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MPLS Network Action (MNA) Sub-Stack Solution
draft-jags-mpls-mna-hdr-05

Document Type Active Internet-Draft (mpls WG)
Authors Jaganbabu Rajamanickam , Rakesh Gandhi , Royi Zigler , Haoyu Song , Kireeti Kompella
Last updated 2023-01-10
Replaces draft-jags-mpls-ext-hdr
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draft-jags-mpls-mna-hdr-05
MPLS Working Group                                  J. Rajamanickam, Ed.
Internet-Draft                                            R. Gandhi, Ed.
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: 14 July 2023                                     R. Zigler, Ed.
                                                                Broadcom
                                                            H. Song, Ed.
                                                  Futurewei Technologies
                                                        K. Kompella, Ed.
                                                        Juniper Networks
                                                         10 January 2023

              MPLS Network Action (MNA) Sub-Stack Solution
                       draft-jags-mpls-mna-hdr-05

Abstract

   This document defines the MPLS Network Action (MNA) sub-stack
   solution for carrying Network Actions and Ancillary Data in the label
   stack.  MPLS Network Actions can be used to influence packet
   forwarding decisions, carry additional OAM information in the MPLS
   packet, or perform user-defined operations.  This document addresses
   the MNA requirements specified in draft-ietf-mpls-mna-requirements.
   This document follows the MNA framework specified in draft-ietf-mpls-
   mna-fwk.

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 14 July 2023.

Copyright Notice

   Copyright (c) 2023 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 . . . . . . . . . . . . . . . . . .   4
     2.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Label Stack Entry Formats . . . . . . . . . . . . . . . . . .   5
     4.1.  LSE Format A: The MNA Sub-Stack Indicator . . . . . . . .   5
     4.2.  LSE Format B: The initial opcode  . . . . . . . . . . . .   6
     4.3.  LSE Format C: Subsequent opcodes  . . . . . . . . . . . .   6
     4.4.  LSE Format D: Additional Data . . . . . . . . . . . . . .   7
   5.  The MNA Sub-Stack . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Opcodes . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Data  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.4.  Unknown Action Handling . . . . . . . . . . . . . . . . .   9
     5.5.  Ordering  . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Special Opcodes . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  bSPL Protection . . . . . . . . . . . . . . . . . . . . .  11
     6.2.  PSD Offset  . . . . . . . . . . . . . . . . . . . . . . .  11
     6.3.  Flag-Based NAIs without AD  . . . . . . . . . . . . . . .  12
     6.4.  Flag based NAIs with AD . . . . . . . . . . . . . . . . .  12
     6.5.  PSD-ISD Ordering  . . . . . . . . . . . . . . . . . . . .  13
     6.6.  Extension Opcode  . . . . . . . . . . . . . . . . . . . .  13
   7.  NAS placement in the Label Stack  . . . . . . . . . . . . . .  13
   8.  Node Capability Signaling . . . . . . . . . . . . . . . . . .  14
   9.  Processing the Network Action Sub-Stack . . . . . . . . . . .  14
     9.1.  Encapsulating Node Responsibilities . . . . . . . . . . .  14
     9.2.  Transit Node Responsibilities . . . . . . . . . . . . . .  15
     9.3.  Penultimate Node Responsibilities . . . . . . . . . . . .  15
     9.4.  Decapsulating Node Responsibilities . . . . . . . . . . .  15
   10. Network Action Indicator Allocation Procedures  . . . . . . .  15
   11. Backward Compatibility  . . . . . . . . . . . . . . . . . . .  16
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     13.1.  MNA bSPL Label . . . . . . . . . . . . . . . . . . . . .  17
     13.2.  MPLS Network Actions Parameters  . . . . . . . . . . . .  18

