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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" version="3" category="std" consensus="true" docName="draft-ietf-lsr-flex-algo-26" indexInclude="true" ipr="trust200902" number="9350" prepTime="2023-02-22T10:15:15" scripts="Common,Latin" sortRefs="true" submissionType="IETF" symRefs="true" tocDepth="3" tocInclude="true" xml:lang="en">
  <link href="https://datatracker.ietf.org/doc/draft-ietf-lsr-flex-algo-26" rel="prev"/>
  <link href="https://dx.doi.org/10.17487/rfc9350" rel="alternate"/>
  <link href="urn:issn:2070-1721" rel="alternate"/>
  <front>
    <title abbrev="IGP Flexible Algorithm">IGP Flexible Algorithm</title>
    <seriesInfo name="RFC" value="9350" stream="IETF"/>
    <author fullname="Peter Psenak" initials="P." role="editor" surname="Psenak">
      <organization showOnFrontPage="true">Cisco Systems, Inc.</organization>
      <address>
        <postal>
          <extaddr>Apollo Business Center</extaddr>
          <street>Mlynske nivy 43</street>
          <city>Bratislava</city>
          <code>82109</code>
          <country>Slovakia</country>
          <code/>
        </postal>
        <email>ppsenak@cisco.com</email>
      </address>
    </author>
    <author fullname="Shraddha Hegde" initials="S" surname="Hegde">
      <organization showOnFrontPage="true">Juniper Networks, Inc.</organization>
      <address>
        <postal>
          <extaddr>Embassy Business Park</extaddr>
          <street/>
          <city>Bangalore</city>
          <region>KA</region>
          <code>560093</code>
          <country>India</country>
          <code/>
        </postal>
        <email>shraddha@juniper.net</email>
      </address>
    </author>
    <author fullname="Clarence Filsfils" initials="C." surname="Filsfils">
      <organization showOnFrontPage="true">Cisco Systems, Inc.</organization>
      <address>
        <postal>
          <street/>
          <city>Brussels</city>
          <region/>
          <code/>
          <country>Belgium</country>
        </postal>
        <email>cfilsfil@cisco.com</email>
      </address>
    </author>
    <author fullname="Ketan Talaulikar" initials="K." surname="Talaulikar">
      <organization showOnFrontPage="true">Cisco Systems, Inc</organization>
      <address>
        <postal>
          <street/>
          <city/>
          <code/>
          <country>India</country>
        </postal>
        <email>ketant.ietf@gmail.com</email>
      </address>
    </author>
    <author fullname="Arkadiy Gulko" initials="A." surname="Gulko">
      <organization showOnFrontPage="true">Edward Jones</organization>
      <address>
        <postal>
          <street/>
          <city/>
          <region/>
          <code/>
          <country/>
        </postal>
        <email>arkadiy.gulko@edwardjones.com</email>
      </address>
    </author>
    <date month="02" year="2023"/>
    <area>rtg</area>
    <workgroup>lsr</workgroup>
    <abstract pn="section-abstract">
      <t indent="0" pn="section-abstract-1">IGP protocols historically compute the best paths over the network based
      on the IGP metric assigned to the links. Many network deployments use
      RSVP-TE or Segment Routing - Traffic Engineering (SR-TE) to steer
      traffic over a path that is computed using different metrics or
      constraints than the shortest IGP path. This document specifies a
      solution that allows IGPs themselves to compute constraint-based paths
      over the network. This document also specifies a way of using Segment
      Routing (SR) Prefix-SIDs and SRv6 locators to steer packets along the
      constraint-based paths.</t>
    </abstract>
    <boilerplate>
      <section anchor="status-of-memo" numbered="false" removeInRFC="false" toc="exclude" pn="section-boilerplate.1">
        <name slugifiedName="name-status-of-this-memo">Status of This Memo</name>
        <t indent="0" pn="section-boilerplate.1-1">
            This is an Internet Standards Track document.
        </t>
        <t indent="0" pn="section-boilerplate.1-2">
            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.
        </t>
        <t indent="0" pn="section-boilerplate.1-3">
            Information about the current status of this document, any
            errata, and how to provide feedback on it may be obtained at
            <eref target="https://www.rfc-editor.org/info/rfc9350" brackets="none"/>.
        </t>
      </section>
      <section anchor="copyright" numbered="false" removeInRFC="false" toc="exclude" pn="section-boilerplate.2">
        <name slugifiedName="name-copyright-notice">Copyright Notice</name>
        <t indent="0" pn="section-boilerplate.2-1">
            Copyright (c) 2023 IETF Trust and the persons identified as the
            document authors. All rights reserved.
        </t>
        <t indent="0" pn="section-boilerplate.2-2">
            This document is subject to BCP 78 and the IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<eref target="https://trustee.ietf.org/license-info" brackets="none"/>) in effect on the date of
            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 Revised BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Revised BSD License.
        </t>
      </section>
    </boilerplate>
    <toc>
      <section anchor="toc" numbered="false" removeInRFC="false" toc="exclude" pn="section-toc.1">
        <name slugifiedName="name-table-of-contents">Table of Contents</name>
        <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1">
          <li pn="section-toc.1-1.1">
            <t indent="0" keepWithNext="true" pn="section-toc.1-1.1.1"><xref derivedContent="1" format="counter" sectionFormat="of" target="section-1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-introduction">Introduction</xref></t>
          </li>
          <li pn="section-toc.1-1.2">
            <t indent="0" keepWithNext="true" pn="section-toc.1-1.2.1"><xref derivedContent="2" format="counter" sectionFormat="of" target="section-2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-requirements-language">Requirements Language</xref></t>
          </li>
          <li pn="section-toc.1-1.3">
            <t indent="0" keepWithNext="true" pn="section-toc.1-1.3.1"><xref derivedContent="3" format="counter" sectionFormat="of" target="section-3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-terminology">Terminology</xref></t>
          </li>
          <li pn="section-toc.1-1.4">
            <t indent="0" pn="section-toc.1-1.4.1"><xref derivedContent="4" format="counter" sectionFormat="of" target="section-4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-flexible-algorithm">Flexible Algorithm</xref></t>
          </li>
          <li pn="section-toc.1-1.5">
            <t indent="0" pn="section-toc.1-1.5.1"><xref derivedContent="5" format="counter" sectionFormat="of" target="section-5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-flexible-algorithm-definiti">Flexible Algorithm Definition Advertisement</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.5.2">
              <li pn="section-toc.1-1.5.2.1">
                <t indent="0" pn="section-toc.1-1.5.2.1.1"><xref derivedContent="5.1" format="counter" sectionFormat="of" target="section-5.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-is-is-flexible-algorithm-de">IS-IS Flexible Algorithm Definition Sub-TLV</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.2">
                <t indent="0" pn="section-toc.1-1.5.2.2.1"><xref derivedContent="5.2" format="counter" sectionFormat="of" target="section-5.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-flexible-algorithm-def">OSPF Flexible Algorithm Definition TLV</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.3">
                <t indent="0" pn="section-toc.1-1.5.2.3.1"><xref derivedContent="5.3" format="counter" sectionFormat="of" target="section-5.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-common-handling-of-the-flex">Common Handling of the Flexible Algorithm Definition TLV</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.6">
            <t indent="0" pn="section-toc.1-1.6.1"><xref derivedContent="6" format="counter" sectionFormat="of" target="section-6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-sub-tlvs-of-is-is-fad-sub-t">Sub-TLVs of IS-IS FAD Sub-TLV</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.6.2">
              <li pn="section-toc.1-1.6.2.1">
                <t indent="0" pn="section-toc.1-1.6.2.1.1"><xref derivedContent="6.1" format="counter" sectionFormat="of" target="section-6.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-is-is-flexible-algorithm-ex">IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV</xref></t>
              </li>
              <li pn="section-toc.1-1.6.2.2">
                <t indent="0" pn="section-toc.1-1.6.2.2.1"><xref derivedContent="6.2" format="counter" sectionFormat="of" target="section-6.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-is-is-flexible-algorithm-in">IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV</xref></t>
              </li>
              <li pn="section-toc.1-1.6.2.3">
                <t indent="0" pn="section-toc.1-1.6.2.3.1"><xref derivedContent="6.3" format="counter" sectionFormat="of" target="section-6.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-is-is-flexible-algorithm-inc">IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV</xref></t>
              </li>
              <li pn="section-toc.1-1.6.2.4">
                <t indent="0" pn="section-toc.1-1.6.2.4.1"><xref derivedContent="6.4" format="counter" sectionFormat="of" target="section-6.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-is-is-flexible-algorithm-def">IS-IS Flexible Algorithm Definition Flags Sub-TLV</xref></t>
              </li>
              <li pn="section-toc.1-1.6.2.5">
                <t indent="0" pn="section-toc.1-1.6.2.5.1"><xref derivedContent="6.5" format="counter" sectionFormat="of" target="section-6.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-is-is-flexible-algorithm-exc">IS-IS Flexible Algorithm Exclude SRLG Sub-TLV</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.7">
            <t indent="0" pn="section-toc.1-1.7.1"><xref derivedContent="7" format="counter" sectionFormat="of" target="section-7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-sub-tlvs-of-the-ospf-fad-tl">Sub-TLVs of the OSPF FAD TLV</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.7.2">
              <li pn="section-toc.1-1.7.2.1">
                <t indent="0" pn="section-toc.1-1.7.2.1.1"><xref derivedContent="7.1" format="counter" sectionFormat="of" target="section-7.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-flexible-algorithm-exc">OSPF Flexible Algorithm Exclude Admin Group Sub-TLV</xref></t>
              </li>
              <li pn="section-toc.1-1.7.2.2">
                <t indent="0" pn="section-toc.1-1.7.2.2.1"><xref derivedContent="7.2" format="counter" sectionFormat="of" target="section-7.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-flexible-algorithm-inc">OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV</xref></t>
              </li>
              <li pn="section-toc.1-1.7.2.3">
                <t indent="0" pn="section-toc.1-1.7.2.3.1"><xref derivedContent="7.3" format="counter" sectionFormat="of" target="section-7.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-flexible-algorithm-incl">OSPF Flexible Algorithm Include-All Admin Group Sub-TLV</xref></t>
              </li>
              <li pn="section-toc.1-1.7.2.4">
                <t indent="0" pn="section-toc.1-1.7.2.4.1"><xref derivedContent="7.4" format="counter" sectionFormat="of" target="section-7.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-flexible-algorithm-defi">OSPF Flexible Algorithm Definition Flags Sub-TLV</xref></t>
              </li>
              <li pn="section-toc.1-1.7.2.5">
                <t indent="0" pn="section-toc.1-1.7.2.5.1"><xref derivedContent="7.5" format="counter" sectionFormat="of" target="section-7.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-flexible-algorithm-excl">OSPF Flexible Algorithm Exclude SRLG Sub-TLV</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.8">
            <t indent="0" pn="section-toc.1-1.8.1"><xref derivedContent="8" format="counter" sectionFormat="of" target="section-8"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-is-is-flexible-algorithm-pr">IS-IS Flexible Algorithm Prefix Metric Sub-TLV</xref></t>
          </li>
          <li pn="section-toc.1-1.9">
            <t indent="0" pn="section-toc.1-1.9.1"><xref derivedContent="9" format="counter" sectionFormat="of" target="section-9"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-flexible-algorithm-pre">OSPF Flexible Algorithm Prefix Metric Sub-TLV</xref></t>
          </li>
          <li pn="section-toc.1-1.10">
            <t indent="0" pn="section-toc.1-1.10.1"><xref derivedContent="10" format="counter" sectionFormat="of" target="section-10"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-flexible-algorithm-asb">OSPF Flexible Algorithm ASBR Reachability Advertisement</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.10.2">
              <li pn="section-toc.1-1.10.2.1">
                <t indent="0" pn="section-toc.1-1.10.2.1.1"><xref derivedContent="10.1" format="counter" sectionFormat="of" target="section-10.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospfv2-extended-inter-area-">OSPFv2 Extended Inter-Area ASBR LSA</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.10.2.1.2">
                  <li pn="section-toc.1-1.10.2.1.2.1">
                    <t indent="0" pn="section-toc.1-1.10.2.1.2.1.1"><xref derivedContent="10.1.1" format="counter" sectionFormat="of" target="section-10.1.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospfv2-extended-inter-area-a">OSPFv2 Extended Inter-Area ASBR TLV</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.10.2.2">
