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Support MPLS Network Actions using Post-Stack Extension Headers

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Haoyu Song , Zhenbin Li , Tianran Zhou , Loa Andersson , Zhaohui (Jeffrey) Zhang , Rakesh Gandhi , Jaganbabu Rajamanickam , Jisu Bhattacharya
Last updated 2022-09-01
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Candidate for WG Adoption
Document shepherd Tarek Saad
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MPLS                                                        H. Song, Ed.
Internet-Draft                                    Futurewei Technologies
Intended status: Standards Track                                   Z. Li
Expires: 5 March 2023                                            T. Zhou
                                                            L. Andersson
                                                Bronze Dragon Consulting
                                                                Z. Zhang
                                                        Juniper Networks
                                                               R. Gandhi
                                                         J. Rajamanickam
                                                         J. Bhattacharya
                                                           Cisco Systems
                                                        1 September 2022

    Support MPLS Network Actions using Post-Stack Extension Headers


   Motivated by the need to support multiple in-network services and
   functions in an MPLS network (a.k.a.  MPLS Network Actions (MNA)),
   this document describes a generic and extensible method to
   encapsulate MNA instructions as well as possible ancillary data in an
   MPLS packet.  All the post-stack MNAs are encapsulated in a structure
   called Post-stack MNA Header (PAH).  A PAH is composed of a common
   header plus a chain of extension headers; each extension header is a
   container for an MNA.  The encapsulation method allows chaining
   multiple post-stack extension headers and provides the means to
   enable fast access to them as well as the original upper layer
   headers.  This document confines to the solution of PAH encoding and
   leaves the specification of PAH indicator to the overall MNA
   solution.  We show how PAH can be used to support several new MNAs as
   a generic post-stack mechanism.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 5 March 2023.

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   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  MPLS Post-stack Network Action Header . . . . . . . . . . . .   4
   3.  Scope of MPLS Extension Headers . . . . . . . . . . . . . . .   8
   4.  Operation on MPLS PAH . . . . . . . . . . . . . . . . . . . .   9
   5.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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1.  Motivation

   Some applications require adding sizable action instructions and/or
   ancillary data to packets within an MPLS network.  Such examples
   include In-situ OAM (IOAM) [RFC9197] and Service Function Chaining
   (SFC) [RFC7665].  New applications are emerging.  It is possible that
   the instructions and/or ancillary data for multiple MNAs are stacked
   together in one packet to support a compound service.

   Such instructions and/or ancillary data would need to be encoded and
   encapsulated as new headers in packets.  Such headers may require to
   be processed in fast path due to performance considerations.
   Moreover, such headers may require being attended at each hop on the
   forwarding path (i.e., hop-by-hop or HBH) or at designated end nodes
   (i.e., end-to-end or E2E).

   The need and requirements to support such applications in MPLS
   networks, i.e., MPLS Network Actions (MNA), are described in
   [I-D.ietf-mpls-mna-usecases] and [I-D.ietf-mpls-mna-requirements].
   It is clear that some header should be located after the MPLS label
   stack.  We call such a header Post-stack MNA Header (PAH).  The
   encapsulation of PAH poses some challenges to MPLS networks, because
   the MPLS label stack contains no explicit indicator for the upper
   layer protocols by design.

   The mechanism to indicate the presence of the PAH is outside the
   scope of this document.  The indication for the presence of the PAH
   can be achieved using several mechanisms, including carrying a
   Special Purpose Label (SPL) or signaling it with the label Forwarding
   Equivalence Class (FEC) as described in [I-D.ietf-mpls-mna-fwk].  In
   this document, we focus on the encoding and encapsulation of the PAH
   in an MPLS packet.

   The conventional header encoding and encapsulation methods face some
   challenges in the case of post-stack MNA:

   *  A solution may rely on either the built-in next-protocol indicator
      in the header or the knowledge of the format and size of the
      header to access the following packet headers.  This method
      requires each node to be able to parse the new header, which is
      unrealistic in an incremental deployment environment.

   *  Some works provide only piecemeal solutions which assume the new
      header is the only extra header and its location in the packet is
      fixed by default (e.g., Encapsulation of SFC NSH in MPLS
      [RFC8596]).  It is impossible or difficult to support multiple new
      headers in one packet due to the conflicting location assumption.

