15.1.  Network Action Encoding Examples . . . . . . . . . . . .  21
       15.1.1.  Network Action Flags without AD  . . . . . . . . . .  21
       15.1.2.  Network Action Opcode with AD  . . . . . . . . . . .  22
       15.1.3.  Network Action Opcode with more AD with Format-B . .  23
       15.1.4.  Network Action Opcode with more AD with Format C . .  23
     15.2.  Network Action Processing Order  . . . . . . . . . . . .  24
       15.2.1.  Network Action Processing Order  . . . . . . . . . .  24
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     16.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  27
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

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.  Ancillary Data can be used to carry additional
   information, such as a network slice identifier or an entropy value
   for load-balancing.  Several MNA applications are described in
   [RFC9791].

   This solution document specifies In-stack network action and In-stack
   data (ISD) specific requirements found in [RFC9613].

   This document defines the syntax and semantics of network actions and
   ancillary data encoded in an MPLS Label Stack.  In-stack actions and
   ancillary data are contained in a Network Action Sub-Stack (NAS),
   which is recognized by a new base Special Purpose Label (bSPL).  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] is used in this
   document.















































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      +==============+=============================+===============+
      | Abbreviation | Meaning                     | Reference     |
      +==============+=============================+===============+
      | AD           | Ancillary Data              | [RFC9613]     |
      +--------------+-----------------------------+---------------+
      | bSPL         | Base Special Purpose Label  | [RFC9017]     |
      +--------------+-----------------------------+---------------+
      | BOS          | Bottom Of Stack             | [RFC3032]     |
      +--------------+-----------------------------+---------------+
      | HBH          | Hop-By-Hop Scope            | [RFC9789]     |
      +--------------+-----------------------------+---------------+
      | I2E          | Ingress-To-Egress Scope     | [RFC9789]     |
      +--------------+-----------------------------+---------------+
      | IHS          | I2E, HBH, or Select Scope   | [RFC9789],    |
      |              |                             | This document |
      +--------------+-----------------------------+---------------+
      | ISD          | In-stack Data               | [RFC9613]     |
      +--------------+-----------------------------+---------------+
      | LSE          | Label Stack Entry           | [RFC3032]     |
      +--------------+-----------------------------+---------------+
      | MNA          | MPLS Network Actions        | [RFC9789]     |
      +--------------+-----------------------------+---------------+
      | NAI          | Network Action Indicator    | [RFC9613]     |
      +--------------+-----------------------------+---------------+
      | NAL          | Network Action Length       | This document |
      +--------------+-----------------------------+---------------+
      | NAS          | Network Action Sub-Stack    | [RFC9789]     |
      +--------------+-----------------------------+---------------+
      | NASI         | Network Action Sub-Stack    | This document |
      |              | Indicator                   |               |
      +--------------+-----------------------------+---------------+
      | NASL         | Network Action Sub-Stack    | This document |
      |              | Length                      |               |
      +--------------+-----------------------------+---------------+
      | OAM          | Operations, Administration, | [RFC6291]     |
      |              | and Maintenance             |               |
      +--------------+-----------------------------+---------------+
      | RLD          | Readable Label Depth        | [RFC9789]     |
      +--------------+-----------------------------+---------------+
      | TC           | Traffic Class               | [RFC5462]     |
      +--------------+-----------------------------+---------------+
      | TTL          | Time To Live                | [RFC3032]     |
      +--------------+-----------------------------+---------------+

                          Table 1: Abbreviations






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3.  Overview

   The MPLS Network Action Sub-Stack (NAS) is a set of Label Stack
   Entries (LSEs) that appear as part of an MPLS Label Stack and serve
   to encode information about the network actions that should be
   invoked for the packet.  Multiple NASes may appear in a label stack.

   This document describes how network actions and their optional
   ancillary data are encoded as part of an NAS as a stack of LSEs.
   Mechanisms that allow sharing of ancillary data AD between multiple
   network actions encoded in the same NAS can be described in other
   documents and do not rely on any explicit provision in the encodings
   described in this document.

4.  Label Stack Entry Formats

   The NAS uses a variety of different formats of LSEs for different
   purposes.  This section describes the syntax of the various formats
   while the overall structure of the NAS and the semantics of the
   various LSEs are described in the sections below.

4.1.  LSE Format A: The MNA Sub-Stack Indicator

   LSE Format A is an LSE as described in [RFC3032] and [RFC5462].  The
   label value is an IANA-assigned value (TBA) for the MNA bSPL label
   from the "Base Special-Purpose MPLS Label Values" registry to
   indicate the presence of MNA in the packet and the beginning an MNA
   Sub-Stack in the label stack.

    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=bSPL                   | TC  |S|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 1: LSE Format A: The MNA Sub-Stack Indicator

   *  S (1 bit): The Bottom of Stack [RFC3032].  MUST be set to 0 on
      transmitted packets.  If a packet is received with S bit set to 1,
      then the packet MUST be dropped.

