MNA framework: MPLS Network Actions (MNAs) Framework

Internet Engineering Task Force (IETF)                      L. Andersson
Request for Comments: 9789                           Huawei Technologies
Category: Informational                                        S. Bryant
ISSN: 2070-1721                                University of Surrey 5GIC
                                                                M. Bocci
                                                                   Nokia
                                                                   T. Li
                                                        Juniper Networks
                                                               July 2025


                 MPLS Network Actions (MNAs) Framework

Abstract

   This document describes an architectural framework for MPLS Network
   Action (MNA) technologies.  MNA technologies are used to indicate
   actions that impact the forwarding or other processing (such as
   monitoring) of the packet along the Label Switched Path (LSP) of the
   packet and to transfer any additional data needed for these actions.

   This document provides the foundation for the development of a common
   set of network actions and information elements supporting additional
   operational models and capabilities of MPLS networks.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9789.

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Table of Contents

   1.  Introduction
     1.1.  Requirements Language
     1.2.  Normative Definitions
     1.3.  Abbreviations
   2.  Structure
     2.1.  Scopes
     2.2.  Partial Processing
     2.3.  Signaling
       2.3.1.  Readable Label Depth
     2.4.  State
   3.  Encoding
     3.1.  The MNA Label
       3.1.1.  Existing Base SPL
       3.1.2.  New Base SPL
       3.1.3.  New Extended SPL
       3.1.4.  User-Defined Label
     3.2.  TC and TTL
       3.2.1.  TC and TTL Retained
       3.2.2.  TC and TTL Repurposed
     3.3.  Length of the NAS
       3.3.1.  Last/Continuation Bits
       3.3.2.  Length Field
     3.4.  Encoding of Scopes
     3.5.  Encoding a Network Action
       3.5.1.  Bit Catalogs
       3.5.2.  Operation Codes
     3.6.  Encoding of Post-Stack Data
       3.6.1.  First Nibble Considerations
   4.  Semantics
   5.  Definition of a Network Action
   6.  Management Considerations
   7.  Security Considerations
   8.  IANA Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   This document describes an architectural framework for MPLS Network
   Action (MNA) technologies.  MNA technologies are used to indicate
   actions for Label Switched Paths (LSPs) and/or MPLS packets and to
   transfer data needed for these actions.

   This document provides the foundation for the development of a common
   set of network actions and information elements supporting additional
   operational models and capabilities of MPLS networks.  MNA solutions
   derived from this framework are intended to address the requirements
   found in [RFC9613].  In addition, MNA may support actions that
   overlap existing MPLS functionality.  This may be beneficial for
   numerous reasons, such as making it more efficient to combine
   existing functionality and new functions in the same MPLS packet.

   MPLS forwarding actions are instructions to MPLS routers to apply
   additional actions when forwarding a packet.  These might include
   load-balancing a packet given its entropy, indicating whether or not
   to perform Fast Reroute on a failure, and indicating whether or not a
   packet has metadata relevant to the forwarding actions along the
   path.

   This document generalizes the concept of MPLS "forwarding actions" to
   "network actions" that include any action that an MPLS router is
   requested to take on the packet.  Network actions include any MPLS
   forwarding actions but may also include other operations (such as
   security functions, Operations, Administration, and Maintenance (OAM)
   procedures, etc.) that are not directly related to forwarding of the
   packet.  MPLS network actions are always triggered by an MNA packet
   but may have implications for subsequent traffic, including non-MNA
   packets, as discussed in Section 2.4.

   MNA technologies may redefine the semantics of the Label, Traffic
   Class (TC), and Time to Live (TTL) fields in an MPLS Label Stack
   Entry (LSE) within a Network Action Sub-Stack (NAS).

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Although this is an Informational document, these conventions are
   applied to achieve clarity in the requirements that are presented.

1.2.  Normative Definitions

   This document adopts the definitions of the following terms and
   abbreviations from [RFC9613] as normative: "Network Action", "Network
   Action Indicator (NAI)", "Ancillary Data (AD)", and "Scope".

   In addition, this document defines the following terms:

   Network Action Sub-Stack (NAS):  A set of related, contiguous LSEs in
      the MPLS label stack for carrying information related to network
      actions.  The Label, TC, and TTL values in the LSEs in the NAS may
      be redefined, but the meaning of the S bit is unchanged.