                <t indent="0" pn="section-toc.1-1.10.2.2.1"><xref derivedContent="10.2" format="counter" sectionFormat="of" target="section-10.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-flexible-algorithm-asbr">OSPF Flexible Algorithm ASBR Metric Sub-TLV</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.11">
            <t indent="0" pn="section-toc.1-1.11.1"><xref derivedContent="11" format="counter" sectionFormat="of" target="section-11"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-advertisement-of-node-parti">Advertisement of Node Participation in a Flex-Algorithm</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.11.2">
              <li pn="section-toc.1-1.11.2.1">
                <t indent="0" pn="section-toc.1-1.11.2.1.1"><xref derivedContent="11.1" format="counter" sectionFormat="of" target="section-11.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-advertisement-of-node-partic">Advertisement of Node Participation for Segment Routing</xref></t>
              </li>
              <li pn="section-toc.1-1.11.2.2">
                <t indent="0" pn="section-toc.1-1.11.2.2.1"><xref derivedContent="11.2" format="counter" sectionFormat="of" target="section-11.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-advertisement-of-node-partici">Advertisement of Node Participation for Other Data Planes</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.12">
            <t indent="0" pn="section-toc.1-1.12.1"><xref derivedContent="12" format="counter" sectionFormat="of" target="section-12"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-advertisement-of-link-attri">Advertisement of Link Attributes for Flex-Algorithm</xref></t>
          </li>
          <li pn="section-toc.1-1.13">
            <t indent="0" pn="section-toc.1-1.13.1"><xref derivedContent="13" format="counter" sectionFormat="of" target="section-13"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-calculation-of-flexible-alg">Calculation of Flexible Algorithm Paths</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.13.2">
              <li pn="section-toc.1-1.13.2.1">
                <t indent="0" pn="section-toc.1-1.13.2.1.1"><xref derivedContent="13.1" format="counter" sectionFormat="of" target="section-13.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-multi-area-and-multi-domain">Multi-area and Multi-domain Considerations</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.14">
            <t indent="0" pn="section-toc.1-1.14.1"><xref derivedContent="14" format="counter" sectionFormat="of" target="section-14"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-flex-algorithm-and-forwardi">Flex-Algorithm and Forwarding Plane</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.14.2">
              <li pn="section-toc.1-1.14.2.1">
                <t indent="0" pn="section-toc.1-1.14.2.1.1"><xref derivedContent="14.1" format="counter" sectionFormat="of" target="section-14.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-segment-routing-mpls-forwar">Segment Routing MPLS Forwarding for Flex-Algorithm</xref></t>
              </li>
              <li pn="section-toc.1-1.14.2.2">
                <t indent="0" pn="section-toc.1-1.14.2.2.1"><xref derivedContent="14.2" format="counter" sectionFormat="of" target="section-14.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-srv6-forwarding-for-flex-al">SRv6 Forwarding for Flex-Algorithm</xref></t>
              </li>
              <li pn="section-toc.1-1.14.2.3">
                <t indent="0" pn="section-toc.1-1.14.2.3.1"><xref derivedContent="14.3" format="counter" sectionFormat="of" target="section-14.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-other-data-planes-forwardin">Other Data Planes' Forwarding for Flex-Algorithm</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.15">
            <t indent="0" pn="section-toc.1-1.15.1"><xref derivedContent="15" format="counter" sectionFormat="of" target="section-15"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-operational-considerations">Operational Considerations</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.15.2">
              <li pn="section-toc.1-1.15.2.1">
                <t indent="0" pn="section-toc.1-1.15.2.1.1"><xref derivedContent="15.1" format="counter" sectionFormat="of" target="section-15.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-inter-area-considerations">Inter-area Considerations</xref></t>
              </li>
              <li pn="section-toc.1-1.15.2.2">
                <t indent="0" pn="section-toc.1-1.15.2.2.1"><xref derivedContent="15.2" format="counter" sectionFormat="of" target="section-15.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-usage-of-the-srlg-exclude-r">Usage of the SRLG Exclude Rule with Flex-Algorithm</xref></t>
              </li>
              <li pn="section-toc.1-1.15.2.3">
                <t indent="0" pn="section-toc.1-1.15.2.3.1"><xref derivedContent="15.3" format="counter" sectionFormat="of" target="section-15.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-max-metric-consideration">Max-Metric Consideration</xref></t>
              </li>
              <li pn="section-toc.1-1.15.2.4">
                <t indent="0" pn="section-toc.1-1.15.2.4.1"><xref derivedContent="15.4" format="counter" sectionFormat="of" target="section-15.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-flexible-algorithm-definitio">Flexible Algorithm Definition and Changes</xref></t>
              </li>
              <li pn="section-toc.1-1.15.2.5">
                <t indent="0" pn="section-toc.1-1.15.2.5.1"><xref derivedContent="15.5" format="counter" sectionFormat="of" target="section-15.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-number-of-flex-algorithms">Number of Flex-Algorithms</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.16">
            <t indent="0" pn="section-toc.1-1.16.1"><xref derivedContent="16" format="counter" sectionFormat="of" target="section-16"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-backward-compatibility">Backward Compatibility</xref></t>
          </li>
          <li pn="section-toc.1-1.17">
            <t indent="0" pn="section-toc.1-1.17.1"><xref derivedContent="17" format="counter" sectionFormat="of" target="section-17"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-security-considerations">Security Considerations</xref></t>
          </li>
          <li pn="section-toc.1-1.18">
            <t indent="0" pn="section-toc.1-1.18.1"><xref derivedContent="18" format="counter" sectionFormat="of" target="section-18"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-iana-considerations">IANA Considerations</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.18.2">
              <li pn="section-toc.1-1.18.2.1">
                <t indent="0" pn="section-toc.1-1.18.2.1.1"><xref derivedContent="18.1" format="counter" sectionFormat="of" target="section-18.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-igp-iana-considerations">IGP IANA Considerations</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.18.2.1.2">
                  <li pn="section-toc.1-1.18.2.1.2.1">
                    <t indent="0" pn="section-toc.1-1.18.2.1.2.1.1"><xref derivedContent="18.1.1" format="counter" sectionFormat="of" target="section-18.1.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-igp-algorithm-types-registr">IGP Algorithm Types Registry</xref></t>
                  </li>
                  <li pn="section-toc.1-1.18.2.1.2.2">
                    <t indent="0" pn="section-toc.1-1.18.2.1.2.2.1"><xref derivedContent="18.1.2" format="counter" sectionFormat="of" target="section-18.1.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-igp-metric-type-registry">IGP Metric-Type Registry</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.18.2.2">
                <t indent="0" pn="section-toc.1-1.18.2.2.1"><xref derivedContent="18.2" format="counter" sectionFormat="of" target="section-18.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-igp-flexible-algorithm-defi">IGP Flexible Algorithm Definition Flags Registry</xref></t>
              </li>
              <li pn="section-toc.1-1.18.2.3">
                <t indent="0" pn="section-toc.1-1.18.2.3.1"><xref derivedContent="18.3" format="counter" sectionFormat="of" target="section-18.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-is-is-iana-considerations">IS-IS IANA Considerations</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.18.2.3.2">
                  <li pn="section-toc.1-1.18.2.3.2.1">
                    <t indent="0" pn="section-toc.1-1.18.2.3.2.1.1"><xref derivedContent="18.3.1" format="counter" sectionFormat="of" target="section-18.3.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-is-is-sub-tlvs-for-is-is-ro">IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV Registry</xref></t>
                  </li>
                  <li pn="section-toc.1-1.18.2.3.2.2">
                    <t indent="0" pn="section-toc.1-1.18.2.3.2.2.1"><xref derivedContent="18.3.2" format="counter" sectionFormat="of" target="section-18.3.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-is-is-sub-tlvs-for-tlvs-adv">IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability Registry</xref></t>
                  </li>
                  <li pn="section-toc.1-1.18.2.3.2.3">
                    <t indent="0" pn="section-toc.1-1.18.2.3.2.3.1"><xref derivedContent="18.3.3" format="counter" sectionFormat="of" target="section-18.3.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-is-is-sub-sub-tlvs-for-flex">IS-IS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV Registry</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.18.2.4">
                <t indent="0" pn="section-toc.1-1.18.2.4.1"><xref derivedContent="18.4" format="counter" sectionFormat="of" target="section-18.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-iana-considerations">OSPF IANA Considerations</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.18.2.4.2">
                  <li pn="section-toc.1-1.18.2.4.2.1">
                    <t indent="0" pn="section-toc.1-1.18.2.4.2.1.1"><xref derivedContent="18.4.1" format="counter" sectionFormat="of" target="section-18.4.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-router-information-ri-">OSPF Router Information (RI) TLVs Registry</xref></t>
                  </li>
                  <li pn="section-toc.1-1.18.2.4.2.2">
                    <t indent="0" pn="section-toc.1-1.18.2.4.2.2.1"><xref derivedContent="18.4.2" format="counter" sectionFormat="of" target="section-18.4.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospfv2-extended-prefix-tlv-">OSPFv2 Extended Prefix TLV Sub-TLVs Registry</xref></t>
                  </li>
                  <li pn="section-toc.1-1.18.2.4.2.3">
                    <t indent="0" pn="section-toc.1-1.18.2.4.2.3.1"><xref derivedContent="18.4.3" format="counter" sectionFormat="of" target="section-18.4.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospfv3-extended-lsa-sub-tlv">OSPFv3 Extended-LSA Sub-TLVs Registry</xref></t>
                  </li>
                  <li pn="section-toc.1-1.18.2.4.2.4">
                    <t indent="0" pn="section-toc.1-1.18.2.4.2.4.1"><xref derivedContent="18.4.4" format="counter" sectionFormat="of" target="section-18.4.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-flex-algorithm-prefix-">OSPF Flex-Algorithm Prefix Metric Bits Registry</xref></t>
                  </li>
                  <li pn="section-toc.1-1.18.2.4.2.5">
                    <t indent="0" pn="section-toc.1-1.18.2.4.2.5.1"><xref derivedContent="18.4.5" format="counter" sectionFormat="of" target="section-18.4.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-opaque-link-state-advertise">Opaque Link-State Advertisements (LSA) Option Types Registry</xref></t>
                  </li>
                  <li pn="section-toc.1-1.18.2.4.2.6">
                    <t indent="0" pn="section-toc.1-1.18.2.4.2.6.1"><xref derivedContent="18.4.6" format="counter" sectionFormat="of" target="section-18.4.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospfv2-extended-inter-area-as">OSPFv2 Extended Inter-Area ASBR TLVs Registry</xref></t>
                  </li>
                  <li pn="section-toc.1-1.18.2.4.2.7">
                    <t indent="0" pn="section-toc.1-1.18.2.4.2.7.1"><xref derivedContent="18.4.7" format="counter" sectionFormat="of" target="section-18.4.7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospfv2-extended-inter-area-asbr">OSPFv2 Extended Inter-Area ASBR Sub-TLVs Registry</xref></t>
                  </li>
                  <li pn="section-toc.1-1.18.2.4.2.8">
                    <t indent="0" pn="section-toc.1-1.18.2.4.2.8.1"><xref derivedContent="18.4.8" format="counter" sectionFormat="of" target="section-18.4.8"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-ospf-flexible-algorithm-defin">OSPF Flexible Algorithm Definition TLV Sub-TLVs Registry</xref></t>
                  </li>
                  <li pn="section-toc.1-1.18.2.4.2.9">
                    <t indent="0" pn="section-toc.1-1.18.2.4.2.9.1"><xref derivedContent="18.4.9" format="counter" sectionFormat="of" target="section-18.4.9"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-link-attribute-application-">Link Attribute Application Identifiers Registry</xref></t>
                  </li>
                </ul>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.19">
            <t indent="0" pn="section-toc.1-1.19.1"><xref derivedContent="19" format="counter" sectionFormat="of" target="section-19"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-references">References</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.19.2">
              <li pn="section-toc.1-1.19.2.1">
                <t indent="0" pn="section-toc.1-1.19.2.1.1"><xref derivedContent="19.1" format="counter" sectionFormat="of" target="section-19.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-normative-references">Normative References</xref></t>
              </li>
              <li pn="section-toc.1-1.19.2.2">