4.2.  LSE Format B: The initial opcode

   LSE Format B is used to encode the first opcode in the NAS, plus a
   number of other fields about the NAS.  This LSE cannot carry more
   than 13 bits of data.





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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Opcode    |        13-bit Data      |R|IHS|S|  NASL |U| NAL |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 2: LSE Format B: The initial opcode

   *  Opcode (7 bits): The operation code for this LSE.  See
      Section 5.1.

   *  Data (13 bits): Opcode-specific data.

   *  R (1 bit): Reserved.  This bit MUST be transmitted as zero and
      ignored upon receipt.

   *  IHS (2 bits): The scope of the sub-stack.  See Section 5.3.

   *  S (1 bit): The Bottom of Stack [RFC3032].  If NASL value is non-
      zero, then S bit MUST be 0.  If a packet is received with S bit
      set to 1 and a non-zero NASL value, then the packet MUST be
      dropped.  The encapsulating node MUST ensure that S bit is set to
      1 only in the Last LSE in the MPLS header.

   *  NASL (4 bits): The Network Action Sub-Stack Length (NASL).  The
      number of additional LSEs in the sub-stack, not including the
      leading Format A LSE and the Format B LSE.

   *  U (1 bit): Unknown Network Action Handling.  See Section 5.4.

   *  NAL (3 bits): Network Action Length.  The number of LSEs of
      additional data, encoded in LSE Format D (Section 4.4) following
      this LSE.  The NAL value MUST be less than or equal to the NASL
      value in Format B, if not the packet MUST be dropped.

   NOTE: Format A and B LSEs MUST be present when a Format C or D LSE is
   to be carried in the NAS.

4.3.  LSE Format C: Subsequent opcodes

   LSE Format C is used to encode the subsequent opcodes in the NAS.

    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    |        16-bit Data            |S|4b Data|U| NAL |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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                 Figure 3: LSE Format C: Subsequent opcodes

   *  Opcode (7 bits): The operation code for this LSE.  See
      Section 5.1.

   *  Data (16 bits + 4 bits): Opcode specific data

   *  S (1 bit): The Bottom of Stack [RFC3032].  If NAL value is non-
      zero and if S bit is set to 1, then the packet MUST be dropped.
      If this is not the last LSE in the NAS and if S bit is set to 1
      then the packet MUST be dropped.  The encapsulating node MUST
      ensure that S bit is set to 1 only in the Last LSE.

   *  U (1 bit): Unknown Network Action Handling.  See Section 5.4.

   *  NAL (3 bits): Network Action Length.  The number of LSEs of
      additional data, encoded in LSE Format D (Section 4.4) following
      this LSE.  The NAL value MUST be less than or equal to the NASL
      value in Format B, if not the packet MUST be dropped.

4.4.  LSE Format D: Additional Data

   LSE Format D is used to encode additional data that did not fit in
   the LSE with the preceding opcode.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|                    22-bit Data            |S|  8-bit Data   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 4: LSE Format D: Additional Data

   *  1 (1 bit): The most significant bit MUST be set.  This prevents
      legacy implementations from misinterpreting this LSE as containing
      a special purpose label if the data begins with zeros.

   *  S (1 bit): The Bottom of Stack [RFC3032].  If this is not the last
      LSE for the Network Action based on the NAL value and if S bit is
      set to 1 then the packet MUST be dropped.  If this is not the last
      LSE in the NAS and if S bit is set to 1 then the packet MUST be
      dropped.  The encapsulating node MUST ensure that the S bit is set
      to 1 only in the Last LSE.

   *  Data (22 bits + 8 bits): Opcode specific data






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5.  The MNA Sub-Stack

   The MNA Sub-Stack MUST begin with a Format A LSE (Section 4.1).  The
   label value of the LSE contains the MNA bSPL (value TBA) to indicate
   the presence of the MNA Sub-Stack.

   The TC and TTL values of the Format A LSE retain their semantics as
   defined in [RFC3032] and [RFC5462].  The TTL and TC values in the
   Format A LSE are copied from the forwarding label at the top of the
   label stack.  The penultimate node on the path copies the TTL and TC
   values from the preceding LSE to the next LSE on the label stack,
   overwriting the TTL and TC values of the next LSE, as specified in
   Section 3.5 of [RFC3443] and Section 2.6.3 of [RFC3270] in the
   Uniform Mode LSPs.  If the node performing this copy is not aware of
   MNA, this could overwrite the values in the first LSE of the MNA sub-
   stack.

   The second LSE in an NAS MUST be a Format B LSE (Section 4.2).  This
   LSE contains an initial opcode plus additional fields that describe
   the NAS.

