



Internet Engineering Task Force (IETF)                     M. Bocci, Ed.
Request for Comments: 9613                                         Nokia
Category: Informational                                        S. Bryant
ISSN: 2070-1721                                 University of Surrey ICS
                                                                J. Drake
                                                             Independent
                                                             August 2024


  Requirements for Solutions that Support MPLS Network Actions (MNAs)

Abstract

   This document specifies requirements for the development of MPLS
   Network Actions (MNAs) that affect the forwarding or other processing
   of MPLS packets.  These requirements are informed by a number of
   proposals for additions to the MPLS information in the labeled packet
   to allow such actions to be performed, either by a transit or
   terminating Label Switching Router (i.e., the Label Edge Router -
   LER).

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/rfc9613.

Copyright Notice

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   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  Requirements Language
   3.  MPLS Network Action Requirements
     3.1.  General Requirements
     3.2.  Requirements on the MNA Alert Mechanism
     3.3.  Requirements on Network Actions
     3.4.  Requirements on Network Action Indicators
     3.5.  Requirements on Ancillary Data
   4.  IANA Considerations
   5.  Security Considerations
   6.  Acknowledgements
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Authors' Addresses

1.  Introduction

   There is significant interest in developing the MPLS data plane to
   address the requirements of new use cases [MNA-USECASES].  This
   requires a general mechanism, termed MPLS Network Actions (MNAs), to
   allow the network to make a forwarding or processing decision based
   on information other than the top label and Traffic Class (TC) bits,
   and to also make use of the Network Action Indicator (NAI) and
   ancillary data (MNA information).  These use cases require the
   definition of extensions to the MPLS architecture and label-stack
   operations that can be used across these use cases in order to
   minimize implementation complexity and promote interoperability and
   extensibility.  These protocol extensions need to conform to the
   existing MPLS architecture as specified by [RFC3031], [RFC3032], and
   [RFC6790].

   Note that the MPLS architecture specified in [RFC3031] describes a
   mechanism for forwarding MPLS packets through a network without
   requiring any analysis of the MPLS packet payload's network layer
   header by intermediate nodes (Label Switching Routers - LSRs).
   Formally, inspection may only occur at network ingress (the Label
   Edge Router - LER) where the MPLS packet is assigned to a Forwarding
   Equivalence Class (FEC).

   This document specifies the requirements for solutions that encode
   MNAs and ancillary data that may be needed to process those actions.
   These requirements are informed by a number of proposals to allow
   additions to the MPLS information in the labeled packet so that such
   actions can be performed, either by a transit or terminating LSR.  It
   is anticipated that these will result in two types of solution
   specifications:

   MNA solution specification:  A specification that describes a common
      protocol that supports all forms of MNAs.

   Network Action solution specifications:  One or more specifications
      describing the protocol extensions for the MNA solution to address
      a use case.

   The term 'solutions', in isolation, refers to both MNA and Network
   Action solutions.  The requirements constrain the MNA solution design
   to enable interoperability between implementations.

1.1.  Terminology

   Network Action (NA):  An operation to be performed on an MPLS packet
      or as a consequence of an MPLS packet being processed by a router.
      An NA may affect router state or MPLS packet forwarding, or it may
      affect the MPLS packet in some other way.

   Network Action Indicator (NAI):  An indication in the MPLS packet
      that a certain NA is to be performed.

   Ancillary Data (AD):  Data in an MPLS packet associated with a given
      NA that may be used as input to process the NA or may result from
      processing the NA.  Ancillary data may be associated with:

      *  Both the control or maintenance information and the data
         traffic carried by the Label Switched Path (LSP).

      *  Only the control or maintenance information.

      *  Only the data traffic carried by the LSP.

   In-Stack Data:  Ancillary data carried within the MPLS label stack.

   Post-Stack Data:  Ancillary data carried in an MPLS packet between
      the bottom of the MPLS label stack and the first octet of the user
      payload.  This document does not prescribe whether post-stack data
      precedes or follows any other post-stack header such as a Control
      Word or Associated Channel Header (ACH).