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   *  Some previous work such as G-ACH [RFC5586] was explicitly defined
      for control channel only, but we need the mechanism to also work
      for user packets.

   To solve the aforementioned problems, we introduce PAH as a general
   and extensible means to support new MNAs which involve instructions
   and/or ancillary data for each MNA.  The concept is similar to IPv6
   extension headers which offer a huge potential for extending IPv6's
   capability (e.g, network security, SRv6 [RFC8754], network
   programming [RFC8986], SFC [I-D.ietf-spring-sr-service-programming],
   etc.).  Thanks to the mechanism of extension headers, it is
   straightforward to continue introducing new network services into
   IPv6 networks.

   Nevertheless, when applying the extension headers to MPLS, some
   issues of the IPv6 EH should be avoided:

   *  IPv6's extension headers are chained with the original upper layer
      protocol headers in a flat stack.  One must scan all the extension
      headers to access the upper layer protocol headers and the
      payload.  This is inconvenient and raises some performance
      concerns for some applications (e.g., Deep Packet Inspection (DPI)
      and Equal Cost Multi Path (ECMP)).  The new PAH scheme for MPLS
      needs to improve this.

   *  [RFC8200] enforces many constraints to IPv6 extension headers
      (e.g., EH can only be added or deleted by the end nodes specified
      by the IP addresses in the IPv6 header, and there is only one Hop-
      by-Hop EH that can be processed on the path nodes), which are not
      suitable for MPLS networks.  For example, MPLS label stacks are
      added and changed in network, and there could be tunnel within
      tunnel, so the extension headers need more flexibility.

2.  MPLS Post-stack Network Action Header

   The concept and design of the PAH comply with the requirements laid
   out in [I-D.ietf-mpls-mna-requirements].  All the post-stack MNAs are
   encapsulated in a PAH.  A PAH is composed of a common header plus a
   chain of extension headers; each extension header is a container for
   an MNA.  Here we highlight some design objectives of PAH (Note: these
   should be covered by the MNA requirement document):

   Performance:  Unnecessary full extension header chain scanning for
      all MNAs or the upper layer headers should be avoided.  The
      extension headers should be ordered according to the access need.
      Each extension header should serve only one MNA to avoid the need
      of packing multiple TLV options in one extension header.

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   Scalability:  New MNAs can be supported by introducing new extension
      headers.  Multiple extension headers can be easily stacked
      together to support multiple services simultaneously.

   Backward Compatibility:  Legacy devices which do not recognize the
      PAH should still be able to forward the packets based on the top
      label as usual.  If a PAH-aware device recognizes some of the MNAs
      but not the others in an extension header chain, it can process
      the known MNAs only while ignoring the others.

   Flexibility:  A node (i.e., an LER or LSR) can be configured to
      process or not process any EH.  Any tunnel end nodes in the MPLS
      domain can add new EH to the packets which shall be removed on the
      other end of the tunnel.

   We assume the MPLS label stack has included some indicator of the
   PAH.  The actual PAH is inserted between the MPLS label stack and the
   original upper layer header.  The format of the MPLS packets with PAH
   is shown in Figure 1.

    0                                  31
    |                                   |
    ~         MPLS Label Stack          ~
    |                                   |
    |             BoS Label             |
    |    Common Header of PAH (CH)      |  \
    +--------+--------+--------+--------+  |
    |                                   |  |
    ~     Extension Header (EH) 1       ~  |
    |                                   |  |
    +--------+--------+--------+--------+   >MPLS PAH
    ~             ... ...               ~  |
    +--------+--------+--------+--------+  |
    |                                   |  |
    ~     Extension Header (EH) n       ~  |
    |                                   |  /
    |             Original              |
    ~    Upper Layer Headers/Payload    ~
    |                                   |

            Figure 1: MPLS with Post Stack Network Action Header

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   Following the MPLS label stack is the 4-octet Common Header of PAH
   (CH), which indicates the total number of extension headers in this
   packet, the overall length of the PAH, the type of the original upper
   layer header, and the type of the next extension header.  The format
   of the CH is shown in Figure 2.