   The Format B LSE (Section 4.2) could optionally carry additional data
   in Format D (Section 4.4) LSEs, up to the length encoded in the LSE's
   NAL value.

   An NAS MAY contain more Format C (Section 4.3) and Format D
   (Section 4.4) LSEs, up to the length encoded in the NASL value.  All
   Format D LSEs MUST follow a Format C or B LSE and be included in that
   LSE's NAL value.

5.1.  Opcodes

   The opcode is a 7-bit field that indicates the semantics of its LSE.
   Several opcodes are assigned special semantics (Section 6), others
   act as Network Action Indicators and are assigned through IANA
   (Section 10 and Section 14.4).

5.2.  Data

   The data field carries opcode-specific data that is ancillary data
   for a network action.  In the case of opcode 1, the data field
   carries Flag-Based Network Action Indicators without ancillary data.

   Legacy implementations might use the label value (most significant 20
   bits) in one or more consecutive LSEs when load-balancing data flows
   in an ECMP environment.  Modifying the first 20 bits in an LSE might
   alter that packet's path and result in out-of-order delivery of
   packets.  To preserve the stability of the deployed services in ECMP



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   environments that use information in the label value to load-balance
   data flows, if a network action encodes data in the given LSE that
   may be different among packets in the same flow or might change
   during packet forwarding across the MPLS network, then that data MUST
   NOT be carried in the most significant 20 bits of a Format B LSE
   (Section 4.2), a Format C LSE (Section 4.3), or a Format D LSE
   (Section 4.4).  Thus, the available bits for data that can change by
   a transit node or differ among packets of the same flow in Format A
   and Format B LSEs are 0, Format C LSE is 7 (bits 20-22 and 25-28) and
   Format D LSE is 11 (bits 20-22 and 24-31).

   Similarly, to preserve service stability, such data also MUST NOT be
   carried in the most significant 23 bits of these LSEs when the legacy
   implementation also uses the TC value, in addition to the label
   value, in all LSEs when computing ECMP decisions.

   The available mitigations for these problems are to use additional
   Format D LSEs to carry the data, or to place the data in Post-Stack
   Data as described in [RFC9789].

   In network deployments where it is known that a load-balancing of
   data flows is not used, or, otherwise, if only the explicitly
   signaled entropy value is used, and it is certain that the load-
   balancing path selection will not be based on the label value of the
   LSEs, then the data in the label value of the LSEs in ISD MAY be
   mutable within the data flow without causing the out-of-order
   delivery of packets.

5.3.  Scope

   The IHS field in the Format B LSE indicates the scope of the In-stack
   NAIs encoded in the NAS.  Scope defines which nodes along the MPLS
   path should perform the network actions found within the NAS.  The
   specific values of the IHS field are as follows:

                            +======+==========+
                            | Bits | Scope    |
                            +======+==========+
                            | 00   | I2E      |
                            +------+----------+
                            | 01   | HBH      |
                            +------+----------+
                            | 10   | Select   |
                            +------+----------+
                            | 11   | Reserved |
                            +------+----------+

                         Table 2: IHS Scope Values



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      Ingress To Egress (I2E) - The NAS MUST NOT be processed by any
      node the except the egress node.

      Hop-By-Hop (HBH) - All nodes along the path MUST process the NAS.

      Select - Only specific nodes along the path that brings NAS to top
      of the stack will perform the action.

   A single NAS carries only one of the three scopes (I2E/HBH/Select).
   To support multiple scopes for a single packet, multiple NASes MAY be
   included in a single label stack.

   The egress node is included in the HBH scope.  This implies that the
   penultimate node MUST NOT remove a HBH NAS.  The egress node MAY
   receive an NAS at the top of the label stack as discussed in
   Section 10.

   An I2E scope NAS, if present, MUST be encoded after any HBH or
   Select-scope NASes.  This makes it easier for the transit nodes to
   process a NAS with HBH or Select scope.

5.4.  Unknown Network Action Handling

   The Unknown Network Action Handling (U) field in a Format B LSE
   (Section 4.2) and Format C LSE (Section 4.3) is a 1-bit value that
   defines the action to be taken by a node that does not understand an
   action within the NAS.  The different types of Unknown Network Action
   Handling actions are defined below.

                       +=====+=====================+
                       | Bit | Action              |
                       +=====+=====================+
                       | 0   | Skip to the next NA |
                       +-----+---------------------+
                       | 1   | Drop the packet     |
                       +-----+---------------------+

                          Table 3: Unknown Network
                              Action Handling

   When a packet with an unknown Network Action is dropped, the node
   SHOULD maintain a local counter for this event, and MAY send a rate-
   limited notification to the operator.








