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<rfc category="std" xmlns:xi="http://www.w3.org/2001/XInclude" docName="draft-ietf-mpls-spl-terminology-06" number="9017" ipr="trust200902" updates="3032, 7274"> 7274" obsoletes="" submissionType="IETF" category="std" consensus="true" xml:lang="en" tocInclude="true" tocDepth="3" symRefs="true" sortRefs="true" version="3">

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  <front>
    <title abbrev="SPL Terminology">Special Purpose Terminology">Special-Purpose Label terminology</title> Terminology</title>
    <seriesInfo name="RFC" value="9017"/>
    <author fullname="Loa Andersson" initials="L." surname="Andersson">
      <organization>Bronze Dragon Consulting</organization>
      <address>
        <email>loa@pi.nu</email>
      </address>
    </author>
    <author fullname="Kireeti Kompella" initials="K." surname="Kompella">
      <organization>Juniper Networks</organization>
      <address>
        <email>kireeti@juniper.net</email>
      </address>
    </author>
    <author fullname="Adrian Farrel" initials="A." surname="Farrel">
      <organization>Old Dog Consulting</organization>
      <address>
        <email>adrian@olddog.co.uk</email>
      </address>
    </author>

    <date/>

     <workgroup>MPLS Working Group</workgroup>
    <date year="2021" month="April"/>

<keyword>MPLS</keyword>
<keyword>Extended Special-Purpose Label</keyword>
<keyword>Base Special-Purpose Label</keyword>
<keyword>Reserved Label</keyword>
<keyword>Entropy Label Indicator</keyword>

    <abstract>
      <t>This document discusses and recommends a terminology that may be used when
      MPLS Special Purpose Special-Purpose Labels (SPL) (SPLs) are specified and documented.</t>

      <t>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).</t> (15).
</t>
      <t>This document updates RFC 7274 RFCs 3032 and RFC 3032.</t> 7274.</t>
    </abstract>
  </front>
  <middle>
    <section anchor="intro" title="Introduction"> numbered="true" toc="default">
      <name>Introduction</name>
      <t>RFC 7274 <xref target="RFC7274"/> target="RFC7274" format="default"/> made some changes to the terminology
         used for MPLS Special Purpose Labels, Special-Purpose Labels but did not define consistent
         terminology. </t>
      <t>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 "Special-Purpose Labels" was introduced in RFC
         7274 in place of the previous term, and the abbreviation SPL "SPL" was
         recommended.</t>
      <t>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 <xref target="RFC8595"/>. target="RFC8595" format="default"/>.  This document discusses and recommends
         terminology and abbreviations to be used when talking about and
         documenting Special Purpose Special-Purpose Labels.</t>
      <t>This document updates RFC 3032  <xref target="RFC3032"/> target="RFC3032" format="default"/> and RFC 7274
         <xref target="RFC7274"/> target="RFC7274" format="default"/> in that it changes the terminology for both
         Base SPLs (previously referred to simply as "Special-Purpose Labels") and Extended SPLs.</t>
      <t>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).</t>
      <section anchor="terms" title="Terminology"> numbered="true" toc="default">
        <name>Terminology</name>
       <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
            "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
            "MAY", "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
       "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>",
       "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>",
       "<bcp14>SHOULD NOT</bcp14>",
       "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
       "<bcp14>MAY</bcp14>", and "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document
       are to be interpreted as described in BCP 14 BCP&nbsp;14
       <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only
       when, they appear in all capitals, as shown here.</t>
      </section>
    </section>
    <section anchor="bckground" title="Background"> numbered="true" toc="default">
      <name>Background</name>
      <t>Two sets of SPLs are defined for use in MPLS:
         <list style="symbol">
           <t>The
      </t>
      <ul spacing="normal">
        <li>The range of 0-15, Base Special Purpose 0-15 (Base Special-Purpose Labels (bSPLs), (bSPLs) as described
   in this document) is specified in RFC 3032
              <xref target="RFC3032"/>.</t> target="RFC3032" format="default"/>.</li>
        <li>
          <t>The range 0-1048575 of eSPLs is specified in RFC 7274 <xref target="RFC7274"/>.
              <list style="symbols">
                <t>the target="RFC7274" format="default"/>.
          </t>
          <ul spacing="normal">
            <li>The values 0-15 have been reserved and are never to be allocated</t>
                <t>the allocated.</li>
            <li>The values 16-239 are available for allocation</t>
                <t>the allocation.</li>
            <li>The values 240-255 are for experimental use</t>
                <t>the use.</li>
            <li>The values 256-1048575 are currently not available for
     allocation. A standard track Standards Track RFC will would be needed to
                   allocate any labels from change
     this range.</t>
              </list>
           </t>
         </list>
      </t> rule, and that RFC would need to define the ranges that are
     made available for allocation and the registration policy for those ranges.</li>
          </ul>
        </li>
      </ul>
      <section title="GMPLS Special Purpose Labels" anchor="GMPLS"> anchor="GMPLS" numbered="true" toc="default">
        <name>GMPLS Special-Purpose Labels</name>
        <t>Note that IANA maintains a registry that is called "Special Purpose "Special-Purpose Generalized Label Values".
  Labels in that registry have special meaning when present in certain
            signalling
            signaling objects, are 32 bits long, and are not to be confused with MPLS
            forwarding plane
            forwarding-plane labels.  This document does not make any changes to the GMPLS
            registry or to how labels from that registry are described.</t> described.