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     13.3.  Network Action Flags With Ancillary Data . . . . . . . .  18
     13.4.  Network Action Flags Without Ancillary Data  . . . . . .  18
     13.5.  Network Action Opcodes . . . . . . . . . . . . . . . . .  19
   14. Appendix A: Examples  . . . . . . . . . . . . . . . . . . . .  20
     14.1.  Network Action Encoding Examples . . . . . . . . . . . .  20
       14.1.1.  Network Action Flags without AD  . . . . . . . . . .  20
       14.1.2.  Network Action Opcode with AD  . . . . . . . . . . .  21
       14.1.3.  Network Action Opcode with more AD . . . . . . . . .  22
     14.2.  Post-Stack Network Action Encoding . . . . . . . . . . .  22
       14.2.1.  NAS that only indicates Post-Stack NAs . . . . . . .  23
       14.2.2.  NAS with both in-stack and post-stack NAs  . . . . .  23
       14.2.3.  NASes with multiple scopes . . . . . . . . . . . . .  24
     14.3.  Network Action Processing Order  . . . . . . . . . . . .  24
       14.3.1.  Network Action Processing Order  . . . . . . . . . .  24
       14.3.2.  Post-Stack NA Processing Order . . . . . . . . . . .  25
       14.3.3.  A mix of in-stack and post-stack NAs . . . . . . . .  26
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  26
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  28
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

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 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 [I-D.ietf-mpls-mna-usecases].  User-
   defined network actions allow new, local actions to be defined.

   This document defines the syntax and semantics of network actions
   encoded within an MPLS Label Stack.  Network actions can be encoded
   with or without Ancillary Data (AD), either in or after the 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) (value TBA).  This document addresses
   the requirements specified in [I-D.ietf-mpls-mna-requirements].  This
   document follows the framework specified in [I-D.ietf-mpls-mna-fwk].

2.  Conventions Used in This Document

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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.

2.2.  Abbreviations

   The terminology defined in [I-D.ietf-mpls-mna-fwk] and
   [I-D.ietf-mpls-mna-requirements] are used in this document.

   +============+===================+==================================+
   |Abbreviation|Meaning            | Reference                        |
   +============+===================+==================================+
   |AD          |Ancillary Data     | [I-D.ietf-mpls-mna-requirements] |
   +------------+-------------------+----------------------------------+
   |bSPL        |Base Special       | [RFC9017]                        |
   |            |Purpose Label      |                                  |
   +------------+-------------------+----------------------------------+
   |BOS         |Bottom Of Stack    | [RFC3032]                        |
   +------------+-------------------+----------------------------------+
   |HBH         |Hop-By-Hop Scope   | [I-D.ietf-mpls-mna-fwk]          |
   +------------+-------------------+----------------------------------+
   |I2E         |Ingress-To-Egress  | [I-D.ietf-mpls-mna-fwk]          |
   |            |Scope              |                                  |
   +------------+-------------------+----------------------------------+
   |IHS         |I2E, HBH, or       | This document                    |
   |            |Select Scope       |                                  |
   +------------+-------------------+----------------------------------+
   |ISD         |In-Stack Data      | [I-D.ietf-mpls-mna-requirements] |
   +------------+-------------------+----------------------------------+
   |LSE         |Label Stack Entry  | [RFC3032]                        |
   +------------+-------------------+----------------------------------+
   |MNA         |MPLS Network       | [I-D.ietf-mpls-mna-fwk]          |
   |            |Actions            |                                  |
   +------------+-------------------+----------------------------------+
   |NAI         |Network Action     | [I-D.ietf-mpls-mna-requirements] |
   |            |Indicator          |                                  |
   +------------+-------------------+----------------------------------+
   |NAL         |Network Action     | This document                    |
   |            |Length             |                                  |
   +------------+-------------------+----------------------------------+
   |NAS         |Network Action     | [I-D.ietf-mpls-mna-fwk]          |
   |            |Sub-Stack          |                                  |
   +------------+-------------------+----------------------------------+
   |NASI        |Network Action     | This document                    |
   |            |Sub-Stack          |                                  |