   Network Action Sub-Stack Indicator (NSI):  The first LSE in the NAS,
      which contains a special-purpose label that indicates the start of
      the NAS.

1.3.  Abbreviations

    +==============+=====================+===========================+
    | Abbreviation | Meaning             | Reference                 |
    +==============+=====================+===========================+
    | AD           | Ancillary Data      | [RFC9613]                 |
    +--------------+---------------------+---------------------------+
    | BIER         | Bit Index Explicit  | [RFC8279]                 |
    |              | Replication         |                           |
    +--------------+---------------------+---------------------------+
    | BoS          | Bottom of Stack     | [RFC6790]                 |
    +--------------+---------------------+---------------------------+
    | bSPL         | Base Special-       | [RFC9017]                 |
    |              | Purpose Label       |                           |
    +--------------+---------------------+---------------------------+
    | ECMP         | Equal-Cost          | [RFC9522]                 |
    |              | Multipath           |                           |
    +--------------+---------------------+---------------------------+
    | EL           | Entropy Label       | [RFC6790]                 |
    +--------------+---------------------+---------------------------+
    | ERLD         | Entropy Readable    | [RFC8662]                 |
    |              | Label Depth         |                           |
    +--------------+---------------------+---------------------------+
    | eSPL         | Extended Special-   | [RFC9017]                 |
    |              | Purpose Label       |                           |
    +--------------+---------------------+---------------------------+
    | HbH          | Hop by Hop          | In the MNA context, this  |
    |              |                     | document.                 |
    +--------------+---------------------+---------------------------+
    | I2E          | Ingress to Egress   | In the MNA context, this  |
    |              |                     | document.                 |
    +--------------+---------------------+---------------------------+
    | IGP          | Interior Gateway    |                           |
    |              | Protocol            |                           |
    +--------------+---------------------+---------------------------+
    | ISD          | In-Stack Data       | [RFC9613]                 |
    +--------------+---------------------+---------------------------+
    | LSE          | Label Stack Entry   | [RFC3032]                 |
    +--------------+---------------------+---------------------------+
    | LSP          | Label Switched Path |                           |
    +--------------+---------------------+---------------------------+
    | MNA          | MPLS Network Action | [RFC9613]                 |
    +--------------+---------------------+---------------------------+
    | MSD          | Maximum SID Depth   | [RFC8491]                 |
    +--------------+---------------------+---------------------------+
    | NAI          | Network Action      | [RFC9613]                 |
    |              | Indicator           |                           |
    +--------------+---------------------+---------------------------+
    | NAS          | Network Action Sub- | This document             |
    |              | Stack               |                           |
    +--------------+---------------------+---------------------------+
    | NSI          | Network Action Sub- | This document             |
    |              | Stack Indicator     |                           |
    +--------------+---------------------+---------------------------+
    | PSD          | Post-Stack Data     | [RFC9613] and Section 3.6 |
    |              |                     | of this document          |
    +--------------+---------------------+---------------------------+
    | RLD          | Readable Label      | This document             |
    |              | Depth               |                           |
    +--------------+---------------------+---------------------------+
    | SID          | Segment Identifier  | [RFC8402]                 |
    +--------------+---------------------+---------------------------+
    | SPL          | Special-Purpose     | [RFC9017]                 |
    |              | Label               |                           |
    +--------------+---------------------+---------------------------+
    | TC           | Traffic Class       |                           |
    +--------------+---------------------+---------------------------+
    | TTL          | Time to Live        |                           |
    +--------------+---------------------+---------------------------+

                          Table 1: Abbreviations

2.  Structure

   An MNA solution specifies one or more network actions to apply to an
   MPLS packet.  These network actions and their ancillary data may be
   carried in sub-stacks within the MPLS label stack and/or post-stack
   data.  A solution must specify where the network action sub-stacks
   occur in the label stack, if and how frequently they should be
   replicated within the label stack, and how the network action sub-
   stack and post-stack data are encoded.

   It seems highly likely that some ancillary data will be needed at
   many points along an LSP.  Replication of ancillary data throughout
   the label stack would be highly inefficient, as would a full rewrite
   of the label stack at each hop; thus, MNA allows encoding of network
   actions and ancillary data deeper in the label stack, requiring
   implementations to look past the first LSE.  Processing of the label
   stack past the top-of-stack LSE was first introduced with the Entropy
   Label (EL) [RFC6790].