</t>
      </section>
    </section>
    <section anchor="terminology" title="Terminology numbered="true" toc="default">
      <name>Terminology and Abbreviations">

      <t>IANA maintains Abbreviations</name>
     <t>
  Prior to the publication of this document, IANA maintained a name space
  for 'Special-Purpose "Special-Purpose Multiprotocol Label Switching (MPLS) Label Values' Values"
  code points <xref target="SPL-NAME-SPACE"/>. target="SPL-NAME-SPACE" format="default"/>.  Within this name space space, there are two
  registries.  One is was called the 'Special-Purpose "Special-Purpose MPLS Label Values' Values"
  registry <xref target="bSPL"/>. target="bSPL" format="default"/>.  The other is was called
         'Extended the "Extended Special-Purpose
  MPLS Label Values' Values" registry <xref target="eSPL"/>.</t>

      <t>The target="eSPL" format="default"/>.
     </t>

     <t>
  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 "Base Special-Purpose MPLS
  Label Values', Values" but leaves the name of the latter registry unchanged
  as 'Extended "Extended Special-Purpose MPLS Label Values'.</t> Values".
     </t>

      <t>The following conventions will be used in specifications and when talking
         about SPLs.
         <list style="symbols">
            <t>Collectively,
      </t>
      <ul spacing="normal">
        <li>Collectively, the two (unrelated) ranges (0-15 and 16-1048575) are
               known as Special Purpose Labels (SPL).</t>
            <t>Special purpose labels "Special-Purpose Labels" (SPLs).</li>
        <li>SPLs from the range 0-15 are called Base Special
               Purpose Labels (bSPL).</t>
            <t>Special purpose labels "Base Special-Purpose Labels" (bSPLs).</li>
        <li>SPLs from the range 16-1048575 are called Extended
               Special Purpose Labels (eSPL). "Extended
               Special-Purpose Labels" (eSPLs).  (Note that the reserved values 0-15 from the
               'Extended
               "Extended Special-Purpose MPLS Label Values' Values" registry do not need a
               name
        name, as they are not available for allocation and MUST NOT <bcp14>MUST NOT</bcp14> be used.)</t>
            <t>The used.)</li>
        <li>The combination of the Extension Label (XL) (value 15 15, which is a
               bSPL,
               bSPL and which is also called the xSPL) "xSPL") and an eSPL is called a
               Composite Special Purpose Label (cSPL).</t>
         </list>
      </t>
               "Composite Special-Purpose Label" (cSPL).</li>
      </ul>
      <t>This results in a label stacks such as the illustrative examples shown
         in <xref target="stack-example-1"/> Figures&nbsp;<xref target="stack-example-1" format="counter"/> and <xref target="stack-example-2"/>.</t> target="stack-example-2" format="counter"/>.</t>
      <figure title="Example anchor="stack-example-1">
        <name>Example of Label Stack" align="left" anchor="stack-example-1"> Stack</name>
        <artwork align="left">
            <![CDATA[ align="left" name="" type="" alt=""><![CDATA[
                0                                  31
                |     MPLS Label Stack entry        |
                +--------+--------+--------+--------+
                |     MPLS Label Stack entry        |
                +--------+--------+--------+--------+
      bSPL      |             Base SPL              |
                +--------+--------+--------+--------+
                |  MPLS Label Stack entry (cont.)   |
            ]]>
         </artwork>   |]]></artwork>
      </figure>
      <figure title="Example anchor="stack-example-2">
        <name>Example of Label Stack" align="left" anchor="stack-example-2"> Stack</name>
        <artwork align="left">
            <![CDATA[ align="left" name="" type="" alt=""><![CDATA[
                0                                  31
                |     MPLS Label Stack entry        |
                +--------+--------+--------+--------+
                |     MPLS Label Stack entry        |
                +--------+--------+--------+--------+
      xSPL      |       Extension Label (XL)        | <--+
                +--------+--------+--------+--------+    |--- cSPL
      eSPL      |           Extended SPL            | <--+
                +--------+--------+--------+--------+
                |  MPLS Label Stack entry (cont.)   |
            ]]>
         </artwork>   |]]></artwork>
      </figure>
    </section>
    <section anchor="clarify" title="Clarification numbered="true" toc="default">
      <name>Clarification on Handling of the Entropy Label Indicator">

      <t>Section 3.1 of [RFC7274] Indicator</name>
      <t><xref target="RFC7274" sectionFormat="of" section="3.1"/>
 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]
         <xref target="RFC7274"/> by replacing those paragraphs as follows.</t>
      <t>OLD

         <list style="none">

            <t>Values
      </t>
<blockquote>
     <t>
        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
               <bcp14>MUST NOT</bcp14> 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 <bcp14>MUST</bcp14> drop the packet.</t> packet.
     </t>
        <t>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 <bcp14>MUST</bcp14> insert the ELI as a regular
               special-purpose label, not as an ESPL.</t>
         </list></t>
</blockquote>
      <t>NEW