                <t indent="0" pn="section-toc.1-1.19.2.2.1"><xref derivedContent="19.2" format="counter" sectionFormat="of" target="section-19.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-informative-references">Informative References</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.20">
            <t indent="0" pn="section-toc.1-1.20.1"><xref derivedContent="" format="none" sectionFormat="of" target="section-appendix.a"/><xref derivedContent="" format="title" sectionFormat="of" target="name-acknowledgements">Acknowledgements</xref></t>
          </li>
          <li pn="section-toc.1-1.21">
            <t indent="0" pn="section-toc.1-1.21.1"><xref derivedContent="" format="none" sectionFormat="of" target="section-appendix.b"/><xref derivedContent="" format="title" sectionFormat="of" target="name-authors-addresses">Authors' Addresses</xref></t>
          </li>
        </ul>
      </section>
    </toc>
  </front>
  <middle>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-1">
      <name slugifiedName="name-introduction">Introduction</name>
      <t indent="0" pn="section-1-1">An IGP-computed path based on the shortest IGP metric is often
      replaced by a traffic-engineered path due to requirements
      that are not reflected by the IGP metric. Some networks engineer the
      IGP metric assignments in a way that the IGP metric reflects the link
      bandwidth or delay. If, for example, the IGP metric reflects the
      bandwidth on the link and user traffic is delay sensitive,
      the best IGP path may not reflect the best path from such a user's perspective.</t>
      <t indent="0" pn="section-1-2">To overcome this limitation, various sorts of Traffic Engineering
      have been deployed, including RSVP-TE and SR-TE, in which case the TE
      component is responsible for computing paths based on additional metrics
      and/or constraints. Such paths need to be installed in the forwarding
      tables in addition to, or as a replacement for, the original paths
      computed by IGPs. Tunnels are often used to represent the engineered
      paths and mechanisms, like the one described in <xref target="RFC3906" format="default" sectionFormat="of" derivedContent="RFC3906"/>, and are
      used to replace the original IGP paths with such tunnel paths.</t>
      <t indent="0" pn="section-1-3">This document specifies a set of extensions to IS-IS, OSPFv2, and
      OSPFv3 that enable a router to advertise TLVs that (a) identify a
      calculation-type, (b) specify a metric-type, and (c) describe a set of
      constraints on the topology that are to be used to compute the best
      paths along the constrained topology. A given combination of
      calculation-type, metric-type, and constraints is known as a "Flexible
      Algorithm Definition". A router that sends such a set of TLVs also
      assigns a Flex-Algorithm value to the specified combination of
      calculation-type, metric-type, and constraints.</t>
      <t indent="0" pn="section-1-4">This document also specifies a way for a router to use IGPs to
      associate one or more Segment Routing with the MPLS Data Plane (SR-MPLS)
      Prefix-SIDs <xref target="RFC8660" format="default" sectionFormat="of" derivedContent="RFC8660"/> or Segment Routing over IPv6 (SRv6) 
      locators <xref target="RFC8986" format="default" sectionFormat="of" derivedContent="RFC8986"/> with a particular Flex-Algorithm. Each 
      such Prefix-SID or SRv6 locator then represents a path that is computed 
      according to the identified Flex-Algorithm. In SRv6, it is the locator, not the Segment Identifier (SID),
      that holds the binding to the algorithm.</t>
    </section>
    <section anchor="reqlang" numbered="true" toc="include" removeInRFC="false" pn="section-2">
      <name slugifiedName="name-requirements-language">Requirements Language</name>
      <t indent="0" pn="section-2-1">The key words "<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
      "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as described in BCP 14
      <xref target="RFC2119" format="default" sectionFormat="of" derivedContent="RFC2119"/> <xref target="RFC8174" format="default" sectionFormat="of" derivedContent="RFC8174"/> when, and only when,
      they appear in all capitals, as shown here.</t>
    </section>
    <section anchor="TERMINOLOGY" numbered="true" toc="include" removeInRFC="false" pn="section-3">
      <name slugifiedName="name-terminology">Terminology</name>
      <t indent="0" pn="section-3-1">This section defines terms that are often used in this document.</t>
      <dl newline="false" spacing="normal" indent="3" pn="section-3-2">
        <dt pn="section-3-2.1">Flexible Algorithm Definition (FAD):</dt>
        <dd pn="section-3-2.2">the set consisting of (a) a
      calculation-type, (b) a metric-type, and (c) a set of constraints.</dd>
        <dt pn="section-3-2.3">Flex-Algorithm:</dt>
        <dd pn="section-3-2.4">a numeric identifier in the range 128-255 that
	is associated via configuration with the Flexible Algorithm
	Definition.</dd>
        <dt pn="section-3-2.5">Flexible Algorithm Participation:</dt>
        <dd pn="section-3-2.6">per the data plane configuration
	state that expresses whether the node is participating in a particular
	Flexible Algorithm. Not all routers in a given network need to participate 
	in a given Flexible Algorithm. The Flexible Algorithm(s) that a given router 
  	participates in is determined by configuration.</dd>
        <dt pn="section-3-2.7">IGP Algorithm:</dt>
        <dd pn="section-3-2.8">value from the IANA "IGP Algorithm Types" registry
	defined under the "Interior Gateway Protocol (IGP) Parameters"
	registry group. IGP Algorithms represent the triplet (calculation-type,
	metric-type, and constraints), where the second and third elements of the triplet
	<bcp14>MAY</bcp14> be unspecified.</dd>
        <dt pn="section-3-2.9">ABR:</dt>
        <dd pn="section-3-2.10">Area Border Router. In IS-IS terminology, it is also known as the
	Level 1 (L1) / Level 2 (L2) router.</dd>
        <dt pn="section-3-2.11">ASBR:</dt>
        <dd pn="section-3-2.12">Autonomous System Border Router.</dd>
      </dl>
    </section>
    <section anchor="FLEXALG" numbered="true" toc="include" removeInRFC="false" pn="section-4">
      <name slugifiedName="name-flexible-algorithm">Flexible Algorithm</name>
      <t indent="0" pn="section-4-1">Many possible constraints may be used to compute a path over a
      network. Some networks are deployed as multiple planes. A simple form of
      constraint may be to use a particular plane. A more sophisticated form
      of constraint can include some extended metric, as described in <xref target="RFC8570" format="default" sectionFormat="of" derivedContent="RFC8570"/>. Constraints that restrict paths to links with
      specific affinities or avoid links with specific affinities are also
      possible. Combinations of these are also possible.</t>
      <t indent="0" pn="section-4-2">To provide maximum flexibility, a mechanism is provided that
      allows a router to identify a particular calculation-type and
      metric-type, describe a particular set of constraints, and
      assign a numeric identifier, referred to as Flex-Algorithm, to the
      combination of that calculation-type, metric-type, and those
      constraints. The mapping between the Flex-Algorithm and its
      meaning is flexible and defined by the user. As long as all routers
      in the domain have a common understanding as to what a particular
      Flex-Algorithm represents, the resulting routing computation is
      consistent and traffic is not subject to any looping.</t>
      <t indent="0" pn="section-4-3">The set consisting of (a) a calculation-type, (b) a metric-type, and (c)
      a set of constraints is referred to as a Flexible Algorithm
      Definition.</t>
      <t indent="0" pn="section-4-4">The Flex-Algorithm is a numeric identifier in the range 128-255 that
      is associated via configuration with the Flexible Algorithm
      Definition.</t>
      <t indent="0" pn="section-4-5">The IANA "IGP Algorithm Types" registry defines the set of values for IGP
      Algorithms. The following values are allocated by IANA from this registry for
      Flex-Algorithms: </t>
      <t indent="3" pn="section-4-6">128-255 - Flex-Algorithms</t>
    </section>
    <section anchor="FLEXALGDEF" numbered="true" toc="include" removeInRFC="false" pn="section-5">
      <name slugifiedName="name-flexible-algorithm-definiti">Flexible Algorithm Definition Advertisement</name>
      <t indent="0" pn="section-5-1">To guarantee loop-free forwarding for paths computed for a
      particular Flex-Algorithm, all routers that (a) are configured to
      participate in a particular Flex-Algorithm and (b) are in the same
      Flex-Algorithm Definition advertisement scope <bcp14>MUST</bcp14> agree on the
      definition of the Flex-Algorithm. The following procedures
  	  ensure this condition is fulfilled.</t>
      <section anchor="ISISFLEXALGTLV" numbered="true" toc="include" removeInRFC="false" pn="section-5.1">
        <name slugifiedName="name-is-is-flexible-algorithm-de">IS-IS Flexible Algorithm Definition Sub-TLV</name>
        <t indent="0" pn="section-5.1-1">The IS-IS Flexible Algorithm Definition (FAD) sub-TLV is
        used to advertise the definition of the Flex-Algorithm.</t>
        <t indent="0" pn="section-5.1-2">The IS-IS FAD sub-TLV is advertised as a sub-TLV of the IS-IS Router
        CAPABILITY TLV-242, which is defined in <xref target="RFC7981" format="default" sectionFormat="of" derivedContent="RFC7981"/>.</t>
        <t indent="0" pn="section-5.1-3">The IS-IS FAD sub-TLV has the following format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-5.1-4">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Type     |    Length     |Flex-Algorithm |  Metric-Type  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Calc-Type   |    Priority   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Sub-TLVs                             |
+                                                               +
|                            ...                                |

|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-5.1-5">
          <dt pn="section-5.1-5.1">where:</dt>
          <dd pn="section-5.1-5.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-5.1-5.2.1">
              <dt pn="section-5.1-5.2.1.1">Type:</dt>
              <dd pn="section-5.1-5.2.1.2">26</dd>
              <dt pn="section-5.1-5.2.1.3">Length:</dt>
              <dd pn="section-5.1-5.2.1.4">variable number of octets, dependent on the included sub-TLVs.</dd>
              <dt pn="section-5.1-5.2.1.5">Flex-Algorithm:</dt>
              <dd pn="section-5.1-5.2.1.6">Flexible Algorithm number. Single octet value between 
              128 and 255 inclusive.</dd>
              <dt pn="section-5.1-5.2.1.7">Metric-Type:</dt>
              <dd pn="section-5.1-5.2.1.8">
                <t indent="0" pn="section-5.1-5.2.1.8.1">type of metric from the IANA "IGP Metric-Type" registry
		(<xref target="IGPMETRICTYPE" format="default" sectionFormat="of" derivedContent="Section 18.1.2"/>) to be used during the
		calculation. The following values are defined: </t>
                <dl newline="false" spacing="normal" indent="3" pn="section-5.1-5.2.1.8.2">
                  <dt pn="section-5.1-5.2.1.8.2.1">0:</dt>
                  <dd pn="section-5.1-5.2.1.8.2.2">IGP Metric</dd>
                  <dt pn="section-5.1-5.2.1.8.2.3">1:</dt>
                  <dd pn="section-5.1-5.2.1.8.2.4">Min Unidirectional Link Delay, as defined in <xref target="RFC8570" section="4.2" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8570#section-4.2" derivedContent="RFC8570"/>, encoded as an application-specific link attribute, as specified in <xref target="RFC8919" format="default" sectionFormat="of" derivedContent="RFC8919"/> and <xref target="FLEXALGLINKATTR" format="default" sectionFormat="of" derivedContent="Section 12"/> of this document.</dd>
                  <dt pn="section-5.1-5.2.1.8.2.5">2:</dt>
                  <dd pn="section-5.1-5.2.1.8.2.6">Traffic Engineering Default Metric, as defined in <xref target="RFC5305" section="3.7" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5305#section-3.7" derivedContent="RFC5305"/>, encoded as an
		  application-specific link attribute, as specified in <xref target="RFC8919" format="default" sectionFormat="of" derivedContent="RFC8919"/> and <xref target="FLEXALGLINKATTR" format="default" sectionFormat="of" derivedContent="Section 12"/> of this document.</dd>
                </dl>
              </dd>
              <dt pn="section-5.1-5.2.1.9">Calc-Type:</dt>
              <dd pn="section-5.1-5.2.1.10">calculation-type. Value from 0-127 inclusive from the IANA "IGP
              Algorithm Types" registry defined under the "Interior Gateway Protocol
              (IGP) Parameters" registry. IGP Algorithms in the range of
              0-127 have a defined triplet (calculation-type, metric-type,
              constraints). When used to specify the calculation-type in the FAD
              sub-TLV, only the calculation-type defined for the specified IGP
              Algorithm is used. The Metric/Constraints <bcp14>MUST NOT</bcp14> be
	      inherited.
              If the required calculation-type is Shortest Path First, the value
              0 <bcp14>MUST</bcp14> appear in this field.</dd>
              <dt pn="section-5.1-5.2.1.11">Priority:</dt>
              <dd pn="section-5.1-5.2.1.12">value between 0 and 255 inclusive that specifies the
              priority of the advertisement. Numerically greater values are preferred.
              Usage of the priority is described in <xref target="COMMONLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.3"/>.</dd>
              <dt pn="section-5.1-5.2.1.13">Sub-TLVs:</dt>
              <dd pn="section-5.1-5.2.1.14">optional sub-TLVs.</dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-5.1-6">The IS-IS FAD sub-TLV <bcp14>MAY</bcp14> be advertised in a Label Switched Path (LSP) of any number. 