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5.5.  Ordering

   The network actions encoded in the NAS MUST be processed as if they
   were processed in the order that they appear in the NAS, from the top
   of the NAS to the bottom.  NAI encoded as flags (see Section 6.2)
   MUST be processed as if they were processed from the most significant
   bit to the least significant bit.  If a label stack contains multiple
   NASes, then they MUST be processed as if they were processed in the
   order that they appear in the label stack, subject to the
   restrictions in Section 7.

6.  Special Opcodes

   Below are the special opcodes used to build a basic In-stack MNA
   solution.  In future, additional special opcodes can be defined and
   their code-points can be assigned from the "Network Action Opcodes"
   IANA registry.

6.1.  bSPL Protection

   Opcode: 0

   Purpose: Legacy implementations may scan the label stack looking for
   bSPL values.  As long as the opcode field is non-zero, an LSE cannot
   be misinterpreted as containing a bSPL.  Opcode 0 is therefore
   reserved and is not used.

6.2.  Flag-Based NAIs without AD

   Opcode: 1

   Purpose: Network actions that do not require Ancillary Data do not
   require an entire LSE.  A single flag can be used to indicate each of
   these network actions.

   LSE Formats: B, C, D

   Data: The data field carries Network Action Indicators, which should
   be evaluated from the most significant bit to the least significant
   bit.  If this opcode is used with LSE Format B only, then up to 13
   flags may be carried.  If this opcode is used with LSE Format C only,
   then up to 20 flags may be carried.  Format D LSEs can be used with
   format C LSEs to encode more than 20 flags.  Flags are assigned from
   the "Network Action Flags Without Ancillary Data" registry
   (Section 14.3).  If flags need to be evaluated in a different order,
   multiple LSEs using this opcode may be used to specify the requested
   order.  The Flag-Based Network Action Indicators MUST follow the
   procedure for data specified in Section 5.2.



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   Scope: This opcode can be used with any scope.

6.3.  No-Operation Opcode

   Opcode: 2

   Purpose: This opcode is reserved to indicate that this opcode does
   not perform any Network Action and MUST be skipped.

   Scope: Format B.

6.4.  Extension Opcode

   Opcode: 127

   Purpose: This opcode is reserved to extend the current opcode range
   beyond 127 in future.  If this opcode is not supported, then the
   packet with the opcode 127 MUST be dropped.  Use of this opcode is
   outside the scope of this document.

7.  NAS placement in the Label Stack

   The node adding an NAS to the label stack places a copy of the NAS
   where the relevant nodes can read it.  Each downstream node along the
   path has a Readable Label Depth (RLD) the NAS MUST NOT appear at the
   top of the stack at any MNA incapable node on the path.  If the NAS
   is to be processed by a downstream MNA-capable node, then the entire
   NAS MUST be placed so that it is within RLD by the time the packet
   reaches the downstream MNA-capable node and the NAS MUST NOT appear
   at the top of the stack at any MNA incapable node on the path.  When
   adding the Entropy Label Identifier (bSPL label 7) and Entropy Label
   as defined in [RFC6790], along with an NAS, the RLD MUST be
   considered for the placement of both.

   If the label stack is deep, several copies of the NAS may need to be
   encoded in the label stack.

   For an NAS with HBH scope, every node will process the top copy of
   the NAS.

   For an NAS with Select scope, it is processed by the node that brings
   it to the top of stack (for example, in the case of using MPLS label
   pop operation in Segment Routing) and then the NAS is removed from
   the stack.  The select-scoped NAS needs to be inserted after the
   forwarding label and before the next forwarding label.  It could be
   inserted before or after a HBH NAS.  Note that the case of an NAS
   with Select scope with MPLS label swap operation (for example, with
   RSVP-TE LSPs) is for future study.



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   For I2E scope, only one copy of the NAS needs to be added at the
   bottom of the stack.

   Transit, non-penultimate nodes that pop a forwarding label and expose
   a copy of an NAS MUST remove it.

   A node performing Penultimate Hop Popping (PHP) that pops the
   forwarding label with only the NAS(es) remaining on the stack MUST
   NOT remove the NAS(es).  Instead, it forwards the packet with the
   NAS(es) at the top of stack to the next node.  Note that the behavior
   of the PHP node, as defined in [RFC3270] for TC processing, and as
   defined in [RFC3443] for TTL processing, is not modified regardless
   of whether the PHP node supports MNA.

   The node that receives the NAS at the top of the label stack MUST
   remove it.

7.1.  Actions when Pushing Labels

   An MNA-capable node may need to push additional labels as well as
   push new network actions onto a received packet.

   While pushing additional labels on to the label stack of the receive
   packet, the MNA-capable node MUST verify that the entire top-most NAS
   with HBH scope is still within the RLD of the downstream MNA-capable
   nodes.  If required, the MNA-capable node MAY create a copy of the
   top-most NAS with HBH scope and insert it within the RLD of the
   downstream MNA-capable nodes on the label stack.