   Scope:  The set of nodes that should perform a given action.

2.  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 document is not a protocol specification, this
   convention is adopted for clarity of description of requirements.

3.  MPLS Network Action Requirements

   This document specifies requirements on MNAs and the technology to
   support them in MPLS, such as NAIs, the associated AD, and the alert
   mechanism to indicate to an LSR that NAIs are present in an MPLS
   packet.

   The requirements are for the behavior of the protocol mechanisms and
   procedures that constitute building blocks out of which indicators
   for a NA and associated ancillary data are constructed.  It does not
   specify the detailed actions and processing of any NAs or ancillary
   data by an LSR or LER.

   The size of the ancillary data carried post-stack end to end in an
   MPLS packet is a matter for agreement between the ingress and egress
   provider edges (PEs), and is not part of these requirements.  Since
   in-stack ancillary data and per-hop post-stack data need to be parsed
   and processed by transit LSRs along the Label Switched Path (LSP),
   requirements on the size of such ancillary data are documented in the
   following sections.

3.1.  General Requirements

   1.   Any solutions MUST maintain the properties of extensibility,
        flexibility, and efficiency inherent in the split between the
        control plane context and simple data plane used in MPLS and the
        specification SHOULD describe how this is achieved.

   2.   Any solutions to these requirements MUST be based on and MUST
        NOT restrict the generality of the MPLS architecture [RFC3031]
        [RFC3032] [RFC5331].

   3.   If extensions to the MPLS data plane are required, they MUST be
        consistent with the MPLS architecture [RFC3031] [RFC3032]
        [RFC5331].

   4.   Solutions meeting the requirements set out in this document MUST
        be able to coexist with existing MPLS mechanisms.

   5.   Subject to the constraints in these requirements, a Network
        Action solution MAY carry MNA information in-stack, post-stack,
        or both in-stack and post-stack.

   6.   Solution specifications MUST NOT require an implementation to
        support in-stack ancillary data, unless the implementation
        chooses to support an NA that uses in-stack ancillary data.

   7.   Solution specifications MUST NOT require an implementation to
        support post-stack ancillary data, unless the implementation
        chooses to support an NA that uses post-stack ancillary data.

   8.   The design of any MNA solution MUST minimize the amount of
        processing required to parse the label stack at an LSR.

   9.   Solutions MUST minimize any additions to the size of the MPLS
        label stack.

   10.  Solution specifications that increase the size of the MPLS label
        stack in a way that is not controlled by the ingress LER MUST
        discuss the consequences.

   11.  Solution specifications MUST discuss the ECMP consequences of
        the design.

   12.  A Network Action solution MUST NOT expose information to the
        LSRs that is not already exposed to the LER.

   13.  The design of any NA MUST NOT expose any information that a user
        of any service using the LSP considers confidential [RFC6973]
        [RFC3552].

   14.  Solution specifications MUST document any new security
        considerations that they introduce.

   15.  An MNA solution MUST allow MPLS packets carrying NAI and
        ancillary data (where it exists) to coexist with MPLS packets
        that do not carry this information on the same LSP.

3.2.  Requirements on the MNA Alert Mechanism

   16. An MNA solution specification MUST define how a node determines
       whether NAIs are present in the MPLS packet.

   17. Special Purpose Labels (SPLs) are a mechanism of last resort;
       therefore, an MNA solution specification that defines their use
       MUST minimize the number of new SPLs that are allocated.

3.3.  Requirements on Network Actions

   18. It is RECOMMENDED that an MNA solution include support for NAs
       for Private Use (see Section 4.1 of [RFC8126]).

   19. Network Action solution specifications MUST define if the NA
       needs to be processed as a part of the immediate forwarding
       operation and whether MPLS packet misordering is allowed to occur
       as a result of the time taken to process the NA.

   20. If a Network Action solution specification allows more than one
       scope for an NA, it MUST define a mechanism to indicate the
       precedence of the scopes or any combination of the scopes.

