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Requirements for MPLS Label Stack Indicators and Ancillary Data

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This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Authors Matthew Bocci , Stewart Bryant
Last updated 2022-01-24
Replaced by draft-ietf-mpls-mna-requirements
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MPLS Working Group                                              M. Bocci
Internet-Draft                                                     Nokia
Intended status: Informational                                 S. Bryant
Expires: July 28, 2022                         University of Surrey 5GIC
                                                        January 24, 2022

    Requirements for MPLS Label Stack Indicators and Ancillary Data


   This draft specifies requirements for indicators in the MPLS label
   stack to support ancillary data that exists below the label stack.
   This work is the product of the IETF MPLS Open Design Team.
   Requirements are derived from a number of new proposals for additions
   to the MPLS label stack to allow forwarding or other processing
   decisions to be made, either by a transit or terminating LSR, based
   on application data that may be in or below the bottom of the label

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
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   This Internet-Draft will expire on July 28, 2022.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Background  . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  MPLS Ancillary Data Indicator Requirements  . . . . . . . . .   4
     3.1.  General Requirements  . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   There is significant interest in developing the MPLS data plane to
   address the requirements of new applications.  These applications
   typically require the inclusion of ancillary data in the MPLS packet.
   This data may be encoded either in the label stack or below the
   bottom of the label stack.  This data is then either intercepted and
   processed, or some other forwarding decision is taken by routers
   processing the packet.  The ancillary data is added by the ingress
   LSR, and then makes use of mechanisms implemented by an intermediate
   or egress LSR that complies with the MPLS base architecture and
   potentially its extensions, including (but not limited to) [RFC3031],
   [RFC3032], [RFC6790].

   This draft specifies requirements for indicators in the MPLS label
   stack to support these applications.

1.1.  Terminology

   o  Ancillary Data: Data relating to the MPLS packet that may be used
      to affect the forwarding or other processing of that packet,
      either at the LER or LSR.  This data may be implicit (i.e.
      context-specific), encoded within the label stack (in-stack data),
      and/or after the bottom of the label stack (post-stack data).

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   o  In-Stack Data: Any data within the MPLS label stack including the
      outer label and the bottom of stack (the label with the S-bit

   o  Post-Stack Data: Any data beyond the LSE with the S-Bit set, but
      before the first octet of the user payload.  This document does
      not prescribe whether post-stack data precedes or follows any
      other protocol structure such as a control word or associated
      channel header (ACH).

   o  Ancillary Data Indicator (ADI): A indicator in the MPLS label
      stack that ancillary data exists in this packet.  It MAY also
      indicate the specific type of the ancillary data.

1.2.  Background

   The MPLS architecture is specified in [RFC3031] and provides a
   mechanism for forwarding packets through a network without requiring
   any analysis of the 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 packet is assigned to a forwarding equivalence class

   MPLS uses switching based on a label pushed on the packet to achieve
   efficient forwarding and traffic engineering of flows associated with
   the FEC.  While originally used for IP traffic, MPLS has been
   extended to support point-to-point, point-to-multipoint and
   multipoint-to-multipoint layer 2 and layer 3 services.  An overview
   of the development of MPLS is provided in

   A number of applications have emerged which require LSRs to make
   forwarding or other processing decisions based on inspection of the
   network layer header, or some other ancillary information in the
   protocol stack encapsulated deeper in the packet.  An early example
   of this was generation of a hash of the payload header to be used for
   load balancing over Equal Cost Multipath (ECMP) or Link Aggregation
   Group (LAG) next hops.  This is based on an assumption that the
   network layer protocol is IP.  MPLS was extended to avoid the need
   for LSRs to perform this operation if load balancing was needed based
   on the payload and instead use only the MPLS label stack, using the
   Entropy Label / Entropy Label Indicator [RFC6790] which are inserted
   at the LER.  Other applications where the intermediate LSRs may need
   to inspect and process a packet on an LSP include OAM, which can make
   use of mechanisms such the Router Alert Label [RFC3032] or the
   Generic Associated Channel Label (GAL) [RFC5586] to indicate that an

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   intercepted packet should be processed locally.  See
   [I-D.bryant-mpls-dev-primer] for detailed list of such applications.

   There have been a number of new proposals for how ancillary data is
   carried in MPLS and how its presence is indicated to the LSR or
   egress LER, for example In-situ OAM and Service Function Chaining
   (SFC).  A summary of these proposals is contained in
   [I-D.bryant-mpls-dev-primer], an overview of use cases is provided in
   [Reference to MIAD use cases].  []
   summarises some of the issues with existing solutions to address
   these new applications (note that this document draws on the
   requirements and issues without endorsing a specific solution from

   These 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.

   A piecemeal solution often 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 assumption.  An example of this
   is that the GAL/G-ACH mechanism assumes that if the GAL is
   present, only a single G-ACH header follows.

   New applications therefore require the definition of extensions to
   the MPLS architecture and label stack operations that can be used
   across these applications in order to minimise implementation
   complexity and promote interoperability.

2.  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.

3.  MPLS Ancillary Data Indicator Requirements

   This document specifies requirements of MPLS Indicators for Ancillary
   Data (MIAD) and the Ancillary Data itself.  The requirements are for
   the behavior of the protocol mechanisms and procedures that
   constitute building blocks out of which indicators for ancillary data
   are constructed.  It does not specify the detailed processing that
   may be required by an application of that ancillary data by an LSR or

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   LER.  The requirements in this document do not describe what
   functions an implementation must support.  The purpose of this
   document is to identify the toolkit and any new protocol work that is
   required.  This new protocol work MUST be based on the existing MPLS

3.1.  General Requirements

   1.   MPLS combines extensibility, flexibility and efficiency by using
        control plane context combined with a simple data plane
        mechanism to allow the network to make forwarding decisions
        about a packet.  Any solution MUST maintain these properties of

   2.   Any solutions to these requirements MUST NOT restrict the
        generality of MPLS architecture [RFC3031], [RFC3032].