   When an MNA-capable node needs to push a new NAS with HBH scope on to
   a received packet that already has an NAS with HBH scope, it SHOULD
   copy (and merge) the network actions (including their Ancillary Data)
   from the received top-most NAS with HBH scope in the new NAS with HBH
   scope.  The new NAS MUST be placed within the RLD of the downstream
   MNA-capable nodes.  This behavior can be based on local policy.

   The new network actions added MUST NOT conflict with the network
   actions in the received NAS with HBH scope.  The mechanism to resolve
   such conflicts depend on the network actions and can be based on
   local policy.  The MNA-capable node that pushes entries MUST
   understand any network actions which it is pushing which may result
   in a conflict, and MUST resolve any conflicts between new and
   received network actions.  In the usual case of a conflict of
   duplicating a network action, the definition of the network action
   will generally give guidance on likely resolutions.






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8.  Node Capability Signaling

   Encapsulating Node is the node that pushes an NAS on to the Label
   stack.

   The encapsulating node MUST make sure that the NAS can be processed
   by the transit and egress nodes.

   *  The path computation system needs to know the MSD and RLD that can
      be imposed at the ingress node of a given SR path [RFC8664].  This
      ensures that the label stack depth of a computed path does not
      exceed the maximum number of labels (i.e., MSD) the node is
      capable of imposing and the maximum number of labels that can be
      read by the MNA-processing nodes in the path.  The MSD needs to
      include the MNA Sub-Stacks to be added.

   *  The node responsible for selecting a path through the MPLS network
      needs to know and consider the MNA-capabilities and RLD of the
      transit nodes, and the MNA-capabilities of the end point.  it
      supports.

   *  Information about the capabilities of the nodes may be configured,
      collected through management protocols, or distributed by control
      protocols (such as advertising by routing protocols).

   *  The mechanisms by which the capabilities of the nodes are known by
      the node responsible for selecting a path through the MPLS network
      are out of scope for this document.

9.  Processing the Network Action Sub-Stack

   This section defines the specific responsibilities for nodes along an
   LSP.

9.1.  Encapsulating Node Responsibilities

   The encapsulating node MAY add NASes to the label stack in accordance
   with its policies, the placement restrictions in Section 7, and the
   limitations learned from Section 8.

   The encapsulating node MUST NOT add an NAS to the label stack if the
   egress node does not support MNA.

   If there is an existing label stack, the encapsulating node MUST NOT
   modify the first 20 bits of any LSE in the label stack when the ECMP
   technique in the network is using the hashing of the labels on the
   label stack.




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   If the encapsulating node is also a transit node, then it MUST also
   follow the rules set out in Section 9.2.

9.2.  Transit Node Responsibilities

   Transit Node is the node that process an NAS on to the Label stack
   but does not push any new NAS.

   The transit node MUST follow the procedure for data specified in
   Section 5.2.

   Transit nodes MUST process the NASes in the label stack, according to
   the rules set out in Section 5.5.

   A transit node that processes an NAS and does not recognise the value
   of an opcode MUST follow the rules according to the setting of the
   Unknown Action Handling value in the NAS as described in
   (Section 5.4).

9.3.  Penultimate Node Responsibilities

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

9.4.  Egress Node Responsibilities

   The egress node MUST remove any NAS it receives.

10.  Network Action Indicator Opcode Definition

   The following information MUST be defined for new Network Action
   Indicator opcode request in the document that specifies the Network
   Action.

   A request for a new NAI MUST include the following information:

   *  Format: The definition of the new Network Action MUST specify the
      LSE Formats.  The opcode can define Network Action in Format B or
      C or both B and C.  Both Format B and C LSEs MAY optionally carry
      Format D LSE(s).

   *  Scope: The request MUST specify at-least one scope (I2E, HBH,
      Select) for the Network Action.  The request MAY specify more than
      one scope.




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   *  Ancillary Data: A request MUST specify the quantity, syntax, and
      semantics of any associated Ancillary Data.  The Ancillary Data
      MAY be variable length, but the length MUST be computable based on
      the data present in the NAS.

   *  Processing: The request MUST specify the detailed procedure for
      processing the network action.

   *  Interactions: The definition of the new Network Action MUST
      specify its interaction with other currently defined Network
      Action if there is any.

   An assignment for an NAI MAY make requests from any combination of
   the "Network Action Opcodes" or "Network Action Flags Without
   Ancillary Data" assignments.  This decision should optimize for
   eventual encoding efficiency.  If the NAI does not require any
   ancillary data, then a flag is preferred as only one bit is used in
   the encoding.

11.  Backward Compatibility

   This section discusses interactions between MNA-capable and legacy,
   non-MNA-capable nodes.

   An MNA-encapsulating node MUST ensure that the MPLS Network Action
   Sub-Stack indicator is not at the top of the MPLS Label Stack when
   the packet arrives at a non-MNA-capable node.  If such a packet did
   arrive at a non-MNA-capable node, it will most likely be dropped.