       0           1         2          3
       0123 4567 89012345 67890123 45678901
      | R  |EHC |  EHTL  |  OUL   |   NH   |

                            Figure 2: CH Format

   The meaning of the fields in a CH is as follows:

   R:  reserved nibble.  The nibble value means to avoid any potential
      conflicting with IP version numbers and other well-defined
      semantics [I-D.kbbma-mpls-1stnibble].

   EHC:  4-bit unsigned integer for the Extension Header Counter.  This
      field keeps the total number of extension headers included in this
      packet.  It does not count the original upper layer headers.  At
      most 15 EHs are allowed in one packet.

   EHTL:  8-bit unsigned integer for the Extension Header Total Length
      in 4-octet units.  This field keeps the total length of the EHs in
      this packet, not including the CH itself.

   OUL:  8-bit Original Upper Layer protocol number indicating the
      original upper layer protocol type.  It can be set to "UNKNOWN"
      (value TBD) if unknown.  Sometimes the MPLS FEC may indicate the
      type of payload.  In this case either OUL is redundant or OUL can
      be used to replace the control plane mechanism.

   NH:  8-bit indicator for the Next Header.  This field identifies the
      type of the MNA in the extension header immediately following the

   The value of the reserved nibble needs further consideration.  The
   EHC field can be used to keep track of the number of extension
   headers when some headers are inserted or removed at some network
   nodes.  The EHTL field can help to skip all the EHs in one step if
   the original upper layer headers or payload need to be accessed.  The
   OUL field can help identify the type of the original upper layer

   The format of an Extension Header (EH) is shown in Figure 3.

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       0          1          2          3
       01234567 89012345 67890123 45678901
      |  NH    |  HLEN  |    EXT(opt)    |
      |                                  |
      ~  MNA Instruction/Ancillary Data  ~
      |                                  |

                            Figure 3: EH Format

   The meaning of the fields in an EH is as follows:

   NH:  8-bit indicator for the Next Header type.  This field identifies
      the type of the MNA in the EH immediately following this EH.

   HLEN:  8-bit unsigned integer for the Extension Header Length in
      4-octet units, not including the first 4 octets.

   EXT:  16-bit optional type extension.  To save the Next Header
      numbers and extend the number space, it is possible to use one
      "Next Header" code to cover a set of sub-types.  For example, IOAM
      has several different options, such as trace and DEX.  It is too
      expensive to assign an EH type for each of it.  In this case, it
      is better to have a single EH type value for IOAM, and use the EXT
      to specify the option types.  This field is optional and only
      specified for some specific MNA types.  This field can also be
      used to encode other information.

   MNA Instruction/Ancillary Data:  A variable length field for the
      specification of an MNA.  This field may need to be padded to make
      the EH 4-octet aligned.

   The extension headers as well as the first original upper layer
   protocol header are chained together through the NH field in CH and
   EHs.  The encoding of NH can share the same value registry for IPv4/
   IPv6 protocol numbers.  Values for new MNA types (i.e., NH number)
   shall be assigned by IANA from the same registry as for the ipv4 and
   ipv6 protocol numbers (

   Specifically, the NH field of the last EH in a chain can have some
   special values, which shall be assigned by IANA as well:

   NONE (No Next Header):  Indicates that there is no other header and

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      payload after this EH.  This can be used to transport packets with
      only extension header(s), for example, the control packets for
      control or the probe packets for measurements.  Note that value 59
      was reserved for "IPv6 No Next Header" indicator.  It may be
      possible for MPLS EH to share this value (Note: need to work with

   UNKNOWN (Unknown Next Header):  Indicates that the type of the header
      after this header is unknown.  This is intended to be compatible
      with the original MPLS design in which the upper layer protocol
      type is unknown from the MPLS header alone.

   MPLS:  Indicates that the next protocol header is still of MPLS type
      and another MPLS label stack follows.

   These NH values can only appear in the last EH in an PAH.  Note that
   the original upper layer protocol can be of type "MPLS", which
   implies that a packet may contain multiple logically independent
   label stacks separated by PAH.  Having more than one independent
   label stack is not new.  For example, A Bier header could separate
   the transport/bier labels and the payload labels; An MPLS Pseudo Wire
   (PW) network could be implemented on the top of another
   infrastructure MPLS network.  In such cases, we have the flexibility
   to apply different services to different label stacks.

