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Use Cases for MPLS Network Action Indicators and MPLS Ancillary Data

Document Type Active Internet-Draft (mpls WG)
Authors Tarek Saad , Kiran Makhijani , Haoyu Song , Greg Mirsky
Last updated 2023-03-13
Replaces draft-saad-mpls-miad-usecases
RFC stream Internet Engineering Task Force (IETF)
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Additional resources Mailing list discussion
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MPLS Working Group                                               T. Saad
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Informational                              K. Makhijani
Expires: 14 September 2023                                       H. Song
                                                  Futurewei Technologies
                                                               G. Mirsky
                                                           13 March 2023

  Use Cases for MPLS Network Action Indicators and MPLS Ancillary Data


   This document presents a number of use cases that have a common need
   for encoding network action indicators and associated ancillary data
   inside MPLS packets.  There has been significant recent interest in
   extending the MPLS data plane to carry such indicators and ancillary
   data to address a number of use cases that are described in this

   The use cases described in this document are not an exhaustive set,
   but rather the ones that are actively discussed by members of the
   IETF MPLS, PALS and DETNET working groups participating in the MPLS
   Open Design Team.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 14 September 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Acronyms and Abbreviations  . . . . . . . . . . . . . . .   3
   2.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  No Further Fastreroute  . . . . . . . . . . . . . . . . .   3
     2.2.  In-situ OAM . . . . . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  In-situ OAM Direct Export . . . . . . . . . . . . . .   5
     2.3.  Network Slicing . . . . . . . . . . . . . . . . . . . . .   5
       2.3.1.  Dedicated Identifier as Flow-Aggregate Selector . . .   6
       2.3.2.  Forwarding Label as a Flow-Aggregate Selector . . . .   6
     2.4.  Generic Delivery Functions  . . . . . . . . . . . . . . .   6
     2.5.  Delay Budgets for Time-Bound Applications . . . . . . . .   7
       2.5.1.  Stack Based Methods for Latency Control . . . . . . .   7
     2.6.  NSH-based Service Function Chaining . . . . . . . . . . .   8
     2.7.  Network Programming . . . . . . . . . . . . . . . . . . .   8
   3.  Existing MPLS Use cases . . . . . . . . . . . . . . . . . . .   8
   4.  Co-existence of Usecases  . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   This document describes important cases that require carrying
   additional ancillary data within the MPLS packets, as well as means
   to indicate the ancillary data is present, and a specific action
   needs to be performed on the packet.

   These use cases have been identified by the MPLS Working Group Open
   Design Team working on defining MPLS Network Actions for the MPLS
   data plane.  The MPLS Ancillary Data (AD) can be classified as:

   *  implicit, or "no-data" associated with a Network Action (NA)

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   *  residing within the MPLS label stack and referred to as In Stack
      Data (ISD), and

   *  residing after the Bottom of MPLS label Stack (BoS) and referred
      to as Post Stack Data (PSD).

   The use cases described in this document will be used to assist in
   identifying requirements and issues to be considered for future
   resolution by the working group.

1.1.  Terminology

   The following terminology is used in the document:

   IETF Network Slice:
      a well-defined composite of a set of endpoints, the connectivity
      requirements between subsets of these endpoints, and associated
      requirements; the term 'network slice' in this document refers to
      'IETF network slice' as defined in

   Time-Bound Networking:
      Networks that transport time-bounded traffic.

1.2.  Acronyms and Abbreviations

      ISD: In-stack data

      PSD: Post-stack data

      MNA: MPLS Network Action

      NAI: Network Action Indicator

      AD: Ancillary Data

2.  Use Cases

2.1.  No Further Fastreroute

   MPLS Fast Reroute (FRR) [RFC4090], [RFC5286] and [RFC7490] is a
   useful and widely deployed tool for minimizing packet loss in the
   case of a link or node failure.

   Several cases exist where, once FRR has taken place in an MPLS
   network and resulted in rerouting a packet away from the failure, a
   second FRR that impacts the same packet to rerouting is not helpful,
   and may even be disruptive.

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   For example, in such a case, the packet may continue to loop until
   its TTL expires.  This can lead to link congestion and further packet
   loss.  Thus, the attempt to prevent a packet from being dropped may
   instead affect many other packets.  A proposal to address this is
   presented in [I-D.kompella-mpls-nffrr].