   A network action sub-stack contains:

   *  Network Action Sub-Stack Indicator (NSI): The first LSE in the
      NAS, which contains a special-purpose label, called the MNA label,
      that is used to indicate the start of a network action sub-stack.

   *  Network Action Indicators (NAIs): Optionally, a set of indicators
      that describes the set of network actions.  If the set of
      indicators is not in the sub-stack, a solution could encode them
      in post-stack data.  A network action is said to be present if
      there is an indicator in the packet that invokes the action.

   *  In-Stack Data (ISD): A set of zero or more LSEs that carry
      ancillary data for the network actions that are present.  Network
      action indicators are not considered ancillary data.

   Each network action present in the network action sub-stack may have
   zero or more LSEs of in-stack data.  The ordering of the in-stack
   data LSEs corresponds to the ordering of the network action
   indicators.  The encoding of the in-stack data, if any, for a network
   action must be specified in the document that defines the network
   action.  In-stack data may be referenced by multiple network actions.

   As an example, in-stack data might look like the following label
   stack with an embedded 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Label                   | TC  |0|      TTL      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Label                   | TC  |0|      TTL      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Label                   | TC  |0|      TTL      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Network Action Sub-Stack     |0|               |
      ~                                                               ~
      |         Network Action Sub-Stack continued  |0|               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Label                   | TC  |0|      TTL      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Label                   | TC  |1|      TTL      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Payload                             |

     Figure 1: A Label Stack with an Embedded Network Action Sub-Stack

   Certain network actions may also specify that data is carried after
   the label stack.  This is called post-stack data.  The encoding of
   the post-stack data, if any, for a network action must be specified
   in the document that defines the network action.  If multiple network
   actions are present and have post-stack data, the ordering of their
   post-stack data corresponds to the ordering of the network action
   indicators.

   As an example, a packet containing post-stack data might contain a
   label stack followed by post-stack data, followed by the payload:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Label                   | TC  |0|      TTL      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Label                   | TC  |1|      TTL      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Post-Stack Data                         |
      ~                                                               ~
      |                  Post-Stack Data continued                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Payload                             |

            Figure 2: A Label Stack Followed by Post-Stack Data

   A solution must specify the order for network actions to be applied
   to the packet for the actions to have consistent semantics.  Since
   there are many possible orderings, especially with bit catalogs
   (Section 3.5.1), the solution must provide an unambiguous
   specification.  The precise semantics of an action are dependent on
   the contents of the packet, including any ancillary data, and the
   state of the router.

   This document assumes that the MPLS WG will select a single solution
   for the encoding of ISD and not more than one solution for the
   encoding of PSD.

2.1.  Scopes

   A network action may need to be processed by every node along the
   path or some subset of the nodes along its path.  Some of the scopes
   that an action may have are:

   *  Hop by Hop (HbH): Every node along the path will perform the
      action.

   *  Ingress to Egress (I2E): Only the last node on the path will
      perform the action.

   *  Select: Only specific nodes along the path will perform the
      action.

   If a solution supports the select scope, it must describe how it
   specifies the set of nodes to perform the actions.

   This framework does not place any constraints on the scope of, or the
   ancillary data for, a network action.  Any network action may appear
   in any scope or combination of scopes, may have no ancillary data,
   and may require in-stack data and/or post-stack data.  Some
   combinations may be suboptimal, but this framework does not restrict
   the combinations in an MNA solution.  A specific MNA solution may
   define such constraints.

2.2.  Partial Processing

   As described in [RFC3031], legacy devices that do not recognize the
   MNA label will discard the packet if the top label is the MNA label.

   Devices that do recognize the MNA label might not implement all of
   the network actions that are present.  A solution must specify how
   unrecognized network actions that are present should be handled.

   One alternative is that an implementation should stop processing
   network actions when it encounters an unrecognized network action.
   Subsequent present network actions would not be applied.  The result
   is dependent on the solution's order of operations.

   Another alternative is that an implementation should drop any packet
   that contains any unrecognized present network actions.

   A third alternative is that an implementation should perform all
   recognized present network actions but ignore all unrecognized
   present network actions.

   Other alternatives may also be possible.  The solution should specify
   the alternative adopted.