   Legacy nodes may scan the label stack, potentially looking for a
   label value containing a bSPL.  To ensure that the LSE formats
   described herein do not appear to contain a bSPL value, the opcode
   value of 0 has been reserved.  By ensuring that there is a non-zero
   value in the high order 7 bits, we are assured that the high order 20
   bits cannot be misinterpreted as containing a bSPL value (0-15).

   The TC and TTL values of the Format A LSE are not re-purposed for
   encoding, as the penultimate node on the MPLS packet path may
   propagate TTL from the transport (or forwarding) label to the next
   label on the label stack, overwriting the TTL on the next label.  If
   the penultimate node is a legacy node, it might perform this action,
   potentially corrupting other values stored in the TC and TTL values.
   To protect against this, we retain the TC and TTL values in the
   Format A LSE.







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12.  Implementation Status

   [Note to the RFC Editor - remove this section before publication, as
   well as remove the reference to [RFC7942]]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

12.1.  University of Tuebingen Implementation

   The solution defined in the document draft-ietf-mpls-mna-hdr-08 has
   been implemented using P4 pipeline.  The implementation code can be
   found at https://github.com/uni-tue-kn/P4-MNA.

13.  Security Considerations

   The security considerations in [RFC3032] and [RFC9789] also apply to
   this document.

   In addition, MNA-creates a new dimension in security concerns:

   *  The actions of an encapsulating node can affect any or all of the
      nodes along the path.  In the most common and benign situations,
      such as a syntactically incorrect packet could result in packet
      loss or corruption.

   *  The semantics of a network action are unbounded and may be
      insecure.  A network action could be defined that made arbitrary
      changes to the memory of the forwarding router, which could then
      be used by the encapsulating node to compromise every MNA-capable
      router in the network.  The IETF needs to ensure that only secure
      network actions are defined.

   *  The MNA architecture supports locally-defined network actions.
      For such actions, there will be limited oversight to ensure that
      the semantics do not create security issues.  Implementors and
      network operators will need to ensure that locally-defined network
      actions do not compromise the security of the network.



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   *  The MPLS domain border nodes MUST ensure that the MPLS packets
      with MNA from any domain with a different administrative control
      can be filtered to prevent entering the provider MPLS domain.  The
      filtering capability MAY be enabled on a per network action basis
      and it can be based on a local policy.  The filtering capability
      MUST be implemented on those nodes before deploying MNA in the
      provider MPLS domain.  The RLD on the filtering node MUST be
      higher than the RLD on all other nodes in the provider MPLS
      domain.

   *  The MNA architecture supports modifying the AD on the intermediate
      nodes, so the critical network functions should either not rely on
      the data or should be aware of the risks and use other means to
      verify the security of the whole network.

   *  The "private Use" opcodes in "Network Action Opcodes" Section 14.4
      and "Network Action Flags Without Ancillary Data" Section 14.3
      Registry are subject to the considerations described in [RFC8126].

14.  IANA Considerations

14.1.  MNA bSPL Label

   This document requests that IANA allocate a value (TBA) for the MNA
   bSPL label from the "Base Special-Purpose MPLS Label Values" registry
   to indicate the presence of an MNA Sub-Stack in the label stack.  The
   description of the value should be "MPLS Network Actions".  The
   reference should be this document.

14.2.  MPLS Network Actions Parameters

   This document requests that IANA create a new category called "MPLS
   Network Actions Parameters" within the "Multiprotocol Label Switching
   Architecture (MPLS)" category.  The registries described below should
   belong to this new category.

14.3.  Network Action Flags Without Ancillary Data

   This document requests that IANA create a new registry with the name
   "Network Action Flags Without Ancillary Data".  Registration requests
   should comply with Section 10.  The registration procedure for this
   registry is "IETF Review", "Experimental Use" and "Private Use" as
   defined in [RFC8126].  The fields in this registry are "Bit Position"
   (integer), "Description" (string), and "Reference" (string).

   Bit Position refers to the position relative to the most significant
   bit in LSE Format B or C Data fields and any subsequent Format D
   LSEs.  Bit Position 0 is the most significant bit in an LSE Format B



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   or C Data field.  Bit Position 20 is the most significant bit in the
   first LSE Format D Data field.  There are 20 bits available in LSE
   Format C and 30 bits available in LSE Format D.  There are at most 14
   Format D LSEs per opcode (due to NASL limit of 15 and Format D
   requires Format C LSE), so there are at most 20 + 14 * 30 = 440 bit
   positions.  The Bit Position is an integer with value 0-469.