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   |            |Indicator          |                                  |
   +------------+-------------------+----------------------------------+
   |NASL        |Network Action     | This document                    |
   |            |Sub-Stack Length   |                                  |
   +------------+-------------------+----------------------------------+
   |OAM         |Operations And     | [RFC4377]                        |
   |            |Management         |                                  |
   +------------+-------------------+----------------------------------+
   |P           |Post-Stack Network | This document                    |
   |            |Action Presence    |                                  |
   |            |Indicator          |                                  |
   +------------+-------------------+----------------------------------+
   |PSD         |Post-stack data    | [I-D.ietf-mpls-mna-requirements] |
   |            |                   | and [I-D.ietf-mpls-mna-fwk]      |
   +------------+-------------------+----------------------------------+
   |TC          |Traffic Class      | [RFC5462]                        |
   +------------+-------------------+----------------------------------+
   |TTL         |Time To Live       | [RFC3032]                        |
   +------------+-------------------+----------------------------------+

                           Table 1: Abbreviations

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 encapsulated packet.  Multiple NASes may appear in a
   label stack and the packet may contain Post-Stack Data, including
   additional network actions, as specified in
   [I-D.song-mpls-extension-header].

   Network actions and their optional Ancillary Data (AD) may be encoded
   as part of the NAS as a series of LSEs.

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 a traditional LSE, as described in [RFC3032] and
   [RFC5462].

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    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Label                 | TC  |S|      TTL      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                  Figure 1

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.

    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Opcode    |        Data             |P|IHS|S| Res |U|  NASL |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                  Figure 2

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

   *  Data (13 bits) : Opcode specific data

   *  P (1 bit) : If set, it indicates the presence of post-stack
      network actions.

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

   *  S (1 bit) : The Bottom of Stack [RFC3032].

   *  Res (3 bits) : Reserved bits.  These must be transmitted as zero
      and ignored upon receipt.

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

   *  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.

4.3.  LSE Format C: Subsequent opcodes

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

    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Opcode    |             Data              |S|  Data |  NAL  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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                                  Figure 3

   *  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].

   *  NAL (4 bits): Network Action Length.  The number of LSEs of
      additional data, encoded in LSE Format D (Section 4.4) following
      this LSE.

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 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|                   Data                    |S|     Data      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                  Figure 4

   *  1 (1 bit) : The most significant bit MUST be set.  This prevents
      legacy implementations from misinterpreting this LSE as containing
      a special label.

   *  S (1 bit) : The Bottom of Stack [RFC3032].

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

5.  The MNA Sub-Stack

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

   The TC and TTL fields of the first LSE retain their traditional
   semantics, as the penultimate node on the path may copy the TTL and
   TC fields from the preceding LSE to the next LSE on the label stack,
   overwriting the TTL and TC fields of the next LSE, as specified in
   Section 3.5 of [RFC3443].  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.

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

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

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 allocated through IANA
   (Section 10 and Section 13.5).

5.2.  Data

   The data field carries opcode specific data.  This may be ancillary
   data for a network action.

   To preserve backward compatibility, if a network action encodes data
   that will change during packet forwarding, then that data MUST be in
   the least significant 4 bits in the data field of a Format C LSE
   (Section 4.3) or the least significant 8 bits of a Format D LSE
   (Section 4.4).  Some legacy implementations may use the label field
   in all LSEs when computing ECMP decisions and modifying the label
   field might disrupt that packet's flow.

5.3.  Scope

   The IHS field in the Format B LSE indicates the scope of the In-Stack
   and Post-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:

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

                         Table 2: IHS Scope Values

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

      Select - Only specific nodes along the path will perform the
      action.

      Ingress To Egress (I2E) - The NAS MUST be processed only by the
      egress node.

   A single NAS carries only one of the three scopes (HBH/Select/I2E).
   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 a NAS at the top of the label stack.