        The IS-IS router <bcp14>MAY</bcp14> advertise more than one IS-IS FAD sub-TLV for a given
        Flexible Algorithm (see <xref target="ISISFADLTLVS" format="default" sectionFormat="of" derivedContent="Section 6"/>).</t>
        <t indent="0" pn="section-5.1-7">The IS-IS FAD sub-TLV has an area/level scope. The Router Capability TLV
        in which the FAD sub-TLV is present <bcp14>MUST</bcp14> have the S bit clear.</t>
        <t indent="0" pn="section-5.1-8">An IS-IS L1/L2 router <bcp14>MAY</bcp14> be configured to regenerate the winning FAD
        from level 2, without any modification to it, to the level 1 area. The
        regeneration of the FAD sub-TLV from level 2 to level 1 is determined
        by the L1/L2 router, not by the originator of the FAD advertisement in
        level 2. In such a case, the regenerated FAD sub-TLV will be
        advertised in the level 1 Router Capability TLV originated by the
        L1/L2 router.</t>
        <t indent="0" pn="section-5.1-9">An L1/L2 router <bcp14>MUST NOT</bcp14> regenerate any FAD sub-TLV from level 1 to
        level 2.</t>
      </section>
      <section anchor="OSPFFLEXALGTLV" numbered="true" toc="include" removeInRFC="false" pn="section-5.2">
        <name slugifiedName="name-ospf-flexible-algorithm-def">OSPF Flexible Algorithm Definition TLV</name>
        <t indent="0" pn="section-5.2-1">The OSPF FAD TLV is advertised as a top-level TLV of the Router Information (RI)
        Link State Advertisement (LSA), which is defined in <xref target="RFC7770" format="default" sectionFormat="of" derivedContent="RFC7770"/>.</t>
        <t indent="0" pn="section-5.2-2">The OSPF FAD TLV has the following format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-5.2-3">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Type             |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm |   Metric-Type |   Calc-Type   |    Priority   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Sub-TLVs                           |
+                                                               +
|                               ...                             |

|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  </artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-5.2-4">
          <dt pn="section-5.2-4.1">where:</dt>
          <dd pn="section-5.2-4.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-5.2-4.2.1">
              <dt pn="section-5.2-4.2.1.1">Type:</dt>
              <dd pn="section-5.2-4.2.1.2">16</dd>
              <dt pn="section-5.2-4.2.1.3">Length:</dt>
              <dd pn="section-5.2-4.2.1.4">variable number of octets, dependent on the included sub-TLVs.</dd>
              <dt pn="section-5.2-4.2.1.5">Flex-Algorithm:</dt>
              <dd pn="section-5.2-4.2.1.6">Flexible Algorithm number. Single octet value between 
              128 and 255 inclusive.</dd>
              <dt pn="section-5.2-4.2.1.7">Metric-Type: </dt>
              <dd pn="section-5.2-4.2.1.8">
                <t indent="0" pn="section-5.2-4.2.1.8.1">type of metric from the IANA "IGP Metric-Type" registry
		(<xref target="IGPMETRICTYPE" format="default" sectionFormat="of" derivedContent="Section 18.1.2"/>) to be used during the
		calculation. The following values are defined: </t>
                <dl newline="false" spacing="normal" indent="3" pn="section-5.2-4.2.1.8.2">
                  <dt pn="section-5.2-4.2.1.8.2.1">0:</dt>
                  <dd pn="section-5.2-4.2.1.8.2.2">IGP Metric</dd>
                  <dt pn="section-5.2-4.2.1.8.2.3">1:</dt>
                  <dd pn="section-5.2-4.2.1.8.2.4">Min Unidirectional Link Delay, as defined in <xref target="RFC7471" section="4.2" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7471#section-4.2" derivedContent="RFC7471"/>, encoded as an
	      application-specific link attribute, as specified in <xref target="RFC8920" format="default" sectionFormat="of" derivedContent="RFC8920"/> and <xref target="FLEXALGLINKATTR" format="default" sectionFormat="of" derivedContent="Section 12"/> of
              this document.</dd>
                  <dt pn="section-5.2-4.2.1.8.2.5">2:</dt>
                  <dd pn="section-5.2-4.2.1.8.2.6">Traffic Engineering Metric, as defined in <xref target="RFC3630" section="2.5.5" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3630#section-2.5.5" derivedContent="RFC3630"/>, encoded as an
	      application-specific link attribute, as specified in <xref target="RFC8920" format="default" sectionFormat="of" derivedContent="RFC8920"/> and <xref target="FLEXALGLINKATTR" format="default" sectionFormat="of" derivedContent="Section 12"/> of
              this document.</dd>
                </dl>
              </dd>
              <dt pn="section-5.2-4.2.1.9">Calc-Type:</dt>
              <dd pn="section-5.2-4.2.1.10">as described in <xref target="ISISFLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.1"/>.</dd>
              <dt pn="section-5.2-4.2.1.11">Priority: </dt>
              <dd pn="section-5.2-4.2.1.12">as described in <xref target="ISISFLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.1"/>.</dd>
              <dt pn="section-5.2-4.2.1.13">Sub-TLVs:</dt>
              <dd pn="section-5.2-4.2.1.14">optional sub-TLVs.</dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-5.2-5">When multiple OSPF FAD TLVs, for the same Flexible Algorithm, are
        received from a given router, the receiver <bcp14>MUST</bcp14> use the first
        occurrence of the TLV in the RI LSA. If the OSPF FAD
        TLV, for the same Flex-Algorithm, appears in multiple RI
        LSAs that have different flooding scopes, the OSPF FAD TLV
        in the RI LSA with the area-scoped flooding scope <bcp14>MUST</bcp14>
        be used. If the OSPF FAD TLV, for the same algorithm, appears in
        multiple RI LSAs that have the same flooding scope,
        the OSPF FAD TLV in the RI LSA with the
        numerically smallest Instance ID <bcp14>MUST</bcp14> be used and subsequent
	instances of the OSPF FAD TLV <bcp14>MUST</bcp14> be ignored.</t>
        <t indent="0" pn="section-5.2-6">The RI LSA can be advertised at any of the defined opaque flooding
        scopes (link, area, or Autonomous System (AS)). For the purpose of
        OSPF FAD TLV advertisement, area-scoped flooding is <bcp14>REQUIRED</bcp14>. The
        AS flooding scope <bcp14>SHOULD NOT</bcp14> be used unless
        local configuration policy on the originating router indicates domain-wide
	flooding.</t>
      </section>
      <section anchor="COMMONLEXALGTLV" numbered="true" toc="include" removeInRFC="false" pn="section-5.3">
        <name slugifiedName="name-common-handling-of-the-flex">Common Handling of the Flexible Algorithm Definition TLV</name>
        <t indent="0" pn="section-5.3-1">This section describes the protocol-independent handling of the FAD
        TLV (OSPF) or FAD sub-TLV (IS-IS). We will refer to it as FAD TLV in
        this section, even though, in the case of IS-IS, it is a sub-TLV.</t>
        <t indent="0" pn="section-5.3-2">The value of the Flex-Algorithm <bcp14>MUST</bcp14> be between 128 and 255
        inclusive. If it is not, the FAD TLV <bcp14>MUST</bcp14> be ignored.</t>
        <t indent="0" pn="section-5.3-3">Only a subset of the routers participating in the particular
        Flex-Algorithm need to advertise the definition of the
        Flex-Algorithm.</t>
        <t indent="0" pn="section-5.3-4">Every router that is configured to participate in a particular
        Flex-Algorithm <bcp14>MUST</bcp14> select the Flex-Algorithm Definition based on the
        following ordered rules. This allows for the consistent Flex-Algorithm
        Definition selection in cases where different routers advertise
        different definitions for a given Flex-Algorithm: </t>
        <ol spacing="normal" type="1" indent="adaptive" start="1" pn="section-5.3-5">
          <li pn="section-5.3-5.1" derivedCounter="1.">From the advertisements of the FAD in the area (including
            both locally generated advertisements and received advertisements),
            select the one(s) with the numerically greatest priority value.</li>
          <li pn="section-5.3-5.2" derivedCounter="2.">If there are multiple advertisements of the FAD with the
          same numerically greatest priority, select the one that is originated from
	  the router with the numerically greatest System-ID, in the case of IS-IS, or
	  Router ID, in the case of OSPFv2 and OSPFv3. For IS-IS, the System-ID is
          described in <xref target="ISO10589" format="default" sectionFormat="of" derivedContent="ISO10589"/>. For OSPFv2 and OSPFv3,
          the standard Router ID is described in <xref target="RFC2328" format="default" sectionFormat="of" derivedContent="RFC2328"/> and <xref target="RFC5340" format="default" sectionFormat="of" derivedContent="RFC5340"/>,
	  respectively.</li>
        </ol>
        <t indent="0" pn="section-5.3-6">The FAD selected according to these rules is also known as the 
  		"winning FAD".</t>
        <t indent="0" pn="section-5.3-7">A router that is not configured to participate in a particular
        Flex-Algorithm <bcp14>MUST</bcp14> ignore FAD sub-TLV advertisements for such
        Flex-Algorithm.</t>
        <t indent="0" pn="section-5.3-8">A router that is not participating in a particular Flex-Algorithm
        <bcp14>MAY</bcp14> advertise the FAD for such Flex-Algorithm. Receiving routers
        <bcp14>MUST</bcp14> consider a received FAD advertisement regardless of the Flex-Algorithm
        participation of that FAD advertisement's originator.</t>
        <t indent="0" pn="section-5.3-9">Any change in the Flex-Algorithm Definition may result in a temporary
        disruption of traffic that is forwarded based on such Flex-Algorithm
        paths. The impact is similar to any other event that requires
        network-wide convergence.</t>
        <t indent="0" pn="section-5.3-10">If a node is configured to participate in a particular
        Flexible Algorithm, but there is no valid Flex-Algorithm Definition available for
        it or the selected Flex-Algorithm Definition includes calculation-type, metric-type, 
        constraint, flag, or sub-TLV that is not supported by the node, it <bcp14>MUST</bcp14> stop 
        participating in such Flexible Algorithm. That implies that it <bcp14>MUST NOT</bcp14> announce
        participation for such Flexible Algorithm, as specified in <xref target="FLEXALGPART" format="default" sectionFormat="of" derivedContent="Section 11"/>, and it <bcp14>MUST</bcp14> remove any forwarding state
        associated with it.</t>
        <t indent="0" pn="section-5.3-11">The Flex-Algorithm Definition is topology independent. It applies to
        all topologies that a router participates in.</t>
      </section>
    </section>
    <section anchor="ISISFADLTLVS" numbered="true" toc="include" removeInRFC="false" pn="section-6">
      <name slugifiedName="name-sub-tlvs-of-is-is-fad-sub-t">Sub-TLVs of IS-IS FAD Sub-TLV</name>
      <t indent="0" pn="section-6-1">One of the limitations of IS-IS <xref target="ISO10589" format="default" sectionFormat="of" derivedContent="ISO10589"/> is that the length of 
    a TLV/sub-TLV is limited to a maximum of 255 octets. For the FAD sub-TLV, there are 
    a number of sub-sub-TLVs (defined below) that are supported. For a given 
    Flex-Algorithm, it is possible that the total number of octets required to 
    completely define a FAD exceeds the maximum length supported by a single FAD sub-TLV. 
    In such cases, the FAD <bcp14>MAY</bcp14> be split into multiple such sub-TLVs, and the content of 
    the multiple FAD sub-TLVs are combined to provide a complete FAD for the Flex-Algorithm. 
    In such a case, the fixed portion of the FAD (see <xref target="ISISFLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.1"/>) 
    <bcp14>MUST</bcp14> be identical in all FAD sub-TLVs for a given Flex-Algorithm from a given IS. 
    In case the fixed portion of such FAD sub-TLVs differ, the values
    in the fixed portion in the FAD sub-TLV in the first occurrence in the lowest-numbered LSP from a given IS <bcp14>MUST</bcp14> be used.</t>
      <t indent="0" pn="section-6-2">Any specification that introduces a new IS-IS FAD sub-sub-TLV <bcp14>MUST</bcp14> specify whether 
    the FAD sub-TLV may appear multiple times in the set of FAD sub-TLVs for a given 
    Flex-Algorithm from a given IS and how to handle them if multiple are allowed.</t>
      <section anchor="ISISFLEXALGEXLTLV" numbered="true" toc="include" removeInRFC="false" pn="section-6.1">
        <name slugifiedName="name-is-is-flexible-algorithm-ex">IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV</name>
        <t indent="0" pn="section-6.1-1">The Flexible Algorithm Definition can specify "colors" that are
        used by the operator to exclude links during the Flex-Algorithm path
        computation.</t>
        <t indent="0" pn="section-6.1-2">The IS-IS Flexible Algorithm Exclude Admin Group (FAEAG) sub-TLV is used to
        advertise the exclude rule that is used during the Flex-Algorithm path
        calculation, as specified in <xref target="FLEXALGPATHCALC" format="default" sectionFormat="of" derivedContent="Section 13"/>.</t>
        <t indent="0" pn="section-6.1-3">The IS-IS FAEAG sub-TLV
        is a sub-TLV of the IS-IS FAD sub-TLV. It has the following
        format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-6.1-4">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Type     |    Length     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Extended Admin Group                     |
+-                                                             -+
|                            ...                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-6.1-5">
          <dt pn="section-6.1-5.1">where:</dt>
          <dd pn="section-6.1-5.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-6.1-5.2.1">
              <dt pn="section-6.1-5.2.1.1">Type:</dt>
              <dd pn="section-6.1-5.2.1.2">1</dd>
              <dt pn="section-6.1-5.2.1.3">Length:</dt>
              <dd pn="section-6.1-5.2.1.4">variable, dependent on the size of the Extended Admin
              Group. <bcp14>MUST</bcp14> be a multiple of 4 octets.</dd>
              <dt pn="section-6.1-5.2.1.5">Extended Administrative Group:</dt>
              <dd pn="section-6.1-5.2.1.6">Extended Administrative Group, as
              defined in <xref target="RFC7308" format="default" sectionFormat="of" derivedContent="RFC7308"/>.</dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-6.1-6">The IS-IS FAEAG sub-TLV <bcp14>MUST NOT</bcp14> appear more than once in a single IS-IS
        FAD sub-TLV. If it appears more than once, the IS-IS FAD sub-TLV <bcp14>MUST</bcp14>
        be ignored by the receiver.</t>
        <t indent="0" pn="section-6.1-7">The IS-IS FAEAG sub-TLV <bcp14>MUST NOT</bcp14> appear more than once in the set of FAD 
        sub-TLVs for a given Flex-Algorithm from a given IS. If it appears more than once 
        in such a set, the IS-IS FAEAG sub-TLV in the first occurrence in the lowest-numbered 
        LSP from a given IS <bcp14>MUST</bcp14> be used, and any other occurrences <bcp14>MUST</bcp14> be ignored.</t>
      </section>
      <section anchor="ISISFLEXALGINCANYTLV" numbered="true" toc="include" removeInRFC="false" pn="section-6.2">
        <name slugifiedName="name-is-is-flexible-algorithm-in">IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV</name>
        <t indent="0" pn="section-6.2-1">The Flexible Algorithm Definition can specify "colors" that are
        used by the operator to include links during the Flex-Algorithm path
        computation.</t>
        <t indent="0" pn="section-6.2-2">The IS-IS Flexible Algorithm Include-Any Admin Group sub-TLV is used
        to advertise the include-any rule that is used during the Flex-Algorithm
        path calculation, as specified in <xref target="FLEXALGPATHCALC" format="default" sectionFormat="of" derivedContent="Section 13"/>.</t>
        <t indent="0" pn="section-6.2-3">The IS-IS Flexible Algorithm Include-Any Admin Group
        sub-TLV is a sub-TLV of the IS-IS FAD sub-TLV. It has the following
        format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-6.2-4">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Type     |    Length     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Extended Admin Group                     |
+-                                                             -+
|                            ...                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-6.2-5">
          <dt pn="section-6.2-5.1">where:</dt>
          <dd pn="section-6.2-5.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-6.2-5.2.1">
              <dt pn="section-6.2-5.2.1.1">Type: </dt>
              <dd pn="section-6.2-5.2.1.2">2</dd>
              <dt pn="section-6.2-5.2.1.3">Length:</dt>
              <dd pn="section-6.2-5.2.1.4">variable, dependent on the size of the Extended Admin
              Group. <bcp14>MUST</bcp14> be a multiple of 4 octets.</dd>
              <dt pn="section-6.2-5.2.1.5">Extended Administrative Group:</dt>
              <dd pn="section-6.2-5.2.1.6">Extended Administrative Group, as
              defined in <xref target="RFC7308" format="default" sectionFormat="of" derivedContent="RFC7308"/>.</dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-6.2-6">The IS-IS Flexible Algorithm Include-Any Admin Group sub-TLV <bcp14>MUST NOT</bcp14> appear more than once in a single IS-IS FAD sub-TLV. If it appears more
        than once, the IS-IS FAD sub-TLV <bcp14>MUST</bcp14> be ignored by the receiver.</t>
        <t indent="0" pn="section-6.2-7">The IS-IS Flexible Algorithm Include-Any Admin Group sub-TLV <bcp14>MUST NOT</bcp14> appear 
        more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. 