   In some solutions, an indication may be provided in the packet or in
   the action as to how the forwarder should proceed if it does not
   recognize the action.  Where an action needs to be processed at every
   hop, it is recommended that care be taken not to construct an LSP
   that traverses nodes that do not support that action.  It is
   recognized that, in some circumstances, it may not be possible to
   construct an LSP that avoids such nodes, such as when a network is
   reconverging following a failure or when IP Fast Reroute (IPFRR)
   [RFC5714] is taking place.

2.3.  Signaling

   A node that wishes to make use of MNA and apply network actions to a
   packet must understand the nodes that the packet will transit,
   whether or not the nodes support MNA, and the network actions that
   are to be invoked.  These capabilities are presumed to be signaled by
   protocols that are out of scope for this document and are presumed to
   have per-network-action granularity.  If a solution requires
   alternate signaling, it must specify that explicitly.

2.3.1.  Readable Label Depth

   Readable Label Depth (RLD) is defined as the number of LSEs, starting
   from the top of the stack, that a router can read in an incoming MPLS
   packet with no performance impact.  [RFC8662] introduced Entropy
   Readable Label Depth (ERLD).  Readable Label Depth is the same
   concept, but it is generalized and not specifically associated with
   the Entropy Label (EL) or MNA.

   ERLD is not redundant with RLD because ERLD specifies a value of zero
   if a system does not support the Entropy Label.  Since a system could
   reasonably support MNA or other MPLS functions and needs to advertise
   an RLD value but not support the Entropy Label, another advertised
   value is required.

   A node that pushes a NAS onto the label stack is responsible for
   ensuring that all nodes that are expected to process the NAS will
   have the entire NAS within their RLD.  A node SHOULD use signaling
   (e.g., the signaling described in [RFC9088] and [RFC9089]) to
   determine this.  An exception might be, for example, when the node
   has out-of-band knowledge that all nodes along the path do not have
   RLD limitations and thus could avoid the unnecessary overhead of
   using signaling.

   Per [RFC8662], a node that does not support EL will advertise a value
   of zero for its ERLD, so advertising ERLD alone does not suffice in
   all cases.  A node MAY advertise both ERLD and RLD, and it SHOULD do
   so if its ERLD and RLD values are different.  Again, if a node has
   out-of-band knowledge that all nodes do not have RLD limitations,
   then signaling can be avoided.  If a node's ERLD and RLD values are
   the same, it MAY only advertise ERLD for efficiency reasons.  If a
   node supports MNA but does not support EL, then it SHOULD advertise
   RLD unless it has out-of-band knowledge that no nodes in the domain
   have RLD restrictions.

   RLD is advertised by an IGP MSD-Type value of 3 and MAY be advertised
   as a Node MSD, Link MSD, or both.

   An MNA node MUST use the RLD determined by selecting the first
   advertised non-zero value from the following:

   *  The RLD advertised for the link

   *  The RLD advertised for the node

   *  The non-zero ERLD for the node

   A node's RLD is a function of its hardware capabilities and is not
   expected to depend on the specifics of the MNA solution.

2.4.  State

   A network action can affect the state stored in the network.  This
   implies that a packet may affect how subsequent packets are handled.
   In particular, one packet may affect subsequent packets in the same
   LSP.

3.  Encoding

   Several possible ways to encode NAIs have been proposed.  This
   section summarizes the proposals and some considerations for the
   various alternatives.

   When network actions are carried in the MPLS label stack, then
   regardless of their type, they are represented by a set of LSEs
   termed a Network Action Sub-Stack (NAS).  A NAS consists of a
   special-purpose label, optionally followed by LSEs that specify which
   network actions are to be performed on the packet and the in-stack
   ancillary data for each indicated network action.  Different network
   actions may be placed together in one NAS or may be carried in
   different sub-stacks.

   [RFC9613] requires that a solution not add unnecessary LSEs to the
   sub-stack (see requirement 9 in Section 3.1 of [RFC9613]).
   Accordingly, solutions should also make efficient use of the bits
   within the sub-stack (except the S-bit), as inefficient use of the
   bits could result in the addition of unnecessary LSEs.

3.1.  The MNA Label

   The first LSE in a network action sub-stack contains a special-
   purpose label that indicates a network action sub-stack.  A solution
   has several choices for this special-purpose label.

3.1.1.  Existing Base SPL

   A solution may reuse an existing Base SPL (bSPL).  If it elects to do
   so, it must explain how the usage is backward compatible, including
   in the case where there is ISD.

   If an existing inactive bSPL is selected that will not be backward
   compatible, then it must first be retired per [RFC7274] and then
   reallocated.