   An I2E scope NAS 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.

   Forwarding and egress nodes should process at most a single NAS per
   scope.  If a node is to process multiple NASes, it should process
   them in the order that they appear in the label stack.

5.4.  Unknown Action Handling

   The Unknown Action Handling (U) field in a Format B 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 Action Handling actions are defined below.

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

                          Table 3: Unknown Action
                                  Handling

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 MUST be processed as
   if they were processed from the most significant bit to the least
   significant bit.

5.6.  Examples

   A minimal NAS would have the following format, where the Label field
   would contain the MNA bSPL and the NASL value would be 0:

    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Label                 | TC  |S|      TTL      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Opcode    |        Data             |P|IHS|S| Res |U|  NASL |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                  Figure 5

   A more complex NAS might have multiple opcodes and additional
   Ancillary Data.  This example has two opcodes and two additional LSEs
   of AD.

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    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Label                 | TC  |S|      TTL      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Opcode    |        Data             |P|IHS|S| Res |U|  NASL |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Opcode    |             Data              |S|  Data |  NAL  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|                   Data                    |S|     Data      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|                   Data                    |S|     Data      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                  Figure 6

   In this example, the NASL field would have value 3 and the NAL field
   would have value 2.

6.  Special Opcodes

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.  PSD Offset

   Opcode: 1

   Purpose: This opcode carries the start offset of the Post-Stack
   Action Header (PAH) ([I-D.song-mpls-extension-header] ) within the
   PSD.

   LSE Format: B

   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 PSD is encoded
   immediately after the MPLS BOS.  A data value of 1 indicates that the
   PAH starts 4 octets after the BOS.

   Scope: This opcode can be used with any scope.

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6.3.  Flag-Based NAIs without AD

   Opcode: 2

   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 there are sufficient NAI, then Format D LSEs may be used to
   encode more flags for more network actions.  Flags are allocated from
   the "Network Action Flags Without Ancillary Data" registry
   (Section 13.3).  If flags need to be evaluated in a different order,
   multiple LSEs using this opcode may be used to specify the requested
   order.  If this opcode is used with LSE Format B, then only 13 flags
   may be carried.

   Scope: This opcode can be used with any scope.

   This opcode MAY be used with no flags set in the data field to
   signify that no operation is to be performed.  This can be used, for
   example, if the first action to be performed cannot be encoded in a
   Format B LSE.

6.4.  Flag based NAIs with AD

   Opcode: 3

   Purpose: This opcode supports flag-based network actions that have
   Ancillary Data.

   LSE Formats: C, D

   Data: A format C LSE carries the Network Action Indicators, which
   should be evaluated from the most significant bit to the least
   significant bit.  Flags are allocated from the "Network Action Flags
   With Ancillary Data" registry (Section 13.4).  If flags need to be
   evaluated in a different order, multiple LSEs using this opcode may
   be used to specify the requested order.  Format D LSEs are used to
   encode the associated Ancillary Data, which appears in the same order
   as the flags.

   Scope: This opcode can be used with any scope.

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   If a flag contained within this opcode is unknown and is skipped per
   Section 5.4, then the length of its associated ancillary data will
   also be unknown.  Any subsequent flags within the opcode will not
   have the correct associated ancillary data, so all subsequent flags
   SHOULD be treated as unknown actions and also skipped.

6.5.  PSD-ISD Ordering

   Opcode: 4

   Purpose: In cases where the ordering of network action is significant
   and where some of the network actions reside in PSD, this opcode can
   be used to insert PSD network actions into the order of execution.
   The 'P' bit and 'O' bit MUST be set in the NAS's Format B LSE if this
   opcode is used.