   The initial assignments for this registry are:

            +==============+==================+===============+
            | Bit Position | Description      | Reference     |
            +==============+==================+===============+
            | 0-14         | IETF Review      | This document |
            +--------------+------------------+---------------+
            | 15-16        | Experimental Use | This document |
            +--------------+------------------+---------------+
            | 17-19        | Private Use      | This document |
            +--------------+------------------+---------------+
            | 20-469       | IETF Review      | This document |
            +--------------+------------------+---------------+

              Table 4: Network Action Flags Without Ancillary
                               Data Registry

14.4.  Network Action Opcodes

   This document requests that IANA create a new registry with the name
   "Network Action Opcodes".  Registration requests should comply with
   Section 10.  The registration procedure for this registry is "IETF
   Review", "Experimental Use" and "Private Use" as defined in
   [RFC8126].  The fields are "Opcode" (integer), "Description"
   (string), and "Reference" (string).  Opcode is an integer with value
   1-126.

              +=========+==================+===============+
              | Opcode  |   Description    | Reference     |
              +=========+==================+===============+
              | 1-110   | IETF Review      | This document |
              +---------+------------------+---------------+
              | 111-114 | Experimental Use | This document |
              +---------+------------------+---------------+
              | 115-126 | Private Use      | This document |
              +---------+------------------+---------------+

                     Table 5: Network Action Opcodes

   IANA has allocated values for the following Network Action Opcodes
   from "Network Action Opcodes".



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          +========+===========================+===============+
          | Opcode |        Description        | Reference     |
          +========+===========================+===============+
          | 0      | Reserved                  | This document |
          +--------+---------------------------+---------------+
          | 1      | Flag-Based Network Action | This document |
          |        | Indicators without AD     |               |
          +--------+---------------------------+---------------+
          | 2      | No operation Opcode       | This document |
          +--------+---------------------------+---------------+
          | 127    | Opcode Range Extension    | This document |
          |        | Beyond 127                |               |
          +--------+---------------------------+---------------+

                     Table 6: Network Action Opcodes

15.  Examples

15.1.  Network Action Encoding Examples

15.1.1.  Network Action Flags without AD

    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=bSPL               | TC  |S|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opcode=1   |        13-bit Flags     |R|IHS|S|NASL=0 |U|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 5: NAS with Network Action Flags

   This is an example of an NAS with Flag-Based NAIs without Ancillary
   Data.

   Details:

      Opcode=1: This opcode to indicates that the LSE carries Flag-Based
      NAIs without AD.

      Data: The data field carries the Flag-Based NAIs.

      S: This is the bottom of stack bit.  Set if and only if this LSE
      is the bottom of the stack.

      U: Action to be taken if one of the NAIs are not recognized by the
      processing node.




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      NASL: The NASL value is set to 0, as there are no additional LSEs.

      NAL: The NAL value is set to 0, as there are no additional AD
      encoded using Format D.

    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=bSPL                   | TC  |S|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opcode=2   |        Data=0           |R|IHS|S|NASL=2 |U|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opcode=1   |        Flag-Based NAIs        |S| NAIs  |U|NAL=1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1| Additional Flag-Based NAIs                |S|Flag-Based-NAIs|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 6: Network Action Flags without AD using LSE Format D

   In this example, the NAS contains a Format B LSE with No-Operation
   Opcode value 2.  The next LSE uses Format C, but the Network Action
   Flag is not in a bit position contained within the Format C LSE, so a
   single Format D LSE has been added to the NAS to carry the flag.

   NAL is set to 1 to indicate that Flag-Based NAIs are also encoded in
   the next LSE.

   NASL is set to 2 to indicate that 2 additional LSEs are used.

15.1.2.  Network Action Opcode with AD

    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=bSPL                   | TC  |S|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opcode=8   |      Ancillary Data     |R|IHS|S|NASL=0 |U|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 7: Network action opcode with Ancillary Data

   In this example, the NAS is carrying only one Network Action that
   requires 13 bits of Ancillary Data.

   Details on the Second LSE

      Opcode=8: A network action allocation is outside of this document.




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      Data: The data field contains 13 bits of ancillary data.

15.1.3.  Network Action Opcode with more AD with Format-B

   A network action may require more Ancillary Data than can fit in a
   single LSE.  In this example, a Format D LSE is added to carry
   additional Ancillary Data.

    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=bSPL               | TC  |S|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opcode=10  |      Ancillary Data     |R|IHS|S|NASL=1 |U|NAL=1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|            Ancillary Data                 |S|Ancillary Data |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 8: Network Action With Additional Ancillary Data

   In this example, opcode 10 is encoded in Format B and it requires
   more than one LSE's worth of Ancillary Data, so a Format D LSE is
   added.

   Details on the second LSE:

      Opcode=10: An opcode allocation is outside of this document

      Ancillary Data: Ancillary data required to process the Network
      Action opcode 10

      NAL: Length of additional LSEs used to encode its Ancillary data

   Details on the third LSE:

      Ancillary Data: 22 bits of additional Ancillary data.