2.2.  In-situ OAM

   In-situ Operations, Administration, and Maintenance (IOAM) is used to
   collect operational and telemetry information while packets traverses
   a particular path in a network domain.

   The term "in-situ" refers to the fact that the IOAM data fields are
   added to the data packets rather than being sent within the probe
   packets specifically dedicated to OAM or Performance Measurement

   IOAM can run in two modes Edge-to-Edge (E2E) and Hop-by-Hop (HbH).
   In E2E mode, only the encapsulating and decapsulating nodes will
   process IOAM data fields.  In HbH mode, the encapsulating and
   decapsulating nodes as well as intermediate IOAM-capable nodes
   process IOAM data fields.  The IOAM data fields are defined in
   [I-D.ietf-ippm-ioam-data], and can be used for various OAM use-cases.

   Several IOAM Options have been defined:

   *  Pre-allocated and Incremental

   *  Edge-to-Edge

   *  Proof-of-Transit

   *  Direct Export (see Section 2.2.1)

   [I-D.gandhi-mpls-ioam-sr] defines how IOAM data fields are
   transported using the MPLS data plane encapsulations, including
   Segment Routing (SR) with MPLS data plane (SR-MPLS).

   The IOAM data may be added after the bottom of the MPLS label stack.
   The IOAM data fields can be of fixed or incremental size as defined
   in [I-D.ietf-ippm-ioam-data].  [I-D.gandhi-mpls-ioam] describes the
   applicability of IOAM to MPLS dataplane.  The encapsulating MPLS node
   needs to know if the decapsulating MPLS node can process the IOAM
   data before adding it in the packet.  In HbH IOAM mode, nodes that
   are capable of processing IOAM will intercept and process the IOAM
   data accordingly.  The presence of IOAM header and optional IOAM data
   will betransparent to nodes that do not support or do not participate
   in the IOAM process.

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2.2.1.  In-situ OAM Direct Export

   IOAM Direct Export (DEX) [I-D.ietf-ippm-ioam-direct-export] is an
   IOAM Option-Type in which the operational state and telemetry
   information is collected according to the specified profile and
   exported in a manner and format defined by a local policy.

   In IOAM DEX, the user data packet is only used to trigger the IOAM
   data to be directly exported or locally aggregated without being
   pushed into in-flight data packets.

2.3.  Network Slicing

   An IETF Network Slice service provides connectivity coupled with a
   set of network resource commitments and is expressed in terms of one
   or more connectivity constructs.  A slice-flow aggregate
   [I-D.bestbar-teas-ns-packet] refers to the set of traffic streams
   from one or more connectivity constructs belonging to one or more
   IETF Network Slices that are mapped to a set of network resources and
   provided the same forwarding treatment.  The packets associated with
   a slice-flow aggregate may carry a marking in the packet's network
   layer header to identify this association and this marking is
   referred to as Flow-Aggregate Selector (FAS).  The FAS is used to map
   the packet to the associated set of network resources and provide the
   corresponding forwarding treatment to the packet.

   A router that requires forwarding of a packet that belongs to a
   slice-flow aggregate may have to decide on the forwarding action to
   take based on selected next-hop(s), and the forwarding treatment
   (e.g., scheduling and drop policy) to enforce based on the associated
   per-hop behavior.

   In this case, the routers that forward traffic over resources that
   are shared by multiple slice-flow aggregates need to identify the
   slice aggregate packets in order to enforce the associated forwarding
   action and treatment.

   MNA can be used to indicate the action and carry ancillary data for
   packets traversing Label Switched Paths (LSPs).  An MNA network
   action can be used to carry the FAS in MPLS packets.

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2.3.1.  Dedicated Identifier as Flow-Aggregate Selector

   A dedicated Identifier that is independent of forwarding can be
   carried in the packet as a Flow-Aggregate Selector (FAS).  This can
   be encoded in the MPLS packet as defined in
   [I-D.kompella-mpls-mspl4fa], [], and
   [I-D.decraene-mpls-slid-encoded-entropy-label-id].  The FAS is used
   to associate the packets belonging to Slice-Flow Aggregate to the
   underlying Network Resource Partition (NRP) as described in

   When MPLS packets carry a dedicated FAS identifier, the MPLS LSRs use
   the forwarding label to select the forwarding next-hop(s), and use
   the FAS in the MPLS packet to infer the specific forwarding treatment
   that needs to be applied on the packet.