   LSE Format: B, C, D

   Data: The data field contains one or more 8-bit Next Header (NH)
   indicators [I-D.song-mpls-extension-header].  When used with LSE
   Format B, only one NH indicator is carried.  Two indicators MAY be
   carried in a Format C LSE, and if Format D LSEs are used, each may
   carry up to three indicators.  The indicators are the stored
   concatenated in the most significant bits of the data field.  If
   multiple indicators are carried, the most significant NH indicator is
   evaluated to the least significant.  Indicators do not span LSEs.  If
   some indicator positions are not to be used, then the indicator
   should be set to No Next Header (NONE).

   Scope: This opcode can be used with any scope.

6.6.  Extension Opcode

   Opcode: 127

   Purpose: This opcode is reserved to extend the current opcode range
   beyond 127.  Future use of this opcode is out of scope.

7.  NAS placement in the Label Stack

   Regardless of whether packets are being forwarded based on Segment
   Routing [RFC8662], LDP [RFC5036], or RSVP-TE [RFC3209], the node
   adding an NAS to the label stack will need to place a copy of the NAS
   where it can be read by the relevant nodes.  Each node along the path
   will have a Maximum MPLS Stack Inspection depth, and if the NAS is to
   be processed by a particular node, then the entire NAS must be placed
   so that it is within this depth by the time the packet reaches the
   node.

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

   For a NAS with HBH scope, every node will processes the top copy of
   the NAS.  Transit, non-penultimate nodes that pop a forwarding label
   and expose a copy of the NAS MUST remove it.  The penultimate node
   that pops the forwarding label that exposes the last copy of the NAS
   MUST NOT remove it.  Instead, it forwards the packet with the NAS at
   the top of stack to the next node (e.g., the segment endpoint node).
   The node that receives the NAS at the top of the label stack has to
   remove it.

   For a NAS with Select scope, it is processed by the node that brings
   it to the top of stack and then the NAS is removed from the stack.

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

8.  Node Capability Signaling

   The head-end node which is adding a NAS MUST make sure that the
   egress node removes the NAS.  The head-end node MUST make sure that
   the NAS can be processed by the appropriate transit and egress nodes.

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

   *  Each participating node MUST signal its Maximum MPLS Stack
      Inspection Depth.  This will allow the head-end node to place a
      copy of an NAS at the correct stack depth.

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

9.  Processing the Network Action Sub-Stack

   This section defines the specific responsibilities for nodes along a
   MPLS path.

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 a NAS to the label stack if the
   decapsulation node does not support MNA.

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   If there is an existing label stack, the encapsulating node SHOULD
   NOT change the first 20 bits of each LSE in the label stack to avoid
   ECMP path change.

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

9.2.  Transit Node Responsibilities

   Transit nodes SHOULD NOT change the first 20 bits in the LSEs in the
   label stack.

   A transit node MAY change the Ancillary Data found in the least
   significant 8 bits of an LSE.

   Transit nodes MUST process the NASes in the label stack, respecting
   Section 5.5.

   A transit node MUST respect the Unknown Action Handling value encoded
   in the NAS.

   A node that removes the only NAS that has the P bit set MUST remove
   all PSD.

9.3.  Penultimate Node Responsibilities

   In addition to the transit node responsibilities above, the
   penultimate node MUST NOT remove the last copy of a 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.  Decapsulating Node Responsibilities

   The decapsulating node MUST remove any NAS it receives.

10.  Network Action Indicator Allocation Procedures

   This section discusses the procedures and requirements for a
   allocating a new opcode or flag as a network action indicator (NAI)
   for a network action.  A request for an NAI MAY make requests from
   any combination of the "Network Action Opcodes", "Network Action
   Flags With Ancillary Data", or "Network Action Flags Without
   Ancillary Data" registries.

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

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

   *  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.

   A request for a new NAI MAY request any combination of flags or an
   opcode.  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.  If
   ancillary data is required, then the optimal choice may depend on how
   the action is likely to be combined with other actions.  If the
   action is unlikely to be used in combination with other actions and
   at most 20 bits of ancillary data is required, then an opcode may be
   preferred as the encoding will only consume a single LSE.  If the
   action is likely to be combined with other actions, then a flag is
   more likely to be optimal.