3.  Scope of MPLS Extension Headers

   Basically, MPLS EHs have two application scopes based on the nature
   of the contained MNA: HBH and E2E.  E2E means that the EH is only
   supposed to be inserted/removed and processed at the MPLS tunnel end
   points where the MPLS header is inserted or removed.  The EHs that
   need to be processed on path nodes within the MPLS tunnel are of the
   HBH type.  However, any node in the tunnel can be configured to
   ignore an HBH EH, even if it is capable of processing it.

   If there are two types of EHs in a packet, the HBH EHs must take
   precedence over the E2E EHs.

   Making a distinction of the EH types and ordering the EHs in a packet
   help improve the forwarding performance.  For example, if a node
   within an MPLS tunnel finds only E2E EHs in a packet, it can avoid
   scanning the EH list.

   The scope of an EH (i.e., HBH or E2E) is an intrinsic property of the
   contained MNA.  In other words, such information can be inferred from
   the NH value.

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4.  Operation on MPLS PAH

   A suitable indication for the presence of PAH is ensured before
   adding the first EH X to an MPLS packet.  Then the PAH is inserted
   after the MPLS label stack.  In the CH of the PAH, EHC is set to 1,
   EHTL is set to the length of X in 4-octet units, OUL is set to a
   proper value, and NH is set to the header type value of X.  At last,
   X is inserted after the CH, in which NH and HLEN are set accordingly.
   Note that if this operation happens at a PE device, the upper layer
   protocol is known before the MPLS encapsulation, so its value can be
   saved in the OUL and NH field if desired.  Otherwise, the NH field is
   filled with the value of "UNKNOWN".

   When an EH Y needs to be added to an MPLS packet which already
   contains the PAH, the EHC and EHTL in the CH are updated accordingly
   (i.e., EHC is incremented by 1 and EHTL is incremented by the size of
   Y in 4-octet units).  Then a proper location for Y in the EH chain is
   located.  Y is inserted at this location.  The NH field of Y is
   copied from the previous EH's NH field (or from the CH's NH field, if
   Y is the first EH in the chain).  The previous EH's NH value, or, if
   Y is the first EH in the chain, the CH's NH value, is set to the NH
   value of Y.

   Deleting an EH simply reverses the above operation.  If the deleted
   EH is the last one, the PAH indicator and the PAH can also be

   When processing an MPLS packet with multiple extension headers in an
   PAH, the node needs to parse through the entire EH chain and process
   the EH one by one (but not necessarily in the parsing order).  The
   node should ignore any EH that is not recognized or is configured as
   "Do not Processing" by the control plane.

   The EH can be categorized into HBH or E2E.  Since EHs are ordered
   based on their type (i.e., HBH EHs are located before E2E EHs), a
   node can avoid some unnecessary EH scan.

5.  Use Cases

   In this section, we show how PAH can be used to support several new
   network applications.

   In-situ OAM:  In-situ OAM (IOAM) records flow OAM information within
      user packets while the packets traverse a network.  The
      instruction and collected data are kept in an IOAM header
      [RFC9197].  When applying IOAM in an MPLS network, the IOAM header
      can be encapsulated in an extension header within an PAH.

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   Network Telemetry and Measurement:  A network telemetry and
      instruction header can be carried as an extension header in PAH to
      instruct a node what type of network measurements should be done.
      For example, the method described in [RFC8321] can be implemented
      in MPLS networks since the EH provides a natural way to color MPLS

   Network Security:  Security related functions often require user
      packets to carry some instruction and ancillary data.  In a DoS
      limiting network architecture, a "packet passport" header is used
      to embed packet authentication information for each node to

   Segment Routing and Network Programming:  MPLS extension header in
      PAH can support the implementation of a new flavor of the MPLS-
      based segment routing, with better performance and richer
      functionalities.  The details will be described in another draft.