3.1.2.  New Base SPL

   A solution may select a new bSPL.

3.1.3.  New Extended SPL

   A solution may select a new Extended SPL (eSPL).  If it elects to do
   so, it must address the requirement for the minimal number of LSEs.

3.1.4.  User-Defined Label

   A solution may allow the network operator to define the label that
   indicates the network action sub-stack.  This creates management
   overhead for the network operator to coordinate the use of this label
   across all nodes on the path using management or signaling protocols.
   The user-defined label could be network-wide or LSP-specific.  If a
   solution elects to use a user-defined label, the solution should
   justify this overhead.

3.2.  TC and TTL

   In the first LSE of the network action sub-stack, only the 20 bits of
   the Label value and the Bottom of Stack bit are used by the NSI; the
   TC field (3 bits) and the TTL (8 bits) are not used.  This could
   leave 11 bits that could be used for MNA purposes.

3.2.1.  TC and TTL Retained

   If the solution elects to retain the TC and TTL fields, then the
   first LSE of the network action sub-stack would appear as described
   in [RFC3032]:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Label                   | TC  |S|      TTL      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 3: A Label Stack Entry with TC and TTL Retained

   Label:  Label value, 20 bits
   TC:  Traffic Class, 3 bits
   S:  Bottom of Stack, 1 bit
   TTL:  Time To Live, 8 bits

   Further LSEs would be needed to encode NAIs.  If a solution elects to
   retain the TC and TTL fields, it must address the requirement for the
   minimal number of LSEs.

3.2.2.  TC and TTL Repurposed

   If the solution elects to reuse the TC and TTL fields, then the first
   LSE of the network action sub-stack would appear as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Label                  |x x x|S|x x x x x x x x|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 4: A Label Stack Entry with TC and TTL Repurposed

   Label:  Label value, 20 bits
   x:  Bit available for use in solution definition
   S:  Bottom of Stack, 1 bit

   The solution may use more LSEs to contain NAIs.  If a solution elects
   to use more LSEs, it must address the requirement for the minimal
   number of LSEs.

3.3.  Length of the NAS

   A solution must have a mechanism (such as an indication of the length
   of the NAS) to enable an implementation to find the end of the NAS.
   This must be easily processed even by implementations that do not
   understand the full contents of the NAS.  Two options are described
   below; other solutions may be possible.

3.3.1.  Last/Continuation Bits

   A solution may use a bit per LSE to indicate whether or not the NAS
   continues into the next LSE.  The bit may indicate continuation by
   being set or by being clear.  The overhead of this approach is one
   bit per LSE and has the advantage that it can effectively encode an
   arbitrarily sized NAS.  This approach is efficient if the NAS is
   small.

3.3.2.  Length Field

   A solution may opt to have a fixed-size Length field at a fixed
   location within the NAS.  The fixed size of the Length field may not
   be large enough to support all possible NAS contents.  This approach
   may be more efficient if the NAS is long, but not longer than can be
   described by the Length field.

   One hardware designer recommends a Length field as this minimizes
   branching in the logic.

3.4.  Encoding of Scopes

   A solution may choose to explicitly encode the scope of each action
   contained in a network action sub-stack.  For example, a NAS might
   contain Action A (HbH), Action B (HbH), and Action C (HbH).  A
   solution may alternately choose to have the scope encoded implicitly,
   based on the actions present in the network action sub-stack.  For
   example, a NAS might contain the following actions with HbH scope: A,
   B, and C.  This choice may have performance implications as an
   implementation might have to parse the network actions that are
   present in a network action sub-stack only to discover that there are
   no actions for it to perform.

   For example, suppose that a NAS is embedded in a label stack at a
   depth of six LSEs and the NAS contains three actions, each with
   Select scope.  These actions are not applicable at the current node
   and should be ignored.  If the scope is encoded explicitly with each
   action, then an implementation must parse each action.  However, if
   the scope is encoded as part of the NAS, then an implementation only
   needs to parse the start of the NAS and not individual actions.

   Solutions need to consider the order of scoped NAIs and their
   associated AD within individual sub-stacks and the order of per-scope
   sub-stacks, so that network actions and the AD can be readily found
   and not be processed by nodes that are not required to handle those
   actions.

3.5.  Encoding a Network Action

   Two options for encoding NAIs are described below; other solutions
   may be possible.  Any solution should allow the encoding of an
   arbitrary number of NAIs.