        If it appears more than once in such a set, the IS-IS Flexible Algorithm 
        Include-Any Admin Group sub-TLV in the first occurrence in the lowest-numbered 
        LSP from a given IS <bcp14>MUST</bcp14> be used, and any other occurrences <bcp14>MUST</bcp14> be ignored.</t>
      </section>
      <section anchor="ISISFLEXALGINCALLTLV" numbered="true" toc="include" removeInRFC="false" pn="section-6.3">
        <name slugifiedName="name-is-is-flexible-algorithm-inc">IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV</name>
        <t indent="0" pn="section-6.3-1">The Flexible Algorithm Definition can specify "colors" that are
        used by the operator to include links during the Flex-Algorithm path
        computation.</t>
        <t indent="0" pn="section-6.3-2">The IS-IS Flexible Algorithm Include-All Admin Group sub-TLV is used
        to advertise the include-all rule that is used during the Flex-Algorithm
        path calculation, as specified in <xref target="FLEXALGPATHCALC" format="default" sectionFormat="of" derivedContent="Section 13"/>.</t>
        <t indent="0" pn="section-6.3-3">The IS-IS Flexible Algorithm Include-All Admin Group
        sub-TLV is a sub-TLV of the IS-IS FAD sub-TLV. It has the following
        format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-6.3-4">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Type     |    Length     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Extended Admin Group                     |
+-                                                             -+
|                            ...                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-6.3-5">
          <dt pn="section-6.3-5.1">where:</dt>
          <dd pn="section-6.3-5.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-6.3-5.2.1">
              <dt pn="section-6.3-5.2.1.1">Type:</dt>
              <dd pn="section-6.3-5.2.1.2">3</dd>
              <dt pn="section-6.3-5.2.1.3">Length:</dt>
              <dd pn="section-6.3-5.2.1.4">variable, dependent on the size of the Extended Admin
              Group. <bcp14>MUST</bcp14> be a multiple of 4 octets.</dd>
              <dt pn="section-6.3-5.2.1.5">Extended Administrative Group:</dt>
              <dd pn="section-6.3-5.2.1.6">Extended Administrative Group, as
              defined in <xref target="RFC7308" format="default" sectionFormat="of" derivedContent="RFC7308"/>.</dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-6.3-6">The IS-IS Flexible Algorithm Include-All Admin Group sub-TLV <bcp14>MUST NOT</bcp14> appear more than once in a single IS-IS FAD sub-TLV. If it appears more
        than once, the IS-IS FAD sub-TLV <bcp14>MUST</bcp14> be ignored by the receiver.</t>
        <t indent="0" pn="section-6.3-7">The IS-IS Flexible Algorithm Include-All Admin Group sub-TLV <bcp14>MUST NOT</bcp14> appear 
        more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. 
        If it appears more than once in such a set, the IS-IS Flexible Algorithm Include-All Admin Group 
        sub-TLV in the first occurrence in the lowest-numbered LSP from a given IS <bcp14>MUST</bcp14> 
        be used, and any other occurrences <bcp14>MUST</bcp14> be ignored.</t>
      </section>
      <section anchor="ISISFLEXALGFLAG" numbered="true" toc="include" removeInRFC="false" pn="section-6.4">
        <name slugifiedName="name-is-is-flexible-algorithm-def">IS-IS Flexible Algorithm Definition Flags Sub-TLV</name>
        <t indent="0" pn="section-6.4-1">The IS-IS Flexible Algorithm Definition Flags (FADF) sub-TLV
        is a sub-TLV of the IS-IS FAD sub-TLV. It has the following format:
        </t>
        <artwork name="" type="" align="left" alt="" pn="section-6.4-2">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Type     |    Length     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Flags                             |
+-                                                             -+
|                            ...                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-6.4-3">
          <dt pn="section-6.4-3.1">where:</dt>
          <dd pn="section-6.4-3.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-6.4-3.2.1">
              <dt pn="section-6.4-3.2.1.1">Type:</dt>
              <dd pn="section-6.4-3.2.1.2">4</dd>
              <dt pn="section-6.4-3.2.1.3">Length:</dt>
              <dd pn="section-6.4-3.2.1.4">variable, number of octets of the Flags field.</dd>
              <dt pn="section-6.4-3.2.1.5">Flags:</dt>
              <dd pn="section-6.4-3.2.1.6">
                <artwork name="" type="" align="left" alt="" pn="section-6.4-3.2.1.6.1">
              0 1 2 3 4 5 6 7...
             +-+-+-+-+-+-+-+-+...
             |M| | |          ...
             +-+-+-+-+-+-+-+-+...
            </artwork>
                <dl newline="false" spacing="normal" indent="3" pn="section-6.4-3.2.1.6.2">
                  <dt pn="section-6.4-3.2.1.6.2.1">M-flag:</dt>
                  <dd pn="section-6.4-3.2.1.6.2.2">when set, the Flex-Algorithm-specific prefix metric
              <bcp14>MUST</bcp14> be used for inter-area and external prefix calculation.
              This flag is not applicable to prefixes advertised as SRv6
              locators.</dd>
                </dl>
              </dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-6.4-4">A new IANA "IGP Flexible Algorithm Definition Flags" registry is
        defined for allocation of bits in the Flags field -- 
        see <xref target="IANAFADFLGAS" format="default" sectionFormat="of" derivedContent="Section 18.2"/>.</t>
        <t indent="0" pn="section-6.4-5">Bits are defined/sent starting with bit 0 defined above. Additional
        bit definitions that may be defined in the future <bcp14>SHOULD</bcp14> be assigned
        in ascending bit order to minimize the number of bits that will
        need to be transmitted.</t>
        <t indent="0" pn="section-6.4-6">Undefined bits <bcp14>MUST</bcp14> be transmitted as 0.</t>
        <t indent="0" pn="section-6.4-7">Bits that are not transmitted <bcp14>MUST</bcp14> be treated as if they are set to
        0 on receipt.</t>
        <t indent="0" pn="section-6.4-8">The IS-IS FADF sub-TLV <bcp14>MUST NOT</bcp14> appear more than once in a single IS-IS FAD
        sub-TLV. If it appears more than once, the IS-IS FAD sub-TLV <bcp14>MUST</bcp14> be
        ignored by the receiver.</t>
        <t indent="0" pn="section-6.4-9">The IS-IS FADF sub-TLV <bcp14>MUST NOT</bcp14> appear more than once in the set of FAD 
        sub-TLVs for a given Flex-Algorithm from a given IS. If it appears more than once
        in such a set, the IS-IS FADF sub-TLV in the first occurrence in the lowest-numbered LSP from a given IS <bcp14>MUST</bcp14> be used, and any other occurrences <bcp14>MUST</bcp14> be ignored.</t>
        <t indent="0" pn="section-6.4-10">If the IS-IS FADF sub-TLV is not present inside the IS-IS FAD
        sub-TLV, all the bits are assumed to be set to 0.</t>
        <t indent="0" pn="section-6.4-11">If a node is configured to participate in a particular Flexible Algorithm, 
        but the selected Flex-Algorithm Definition includes a bit in the IS-IS FADF sub-TLV
        that is not supported by the node, it <bcp14>MUST</bcp14> stop participating in such
        Flexible Algorithm.</t>
        <t indent="0" pn="section-6.4-12">New flag bits may be defined in the future. Implementations <bcp14>MUST</bcp14> check all 
        advertised flag bits in the received IS-IS FADF sub-TLV -- not just the subset 
        currently defined.</t>
        <t indent="0" pn="section-6.4-13">The M-flag <bcp14>MUST</bcp14> not be used when calculating prefix reachability for the SRv6 
        Locator prefix.</t>
      </section>
      <section anchor="ISISFLEXALGEXSRLGTLV" numbered="true" toc="include" removeInRFC="false" pn="section-6.5">
        <name slugifiedName="name-is-is-flexible-algorithm-exc">IS-IS Flexible Algorithm Exclude SRLG Sub-TLV</name>
        <t indent="0" pn="section-6.5-1">The Flexible Algorithm Definition can specify Shared Risk Link
        Groups (SRLGs) that the operator wants to exclude during the
        Flex-Algorithm path computation.</t>
        <t indent="0" pn="section-6.5-2">The IS-IS Flexible Algorithm Exclude SRLG (FAESRLG) sub-TLV is used
        to advertise the exclude rule that is used during the Flex-Algorithm
        path calculation, as specified in <xref target="FLEXALGPATHCALC" format="default" sectionFormat="of" derivedContent="Section 13"/>.</t>
        <t indent="0" pn="section-6.5-3">The IS-IS FAESRLG sub-TLV is a sub-TLV of the IS-IS FAD sub-TLV. It
        has the following format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-6.5-4">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Type     |    Length     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Shared Risk Link Group Value             |
+-                                                             -+
|                            ...                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-6.5-5">
          <dt pn="section-6.5-5.1">where:</dt>
          <dd pn="section-6.5-5.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-6.5-5.2.1">
              <dt pn="section-6.5-5.2.1.1">Type:</dt>
              <dd pn="section-6.5-5.2.1.2">5</dd>
              <dt pn="section-6.5-5.2.1.3">Length:</dt>
              <dd pn="section-6.5-5.2.1.4">variable, dependent on number of SRLG values. <bcp14>MUST</bcp14> be a
              multiple of 4 octets.</dd>
              <dt pn="section-6.5-5.2.1.5">Shared Risk Link Group Value:</dt>
              <dd pn="section-6.5-5.2.1.6">SRLG value, as defined in <xref target="RFC5307" format="default" sectionFormat="of" derivedContent="RFC5307"/>.</dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-6.5-6">The IS-IS FAESRLG sub-TLV <bcp14>MUST NOT</bcp14> appear more than once in a single IS-IS FAD
        sub-TLV. If it appears more than once, the IS-IS FAD sub-TLV <bcp14>MUST</bcp14> be
        ignored by the receiver.</t>
        <t indent="0" pn="section-6.5-7">The IS-IS FAESRLG sub-TLV <bcp14>MAY</bcp14> appear more than once in the set of FAD 
        sub-TLVs for a given Flex-Algorithm from a given IS. This may be necessary in cases 
        where the total number of SRLG values that are specified cause the FAD sub-TLV to 
        exceed the maximum length of a single FAD sub-TLV. In such a case, the receiver <bcp14>MUST</bcp14> 
        use the union of all values across all IS-IS FAESRLG sub-TLVs from such set.</t>
      </section>
    </section>
    <section anchor="OSPFFADLTLVS" numbered="true" toc="include" removeInRFC="false" pn="section-7">
      <name slugifiedName="name-sub-tlvs-of-the-ospf-fad-tl">Sub-TLVs of the OSPF FAD TLV</name>
      <section anchor="OSPFFLEXALGEXLTLV" numbered="true" toc="include" removeInRFC="false" pn="section-7.1">
        <name slugifiedName="name-ospf-flexible-algorithm-exc">OSPF Flexible Algorithm Exclude Admin Group Sub-TLV</name>
        <t indent="0" pn="section-7.1-1">The OSPF Flexible Algorithm Exclude Admin Group (FAEAG) sub-TLV
        is a sub-TLV of the OSPF FAD TLV. Its usage is described in <xref target="ISISFLEXALGEXLTLV" format="default" sectionFormat="of" derivedContent="Section 6.1"/>. It has the following format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-7.1-2">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Type             |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Extended Admin Group                     |
+-                                                             -+
|                            ...                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-7.1-3">
          <dt pn="section-7.1-3.1">where:</dt>
          <dd pn="section-7.1-3.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-7.1-3.2.1">
              <dt pn="section-7.1-3.2.1.1">Type:</dt>
              <dd pn="section-7.1-3.2.1.2">1</dd>
              <dt pn="section-7.1-3.2.1.3">Length:</dt>
              <dd pn="section-7.1-3.2.1.4">variable, dependent on the size of the Extended Admin
		Group. <bcp14>MUST</bcp14> be a multiple of 4 octets.</dd>
              <dt pn="section-7.1-3.2.1.5">Extended Administrative Group:</dt>
              <dd pn="section-7.1-3.2.1.6">Extended Administrative Group, as
		defined in <xref target="RFC7308" format="default" sectionFormat="of" derivedContent="RFC7308"/>.</dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-7.1-4">The OSPF FAEAG sub-TLV <bcp14>MUST NOT</bcp14> appear more than once in an OSPF
        FAD TLV. If it appears more than once, the OSPF FAD TLV <bcp14>MUST</bcp14> be