   With PAH, multiple in-network applications can be chained together as
   extension headers.  For example, IOAM and SFC can be applied at the
   same time to support network OAM and service function chaining.  A
   node can stop scanning the extension header chain if all the known
   headers it can process have been located.  For example, if IOAM is
   the first EH in a chain and a node is configured to process IOAM
   only, it can stop searching the EH chain when the IOAM EH is found.

   Details on some of these use cases and discussions on some other use
   cases are covered in [I-D.ietf-mpls-mna-usecases].

6.  Security Considerations

   The major security concerns may come from the MNAs that encapsulated
   in the PAH.  So we need to be careful to admit actions and take
   measures to avoid the security threats such as information leak or
   DoS attack.

7.  IANA Considerations

   This document requests IANA to assign two new Internet Protocol
   Numbers from the "Protocol Numbers" Registry to indicate "No Next
   Header" and "Unknown Next Header".

   This document does not create any other new registries.  New
   registries for protocol numbers and type extension numbers should be
   requested by each MNA use case document.

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8.  Contributors

   The other contributors of this document are listed as follows.

   *  James Guichard

   *  Stewart Bryant

   *  Andrew Malis

9.  Acknowledgments

   We thank Tarek Saad and the other members of MPLS ODT for helping
   improve this document.

10.  References

10.1.  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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

10.2.  Informative References

              Bhandari, S. and F. Brockners, "In-situ OAM IPv6 Options",
              Work in Progress, Internet-Draft, draft-ietf-ippm-ioam-
              ipv6-options-08, 16 June 2022,

              Andersson, L., Bryant, S., Bocci, M., and T. Li, "MPLS
              Network Actions Framework", Work in Progress, Internet-
              Draft, draft-ietf-mpls-mna-fwk-00, 26 July 2022,

              Bocci, M., Bryant, S., and J. Drake, "Requirements for
              MPLS Network Action Indicators and MPLS Ancillary Data",
              Work in Progress, Internet-Draft, draft-ietf-mpls-mna-

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              requirements-03, 19 August 2022,

              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-00, 19 May 2022,

              Clad, F., Xu, X., Filsfils, C., Bernier, D., Li, C.,
              Decraene, B., Ma, S., Yadlapalli, C., Henderickx, W., and
              S. Salsano, "Service Programming with Segment Routing",
              Work in Progress, Internet-Draft, draft-ietf-spring-sr-
              service-programming-06, 9 June 2022,

              Kompella, K., Bryant, S., Bocci, M., Mirsky, G., and L. O.
              (. Andersson, "IANA Registry for the First Nibble
              Following a Label Stack", Work in Progress, Internet-
              Draft, draft-kbbma-mpls-1stnibble-02, 10 July 2022,

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

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   [RFC8321]  Fioccola, G., Ed., Capello, A., Cociglio, M., Castaldelli,
              L., Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi,
              "Alternate-Marking Method for Passive and Hybrid
              Performance Monitoring", RFC 8321, DOI 10.17487/RFC8321,
              January 2018, <>.

   [RFC8596]  Malis, A., Bryant, S., Halpern, J., and W. Henderickx,
              "MPLS Transport Encapsulation for the Service Function
              Chaining (SFC) Network Service Header (NSH)", RFC 8596,
              DOI 10.17487/RFC8596, June 2019,

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,

   [RFC9197]  Brockners, F., Ed., Bhandari, S., Ed., and T. Mizrahi,
              Ed., "Data Fields for In Situ Operations, Administration,
              and Maintenance (IOAM)", RFC 9197, DOI 10.17487/RFC9197,
              May 2022, <>.

Authors' Addresses

   Haoyu Song (editor)
   Futurewei Technologies
   Santa Clara,
   United States of America

   Zhenbin Li
   P.R. China

   Tianran Zhou
   P.R. China

Song, et al.              Expires 5 March 2023                 [Page 13]
Internet-Draft        MPLS Post-Stack Action Header       September 2022


   Loa Andersson
   Bronze Dragon Consulting

   Zhaohui Zhang
   Juniper Networks
   United States of America

   Rakesh Gandhi
   Cisco Systems

   Jaganbabu Rajamanickam
   Cisco Systems

   Jisu Bhattacharya
   Cisco Systems

Song, et al.              Expires 5 March 2023                 [Page 14]