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 field 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).

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   The TC and TTL fields 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 fields.
   To protect against this, we retain the TC and TTL fields in the
   Format A LSE.

12.  Security Considerations

   The security considerations in [RFC3032] 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, this 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.

13.  IANA Considerations

13.1.  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.

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13.2.  MPLS Network Actions Parameters

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

13.3.  Network Action Flags With Ancillary Data

   This document requests that IANA create a new registry with the name
   "Network Action Flags With Ancillary Data".  Registration requests
   should comply with Section 10.  The registration procedure for this
   registry is "IETF Review".  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 C Data fields.  Bit Position 0 is the most
   significant bit a LSE Format C Data field.  There are 20 bit
   positions currently available, 0-19.  This registry may be extended
   in the future.  Further opcodes would need to be defined to carry
   additional flag ranges.

   The initial assignments for this registry are:

              +==============+=============+===============+
              | Bit Position | Description | Reference     |
              +==============+=============+===============+
              | 0-15         | Unassigned  |               |
              +--------------+-------------+---------------+
              | 16-19        | Private Use | This document |
              +--------------+-------------+---------------+

                    Table 4: Network Action Flags With
                         Ancillary Data Registry

13.4.  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".  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 a LSE Format B or C

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   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 available in LSE Format D.  There are at most 15 Format D LSEs
   per opcode, so there are at most 20 + 15 * 30 = 470 bit positions.
   The Bit Position is an integer with value 0-469.

   The initial assignments for this registry are:

              +==============+=============+===============+
              | Bit Position | Description | Reference     |
              +==============+=============+===============+
              | 0-15         | Unassigned  |               |
              +--------------+-------------+---------------+
              | 16-19        | Private Use | This document |
              +--------------+-------------+---------------+
              | 20-469       | Unassigned  |               |
              +--------------+-------------+---------------+

                  Table 5: Network Action Flags Without
                         Ancillary Data Registry

13.5.  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".  The fields are "Opcode" (integer), "Description" (string),
   and "Reference" (string).  Opcode is an integer 0-127.

   The initial assignments for this registry are:

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         +=========+=============================+===============+
         | Opcode  |         Description         | Reference     |
         +=========+=============================+===============+
         | 0       | Reserved                    | This document |
         +---------+-----------------------------+---------------+
         | 1       | Offset of start of Post-    | This document |
         |         | Stack Network Action Header |               |
         +---------+-----------------------------+---------------+
         | 2       | Flag-Based Network Action   | This document |
         |         | Indicators without AD       |               |
         +---------+-----------------------------+---------------+
         | 3       | Flag-Based Network Action   | This document |
         |         | Indicators with AD          |               |
         +---------+-----------------------------+---------------+
         | 4       | PSD-ISD Ordering            | This document |
         +---------+-----------------------------+---------------+
         | 5-110   | Unassigned                  |               |
         +---------+-----------------------------+---------------+
         | 111-126 | Private Use                 |               |
         +---------+-----------------------------+---------------+
         | 127     | Opcode Range Extension      | This document |
         |         | Beyond 127                  |               |
         +---------+-----------------------------+---------------+

                  Table 6: Network Action Opcodes Registry

14.  Appendix A: Examples

14.1.  Network Action Encoding Examples

14.1.1.  Network Action Flags without AD

    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Label=MNA bSPL               | TC  |0|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opcode=2   |         Flags           |P|IHS|S| Res |U| NASL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 7: NAS with Network Action Flags

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

   Details:

      Opcode=2: This opcode to indicates that the LSE carries Flag-Based
      NAIs without AD.

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      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.