        ignored by the receiver.</t>
      </section>
      <section anchor="OSPFFLEXALGINCANYTLV" numbered="true" toc="include" removeInRFC="false" pn="section-7.2">
        <name slugifiedName="name-ospf-flexible-algorithm-inc">OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV</name>
        <t indent="0" pn="section-7.2-1">The OSPF Flexible Algorithm Include-Any Admin Group
        sub-TLV is a sub-TLV of the OSPF FAD TLV. The usage of this sub-TLV is described in <xref target="ISISFLEXALGINCANYTLV" format="default" sectionFormat="of" derivedContent="Section 6.2"/>. It has the following format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-7.2-2">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Type             |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Extended Admin Group                     |
+-                                                             -+
|                            ...                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-7.2-3">
          <dt pn="section-7.2-3.1">where:</dt>
          <dd pn="section-7.2-3.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-7.2-3.2.1">
              <dt pn="section-7.2-3.2.1.1">Type: </dt>
              <dd pn="section-7.2-3.2.1.2">2</dd>
              <dt pn="section-7.2-3.2.1.3">Length:</dt>
              <dd pn="section-7.2-3.2.1.4">variable, dependent on the size of the Extended Admin
              Group. <bcp14>MUST</bcp14> be a multiple of 4 octets.</dd>
              <dt pn="section-7.2-3.2.1.5">Extended Administrative Group:</dt>
              <dd pn="section-7.2-3.2.1.6">Extended Administrative Group, as
              defined in <xref target="RFC7308" format="default" sectionFormat="of" derivedContent="RFC7308"/>.</dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-7.2-4">The OSPF Flexible Algorithm Include-Any Admin Group sub-TLV <bcp14>MUST NOT</bcp14> appear more than once in an OSPF FAD TLV. If it appears more than
        once, the OSPF FAD TLV <bcp14>MUST</bcp14> be ignored by the receiver.</t>
      </section>
      <section anchor="OSPFFLEXALGINCALLTLV" numbered="true" toc="include" removeInRFC="false" pn="section-7.3">
        <name slugifiedName="name-ospf-flexible-algorithm-incl">OSPF Flexible Algorithm Include-All Admin Group Sub-TLV</name>
        <t indent="0" pn="section-7.3-1">The OSPF Flexible Algorithm Include-All Admin Group
        sub-TLV is a sub-TLV of the OSPF FAD TLV. The usage of this sub-TLV is 
        described in <xref target="ISISFLEXALGINCALLTLV" format="default" sectionFormat="of" derivedContent="Section 6.3"/>. It has the following format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-7.3-2">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Type             |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Extended Admin Group                     |
+-                                                             -+
|                            ...                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-7.3-3">
          <dt pn="section-7.3-3.1">where:</dt>
          <dd pn="section-7.3-3.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-7.3-3.2.1">
              <dt pn="section-7.3-3.2.1.1">Type:</dt>
              <dd pn="section-7.3-3.2.1.2">3</dd>
              <dt pn="section-7.3-3.2.1.3">Length: </dt>
              <dd pn="section-7.3-3.2.1.4">variable, dependent on the size of the Extended Admin
              Group. <bcp14>MUST</bcp14> be a multiple of 4 octets.</dd>
              <dt pn="section-7.3-3.2.1.5">Extended Administrative Group:</dt>
              <dd pn="section-7.3-3.2.1.6">Extended Administrative Group, as
              defined in <xref target="RFC7308" format="default" sectionFormat="of" derivedContent="RFC7308"/>.</dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-7.3-4">The OSPF Flexible Algorithm Include-All Admin Group sub-TLV <bcp14>MUST NOT</bcp14> appear more than once in an OSPF FAD TLV. If it appears more than
        once, the OSPF FAD TLV <bcp14>MUST</bcp14> be ignored by the receiver.</t>
      </section>
      <section anchor="OSPFFLEXALGFLAG" numbered="true" toc="include" removeInRFC="false" pn="section-7.4">
        <name slugifiedName="name-ospf-flexible-algorithm-defi">OSPF Flexible Algorithm Definition Flags Sub-TLV</name>
        <t indent="0" pn="section-7.4-1">The OSPF Flexible Algorithm Definition Flags (FADF) sub-TLV
        is a sub-TLV of the OSPF FAD TLV. It has the following format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-7.4-2">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Type             |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Flags                             |
+-                                                             -+
|                            ...                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-7.4-3">
          <dt pn="section-7.4-3.1">where:</dt>
          <dd pn="section-7.4-3.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-7.4-3.2.1">
              <dt pn="section-7.4-3.2.1.1">Type:</dt>
              <dd pn="section-7.4-3.2.1.2">4</dd>
              <dt pn="section-7.4-3.2.1.3">Length:</dt>
              <dd pn="section-7.4-3.2.1.4">variable, dependent on the size of the Flags field.
            <bcp14>MUST</bcp14> be a multiple of 4 octets.</dd>
              <dt pn="section-7.4-3.2.1.5">Flags:</dt>
              <dd pn="section-7.4-3.2.1.6">
                <artwork name="" type="" align="left" alt="" pn="section-7.4-3.2.1.6.1">
              0 1 2 3 4 5 6 7...
             +-+-+-+-+-+-+-+-+...
             |M| | |          ...
             +-+-+-+-+-+-+-+-+...
            </artwork>
                <dl newline="false" spacing="normal" indent="3" pn="section-7.4-3.2.1.6.2">
                  <dt pn="section-7.4-3.2.1.6.2.1">M-flag:</dt>
                  <dd pn="section-7.4-3.2.1.6.2.2">when set, the Flex-Algorithm-specific prefix and
              ASBR metric <bcp14>MUST</bcp14> be used for inter-area and external
	      prefix calculation. This flag is not applicable to prefixes
              advertised as SRv6 locators.</dd>
                </dl>
              </dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-7.4-4">A new IANA "IGP Flexible Algorithm Definition Flags" registry is
        defined for allocation of bits in the Flags field -- 
        see <xref target="IANAFADFLGAS" format="default" sectionFormat="of" derivedContent="Section 18.2"/>.</t>
        <t indent="0" pn="section-7.4-5">Bits are defined/sent starting with bit 0 defined above. Additional
        bit definitions that may be defined in the future <bcp14>SHOULD</bcp14> be assigned
        in ascending bit order to minimize the number of bits that will
        need to be transmitted.</t>
        <t indent="0" pn="section-7.4-6">Undefined bits <bcp14>MUST</bcp14> be transmitted as 0.</t>
        <t indent="0" pn="section-7.4-7">Bits that are not transmitted <bcp14>MUST</bcp14> be treated as if they are set to
        0 on receipt.</t>
        <t indent="0" pn="section-7.4-8">The OSPF FADF sub-TLV <bcp14>MUST NOT</bcp14> appear more than once in an OSPF FAD
        TLV. If it appears more than once, the OSPF FAD TLV <bcp14>MUST</bcp14> be ignored by
        the receiver.</t>
        <t indent="0" pn="section-7.4-9">If the OSPF FADF sub-TLV is not present inside the OSPF FAD TLV,
        all the bits are assumed to be set to 0.</t>
        <t indent="0" pn="section-7.4-10">If a node is configured to participate in a particular Flexible Algorithm, 
        but the selected Flex-Algorithm Definition includes a bit in the OSPF FADF sub-TLV
        that is not supported by the node, it <bcp14>MUST</bcp14> stop participating in such
        Flexible Algorithm.</t>
        <t indent="0" pn="section-7.4-11">New flag bits may be defined in the future. Implementations <bcp14>MUST</bcp14> check all 
        advertised flag bits in the received OSPF FADF sub-TLV -- not just the subset 
        currently defined.</t>
        <t indent="0" pn="section-7.4-12">The M-flag <bcp14>MUST</bcp14> not be used when calculating prefix reachability for the SRv6 
        Locator prefix.</t>
      </section>
      <section anchor="OSPFFLEXALGEXSRLGTLV" numbered="true" toc="include" removeInRFC="false" pn="section-7.5">
        <name slugifiedName="name-ospf-flexible-algorithm-excl">OSPF Flexible Algorithm Exclude SRLG Sub-TLV</name>
        <t indent="0" pn="section-7.5-1">The OSPF Flexible Algorithm Exclude SRLG (FAESRLG) sub-TLV
        is a sub-TLV of the OSPF FAD TLV. Its usage is described in <xref target="ISISFLEXALGEXSRLGTLV" format="default" sectionFormat="of" derivedContent="Section 6.5"/>. It has the following format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-7.5-2">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Type             |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Shared Risk Link Group Value                |
+-                                                             -+
|                            ...                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-7.5-3">
          <dt pn="section-7.5-3.1">where:</dt>
          <dd pn="section-7.5-3.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-7.5-3.2.1">
              <dt pn="section-7.5-3.2.1.1">Type:</dt>
              <dd pn="section-7.5-3.2.1.2">5</dd>
              <dt pn="section-7.5-3.2.1.3">Length:</dt>
              <dd pn="section-7.5-3.2.1.4">variable, dependent on the number of SRLGs. <bcp14>MUST</bcp14> be a
              multiple of 4 octets.</dd>
              <dt pn="section-7.5-3.2.1.5">Shared Risk Link Group Value:</dt>
              <dd pn="section-7.5-3.2.1.6"> SRLG value, as defined in <xref target="RFC4203" format="default" sectionFormat="of" derivedContent="RFC4203"/>.</dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-7.5-4">The OSPF FAESRLG sub-TLV <bcp14>MUST NOT</bcp14> appear more than once in an OSPF
        FAD TLV. If it appears more than once, the OSPF FAD TLV <bcp14>MUST</bcp14> be
        ignored by the receiver.</t>
      </section>
    </section>
    <section anchor="ISISFLEXMETRIC" numbered="true" toc="include" removeInRFC="false" pn="section-8">
      <name slugifiedName="name-is-is-flexible-algorithm-pr">IS-IS Flexible Algorithm Prefix Metric Sub-TLV</name>
      <t indent="0" pn="section-8-1">The IS-IS Flexible Algorithm Prefix Metric (FAPM) sub-TLV supports the
      advertisement of a Flex-Algorithm-specific prefix metric associated with
      a given prefix advertisement.</t>
      <t indent="0" pn="section-8-2">The IS-IS FAPM sub-TLV is a sub-TLV of TLVs 135, 235, 236, and 237 and
      has the following format: </t>
      <artwork name="" type="" align="left" alt="" pn="section-8-3">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Type     |    Length     |Flex-Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Metric                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
      <dl newline="true" spacing="normal" indent="3" pn="section-8-4">
        <dt pn="section-8-4.1">where:</dt>
        <dd pn="section-8-4.2">
          <dl newline="false" spacing="normal" indent="3" pn="section-8-4.2.1">
            <dt pn="section-8-4.2.1.1">Type:</dt>
            <dd pn="section-8-4.2.1.2">6</dd>
            <dt pn="section-8-4.2.1.3">Length:</dt>
            <dd pn="section-8-4.2.1.4">5 octets</dd>
            <dt pn="section-8-4.2.1.5">Flex-Algorithm:</dt>
            <dd pn="section-8-4.2.1.6">single octet value between 128 and 255
            inclusive.</dd>
            <dt pn="section-8-4.2.1.7">Metric:</dt>
            <dd pn="section-8-4.2.1.8">4 octets of metric information.</dd>
          </dl>
        </dd>
      </dl>
      <t indent="0" pn="section-8-5">The IS-IS FAPM sub-TLV <bcp14>MAY</bcp14> appear multiple times in its parent TLV. If
      it appears more than once with the same Flex-Algorithm value, the first
      instance <bcp14>MUST</bcp14> be used and any subsequent instances <bcp14>MUST</bcp14> be ignored.</t>
      <t indent="0" pn="section-8-6">If a prefix is advertised with a Flex-Algorithm prefix metric larger
      than MAX_PATH_METRIC, as defined in <xref target="RFC5305" format="default" sectionFormat="of" derivedContent="RFC5305"/>, this prefix
      <bcp14>MUST NOT</bcp14> be considered during the Flexible Algorithm computation.</t>
      <t indent="0" pn="section-8-7">The usage of the Flex-Algorithm prefix metric is described in <xref target="FLEXALGPATHCALC" format="default" sectionFormat="of" derivedContent="Section 13"/>.</t>
