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MPLS Extension Header

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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
Last updated 2022-01-10
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MPLS                                                        H. Song, Ed.
Internet-Draft                                    Futurewei Technologies
Intended status: Standards Track                                   Z. Li
Expires: 14 July 2022                                            T. Zhou
                                                            L. Andersson
                                                Bronze Dragon Consulting
                                                                Z. Zhang
                                                        Juniper Networks
                                                         10 January 2022

                         MPLS Extension Header


   Motivated by the need to support multiple in-network services and
   functions in an MPLS network, this document describes a generic and
   extensible method to encapsulate extension headers into MPLS packets.
   The encapsulation method allows stacking multiple extension headers
   and quickly accessing any of them as well as the original upper layer
   protocol header and payload.  We show how the extension header can be
   used to support several new network applications and optimize some
   existing network services.

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.

   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."

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   This Internet-Draft will expire on 14 July 2022.

Copyright Notice

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

   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.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  MPLS Extension Header . . . . . . . . . . . . . . . . . . . .   4
   3.  Type of MPLS Extension Headers  . . . . . . . . . . . . . . .   8
   4.  Operation on MPLS Extension Headers . . . . . . . . . . . . .   8
   5.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Motivation

   Some applications require adding instructions and/or metadata to user
   packets within a network.  Such examples include In-situ OAM (IOAM)
   [I-D.ietf-ippm-ioam-data] and Service Function Chaining (SFC)
   [RFC7665].  New applications are emerging.  It is possible that the
   instructions and/or metadata for multiple applications are stacked
   together in one packet to support a compound service.

   Conceivably, such instructions and/or metadata would be encoded as
   new headers and encapsulated in user packets.  Such headers may
   require to be processed in fast path or in slow path.  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).

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   The encapsulation of the new header(s) poses some challenges to MPLS
   networks, because the MPLS protocol header contains no explicit
   indicator for the upper layer protocols by design.  We leave the
   discussion on the indicator of new header(s) in an MPLS packet to
   another companion document [].  In this
   document, we focus on the encode and encapsulation of new headers in
   an MPLS packet.

   The similar problem has been tackled for some particular application
   before.  However, these solutions have some drawbacks:

   *  The solutions 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 data.  The node is required
      to be able to parse the new header, which is unrealistic in an
      incremental deployment environment.

   *  These works only provide piecemeal solution which assumes the new
      header is the only extra header and its location in the packet is
      fixed by default.  It is impossible or difficult to support
      multiple new headers in one packet due to the conflicted

   *  Some previous work such as G-ACH [RFC5586] was explicitly defined
      for control channel only but what we need is the user packet

   To solve these problems, we introduce extension header as a general
   and extensible means to support new in-network functions and
   applications in MPLS networks.  The idea is similar to IPv6 extension
   headers which offer a huge innovation potential (e.g, network
   security, SRv6 [RFC8754], network programming
   [I-D.ietf-spring-srv6-network-programming], SFC
   [I-D.xu-clad-spring-sr-service-chaining], etc.).  Thanks to the
   existence of extension headers, it is straightforward to introduce
   new in-network services into IPv6 networks.  For example, it has been
   proposed to carry IOAM header [I-D.brockners-inband-oam-transport] as
   a new extension header option in IPv6 networks.

   Nevertheless, IPv6 EH is not perfect either.  It has three issues:

   *  IPv6's header is large compared to MPLS, claiming extra bandwidth
      overhead and complicating the packet processing.  We prefer to
      retain the header compactness in MPLS networks.

   *  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

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      payload.  This is inconvenient and raises some performance
      concerns for some applications(e.g., DPI and ECMP).  The new
      scheme for MPLS header extension needs to address these issues

   *  [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 Extension Header

   From the previous discussion, we have laid out the design
   requirements to support extension headers in MPLS networks:

   Performance:  Unnecessary label stack scanning for a label and the
      full extension header stack scanning for the upper layer protocol
      should be avoided.  The extension headers a node needs to process
      should be located as close to the MPLS label stack as possible.
      Each extension header is better to serve only one application to
      avoid the need of packing multiple TLV options in one extension

   Scalability:  New applications 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
      extension header option should still be able to forward the
      packets as usual.  If a device recognize some of the extension
      headers but not the others in an extension header stack, it can
      process the known headers only while ignoring the others.

   Flexibility:  A node 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
   extension header(s).  The actual extension headers are inserted
   between the MPLS label stack and the original upper layer packet
   header.  The format of the MPLS packets with extension headers is
   shown in Figure 1.

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      0                                  31
      +--------+--------+--------+--------+  \
      |                                   |  |
      ~     MPLS Label Stack              ~  |
      |                                   |  |
      +--------+--------+--------+--------+  |
      |     EH Indicator (TBD)            |   > MPLS Label Stack
      +--------+--------+--------+--------+  |  (extended with EHI)
      |                                   |  |
      ~     MPLS Label Stack              ~  |
      |                                   |  |
      +--------+--------+--------+--------+ <
      | Header of Extension Headers (HEH) |  |
      +--------+--------+--------+--------+  |
      |                                   |  |
      ~     Extension Header (EH) 1       ~  |
      |                                   |  |
      +--------+--------+--------+--------+   > MPLS EH Fields
      ~                                   ~  |  (new)
      +--------+--------+--------+--------+  |
      |                                   |  |
      ~     Extension Header (EH) N       ~  |
      |                                   |  |
      +--------+--------+--------+--------+ <
      |                                   |  |
      ~    Upper Layer Headers/Payload    ~   > MPLS Payload
      |                                   |  |  (as is)
      +--------+--------+--------+--------+  /

                   Figure 1: MPLS with Extension Headers

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

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

                            Figure 2: HEH Format

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

   R:  4-bit reserved.  The nibble value means to avoid conflicting with

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      IP version numbers.

