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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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 Provisions Relating to IETF Documents
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 described in the Simplified BSD License.

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