   The FAS can be encoded within an MPLS label carried in the packet's
   MPLS label stack.  All MPLS packets that belong to the same flow
   aggregate MAY carry the same FAS identifier.

2.3.2.  Forwarding Label as a Flow-Aggregate Selector

   [RFC3031] states in Section 2.1 that: 'Some routers analyze a
   packet's network layer header not merely to choose the packet's next
   hop, but also to determine a packet's "precedence" or "class of

   It is possible by assigning a unique MPLS forwarding label to each
   flow aggregate (FEC) to distinguish the packets forwarded to the same
   destination. from other flow aggregates.  In this case, LSRs can use
   the top forwarding label to infer both the forwarding action and the
   forwarding treatment to be invoked on the packets.

2.4.  Generic Delivery Functions

   The Generic Delivery Functions (GDF), defined in
   [I-D.zzhang-intarea-generic-delivery-functions], provide a new
   mechanism to support functions analogous to those supported through
   the IPv6 Extension Headers mechanism.  For example, GDF can support
   fragmentation/reassembly functionality in the MPLS network by using
   the Generic Fragmentation Header.  MNA can support GDF by placing a
   GDF header in an MPLS packet within the Post-Stack Data block
   [I-D.ietf-mpls-mna-fwk].  Multiple GDF headers can also be present in
   the same MPLS packet organized as a list of headers.

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2.5.  Delay Budgets for Time-Bound Applications

   The routers in a network can perform two distinct functions on
   incoming packets, namely forwarding (where the packet should be sent)
   and scheduling (when the packet should be sent).  IEEE-802.1 Time
   Sensitive Networking (TSN) and Deterministic Networking provide
   several mechanisms for scheduling under the assumption that routers
   are time-synchronized.  The most effective mechanisms for delay
   minimization involve per-flow resource allocation.

   Segment Routing (SR) is a forwarding paradigm that allows encoding
   forwarding instructions in the packet in a stack data structure,
   rather than being programmed into the routers.  The SR instructions
   are contained within a packet in the form of a First-in First-out
   stack dictating the forwarding decisions of successive routers.
   Segment routing may be used to choose a path sufficiently short to be
   capable of providing a bounded end-to-end latency but does not
   influence the queueing of individual packets in each router along
   that path.

   When carried over the MPLS data plane, a solution is required to
   enable the delivery of such packets that can be delivered to their
   final destination by a given time budget.  One approach to address
   this usecase in SR-MPLS was described in [I-D.stein-srtsn].

2.5.1.  Stack Based Methods for Latency Control

   One efficient data structure for inserting local deadlines into the
   headers is a "stack", similar to that used in Segment Routing to
   carry forwarding instructions.  The number of deadline values in the
   stack equals the number of routers the packet needs to traverse in
   the network, and each deadline value corresponds to a specific
   router.  The Top-of-Stack (ToS) corresponds to the first router's
   deadline while the Bottom-of-Stack (BoS) refers to the last's.  All
   local deadlines in the stack are later or equal to the current time
   (upon which all routers agree), and times closer to the ToS are
   always earlier or equal to times closer to the BoS.

   The ingress router inserts the deadline stack into the packet
   headers; no other router needs to be aware of the requirements of the
   time-bound flows.  Hence admitting a new flow only requires updating
   the information base of the ingress router.

   MPLS LSRs that expose the Top of Stack (ToS) label can also inspect
   the associated "deadline" carried in the packet (either in MPLS stack
   as ISD or after BoS as PSD).

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2.6.  NSH-based Service Function Chaining

   [RFC8595] describes how Service Function Chaining (SFC) can be
   realized in an MPLS network by emulating the NSH by using only MPLS
   label stack elements.