      <t indent="0" pn="section-8-8">The IS-IS FAPM sub-TLV <bcp14>MUST NOT</bcp14> be advertised as a sub-TLV of the IS-IS
      SRv6 Locator TLV <xref target="RFC9352" format="default" sectionFormat="of" derivedContent="RFC9352"/>. The
      IS-IS SRv6 Locator TLV includes the Algorithm and Metric fields, which
      <bcp14>MUST</bcp14> be used instead. If the FAPM sub-TLV is present as a sub-TLV of the
      IS-IS SRv6 Locator TLV in the received LSP, such FAPM sub-TLV <bcp14>MUST</bcp14> be
      ignored.</t>
    </section>
    <section anchor="OSPFFLEXMETRIC" numbered="true" toc="include" removeInRFC="false" pn="section-9">
      <name slugifiedName="name-ospf-flexible-algorithm-pre">OSPF Flexible Algorithm Prefix Metric Sub-TLV</name>
      <t indent="0" pn="section-9-1">The OSPF Flexible Algorithm Prefix Metric (FAPM) sub-TLV supports the
      advertisement of a Flex-Algorithm-specific prefix metric associated with
      a given prefix advertisement.</t>
      <t indent="0" pn="section-9-2">The OSPF FAPM sub-TLV is a sub-TLV of the:</t>
      <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-9-3">
        <li pn="section-9-3.1">OSPFv2 Extended Prefix TLV <xref target="RFC7684" format="default" sectionFormat="of" derivedContent="RFC7684"/>
	and</li>
        <li pn="section-9-3.2">
          <t indent="0" pn="section-9-3.2.1">following OSPFv3 TLVs, as defined in <xref target="RFC8362" format="default" sectionFormat="of" derivedContent="RFC8362"/>:</t>
          <ul bare="false" empty="false" indent="3" spacing="normal" pn="section-9-3.2.2">
            <li pn="section-9-3.2.2.1">Inter-Area Prefix TLV</li>
            <li pn="section-9-3.2.2.2">External-Prefix TLV</li>
          </ul>
        </li>
      </ul>
      <t indent="0" pn="section-9-4">The OSPF FAPM sub-TLV has the following format: </t>
      <artwork name="" type="" align="left" alt="" pn="section-9-5">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Type             |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm |     Flags     |            Reserved           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Metric                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
      <dl newline="true" spacing="normal" indent="3" pn="section-9-6">
        <dt pn="section-9-6.1">where:</dt>
        <dd pn="section-9-6.2">
          <dl newline="false" spacing="normal" indent="3" pn="section-9-6.2.1">
            <dt pn="section-9-6.2.1.1">Type:</dt>
            <dd pn="section-9-6.2.1.2">3 for OSPFv2, and 26 for OSPFv3</dd>
            <dt pn="section-9-6.2.1.3">Length:</dt>
            <dd pn="section-9-6.2.1.4">8 octets</dd>
            <dt pn="section-9-6.2.1.5">Flex-Algorithm:</dt>
            <dd pn="section-9-6.2.1.6">single octet value between 128 and 255
            inclusive.</dd>
            <dt pn="section-9-6.2.1.7">Flags:</dt>
            <dd pn="section-9-6.2.1.8">
              <t indent="0" pn="section-9-6.2.1.8.1">1-octet value</t>
              <artwork name="" type="" align="left" alt="" pn="section-9-6.2.1.8.2">
              0 1 2 3 4 5 6 7
             +-+-+-+-+-+-+-+-+
             |E|             |
             +-+-+-+-+-+-+-+-+
            </artwork>
              <dl newline="false" spacing="normal" indent="3" pn="section-9-6.2.1.8.3">
                <dt pn="section-9-6.2.1.8.3.1">E bit:</dt>
                <dd pn="section-9-6.2.1.8.3.2">position 0: The type of external metric. If the bit is
            set, the metric specified is a Type 2 external metric. This bit
            is applicable only to OSPF external and Not-So-Stubby Area (NSSA) external prefixes.
            This is semantically the same as the E bit in <xref target="RFC2328" format="default" sectionFormat="of" section="A.4.5" derivedLink="https://rfc-editor.org/rfc/rfc2328#appendix-A.4.5" derivedContent="RFC2328"/> and <xref target="RFC5340" format="default" sectionFormat="of" section="A.4.7" derivedLink="https://rfc-editor.org/rfc/rfc5340#appendix-A.4.7" derivedContent="RFC5340"/> for OSPFv2 and OSPFv3, respectively.</dd>
                <dt pn="section-9-6.2.1.8.3.3">Bits 1 through 7:</dt>
                <dd pn="section-9-6.2.1.8.3.4">
                  <bcp14>MUST</bcp14> be cleared by the originator and ignored by
            the receiver.</dd>
              </dl>
            </dd>
            <dt pn="section-9-6.2.1.9">Reserved:</dt>
            <dd pn="section-9-6.2.1.10">
              <bcp14>MUST</bcp14> be set to 0 and ignored at reception.</dd>
            <dt pn="section-9-6.2.1.11">Metric:</dt>
            <dd pn="section-9-6.2.1.12">4 octets of metric information.</dd>
          </dl>
        </dd>
      </dl>
      <t indent="0" pn="section-9-7">The OSPF FAPM sub-TLV <bcp14>MAY</bcp14> appear multiple times in its parent TLV. If
      it appears more than once with the same Flex-Algorithm value, the first
      instance <bcp14>MUST</bcp14> be used and any subsequent instances <bcp14>MUST</bcp14> be ignored.</t>
      <t indent="0" pn="section-9-8">The usage of the Flex-Algorithm prefix metric is described in <xref target="FLEXALGPATHCALC" format="default" sectionFormat="of" derivedContent="Section 13"/>.</t>
    </section>
    <section anchor="OSPFASBR" numbered="true" toc="include" removeInRFC="false" pn="section-10">
      <name slugifiedName="name-ospf-flexible-algorithm-asb">OSPF Flexible Algorithm ASBR Reachability Advertisement</name>
      <t indent="0" pn="section-10-1">An OSPF ABR advertises the reachability of ASBRs in its attached
      areas to enable routers within those areas to perform route calculations
      for external prefixes advertised by the ASBRs. OSPF extensions for
      advertisement of Flex-Algorithm-specific reachability and the metric for
      ASBRs is similarly required for Flex-Algorithm external prefix
      computations, as described further in <xref target="FLEXALGPATHCALCINTER" format="default" sectionFormat="of" derivedContent="Section 13.1"/>.</t>
      <section anchor="OSPFEXTASBRLSA" numbered="true" toc="include" removeInRFC="false" pn="section-10.1">
        <name slugifiedName="name-ospfv2-extended-inter-area-">OSPFv2 Extended Inter-Area ASBR LSA</name>
        <t indent="0" pn="section-10.1-1">The OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) LSA is an OSPF
        Opaque LSA <xref target="RFC5250" format="default" sectionFormat="of" derivedContent="RFC5250"/> that is used to advertise
        additional attributes related to the reachability of the OSPFv2 ASBR
        that is external to the area yet internal to the OSPF domain.
        Semantically, the OSPFv2 EIA-ASBR LSA is equivalent to the fixed
        format Type 4 summary-LSA <xref target="RFC2328" format="default" sectionFormat="of" derivedContent="RFC2328"/>. Unlike the Type 4
        summary-LSA, the Link State ID (LSID) of the EIA-ASBR LSA does not carry the ASBR
        Router ID -- the ASBR Router ID is carried in the body of the LSA.
        The OSPFv2 EIA-ASBR LSA is advertised by an OSPFv2 ABR, and its flooding is
        defined to be area-scoped only.</t>
        <t indent="0" pn="section-10.1-2">An OSPFv2 ABR generates the EIA-ASBR LSA for an ASBR when it is
        advertising the Type 4 summary-LSA for it and has the need for
        advertising additional attributes for that ASBR beyond what is
        conveyed in the fixed-format Type 4 summary-LSA. An OSPFv2 ABR <bcp14>MUST NOT</bcp14> advertise the EIA-ASBR LSA for an ASBR for which it is not
        advertising the Type 4 summary-LSA. This ensures that the ABR does not
        generate the EIA-ASBR LSA for an ASBR to which it does not have
        reachability in the base OSPFv2 topology calculation. The OSPFv2 ABR
        <bcp14>SHOULD NOT</bcp14> advertise the EIA-ASBR LSA for an ASBR when it does not
        have additional attributes to advertise for that ASBR.</t>
        <t indent="0" pn="section-10.1-3">The OSPFv2 EIA-ASBR LSA has the following format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-10.1-4">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            LS age             |     Options   |   LS Type     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Opaque Type  |                 Opaque ID                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Advertising Router                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     LS sequence number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         LS checksum           |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+-                            TLVs                             -+
|                             ...                               |
</artwork>
        <t indent="0" pn="section-10.1-5">The LS age and Options fields are as defined in
        <xref target="RFC2328" format="default" sectionFormat="of" section="A.4.1" derivedLink="https://rfc-editor.org/rfc/rfc2328#appendix-A.4.1" derivedContent="RFC2328"/>.</t>
        <t indent="0" pn="section-10.1-6">The LS Type <bcp14>MUST</bcp14> be 10, indicating that the Opaque
        LSA flooding scope is area-local <xref target="RFC5250" format="default" sectionFormat="of" derivedContent="RFC5250"/>.</t>
        <t indent="0" pn="section-10.1-7">The Opaque Type used by the OSPFv2 EIA-ASBR LSA is 11. 
        The Opaque Type is used to differentiate the various types
        of OSPFv2 Opaque LSAs and is described in <xref target="RFC5250" section="3" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5250#section-3" derivedContent="RFC5250"/>.</t>
        <t indent="0" pn="section-10.1-8">The Opaque ID field is an arbitrary value used to maintain multiple
        OSPFv2 EIA-ASBR LSAs. For OSPFv2 EIA-ASBR LSAs, the Opaque ID has no
        semantic significance other than to differentiate OSPFv2 EIA-ASBR LSAs
        originated by the same OSPFv2 ABR. If multiple OSPFv2 EIA-ASBR LSAs
        specify the same ASBR, the attributes from the Opaque LSA with the
        lowest Opaque ID <bcp14>SHOULD</bcp14> be used.</t>
        <t indent="0" pn="section-10.1-9">The Advertising Router, LS sequence number, and LS checksum fields are as defined in 
        <xref target="RFC2328" format="default" sectionFormat="of" section="A.4.1" derivedLink="https://rfc-editor.org/rfc/rfc2328#appendix-A.4.1" derivedContent="RFC2328"/>.</t>
        <t indent="0" pn="section-10.1-10">The Length field is as defined in <xref target="RFC2328" format="default" sectionFormat="of" section="A.4.1" derivedLink="https://rfc-editor.org/rfc/rfc2328#appendix-A.4.1" derivedContent="RFC2328"/>.
        It represents the total length (in octets) of the Opaque LSA, including the 
        LSA header and all TLVs (including padding).</t>
        <t indent="0" pn="section-10.1-11">The format of the TLVs within the body of the OSPFv2 EIA-ASBR LSA
        is the same as the format used by the Traffic Engineering Extensions
        to OSPFv2 <xref target="RFC3630" format="default" sectionFormat="of" derivedContent="RFC3630"/>. The variable TLV section consists
        of one or more nested TLV tuples. Nested TLVs are also referred to as
        sub-TLVs. The TLV Length field defines the length of the value portion in
        octets (thus, a TLV with no value portion would have a length of 0).
        The TLV is padded to 4-octet alignment; padding is not included in the
        Length field (so a 3-octet value would have a length of 3, but the
        total size of the TLV would be 8 octets). Nested TLVs are also 32-bit
        aligned. For example, a 1-octet value would have the Length field set
        to 1, and 3 octets of padding would be added to the end of the value
        portion of the TLV. The padding is composed of zeros.</t>
        <section anchor="OSPFEXTASBRTLV" numbered="true" toc="include" removeInRFC="false" pn="section-10.1.1">
          <name slugifiedName="name-ospfv2-extended-inter-area-a">OSPFv2 Extended Inter-Area ASBR TLV</name>
          <t indent="0" pn="section-10.1.1-1">The OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) TLV is a top-level
          TLV of the OSPFv2 EIA-ASBR LSA and is used to advertise additional
          attributes associated with the reachability of an ASBR.</t>
          <t indent="0" pn="section-10.1.1-2">The OSPFv2 EIA-ASBR TLV has the following format:</t>
          <artwork name="" type="" align="left" alt="" pn="section-10.1.1-3">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             Type              |           Length              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        ASBR Router ID                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.                                                               .
