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Versions: (draft-andersson-mpls-spl-terminology) 00 01 02 03 04 05 06

MPLS Working Group                                          L. Andersson
Internet-Draft                                  Bronze Dragon Consulting
Updates: 3032, 7274 (if approved)                            K. Kompella
Intended status: Standards Track                        Juniper Networks
Expires: March 28, 2021                                        A. Farrel
                                                      Old Dog Consulting
                                                      September 24, 2020


                   Special Purpose Label terminology
                   draft-ietf-mpls-spl-terminology-04

Abstract

   This document discusses and recommends a terminology that may be used
   when MPLS Special Purpose Labels (SPL) 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 RFC 7274 and RFC 3032.

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 https://datatracker.ietf.org/drafts/current/.

   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 March 28, 2021.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of



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   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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  GMPLS Special Purpose Labels  . . . . . . . . . . . . . .   3
   3.  Terminology and Abbreviations . . . . . . . . . . . . . . . .   3
   4.  Clarification on Use of Entropy Label Indicator . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

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 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 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 and Extended SPLs.





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   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 of 0-15, Base Special Purpose Labels (bSPLs), 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 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 standard track RFC will be needed to allocate
         any labels from this range.

2.1.  GMPLS Special Purpose Labels

   Note that IANA maintains a registry called "Special Purpose
   Generalized Label Values".  Labels in that registry have special
   meaning when present in certain signalling 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

   IANA maintains 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 is called the
   'Special-Purpose MPLS Label Values' registry [bSPL].  The other is
   called 'Extended Special-Purpose MPLS Label Values' registry [eSPL].




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   The difference in the name of the name space and the first registry
   is only that the MPLS abbreviation is expanded.  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.

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

   o  Special purpose labels from the range 0-15 are called Base Special
      Purpose Labels (bSPL).

   o  Special purpose labels from the range 16-1048575 are called
      Extended Special Purpose Labels (eSPL).  (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.)

   This results in a label stacks such as the illustrative examples
   shown in Figure 1 and Figure 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















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             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 Use of Entropy Label Indicator

   Section 3.1 of [RFC7274] contains two paragraphs that describe the
   use 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 value 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 value 0-15 MUST drop the packet.




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      When inspecting a label stack to find an Entropy Label Indicator
      (ELI - label 7) a pre-existing implementation may fail to inspect
      the previous label, and so 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 XP 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

   The document describes the terminology to be used when describing and
   specifying the use of SPLs.  It does not effect the forwarding in the
   MPLS data plane, nor does it have any effect on how LSPs 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 is requested to change the name of the registry that today is
   called "Special-Purpose MPLS Label Values" is changed to "Base
   Special- Purpose MPLS Label Values".

7.  Acknowledgements

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

8.  Contributors

   The following people contributed text to this document:


      Stewart Bryant
      Futurewei Technologies Inc.

      Email: stewart.bryant@gmail.com


                                 Figure 3







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9.  References

9.1.  Normative References

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

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

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

9.2.  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>.

Authors' Addresses







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   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



































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