   The approach in [RFC8595] introduces some limitations that are
   discussed in [I-D.lm-mpls-sfc-path-verification].  This approach,
   however, can benefit from the framework introduced with MNA in

   For example, it may be possible to extend NSH emulation using MPLS
   labels [RFC8595] to support the functionality of NSH Context Headers,
   whether fixed or variable-length.  One of the use cases could support
   Flow ID [I-D.ietf-sfc-nsh-tlv] that may be used for load-balancing
   among Service Function Forwarders (SFFs) and/or the Service Function
   (SF) within the same SFP.

2.7.  Network Programming

   In SR, an ingress node steers a packet through an ordered list of
   instructions, called "segments".  Each one of these instructions
   represents a function to be called at a specific location in the
   network.  A function is locally defined on the node where it is
   executed and may range from simply moving forward in the segment list
   to any complex user-defined behavior.

   Network Programming combines Segment Routing (SR) functions to
   achieve a networking objective that goes beyond mere packet routing.

   It may be desirable to encode a pointer to function and its arguments
   within an MPLS packet transport header.  For example, in MPLS we can
   encode the FUNC::ARGs within the label stack or after the Bottom of
   Stack to support the equivalent of FUNC::ARG in SRv6 as described in

3.  Existing MPLS Use cases

   There are serveral services that can be transported over MPLS
   networks today.  These include providing Layer-3 (L3) connectivity
   (e.g. for unicast and multicast L3 services), and Layer-2 (L2)
   connectivity (e.g. for unicast Pseudo-Wires (PWs), multicast E-Tree,
   and broadcast E-LAN L2 services).  In those cases, the user service
   traffic is encapsulated as the payload in MPLS packets.

   For L2 service traffic, it is possible to use A Control Word (CW)
   [RFC4385] and [RFC5085] immediately after the MPLS header to
   disambiguate the type of MPLS payload, prevent possible packet

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   misordering, and allow for fragmentation.  In this case, the first
   nibble the data that immediately follows after MPLS bottom of stack
   is set to 0000b to identify the presence of PW CW.

   In addition to providing connectivity to user traffic, MPLS may also
   transport OAM data (e.g. over MPLS G-AChs [RFC5586]).  In this case,
   the first nibble of the data that immediately follows after MPLS
   bottom of stack is set to 0001b, it indicates the presence of a
   control channel associated witha PW, LSP, or Section.

   Bit Index Explicit Replication (BIER) [RFC8296] traffic can also be
   encapsulated over MPLS.  In this case, BIER has defined 0101b as the
   value for the first nibble in the data that immediately appears after
   the bottom of the label stack for any BIER encapsulated packet over

   For pseudowires, the G-ACh uses the first four bits of the PW control
   word to provide the initial discrimination between data packets and
   packets belonging to the associated channel, as described in

   It is expected that new use cases described in this document and
   within the MNA framework [I-D.ietf-mpls-mna-fwk] will allow for the
   co-existance and backward compatibility with all such existing MPLS

4.  Co-existence of Usecases

   Two or more of the aforementioned use cases MAY co-exist in the same
   packet.  This may require the presence of multiple ancilary data
   (whether In-stack or Post-stack ancillary data) to be present in the
   same MPLS packet.

   For example, IOAM may provide key functions along with network
   slicing to help ensure that critical network slice SLOs are being met
   by the network provider.  In this case, IOAM is able to collect key
   performance measurement parameters of network slice traffic flows as
   it traverses the transport network.

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   This document introduces no new security considerations.

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7.  Acknowledgement

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

8.  Contributors

   The following individuals contributed to this document:

      Loa Andersson
      Bronze Dragon Consulting

9.  Informative References

              Saad, T., Beeram, V. P., Dong, J., Wen, B., Ceccarelli,
              D., Halpern, J. M., Peng, S., Chen, R., Liu, X.,
              Contreras, L. M., Rokui, R., and L. Jalil, "Realizing
              Network Slices in IP/MPLS Networks", Work in Progress,
              Internet-Draft, draft-bestbar-teas-ns-packet-10, 5 May
              2022, <

              Decraene, B., Filsfils, C., Henderickx, W., Saad, T.,
              Beeram, V. P., and L. Jalil, "Using Entropy Label for
              Network Slice Identification in MPLS networks.", Work in
              Progress, Internet-Draft, draft-decraene-mpls-slid-
              encoded-entropy-label-id-05, 12 December 2022,