   3.   Any solution MUST respect the principle that Special Purpose
        Labels are the mechanism of last resort and therefore must
        minimise the number of new SPLs that are allocated.

   4.   Solutions MUST be able to coexist with and MUST NOT obsolete
        existing MPLS mechanisms.

   5.   An ADI or ancillary data MUST NOT be delivered to a node that is
        not capable of processing it.  That is:

        *  An ADI MUST NOT become top of stack at a node that does not
           understand the ADI and is thus is not able to perform a
           disposition operation on it.  Disposition includes both
           processing and ignoring.

        *  When the label stack is popped there MUST NOT exist ancillary
           data that the node is not capable of performing a disposition
           operation on.  Disposition includes both processing and

   6.   Care MUST be taken in the coexistence of ancillary data and
        existing post-stack data mechanisms.

   7.   Mechanisms are needed to determine that all nodes that need to
        process the ancillary data can read the required distance into
        the packet at that node.

   8.   When ancillary data is present in the MPLS label stack, a
        mechanism is REQUIRED to indicate its presence.

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   9.   When ancillary data is present below the MPLS label stack, a
        mechanism is REQUIRED to indicate its presence.

   10.  Ancillary data MAY be associated with control or maintenance
        information for traffic carried by an LSP, and/or it MAY be
        associated with the user traffic itself.

   11.  Ancillary Data Indicators (ADIs) SHOULD make use of existing
        MPLS data plane operations.  If extensions to the MPLS data
        plane are required, they MUST NOT be inconsistent with the MPLS
        architecture [RFC3031], [RFC3032].

   12.  The ADI design MUST support end-to-end (E2E) processing of
        ancillary data.

   13.  The ADI design MUST support hop-by-hop (HBH) processing of
        ancillary data.

   14.  For HBH ancillary data, a mechanism is REQUIRED to enable an LER
        inserting ADIs to determine if the ADI will be processed by LSRs
        along the path.

   15.  For HBH ancillary data, a mechanism is REQUIRED to enable an LER
        inserting ADIs to determine whether LSRs along the path can
        parse the label stack and process the ADI at the depth in the
        label stack where it is inserted.

   16.  If both HBH and E2E ancillary data indicators are present
        together, the precedence must be specified in the design.

   17.  A mechanism is REQUIRED to enable an LER inserting ADIs to
        determine if the far-end LER can accept and process a packet
        containing a given ADI.

   18.  ADIs SHOULD be supported for both P2P and P2MP paths, but any
        specific ADI may only be supported for one or the other.

   19.  Data plane mechanisms for ADIs MUST be independent of the
        control plane type (LDP, RSVP, BGP, static, IGP, etc).

   20.  A mechanism MUST be defined for control planes in use (e.g.
        LDP, RSVP, BGP, static, IGP, etc) to determine the ability of
        downstream LSRs/LERs to accept/process a given ADI.

   21.  A mechanism is REQUIRED to enable an LSR to determine if an ADI
        is present in a packet.

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   22.  The design of this mechanism SHOULD be such that an LSR is able
        to efficiently parse the label stack and MUST NOT add more
        labels to the stack than is absolutely necessary.

   23.  ADIs can only be inserted at LERs, but MAY be processed at LSRs
        and LERs.  If it is required to insert an ADI at a transit
        router on an LSP, then a new label stack MUST be pushed.

   24.  It SHOULD be possible to include indicators for ancillary data
        for multiple applications in the same packet, but each ADI MAY
        only support one application.

   25.  It MUST be possible to insert new ADIs for new applications on
        an established LSP that may have existing ADIs present.

   26.  The solution MUST allow ADI and non-ADI packets to coexist on
        the same LSP.

   27.  The solution MUST support the processing of a subset of the ADIs
        on a packet.

   28.  The solution MUST support slow path processing of ancillary

   29.  The solution MUST support fast path processing of ancillary

   30.  Application specifications MUST specify if the ancillary data
        needs to be processed in the fast path or if it can be processed
        in the slow path.

        (Ed. note: The fast path and slow path terminology may need to
        be reconsidered, and we should perhaps think in terms of
        synchronous and asynchronous operations )

   31.  In order to prevent unnecessary scanning of the packet, care
        needs to be taken in the location of the ancillary data, for
        example it SHOULD be located as close to the label stack as

   32.  A solution MUST be provided to verify the authenticity of
        ancillary data processed to LSRs [RFC3552].

   33.  The design of the ADIs and ancillary data MUST NOT expose
        confidential information [RFC6973] [RFC3552] to the LSRs.

   34.  Any solution that modifies the ADI SHOULD NOT affect ECMP

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4.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

5.  Security Considerations

   The mechanisms required by this document introduce new security
   considerations to MPLS.  Individual solution specifications meeting
   these requirements MUST address any security considerations.

6.  Acknowledgements

   The authors gratefully acknowledge the contributions from Yingzhen
   Qu, Haoyu Song, Tarek Saad, Loa Andersson, and Bruno Decraene.

   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,

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

7.2.  Informative References

              Bryant, S., "A Primer on the Development of MPLS", draft-
              bryant-mpls-dev-primer-01 (work in progress), December

              Song, H., Li, Z., Zhou, T., Andersson, L., and Z. Zhang,
              "MPLS Extension Header", draft-song-mpls-extension-
              header-06 (work in progress), January 2022.

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

   [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,

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,

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

   [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,

   [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,

Authors' Addresses

   Matthew Bocci


   Stewart Bryant
   University of Surrey 5GIC


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