         <list style="none">
      </t>
<blockquote>
        <t>Values 0-15 of the "Extended Special-Purpose MPLS Label Values"
               registry are set aside as reserved.  Furthermore, an implementation
               MUST NOT
               <bcp14>MUST NOT</bcp14> 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 <bcp14>MUST</bcp14> drop the packet.</t>
        <t>When inspecting a label stack to find an Entropy Label Indicator
               (ELI - -- label 7) 7), a pre-existing preexisting implementation may fail to inspect the
               previous label, label and so 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.</t>

         </list></t>
</blockquote>
      <t>END</t>
    </section>
    <section anchor="security" title="Security Considerations">

      <t>The numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>This document describes the terminology to be used when describing and
         specifying the use of SPLs. It does not effect the affect forwarding in the MPLS
         data plane, nor does it have any effect on how LSPs Label Switched Paths are established by an
         MPLS control plane or by a centralized controller.</t>
      <t>This document does not aim to describe existing implementations of SPLs or
         potential vulnerabilities of SPLs.</t>
    </section>
    <section anchor="IANA" title="IANA Considerations"> numbered="true" toc="default">
      <name>IANA Considerations</name>

      <t>IANA is requested to change has changed the name of the registry that today is once called
         "Special-Purpose MPLS Label Values" is changed to now be called "Base Special-
         Purpose Special-Purpose MPLS Label Values". Values" <xref target="bSPL" format="default"/>. </t>

   </section>

   <section anchor="Acknowledgements" title="Acknowledgements">

      <t>We like to thank

      <t>IANA has also updated the Routing Directorate reviwer Eric Gray
         for a detailed, careful and insightful review, and Tom Petch "Base Special-Purpose MPLS Label Values"
      registry by changing the description for
         pointing out several issues of clarity.</t>

   </section>

   <section anchor="Contributors" title="Contributors">

      <t>The following people contributed text value 15 from "Extension Label" to
      "Extension Label (XL)" and also adding this document:</t>
         <figure>
           <artwork align="left">
              <![CDATA[
     Stewart Bryant
     Futurewei Technologies Inc.

     Email: stewart.bryant@gmail.com
              ]]>
           </artwork>
         </figure> document as a reference.
      </t>

<table anchor="iana-reg-update-table-1">
  <name>Updated Entry for Value 15 in the "Base Special-Purpose MPLS Label Values" Registry</name>
  <thead>
    <tr>
      <th align="center">Value</th>
      <th align="center">Description</th>
      <th align="center">Reference</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>15</td>
      <td>Extension Label (XL)</td>
      <td>RFC 7274, RFC 9017</td>
    </tr>
  </tbody>
</table>

    </section>
  </middle>
  <back>

    <references title="Normative References">
      <?rfc include="reference.RFC.2119"?>
      <?rfc include="reference.RFC.3032"?>
      <?rfc include="reference.RFC.7274"?>
      <?rfc include="reference.RFC.8174"?>
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3032.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7274.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>

        <reference anchor="SPL-NAME-SPACE" target="https://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtml/"> target="https://www.iana.org/assignments/mpls-label-values/">
          <front>
            <title>Special-Purpose Multiprotocol Label Switching (MPLS) Label Values</title>
           <author/>
            <author><organization>IANA</organization></author>
            <date/>
          </front>
        </reference>

        <reference anchor="bSPL" target="https://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtml#special-purpose/"> target="https://www.iana.org/assignments/mpls-label-values/">
          <front>
            <title>Special-Purpose
            <title>Base Special-Purpose MPLS Label Values</title>
            <author/>
            <author><organization>IANA</organization></author>
            <date/>
          </front>
        </reference>

        <reference anchor="eSPL" target="https://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtml#extended/"> target="https://www.iana.org/assignments/mpls-label-values/">
          <front>
            <title>Extended Special-Purpose MPLS Label Values</title>
            <author/>
            <author><organization>IANA</organization></author>
            <date/>
          </front>
        </reference>
      </references>

    <references title="Informative References">
      <?rfc include="reference.RFC.8595"?>
      <references>
        <name>Informative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8595.xml"/>
      </references>
    </references>
    <section anchor="Acknowledgements" numbered="false" toc="default">
      <name>Acknowledgements</name>
      <t>We would like to thank the Routing Directorate reviewer, <contact fullname="Eric Gray"/>,
         for a detailed, careful, and insightful review, and <contact fullname="Tom Petch"/> for
         pointing out several issues of clarity.</t>
    </section>
    <section anchor="Contributors" numbered="false" toc="default">
      <name>Contributors</name>
      <t>The following individual contributed text to this document:</t>
        <contact fullname="Stewart Bryant">
        <organization>Futurewei Technologies Inc.</organization>
        <address>
          <postal>
            <street></street>
            <city></city>
            <region></region>
            <country></country>
          </postal>
          <email>stewart.bryant@gmail.com</email>
        </address>
       </contact>
    </section>
  </back>
</rfc>