3.5.1.  Bit Catalogs

   A solution may opt to encode the set of network actions as a list of
   bits, sometimes known as a catalog.  The solution must provide a
   mechanism to determine how many LSEs are devoted to the catalog when
   the NAIs are carried in-stack.  A set bit in the catalog would
   indicate that the corresponding network action is present.

   Catalogs are efficient if the number of present network actions is
   relatively high and if the size of the necessary catalog is small.
   For example, if the first 16 actions are all present, a catalog can
   encode this in 16 bits.  However, if the number of possible actions
   is large, then a catalog can become inefficient.  Selecting only one
   action that is the 256th action would require a catalog of 256 bits,
   which would require more than one LSE when the NAIs are carried in-
   stack.

   A solution may include a bit-remapping mechanism so that a given
   domain may optimize for its commonly used actions.

3.5.2.  Operation Codes

   A solution may opt to encode the set of present network actions as a
   list of operation codes (opcodes).  Each opcode is a fixed number of
   bits.  The size of the opcode bounds the number of network actions
   that the solution can support.

   Opcodes are efficient if there are only one or two active network
   actions.  For example, if an opcode is 8 bits, then two active
   network actions could be encoded in 16 bits.  However, if 16 actions
   are required, then opcodes would consume 128 bits.  Opcodes are
   efficient at encoding a large number of possible actions.  If only
   the 256th action is to be selected, that still requires 8 bits.

3.6.  Encoding of Post-Stack Data

   A solution may carry NAI and AD as PSD.  For ease of parsing, all AD
   should be co-located with its NAI.

   If there are multiple instances of post-stack data, they should occur
   in the same order as their relevant network action sub-stacks and
   then in the same order as their relevant network actions occur within
   the network action sub-stacks.

3.6.1.  First Nibble Considerations

   The first nibble after the label stack has been used to convey
   information in certain cases [RFC4385].  A consolidated view of the
   uses of the first nibble is provided in [RFC9790].

   For example, in [RFC4928], this nibble is investigated to find out if
   it has the value "4" or "6".  If it does not, it is assumed that the
   packet payload is not IPv4 or IPv6, and Equal-Cost Multipath (ECMP)
   is not performed.

   It should be noted that this is an inexact method.  For example, an
   Ethernet pseudowire without a control word might have "4" or "6" in
   the first nibble and thus will be subject to ECMP forwarding.

   Nevertheless, the method is implemented and deployed; it is used
   today and will be for the foreseeable future.

   The use of the first nibble for Bit Index Explicit Replication (BIER)
   is specified in [RFC8296].  BIER sets the first nibble to 5.  The
   same is true for a BIER payload as for any use of the first nibble:
   it is not possible to conclude that the payload is BIER even if the
   first nibble is set to 5 because an Ethernet pseudowire without a
   control word might begin with a 5.  However, the BIER approach meets
   the design goal of [RFC8296] to determine that the payload is IPv4,
   IPv6, or a packet with a header that includes a pseudowire control
   word.

   [RFC4385] allocates 0b0000 for the pseudowire control word and 0b0001
   as the control word for the pseudowire Associated Channel Header
   (ACH).

   A PSD solution should specify the contents of the first nibble, the
   actions to be taken for the value, and the interaction with post-
   stack data used concurrently by other MPLS applications.

4.  Semantics

   For MNA to be consistent across implementations and predictable in
   operational environments, its semantics need to be entirely
   predictable.  An MNA solution MUST specify a deterministic order for
   processing each of the network actions in a packet.  Each network
   action must specify how it interacts with all other previously
   defined network actions.  Private network actions are network actions
   that are not publicly documented.  Private network actions MUST be
   included in the ordering of network actions, but the interactions of
   private actions with other actions are outside of the scope of this
   document.

5.  Definition of a Network Action

   A document defining a network action must contain the following:

   Name:  The name of the network action.

   Network Action Indicator:  The bit position or opcode that indicates
      that the network action is active.

   Scope:  Description of which nodes should perform the network action
      as described in Section 2.1.

   State:  Indication of whether the network action can modify state in
      the network and, if so, the state that may be modified and its
      side effects.

   Required/Optional:  Indication of whether a node is required to
      perform the network action.