   21. If a network action requires an NAI with in-stack ancillary data
       that needs to be imposed at an LSR on an LSP, then the Network
       Action solution MUST specify how this is achieved in all
       circumstances.

   22. If a network action requires an NAI with post-stack ancillary
       data to be imposed at an LSR on an LSP, then the Network Action
       solution specification MUST describe how this is achieved in all
       circumstances.

3.4.  Requirements on Network Action Indicators

   23.  Insertion, parsing, processing, and disposition of NAIs SHOULD
        make use of existing MPLS data plane operations.

   24.  Without constraining the mechanism, an MNA solution MUST enable
        a node inserting or modifying NAIs to determine if the target of
        the NAI, or any other LSR that may expose the NAI, can accept
        and process an MPLS packet containing the NAI.

   25.  An NAI MUST NOT be imposed for delivery to a node unless it is
        known that the node supports processing the NAI.

   26.  The NAI design MUST support setting the scope of network
        actions.

   27.  A given Network Action solution specification MUST define which
        scope or scopes are applicable to the associated NAI.

   28.  An MNA solution specification SHOULD define the support of NAIs
        for both Point-to-Point (P2P) and Point-to-Multipoint (P2MP)
        paths, but the Network Action solution specification MAY limit a
        specific NAI to only one of these path types if there is a clear
        reason to do so.

   29.  An MNA solution specification defining data plane mechanisms for
        NAIs MUST be consistent across different control plane
        protocols.

   30.  An MNA solution MUST allow the deployed MPLS control and
        management planes to determine the ability of downstream LSRs to
        accept and/or process a given NAI.

   31.  An MNA solution MUST allow indicators for multiple network
        actions in the same MPLS packet.

   32.  An MNA solution specification MUST NOT require an implementation
        to process all NAIs present in an MPLS packet.

   33.  NAIs MUST only be inserted at LSRs that push a label onto the
        stack, but they can be processed by LSRs along the path of the
        LSP.  Two examples of LSRs that push a label onto the stack are
        head-end LSRs and points of local repair (PLRs).

   34.  If a network action requires in-stack ancillary data, the NAI
        that indicates this network action MUST be present in the label
        stack.

   35.  All NAIs MUST be encoded in a manner consistent with [RFC3031].

   36.  If there is post-stack ancillary data for an NAI that is present
        in the label stack, it MUST be possible to infer the presence of
        the ancillary data without having to parse below the bottom of
        the label stack.

   37.  Any processing that removes an NAI from the label stack MUST
        also remove all associated ancillary data from the MPLS packet
        unless the ancillary data is required by any remaining NAIs.

   38.  MNA solution specifications MUST request that IANA create
        registries and make allocations from those registries for NAIs
        as necessary to ensure unambiguous identification of
        standardized network actions.  An MNA solution specification MAY
        request that IANA reserve a range of a registry for Private Use.

   39.  A Network Action solution specification MUST state where the
        NAIs are to be placed in the MPLS packet, that is whether they
        are placed in-stack or post-stack.

3.5.  Requirements on Ancillary Data

   40.  Network Action solution specifications MUST state whether
        ancillary data is required to fulfill the action and whether it
        is in-stack and/or post-stack.

   41.  Network Action solution specifications MUST state if in-stack or
        post-stack ancillary data that is already present in the MPLS
        packet MAY be rewritten by an LSR.

   42.  Solutions for in-stack ancillary data MUST be able to coexist
        with and MUST NOT obsolete existing MPLS mechanisms.  Such
        solutions MUST be described in a Standards Track RFC.

   43.  Network Action solutions MUST take care to limit the quantity of
        in-stack ancillary data to the minimum amount required.

   44.  A Network Action solution SHOULD NOT use post-stack ancillary
        data unless the size of that ancillary data could prevent the
        coexistence of the network action with other in-use MPLS network
        functions if it were inserted into the label stack.

   45.  The structure of the NAI and any associated ancillary data MUST
        enable skipping of unknown NAIs and any associated AD.