      Ancillary Data: 8 bits of additional Ancillary Data.

15.1.4.  Network Action Opcode with more AD with Format C

   A network action may require more Ancillary Data than can fit in a
   single LSE.  In this example, a Format D LSE is added to carry
   additional Ancillary Data.







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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=bSPL               | TC  |S|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opcode=2   |      Data=0             |R|IHS|S|NASL=2 |U|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opcode=9   |      Ancillary Data           |S|   AD  |U|NAL=1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|            Ancillary Data                 |S|Ancillary Data |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 9: Network Action With Additional Ancillary Data

   In this example, opcode 9 requires more than one LSE's worth of
   Ancillary Data, so a Format D LSE is added.

   Details on the third LSE:

      Opcode=9: An opcode allocation is outside of this document

      Ancillary Data: Most significant bits of Ancillary data

      AD: 4 bits of additional Ancillary Data

   Details on the fourth LSE:

      Ancillary Data: 22 bits of additional Ancillary data.

      Ancillary Data: 8 bits of additional Ancillary Data.

15.2.  Network Action Processing Order

   The semantics of a network action can vary widely and the results of
   processing one network action may affect the processing of a
   subsequent network action.  See Section 5.5.

15.2.1.  Network Action Processing Order













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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=bSPL              | TC  |S|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=8    |      Ancillary Data     |R|IHS|S|NASL=2 |U|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=7    |      Ancillary Data7          |S|  AD7  |U|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=1    |      Flag-Based NAIs          |S|  NAI  |U|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 10: In-stack NA processing order

   In this example, opcode 8 is processed first, then opcode 7, and then
   the network action flags are processed from most significant to least
   significant.

   In a different case, some Flag-Based NAIs may need to be processed
   before opcode 7 and some Flag-Based NAIs may need to be processed
   after Opcode 7.  This can be done by causing some NAIs to appear
   earlier in the NAS.

    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=bSPL           | TC  |S|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=8    |      Ancillary Data     |R|IHS|S|NASL=3 |U|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=1    |        0x01                   |S|  NAI  |U|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=7    |      Ancillary Data7          |S|  AD7  |U|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=1    |        0x02                   |S|  NAI  |U|NAL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure 11: Interleaving network actions

   In the above example, opcode 8 is processed first, then Flag-Based
   NAI 0x01 is processed, then opcode 7 is processed, and finally NAI
   0x02 is processed.

16.  References

16.1.  Normative References




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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>.

   [RFC3443]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
              in Multi-Protocol Label Switching (MPLS) Networks",
              RFC 3443, DOI 10.17487/RFC3443, January 2003,
              <https://www.rfc-editor.org/info/rfc3443>.

   [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>.

   [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>.

16.2.  Informative References

   [RFC3270]  Le Faucheur, F., Ed., Wu, L., Davie, B., Davari, S.,
              Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen,
              "Multi-Protocol Label Switching (MPLS) Support of
              Differentiated Services", RFC 3270, DOI 10.17487/RFC3270,
              May 2002, <https://www.rfc-editor.org/info/rfc3270>.





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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>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.

   [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 of this document would like to thank the MPLS Working
   Group Open Design Team for the discussions and comments on this
   document.  The authors would also like to thank Amanda Baber for
   reviewing the IANA Considerations and providing many useful
   suggestions.  The authors would like to thank Loa Andersson, Stewart
   Bryant, Greg Mirsky, Joel M.  Halpern and Adrian Farrel for reviewing
   this document and providing many useful suggestions.  The authors
   would like to thank Fabian Ihle and Michael Menth, both from
   University of Tuebingen, for reviewing and implementing the solution
   defined in this document in P4 pipeline.

Contributors

   The following people have substantially contributed to this document:








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


   Bruno Decraene
   Orange
   Email: bruno.decraene@orange.com


   Weiqiang Cheng
   China Mobile
   Email: chengweiqiang@chinamobile.com


   Xiao Min
   ZTE Corp.
   Email: xiao.min2@zte.com.cn


   Luay Jalil
   Verizon
   Email: luay.jalil@verizon.com


   Jie Dong
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China
   Email: jie.dong@huawei.com


   Tianran Zhou
   Huawei Technologies
   China
   Email: zhoutianran@huawei.com


   Bin Wen
   Comcast
   Email: Bin_Wen@cable.comcast.com


   Sami Boutros
   Ciena
   Email: sboutros@ciena.com




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


   John Drake
   Juniper Networks
   United States
   Email: jdrake@juniper.net



                                 Figure 12

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


   Haoyu Song
   Futurewei Technologies
   Email: haoyu.song@futurewei.com


   Kireeti Kompella
   Juniper Networks
   United States
   Email: kireeti.ietf@gmail.com








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