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

    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Label=MNA bSPL                   | TC  |0|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Opcode=2  |        Data=0           |P|IHS|S| Res |U| NASL=2|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Opcode=2  |        Flag-Based NAIs        |0| NAIs  | NAL=1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1| Additional Flag-Based NAIs                |S|Flag-Based-NAIs|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 8: Network Action Flags without AD using LSE Format D

   In this example, the NAS contains a Format B LSE with no flags set,
   indicating no operation.  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.

14.1.2.  Network Action Opcode with AD

    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      MNA-Label=bSPL (TBA)             | TC  |0|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Opcode=8  |      Ancillary Data     |P|IHS|S| Res |U| NASL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 9: Network action opcode with Ancillary Data

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

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   Details on the Second LSE

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

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

14.1.3.  Network Action Opcode with more AD

   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 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Label=MNA bSPL               | TC  |0|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=2  |            Data=0         |P|IHS|0| Res |U| NASL=2|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opcode=9 |        Ancillary Data           |0|  AD   | NAL=1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|            Ancillary Data                 |S|Ancillary Data |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 10: 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 allocated 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.

14.2.  Post-Stack Network Action Encoding

   The details of Post-Stack Network Action Extension Header encodings
   are specified in [I-D.song-mpls-extension-header].

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14.2.1.  NAS that only indicates Post-Stack NAs

    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Label=MNA bSPL              | TC  |S|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opcode=2   |            0            |1|IHS|S| Res |U| NASL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                             |1|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                   Post-Stack Network Actions                  ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   ~                           Payload                             ~
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 11: NAS encoding only Post-Stack NAs

   In some cases the NAS may encode only the presence of Post-Stack NAs.
   In this case, the P-Bit is set.  The IHS field indicates the scope of
   the Post-Stack NAs (I2E, HBH, Select).

14.2.2.  NAS with both in-stack and post-stack NAs

    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Label=MNA bSPL               | TC  |0|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=2    |            0            |1|IHS|0| Res |U| NASL=1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=2    |        Flag-Based NAIs        |S| NAIs  | NAL=0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                             |1|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~              Post-Stack Network Actions Encoding              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   ~                           Payload                             ~
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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              Figure 12: 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 NAs.  In this case, P-Bit is set.  The NASL is
   set to "1", indicating the presence of one additional LSE.  The IHS
   field indicates the scope of both the in-stack and post-stack NAs.

14.2.3.  NASes with multiple scopes

    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Label=MNA bSPL               | TC  |0|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=2    |            0            |0| 1 |0| Res |U| NASL=1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=2    |        Flag-Based NAIs        |0| NAIs  | NAL=0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Label=MNA bSPL               | TC  |0|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Opcode=2   |            0            |1| 0 |1| Res |U| NASL=0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~              Post-Stack Network Actions Encoding              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   ~                           Payload                             ~
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 13: NASes with multiple 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.

14.3.  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.

14.3.1.  Network Action Processing Order

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    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Label=MNA bSPL              | TC  |S|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=8    |      Ancillary Data     |P|IHS|S|Res|U|1| NASL=2|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=7    |      Ancillary Data7          |S|  AD7  | NAL=0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=2    |      Flag-Based NAIs          |S|  NAI  | NAL=0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 14: 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 done by causing some NAIs to appear earlier
   in the NAS.

    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Label=MNA bSPL           | TC  |S|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=8    |      Ancillary Data     |P|IHS|S|Res|U|1| NASL=3|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=2    |        0x01                   |S|  NAI  | NAL=0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=7    |      Ancillary Data7          |S|  AD7  | NAL=0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=2    |        0x02                   |S|  NAI  | NAL=0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 15: Interleaving network actions

   In the above example, opcode 8 is processed first, then Flag-Based
   NAI 0x1 is processed before opcode 7, and finally NAI 0x2 is
   processed.

14.3.2.  Post-Stack NA Processing Order

   By default, post-stack NAs follow the ordering specified in
   [I-D.song-mpls-extension-header].  However, the PSD-ISD ordering
   opcode can be used to override the default ordering and interleave
   PSD actions with in-stack actions.