   In-Stack Data:  The number of LSEs of in-stack data, if any, and the
      encoding of the in-stack data.  If the in-stack data is of a
      variable length, then the solution must specify how an
      implementation can determine the length without implementing the
      network action.

   Post-Stack Data:  The encoding of post-stack data, if any.  If the
      post-stack data is of a variable length, then the solution must
      specify how an implementation can determine the length without
      implementing the network action.

   A solution should create an IANA registry for network actions.

6.  Management Considerations

   Network operators will need to be cognizant of which network actions
   are supported by which nodes and will need to ensure that this is
   signaled.  Some solutions may require network-wide configuration to
   synchronize the use of the labels that indicate the start of a NAS.
   Solution documents must clearly state what management considerations
   apply to the solutions they are describing.  Solution documents must
   describe mechanisms for performing network diagnostics in the
   presence of MNAs.

7.  Security Considerations

   An analysis of the security of MPLS systems is provided in [RFC5920],
   which also notes that the MPLS forwarding plane has no built-in
   security mechanisms.

   Central to the security of MPLS networks is operational security of
   the network, something that operators of MPLS networks are well
   versed in.  The deployment of link-level security (e.g., [MACsec])
   prevents link traffic observation covertly acquiring the label stack
   for an attack.  This is particularly important in the case of a
   network deploying MNA, because the MNA information may be sensitive.
   Thus, the confidentiality and authentication achieved through the use
   of link-level security is particularly advantageous.

   Some additional proposals to add encryption to the MPLS forwarding
   plane have been suggested [MPLS-OPP-SEC], but no mechanisms have been
   agreed upon at the time of publication of this document.
   [MPLS-OPP-SEC] offers hop-by-hop security that encrypts the label
   stack and is functionally equivalent to that provided by [MACsec].
   Alternatively, it also offers end-to-end encryption of the MPLS
   payload with no cryptographic integrity protection of the MPLS label
   stack.

   Particular care is needed when introducing any end-to-end security
   mechanism to allow an in-stack MNA solution that needs to employ on-
   path modification of the MNA data or where post-stack MNA data needs
   to be examined on-path.

   A cornerstone of MPLS security is to protect the network from
   processing MPLS labels that originated outside the network.

   Operators have considerable experience in excluding MPLS-encoded
   packets at the network boundaries, for example, by excluding all MPLS
   packets and all packets that are revealed to be carrying an MPLS
   packet as the payload of IP tunnels.  Where such packets are accepted
   into an MPLS network from an untrusted third party, non-MPLS packets
   are immediately encapsulated in an MPLS label stack specified by the
   MPLS network operator, and MPLS packets have additional label stack
   entries imported as specified by the MPLS network operator.  Thus, it
   is difficult for an attacker to pass an MPLS-encoded packet into a
   network or to present any instructions to the network forwarding
   system.

   Within a single well-managed domain, an adjacent domain may be
   considered to be trusted provided that it is sufficiently shielded
   from third-party traffic ingress and third-party traffic observation.
   In such a situation, no new security vulnerabilities are introduced
   by MNA.

   In some inter-domain applications (including carrier's carrier) where
   a first network's MPLS traffic is encapsulated directly over a second
   MPLS network by simply pushing additional MPLS LSEs, the contents of
   the first network's payload and label stack may be visible to the
   forwarders in the second network.  Historically, this has been benign
   and indeed useful for ECMP.  However, if the first network's traffic
   has MNA information, this may be exposed to MNA-capable forwarders
   and cause unpredictable behavior or modification of the customer MPLS
   label stack or MPLS payload.  This is an increased vulnerability
   introduced by MNA that SHOULD be addressed in any MNA solution.

   Several mitigations are available to an operator:

   a.  Reject all incoming packets containing MNA information that do
       not come from a trusted network.  Note that it may be acceptable
       to accept and process MNA information from a trusted network.

   b.  Fully encapsulate the inbound packet in a new additional MPLS
       label stack such that the forwarder finds a Bottom of Stack (BoS)
       bit imposed by the carrier network and only finds MNA information
       added by the carrier network.

   A mitigation that we reject as unsafe is having the ingress Label
   Switching Router (LSR) push sufficient additional labels such that
   any MNA information received in packets entering the network from a
   third-party network is made inaccessible due to it being below the
   RLD.  This is unsafe in the presence of an overly conservative RLD
   value and can result in the third-party MNA information becoming
   visible to and acted on by an MNA forwarder in the carrier network.