              Gandhi, R., Brockners, F., Wen, B., Decraene, B., and H.
              Song, "MPLS Data Plane Encapsulation for In Situ OAM
              Data", Work in Progress, Internet-Draft, draft-gandhi-
              mpls-ioam-10, 10 March 2023,

              Gandhi, R., Ali, Z., Filsfils, C., Brockners, F., Wen, B.,
              and V. Kozak, "MPLS Data Plane Encapsulation for In-situ
              OAM Data", Work in Progress, Internet-Draft, draft-gandhi-
              mpls-ioam-sr-06, 18 February 2021,

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              Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
              for In Situ Operations, Administration, and Maintenance
              (IOAM)", Work in Progress, Internet-Draft, draft-ietf-
              ippm-ioam-data-17, 13 December 2021,

              Song, H., Gafni, B., Brockners, F., Bhandari, S., and T.
              Mizrahi, "In Situ Operations, Administration, and
              Maintenance (IOAM) Direct Exporting", Work in Progress,
              Internet-Draft, draft-ietf-ippm-ioam-direct-export-11, 23
              September 2022, <

              Andersson, L., Bryant, S., Bocci, M., and T. Li, "MPLS
              Network Actions Framework", Work in Progress, Internet-
              Draft, draft-ietf-mpls-mna-fwk-03, 11 March 2023,

              Wei, Y., Elzur, U., Majee, S., Pignataro, C., and D. E.
              Eastlake, "Network Service Header (NSH) Metadata Type 2
              Variable-Length Context Headers", Work in Progress,
              Internet-Draft, draft-ietf-sfc-nsh-tlv-15, 20 April 2022,

              Farrel, A., Drake, J., Rokui, R., Homma, S., Makhijani,
              K., Contreras, L. M., and J. Tantsura, "A Framework for
              IETF Network Slices", Work in Progress, Internet-Draft,
              draft-ietf-teas-ietf-network-slices-19, 21 January 2023,

              Kompella, K., Beeram, V. P., Saad, T., and I. Meilik,
              "Multi-purpose Special Purpose Label for Forwarding
              Actions", Work in Progress, Internet-Draft, draft-
              kompella-mpls-mspl4fa-03, 10 July 2022,

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              Kompella, K. and W. Lin, "No Further Fast Reroute", Work
              in Progress, Internet-Draft, draft-kompella-mpls-nffrr-03,
              8 July 2022, <

              Li, Z. and J. Dong, "Carrying Virtual Transport Network
              (VTN) Information in MPLS Packet", Work in Progress,
              Internet-Draft, draft-li-mpls-enhanced-vpn-vtn-id-03, 16
              October 2022, <

              Yao, L. and G. Mirsky, "MPLS-based Service Function
              Path(SFP) Consistency Verification", Work in Progress,
              Internet-Draft, draft-lm-mpls-sfc-path-verification-03, 11
              June 2022, <

              Stein, Y. J., "Segment Routed Time Sensitive Networking",
              Work in Progress, Internet-Draft, draft-stein-srtsn-01, 29
              August 2021, <

              Zhang, Z. J., Bonica, R., Kompella, K., and G. Mirsky,
              "Generic Delivery Functions", Work in Progress, Internet-
              Draft, draft-zzhang-intarea-generic-delivery-functions-03,
              11 July 2022, <

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,

   [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
              Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              DOI 10.17487/RFC4090, May 2005,

   [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, <>.

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   [RFC5085]  Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual
              Circuit Connectivity Verification (VCCV): A Control
              Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085,
              December 2007, <>.

   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,

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

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,

   [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, <>.

   [RFC8595]  Farrel, A., Bryant, S., and J. Drake, "An MPLS-Based
              Forwarding Plane for Service Function Chaining", RFC 8595,
              DOI 10.17487/RFC8595, June 2019,

   [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,

Authors' Addresses

   Tarek Saad
   Cisco Systems, Inc.

   Kiran Makhijani
   Futurewei Technologies

Saad, et al.            Expires 14 September 2023              [Page 13]
Internet-Draft                MNA Usecases                    March 2023

   Haoyu Song
   Futurewei Technologies

   Greg Mirsky

Saad, et al.            Expires 14 September 2023              [Page 14]