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14.3.3.  A mix of in-stack and post-stack NAs

   In some cases, post-stack NAs needs to be processed before in-stack
   NAs.  This section shows how to prioritize the post-stack NAs over
   in-stack NAs.

    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Label=MNA bSPL               | TC  |0|    TTL        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=8    |      Ancillary Data     |1|IHS|0| Res |U| NASL=3|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=2    |        Flag-Based NAIs        |0| NAIs  | NAL=0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=4    |       Post-Stack NH 6         |0|PS-NAI | NAL=0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opcode=7    |      Ancillary Data           |S|  AD   | NAL=0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 16: Post-stack and in-stack NA processing order

   In the above example, opcode 8 is processed first, then the Flag-
   Based NAIs, followed by Post-Stack NH 6, and finally opcode 7.

15.  References

15.1.  Normative References

   [I-D.ietf-mpls-mna-fwk]
              Andersson, L., Bryant, S., Bocci, M., and T. Li, "MPLS
              Network Actions Framework", Work in Progress, Internet-
              Draft, draft-ietf-mpls-mna-fwk-02, 21 October 2022,
              <https://www.ietf.org/archive/id/draft-ietf-mpls-mna-fwk-
              02.txt>.

   [I-D.ietf-mpls-mna-requirements]
              Bocci, M., Bryant, S., and J. Drake, "Requirements for
              MPLS Network Actions", Work in Progress, Internet-Draft,
              draft-ietf-mpls-mna-requirements-04, 13 October 2022,
              <https://www.ietf.org/archive/id/draft-ietf-mpls-mna-
              requirements-04.txt>.

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   [I-D.ietf-mpls-mna-usecases]
              Saad, T., Makhijani, K., Song, H., and G. Mirsky, "Use
              Cases for MPLS Network Action Indicators and MPLS
              Ancillary Data", Work in Progress, Internet-Draft, draft-
              ietf-mpls-mna-usecases-01, 24 October 2022,
              <https://www.ietf.org/archive/id/draft-ietf-mpls-mna-
              usecases-01.txt>.

   [I-D.song-mpls-extension-header]
              Song, H., Zhou, T., Andersson, L., Zhang, Z. J., and R.
              Gandhi, "MPLS Network Actions using Post-Stack Extension
              Headers", Work in Progress, Internet-Draft, draft-song-
              mpls-extension-header-11, 15 October 2022,
              <https://www.ietf.org/archive/id/draft-song-mpls-
              extension-header-11.txt>.

   [RFC2119]  Bradner, S. and RFC Publisher, "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>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [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>.

   [RFC4377]  Nadeau, T., Morrow, M., Swallow, G., Allan, D., and S.
              Matsushima, "Operations and Management (OAM) Requirements
              for Multi-Protocol Label Switched (MPLS) Networks",
              RFC 4377, DOI 10.17487/RFC4377, February 2006,
              <https://www.rfc-editor.org/info/rfc4377>.

   [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>.

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   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., Thomas, B., Ed., and
              RFC Publisher, "LDP Specification", RFC 5036,
              DOI 10.17487/RFC5036, October 2007,
              <https://www.rfc-editor.org/info/rfc5036>.

   [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>.

   [RFC8174]  Leiba, B. and RFC Publisher, "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>.

   [RFC8662]  Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
              Shakir, R., and J. Tantsura, "Entropy Label for Source
              Packet Routing in Networking (SPRING) Tunnels", RFC 8662,
              DOI 10.17487/RFC8662, December 2019,
              <https://www.rfc-editor.org/info/rfc8662>.

   [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>.

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 and Greg Mirsky for reviewing our draft and providing many
   useful suggestions.

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 17

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 (editor)
   Broadcom
   Email: royi.zigler@broadcom.com

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

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

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