   46.  Any MNA solution specification MUST describe whether the
        solution can coexist with existing post-stack data mechanisms
        (e.g., control words and the Generic Associated Channel Header
        [RFC5586]), and if so how coexistence operates.

   47.  An MNA solution MUST allow an LER that inserts ancillary data to
        determine whether each node that needs to process the ancillary
        data can read the required distance into the MPLS packet at that
        node (compare with the mechanism in [RFC9088]).

   48.  For scoped in-stack or post-stack ancillary data, any MNA
        solution MUST allow an LER inserting NAIs whose network actions
        make use of that ancillary data to determine if the NAI and
        ancillary data will be processed by LSRs within the scope along
        the path.  Such a solution may need to determine if LSRs along
        the path can process a specific type of AD implied by the NAI at
        the depth in the stack that it will be presented to the LSR.

   49.  A mechanism MUST exist to notify an egress LER of the presence
        of ancillary data so that it can dispose of it appropriately.

   50.  In-stack ancillary data MUST only be inserted in conjunction
        with an operation conforming with [RFC3031].

   51.  Post-stack ancillary data MUST only be inserted in conjunction
        with an operation conforming with [RFC3031].

   52.  Processing of ancillary data below a swapped label MAY include
        rewriting the ancillary data.

   53.  If a Network Action solution needs to change the size of the
        ancillary data, its specification MUST analyze the implications
        on MPLS packet forwarding and specify how these are addressed.

   54.  Not more than one Standards Track solution specification SHOULD
        be defined for encoding in-stack ancillary data.

   55.  Not more than one Standards Track solution specification SHOULD
        be defined for encoding post-stack ancillary data.

4.  IANA Considerations

   This document has no IANA actions.

5.  Security Considerations

   Solutions designed according to the requirements in this document may
   introduce new security considerations to MPLS, whose forwarding plane
   on its own does not provide any built-in security mechanisms
   [RFC5920].

   In particular, such solutions may embed information derived from the
   MPLS payload in the MPLS headers.  This may expose data that a user
   of the MPLS-based service might otherwise assume is opaque to the
   MPLS network.  Furthermore, an LSR may insert information into the
   labeled packet such that the forwarding behavior is no longer purely
   a function of the top label or another label with forwarding context.
   Instead, the forwarding behavior may be the result of a more complex
   heuristic.  This creates an implicit trust relationship between the
   LSR whose forwarding behavior is being changed and the upstream LSR
   inserting the data causing that change.

   Several requirements above address some of these considerations.  The
   MNA framework [MNA-FRAMEWORK] also provides security considerations
   resulting from any extensions to the MPLS architecture, and these
   SHOULD be taken together with the security considerations herein.

   Individual solution specifications meeting the requirements in this
   document MUST address any security considerations introduced by the
   MNA design.

6.  Acknowledgements

   The authors gratefully acknowledge the contributions from Joel
   Halpern, Greg Mirsky, Yingzhen Qu, Haoyu Song, Tarek Saad, Loa
   Andersson, Tony Li, Adrian Farrel, Jie Dong, Bruno Decraene, and
   participants in the MPLS Working Group who have provided comments.

   The authors also gratefully acknowledge the input of the members of
   the MPLS Open Design Team.

7.  References

7.1.  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>.

   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              RFC 5331, DOI 10.17487/RFC5331, August 2008,
              <https://www.rfc-editor.org/info/rfc5331>.

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

   [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>.

7.2.  Informative References

   [MNA-FRAMEWORK]
              Andersson, L., Bryant, S., Bocci, M., and T. Li, "MPLS
              Network Actions (MNA) Framework", Work in Progress,
              Internet-Draft, draft-ietf-mpls-mna-fwk-10, 6 August 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
              mna-fwk-10>.

   [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-10, 20 June 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
              mna-usecases-10>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [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>.

   [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>.

   [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>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/info/rfc6973>.

   [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>.

Authors' Addresses

   Matthew Bocci (editor)
   Nokia
   Email: matthew.bocci@nokia.com


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


   John Drake
   Independent
   Email: je_drake@yahoo.com