8.  IANA Considerations

   IANA has allocated the following code point in the "IGP MSD-Types"
   registry [MSD] within the "Interior Gateway Protocol (IGP)
   Parameters" registry group:

         +=======+======================+============+===========+
         | Value | Name                 | Data Plane | Reference |
         +=======+======================+============+===========+
         | 3     | Readable Label Depth | MPLS       | RFC 9789  |
         +-------+----------------------+------------+-----------+

                         Table 2: New IGP MSD-Type

9.  References

9.1.  Normative References

   [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>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385,
              February 2006, <https://www.rfc-editor.org/info/rfc4385>.

   [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
              <https://www.rfc-editor.org/info/rfc5920>.

   [RFC7274]  Kompella, K., Andersson, L., and A. Farrel, "Allocating
              and Retiring Special-Purpose MPLS Labels", RFC 7274,
              DOI 10.17487/RFC7274, June 2014,
              <https://www.rfc-editor.org/info/rfc7274>.

   [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>.

9.2.  Informative References

   [MACsec]   IEEE, "IEEE Standard for Local and metropolitan area
              networks-Media Access Control (MAC) Security", IEEE Std
              802.1AE-2018, DOI 10.1109/ieeestd.2018.8585421, 26
              December 2018,
              <https://ieeexplore.ieee.org/document/8585421>.

   [MPLS-OPP-SEC]
              Farrel, A. and S. Farrell, "Opportunistic Security in MPLS
              Networks", Work in Progress, Internet-Draft, draft-ietf-
              mpls-opportunistic-encrypt-03, 28 March 2017,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
              opportunistic-encrypt-03>.

   [MSD]      IANA, "IGP MSD-Types",
              <https://www.iana.org/assignments/igp-parameters/>.

   [RFC4928]  Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
              Cost Multipath Treatment in MPLS Networks", BCP 128,
              RFC 4928, DOI 10.17487/RFC4928, June 2007,
              <https://www.rfc-editor.org/info/rfc4928>.

   [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework",
              RFC 5714, DOI 10.17487/RFC5714, January 2010,
              <https://www.rfc-editor.org/info/rfc5714>.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <https://www.rfc-editor.org/info/rfc6790>.

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,
              <https://www.rfc-editor.org/info/rfc8279>.

   [RFC8296]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
              for Bit Index Explicit Replication (BIER) in MPLS and Non-
              MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
              2018, <https://www.rfc-editor.org/info/rfc8296>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8491]  Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
              DOI 10.17487/RFC8491, November 2018,
              <https://www.rfc-editor.org/info/rfc8491>.

   [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>.

   [RFC9088]  Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
              and M. Bocci, "Signaling Entropy Label Capability and
              Entropy Readable Label Depth Using IS-IS", RFC 9088,
              DOI 10.17487/RFC9088, August 2021,
              <https://www.rfc-editor.org/info/rfc9088>.

   [RFC9089]  Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
              and M. Bocci, "Signaling Entropy Label Capability and
              Entropy Readable Label Depth Using OSPF", RFC 9089,
              DOI 10.17487/RFC9089, August 2021,
              <https://www.rfc-editor.org/info/rfc9089>.

   [RFC9522]  Farrel, A., Ed., "Overview and Principles of Internet
              Traffic Engineering", RFC 9522, DOI 10.17487/RFC9522,
              January 2024, <https://www.rfc-editor.org/info/rfc9522>.

   [RFC9790]  Kompella, K., Bryant, S., Bocci, M., Mirsky, G., Ed.,
              Andersson, L., and J. Dong, "IANA Registry and Processing
              Recommendations for the First Nibble Following a Label
              Stack", RFC 9790, DOI 10.17487/RFC9790, July 2025,
              <https://www.rfc-editor.org/info/rfc9790>.

Acknowledgements

   This document is the result of work started in MPLS Open Design Team,
   with participation by the MPLS, PALS, and DETNET Working Groups.

   The authors would like to thank Adrian Farrel for his contributions.
   The authors would also like to thank John Drake, Toerless Eckert, and
   Jie Dong for their comments.

Authors' Addresses

   Loa Andersson
   Huawei Technologies
   Email: loa@pi.nu


   Stewart Bryant
   University of Surrey 5GIC
   Email: sb@stewartbryant.com


   Matthew Bocci
   Nokia
   Email: matthew.bocci@nokia.com


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