   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 protocol
      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
      extension headers in this packet, not including the HEH itself.

   OUL:  8-bit Original Upper Layer protocol number indicating the
      original upper layer protocol type.  It can be set to "UNKNOWN" if

   NH:  8-bit selector for the Next Header.  This field identifies the
      type of the header immediately following the HEH.

   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 extension headers in
   one step if the original upper layer protocol headers or payload need
   to be accessed.  The OUL field can help identify the type of the
   original upper layer protocol.

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

       0          1          2          3
       01234567 89012345 67890123 45678901
      |  NH    |  HLEN  |EXT     |       |
      +--------+--------+--------+       |
      |                                  |
      ~        Header Specific Data      ~
      |                                  |

                            Figure 3: EH Format

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

   NH:  8-bit indicator for the Next Header.  This field identifies the
      type of 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.

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   EXT:  8-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.  Each sub-type is
      assigned a new code in a different name space.  This field is
      optional and it is only specified for some specific EH type.

   Header Specific Data:  Variable length field for the specification of
      the EH.  This field is 4-octet aligned.

   The extension headers as well as the first original upper layer
   protocol header are chained together through the NH field in HEH and
   EHs.  The encoding of NH can share the same value registry for IPv4/
   IPv6 protocol numbers.  Values for new EH types shall be assigned by

   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
      payload after this header.  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.

   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 original upper layer protocol is still MPLS
      and another MPLS label stack follows.

   Note that the original upper layer protocol can be of type "MPLS",
   which implies that in a packet there might be multiple label stacks
   separated by EHs.  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 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.

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3.  Type of MPLS Extension Headers

   Basically, there are two types of MPLS EHs: 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 forwardidng performance.  For example, if a node
   within an MPLS tunnel finds only E2E EHs in a packet, it can avoid
   scanning the EH list.

4.  Operation on MPLS Extension Headers

   When the first EH X needs to be added to an MPLS packet, an EH
   indicator is inserted into the proper location in the MPLS label
   stack.  A HEH is then inserted after the MPLS label stack, in which
   EHCNT is set to 1, EHTLEN is set to the length of X in 4-octet units,
   and NH is set to the header value of X.  At last, X is inserted after
   the HEH, in which NH and HELN 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 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 extension header(s), the EHCNT and EHTLEN in the HEH are
   updated accordingly (i.e., EHCNT is incremented by 1 and EHTLEN 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 HEH'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 HEH's NH, is set to the header value of Y.

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

   When processing an MPLS packet with extension headers, the node needs
   to scan through the entire EH chain and process the EH one by one.
   The node should ignore any unrecognized EH or the EH that is
   configured as "No Processing".

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   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 MPLS extension header 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
      [I-D.ietf-ippm-ioam-data].  When applying IOAM in an MPLS network,
      the IOAM header can be encapsulated as an MPLS extension header.

   Network Telemetry and Measurement:  A network telemetry and
      instruction header can be carried as an extension header 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 metadata.  In a DoS limiting network
      architecture, a "packet passport" header is used to embed packet
      authentication information for each node to verify.

   Segment Routing and Network Programming:  MPLS extension header 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 MPLS extension headers, multiple in-network applications can be
   stacked together.  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 stack if all the known
   headers it can process have been located.  For example, if IOAM is
   the first EH in a stack and a node is configured to process IOAM
   only, it will stop searching the EH stack when the IOAM EH is found.

6.  Security Considerations


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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.

8.  Contributors

   The other contributors of this document are listed as follows.

   *  James Guichard

   *  Stewart Bryant

   *  Andrew Malis

9.  Acknowledgments


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,

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

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

   [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, <>.

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   [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,

10.2.  Informative References

              Brockners, F., Bhandari, S., Govindan, V. P., Pignataro,
              C., Gredler, H., Leddy, J., Youell, S., Mizrahi, T.,
              Mozes, D., Lapukhov, P., and R. Chang, "Encapsulations for
              In-situ OAM Data", Work in Progress, Internet-Draft,
              draft-brockners-inband-oam-transport-05, 3 July 2017,

              Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
              for In-situ OAM", Work in Progress, Internet-Draft, draft-
              ietf-ippm-ioam-data-17, 13 December 2021,

              Filsfils, C., Garvia, P. C., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", Work in Progress, Internet-
              Draft, draft-ietf-spring-srv6-network-programming-28, 29
              December 2020, <

              Song, H., Li, Z., Zhou, T., and L. Andersson, "Options for
              MPLS Extension Header Indicator", Work in Progress,
              Internet-Draft, draft-song-mpls-eh-indicator-04, 3 January
              2022, <

              Clad, F., Xu, X., Filsfils, C., Bernier, D., Decraene, B.,
              Yadlapalli, C., Henderickx, W., Salsano, S., and S. Ma,
              "Segment Routing for Service Chaining", Work in Progress,
              Internet-Draft, draft-xu-clad-spring-sr-service-chaining-
              00, 22 December 2017, <

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

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

Authors' Addresses

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


   Zhenbin Li
   P.R. China


   Tianran Zhou
   P.R. China


   Loa Andersson
   Bronze Dragon Consulting


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   Zhaohui Zhang
   Juniper Networks
   United States of America


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