Internet Engineering Task Force (IETF)                      L. Andersson
Request for Comments: 9017                      Bronze Dragon Consulting
Updates: 3032, 7274                                          K. Kompella
Category: Standards Track                               Juniper Networks
ISSN: 2070-1721                                                A. Farrel
                                                      Old Dog Consulting
                                                              April 2021


                   Special-Purpose Label Terminology

Abstract

   This document discusses and recommends terminology that may be used
   when MPLS Special-Purpose Labels (SPLs) are specified and documented.

   This document applies that terminology change to the relevant IANA
   registry and also clarifies the use of the Entropy Label Indicator
   (7) when immediately preceded by the Extension Label (15).

   This document updates RFCs 3032 and 7274.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9017.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  Background
     2.1.  GMPLS Special-Purpose Labels
   3.  Terminology and Abbreviations
   4.  Clarification on Handling of the Entropy Label Indicator
   5.  Security Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   RFC 7274 [RFC7274] made some changes to the terminology used for MPLS
   Special-Purpose Labels but did not define consistent terminology.

   One thing that RFC 7274 did was to deprecate the use of the term
   "reserved labels" when describing a range of labels allocated from a
   registry maintained by IANA.  The term "Reserved" in such a registry
   means "set aside, not to be used", but that range of labels was
   available for allocation according to the policies set out in that
   registry.  The name "Special-Purpose Labels" was introduced in RFC
   7274 in place of the previous term, and the abbreviation "SPL" was
   recommended.

   At the time of writing the first draft version of this document, the
   IETF was in the process of allocating the very first SPLs from the
   Extended SPL (eSPL) range [RFC8595].  This document discusses and
   recommends terminology and abbreviations to be used when talking
   about and documenting Special-Purpose Labels.

   This document updates RFC 3032 [RFC3032] and RFC 7274 [RFC7274] in
   that it changes the terminology for both Base SPLs (previously
   referred to simply as "Special-Purpose Labels") and Extended SPLs.

   This document applies that terminology change to the relevant IANA
   registry and also clarifies the use of the Entropy Label Indicator
   (7) when immediately preceded by the Extension Label (15).

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Background

   Two sets of SPLs are defined for use in MPLS:

   *  The range 0-15 (Base Special-Purpose Labels (bSPLs) as described
      in this document) is specified in RFC 3032 [RFC3032].

   *  The range 0-1048575 of eSPLs is specified in RFC 7274 [RFC7274].

      -  The values 0-15 have been reserved and are never to be
         allocated.

      -  The values 16-239 are available for allocation.

      -  The values 240-255 are for experimental use.

      -  The values 256-1048575 are currently not available for
         allocation.  A Standards Track RFC would be needed to change
         this rule, and that RFC would need to define the ranges that
         are made available for allocation and the registration policy
         for those ranges.

2.1.  GMPLS Special-Purpose Labels

   Note that IANA maintains a registry that is called "Special-Purpose
   Generalized Label Values".  Labels in that registry have special
   meaning when present in certain signaling objects, are 32 bits long,
   and are not to be confused with MPLS forwarding-plane labels.  This
   document does not make any changes to the GMPLS registry or to how
   labels from that registry are described.

3.  Terminology and Abbreviations

   Prior to the publication of this document, IANA maintained a name
   space for "Special-Purpose Multiprotocol Label Switching (MPLS) Label
   Values" code points [SPL-NAME-SPACE].  Within this name space, there
   are two registries.  One was called the "Special-Purpose MPLS Label
   Values" registry [bSPL].  The other was called the "Extended Special-
   Purpose MPLS Label Values" registry [eSPL].

   The difference in the name of the name space and the first registry
   is only that the MPLS abbreviation is expanded.  This document makes
   no change to the name of the name space itself (i.e., "Special-
   Purpose Multiprotocol Label Switching (MPLS) Label Values").  This
   document changes the name of the first registry to "Base Special-
   Purpose MPLS Label Values" but leaves the name of the latter registry
   unchanged as "Extended Special-Purpose MPLS Label Values".

   The following conventions will be used in specifications and when
   talking about SPLs.

   *  Collectively, the two (unrelated) ranges (0-15 and 16-1048575) are
      known as "Special-Purpose Labels" (SPLs).

   *  SPLs from the range 0-15 are called "Base Special-Purpose Labels"
      (bSPLs).

   *  SPLs from the range 16-1048575 are called "Extended Special-
      Purpose Labels" (eSPLs).  (Note that the reserved values 0-15 from
      the "Extended Special-Purpose MPLS Label Values" registry do not
      need a name, as they are not available for allocation and MUST NOT
      be used.)

   *  The combination of the Extension Label (XL) (value 15, which is a
      bSPL and is also called the "xSPL") and an eSPL is called a
      "Composite Special-Purpose Label" (cSPL).

   This results in label stacks such as the examples shown in Figures 1
   and 2.

                   0                                  31
                   |     MPLS Label Stack entry        |
                   +--------+--------+--------+--------+
                   |     MPLS Label Stack entry        |
                   +--------+--------+--------+--------+
         bSPL      |             Base SPL              |
                   +--------+--------+--------+--------+
                   |  MPLS Label Stack entry (cont.)   |

                      Figure 1: Example of Label Stack

                   0                                  31
                   |     MPLS Label Stack entry        |
                   +--------+--------+--------+--------+
                   |     MPLS Label Stack entry        |
                   +--------+--------+--------+--------+
         xSPL      |       Extension Label (XL)        | <--+
                   +--------+--------+--------+--------+    |--- cSPL
         eSPL      |           Extended SPL            | <--+
                   +--------+--------+--------+--------+
                   |  MPLS Label Stack entry (cont.)   |

                      Figure 2: Example of Label Stack

4.  Clarification on Handling of the Entropy Label Indicator

   Section 3.1 of [RFC7274] contains two paragraphs that describe the
   handling of the Entropy Label Indicator (label 7).  These paragraphs
   have introduced some confusion about whether the Entropy Label
   Indicator can be present when immediately preceded by the Extension
   Label.  This document updates [RFC7274] by replacing those paragraphs
   as follows.

   OLD

   |  Values 0-15 of the "Extended Special-Purpose MPLS Label Values"
   |  registry are set aside as reserved.  Furthermore, values 0-6 and
   |  8-15 MUST NOT appear in the data plane following an XL; an LSR
   |  processing a packet with an XL at the top of the label stack
   |  followed by a label with value 0-6 or 8-15 MUST drop the packet.
   |
   |  Label 7 (when received) retains its meaning as Entropy Label
   |  Indicator (ELI) whether a regular special-purpose label or an
   |  ESPL; this is because of backwards compatibility with existing
   |  implemented and deployed code and hardware that looks for the ELI
   |  without verifying if the previous label is XL or not.  However,
   |  when an LSR inserts an entropy label, it MUST insert the ELI as a
   |  regular special-purpose label, not as an ESPL.

   NEW

   |  Values 0-15 of the "Extended Special-Purpose MPLS Label Values"
   |  registry are set aside as reserved.  Furthermore, an
   |  implementation MUST NOT place a label with a value in the range
   |  0-15 in the label stack immediately following an XL; an LSR
   |  processing a packet with an XL at the top of the label stack
   |  immediately followed by a label with a value in the range 0-15
   |  MUST drop the packet.
   |
   |  When inspecting a label stack to find an Entropy Label Indicator
   |  (ELI -- label 7), a preexisting implementation may fail to inspect
   |  the previous label and thus not notice that it is an XL.  Such
   |  systems can continue to process the entropy information and
   |  forward the packet when the previous label is an XL without
   |  causing harm.  However, the packet will be dropped when the XL
   |  reaches the top of the stack at another LSR.

   END

5.  Security Considerations

   This document describes the terminology to be used when describing
   and specifying the use of SPLs.  It does not affect forwarding in the
   MPLS data plane, nor does it have any effect on how Label Switched
   Paths are established by an MPLS control plane or by a centralized
   controller.

   This document does not aim to describe existing implementations of
   SPLs or potential vulnerabilities of SPLs.

6.  IANA Considerations

   IANA has changed the name of the registry once called "Special-
   Purpose MPLS Label Values" to now be called "Base Special-Purpose
   MPLS Label Values" [bSPL].

   IANA has also updated the "Base Special-Purpose MPLS Label Values"
   registry by changing the description for value 15 from "Extension
   Label" to "Extension Label (XL)" and also adding this document as a
   reference.

           +=======+======================+====================+
           | Value |     Description      |     Reference      |
           +=======+======================+====================+
           | 15    | Extension Label (XL) | RFC 7274, RFC 9017 |
           +-------+----------------------+--------------------+

              Table 1: Updated Entry for Value 15 in the "Base
                Special-Purpose MPLS Label Values" Registry

7.  References

7.1.  Normative References

   [bSPL]     IANA, "Base Special-Purpose MPLS Label Values",
              <https://www.iana.org/assignments/mpls-label-values/>.

   [eSPL]     IANA, "Extended Special-Purpose MPLS Label Values",
              <https://www.iana.org/assignments/mpls-label-values/>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC7274]  Kompella, K., Andersson, L., and A. Farrel, "Allocating
              and Retiring Special-Purpose MPLS Labels", RFC 7274,
              DOI 10.17487/RFC7274, June 2014,
              <https://www.rfc-editor.org/info/rfc7274>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [SPL-NAME-SPACE]
              IANA, "Special-Purpose Multiprotocol Label Switching
              (MPLS) Label Values",
              <https://www.iana.org/assignments/mpls-label-values/>.

7.2.  Informative References

   [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,
              <https://www.rfc-editor.org/info/rfc8595>.

Acknowledgements

   We would like to thank the Routing Directorate reviewer, Eric Gray,
   for a detailed, careful, and insightful review, and Tom Petch for
   pointing out several issues of clarity.

Contributors

   The following individual contributed text to this document:

   Stewart Bryant
   Futurewei Technologies Inc.

   Email: stewart.bryant@gmail.com


Authors' Addresses

   Loa Andersson
   Bronze Dragon Consulting

   Email: loa@pi.nu


   Kireeti Kompella
   Juniper Networks

   Email: kireeti@juniper.net


   Adrian Farrel
   Old Dog Consulting

   Email: adrian@olddog.co.uk