.                            Sub-TLVs                           .
.                                                               .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  </artwork>
          <dl newline="true" spacing="normal" indent="3" pn="section-10.1.1-4">
            <dt pn="section-10.1.1-4.1">where:</dt>
            <dd pn="section-10.1.1-4.2">
              <dl newline="false" spacing="normal" indent="3" pn="section-10.1.1-4.2.1">
                <dt pn="section-10.1.1-4.2.1.1">Type:</dt>
                <dd pn="section-10.1.1-4.2.1.2">1</dd>
                <dt pn="section-10.1.1-4.2.1.3">Length:</dt>
                <dd pn="section-10.1.1-4.2.1.4">variable number of octets.</dd>
                <dt pn="section-10.1.1-4.2.1.5">ASBR Router ID:</dt>
                <dd pn="section-10.1.1-4.2.1.6">4 octets carrying the OSPF Router ID of
		the ASBR whose information is being carried.</dd>
                <dt pn="section-10.1.1-4.2.1.7">Sub-TLVs:</dt>
                <dd pn="section-10.1.1-4.2.1.8">variable</dd>
              </dl>
            </dd>
          </dl>
          <t indent="0" pn="section-10.1.1-5">Only a single OSPFv2 EIA-ASBR TLV <bcp14>MUST</bcp14> be advertised in each
          OSPFv2 EIA-ASBR LSA, and the receiver <bcp14>MUST</bcp14> ignore all instances of
          this TLV other than the first one in an LSA.</t>
          <t indent="0" pn="section-10.1.1-6">The OSPFv2 EIA-ASBR TLV <bcp14>MUST</bcp14> be present inside an OSPFv2 EIA-ASBR LSA
          and <bcp14>MUST</bcp14> include at least a single sub-TLV; otherwise, the OSPFv2
          EIA-ASBR LSA <bcp14>MUST</bcp14> be ignored by the receiver.</t>
        </section>
      </section>
      <section anchor="OSPFFAASBRMETRIC" numbered="true" toc="include" removeInRFC="false" pn="section-10.2">
        <name slugifiedName="name-ospf-flexible-algorithm-asbr">OSPF Flexible Algorithm ASBR Metric Sub-TLV</name>
        <t indent="0" pn="section-10.2-1">The OSPF Flexible Algorithm ASBR Metric (FAAM) sub-TLV supports the
        advertisement of a Flex-Algorithm-specific metric associated with a
        given ASBR reachability advertisement by an ABR.</t>
        <t indent="0" pn="section-10.2-2">The OSPF FAAM sub-TLV is a sub-TLV of
        the: </t>
        <ul bare="false" empty="false" indent="3" spacing="normal" pn="section-10.2-3">
          <li pn="section-10.2-3.1">OSPFv2 Extended Inter-Area ASBR TLV, as defined in <xref target="OSPFEXTASBRTLV" format="default" sectionFormat="of" derivedContent="Section 10.1.1"/>, and</li>
          <li pn="section-10.2-3.2">OSPFv3 Inter-Area-Router TLV, as defined in <xref target="RFC8362" format="default" sectionFormat="of" derivedContent="RFC8362"/>.</li>
        </ul>
        <t indent="0" pn="section-10.2-4">The OSPF FAAM sub-TLV has the following format: </t>
        <artwork name="" type="" align="left" alt="" pn="section-10.2-5">
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Type             |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm |                   Reserved                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Metric                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
        <dl newline="true" spacing="normal" indent="3" pn="section-10.2-6">
          <dt pn="section-10.2-6.1">where:</dt>
          <dd pn="section-10.2-6.2">
            <dl newline="false" spacing="normal" indent="3" pn="section-10.2-6.2.1">
              <dt pn="section-10.2-6.2.1.1">Type:</dt>
              <dd pn="section-10.2-6.2.1.2">1 for OSPFv2, and 33 for OSPFv3</dd>
              <dt pn="section-10.2-6.2.1.3">Length:</dt>
              <dd pn="section-10.2-6.2.1.4">8 octets</dd>
              <dt pn="section-10.2-6.2.1.5">Flex-Algorithm:</dt>
              <dd pn="section-10.2-6.2.1.6">single octet value between 128 and 255
              inclusive.</dd>
              <dt pn="section-10.2-6.2.1.7">Reserved:</dt>
              <dd pn="section-10.2-6.2.1.8">3 octets. <bcp14>MUST</bcp14> be set to 0 and ignored at
	      reception.</dd>
              <dt pn="section-10.2-6.2.1.9">Metric:</dt>
              <dd pn="section-10.2-6.2.1.10">4 octets of metric information.</dd>
            </dl>
          </dd>
        </dl>
        <t indent="0" pn="section-10.2-7">The OSPF FAAM sub-TLV <bcp14>MAY</bcp14> appear multiple times in its parent TLV.
        If it appears more than once with the same Flex-Algorithm value, the
        first instance <bcp14>MUST</bcp14> be used and any subsequent instances <bcp14>MUST</bcp14> be
        ignored.</t>
        <t indent="0" pn="section-10.2-8">The advertisement of the ASBR reachability using the OSPF FAAM
        sub-TLV inside the OSPFv2 EIA-ASBR LSA follows <xref target="RFC2328" section="12.4.3" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc2328#section-12.4.3" derivedContent="RFC2328"/> and inside the OSPFv3 E-Inter-Area-Router-LSA
        follows <xref target="RFC5340" section="4.8.5" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5340#section-4.8.5" derivedContent="RFC5340"/>. The
        reachability of the ASBR is evaluated in the context of the specific
        Flex-Algorithm.</t>
        <t indent="0" pn="section-10.2-9">The FAAM computed by the ABR will be equal to the metric to reach
        the ASBR for a given Flex-Algorithm in a source area or the cumulative
        metric via an ABR(s) when the ASBR is in a remote area. This is
        similar in nature to how the metric is set when the ASBR reachability
        metric is computed in the default algorithm for the metric in the
        OSPFv2 Type 4 ASBR summary-LSA and the OSPFv3 Inter-Area-Router-LSA.</t>
        <t indent="0" pn="section-10.2-10">An OSPF ABR <bcp14>MUST NOT</bcp14> include the OSPF FAAM sub-TLV with a specific
        Flex-Algorithm in its reachability advertisement for an ASBR between
        areas unless that ASBR is reachable for it in the context of that
        specific Flex-Algorithm.</t>
        <t indent="0" pn="section-10.2-11">An OSPF ABR <bcp14>MUST</bcp14> include the OSPF FAAM sub-TLVs as part of the ASBR
        reachability advertisement between areas for any Flex-Algorithm for
        which the winning FAD includes the M-flag and the ASBR is reachable 
        in the context of that specific Flex-Algorithm.</t>
        <t indent="0" pn="section-10.2-12">OSPF routers <bcp14>MUST</bcp14> use the OSPF FAAM sub-TLV to calculate the
        reachability of the ASBRs if the winning FAD for the specific Flex-
        Algorithm includes the M-flag. OSPF routers <bcp14>MUST NOT</bcp14> use the OSPF FAAM
        sub-TLV to calculate the reachability of the ASBRs for the specific
        Flex-Algorithm if the winning FAD for such Flex-Algorithm does not
        include the M-flag. Instead, the OSPFv2 Type 4 summary-LSAs or the
        OSPFv3 Inter-Area-Router-LSAs <bcp14>MUST</bcp14> be used, as specified in
        <xref target="RFC2328" section="16.2" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc2328#section-16.2" derivedContent="RFC2328"/> and <xref target="RFC5340" section="4.8.5" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5340#section-4.8.5" derivedContent="RFC5340"/> for OSPFv2 and OSPFv3, respectively.</t>
        <t indent="0" pn="section-10.2-13">The processing of a new or changed OSPF FAAM sub-TLV triggers the
        processing of external routes similar to what is described in
        <xref target="RFC2328" format="default" sectionFormat="of" section="16.5" derivedLink="https://rfc-editor.org/rfc/rfc2328#section-16.5" derivedContent="RFC2328"/> for OSPFv2 and <xref target="RFC5340" section="4.8.5" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5340#section-4.8.5" derivedContent="RFC5340"/> for OSPFv3 for the specific
        Flex-Algorithm.
	The OSPF external and NSSA external route calculation
        should be limited to a Flex-Algorithm(s) for which the winning FAD(s)
        includes the M-flag.</t>
        <t indent="0" pn="section-10.2-14">Processing of the OSPF FAAM sub-TLV does not require the existence
        of the equivalent OSPFv2 Type 4 summary-LSA or the OSPFv3
        Inter-Area-Router-LSA that is advertised by the same ABR inside the
        area. The presence of the base LSA is not mandatory for the usage of the 
        extended LSA with the OSPF FAAM sub-TLV.</t>
      </section>
    </section>
    <section anchor="FLEXALGPART" numbered="true" toc="include" removeInRFC="false" pn="section-11">
      <name slugifiedName="name-advertisement-of-node-parti">Advertisement of Node Participation in a Flex-Algorithm</name>
      <t indent="0" pn="section-11-1">When a router is configured to participate in a particular Flex-Algorithm and
      is advertising such participation, it is participating in that Flex-Algorithm.</t>
      <t indent="0" pn="section-11-2">Paths for various data planes <bcp14>MAY</bcp14> be computed for a specific Flex-Algorithm. 
      Each data plane uses its own specific forwarding over such Flex-Algorithm paths. 
      To guarantee the presence of the data-plane-specific forwarding, associated with 
      a particular Flex-Algorithm, a router <bcp14>MUST</bcp14> advertise its participation for a
      particular Flex-Algorithm for each data plane. Some data planes may share a common 
      participation advertisement (e.g., SR-MPLS and SRv6).</t>
      <t indent="0" pn="section-11-3">Advertisement of the participation for any particular Flex-Algorithm in any 
      data plane is subject to the condition specified in 
        <xref target="COMMONLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.3"/>.</t>
      <section anchor="FLEXALGPARTSR" numbered="true" toc="include" removeInRFC="false" pn="section-11.1">
        <name slugifiedName="name-advertisement-of-node-partic">Advertisement of Node Participation for Segment Routing</name>
        <t indent="0" pn="section-11.1-1"><xref target="RFC8665" format="default" sectionFormat="of" derivedContent="RFC8665"/>, <xref target="RFC8666" format="default" sectionFormat="of" derivedContent="RFC8666"/>, and <xref target="RFC8667" format="default" sectionFormat="of" derivedContent="RFC8667"/> (IGP Segment Routing extensions) describe how the
        SR-Algorithm is used to compute the IGP best path.</t>
        <t indent="0" pn="section-11.1-2">Routers advertise support for the SR-Algorithm as a node
        capability, as described in the above-mentioned IGP Segment Routing
        extensions. To advertise participation for a particular Flex-Algorithm
        for Segment Routing, including both SR-MPLS and SRv6, the
        Flex-Algorithm value <bcp14>MUST</bcp14> be advertised in the SR-Algorithm TLV (OSPF)
        or sub-TLV (IS-IS).</t>
        <t indent="0" pn="section-11.1-3">Segment Routing Flex-Algorithm participation advertisement is
        topology independent. When a router advertises participation in an
        SR-Algorithm, the participation applies to all topologies in which the
        advertising node participates.</t>
      </section>
      <section anchor="FLEXALGPARTOTHER" numbered="true" toc="include" removeInRFC="false" pn="section-11.2">
        <name slugifiedName="name-advertisement-of-node-partici">Advertisement of Node Participation for Other Data Planes</name>
        <t indent="0" pn="section-11.2-1">This section describes considerations related to how other
        data planes can advertise their participation in a specific
        Flex-Algorithm.</t>
        <t indent="0" pn="section-11.2-2">Data-plane-specific Flex-Algorithm participation advertisements
        <bcp14>MAY</bcp14> be topology specific or <bcp14>MAY</bcp14> be topology independent, depending on
        the data plane itself.</t>
        <t indent="0" pn="section-11.2-3">Data-plane-specific advertisement for Flex-Algorithm participation
        <bcp14>MUST</bcp14> be defined for each data plane and is outside the scope of
        this document.</t>
      </section>
    </section>
    <section anchor="FLEXALGLINKATTR" numbered="true" toc="include" removeInRFC="false" pn="section-12">
      <name slugifiedName="name-advertisement-of-link-attri">Advertisement of Link Attributes for Flex-Algorithm</name>
      <t indent="0" pn="section-12-1">Various link attributes may be used during the Flex-Algorithm path
      calculation. For example, include or exclude rules based on link
      affinities can be part of the Flex-Algorithm Definition, as defined in Sections
      <xref target="ISISFADLTLVS" format="counter" sectionFormat="of" derivedContent="6"/> and <xref target="OSPFFADLTLVS" format="counter" sectionFormat="of" derivedContent="7"/>.</t>
      <t indent="0" pn="section-12-2">Application-specific link attributes, as specified in 
      <xref target="RFC8919" format="default" sectionFormat="of" derivedContent="RFC8919"/> or <xref target="RFC8920" format="default" sectionFormat="of" derivedContent="RFC8920"/>, 
      that are to be used during Flex-Algorithm calculation <bcp14>MUST</bcp14> use the 
      Application-Specific Link Attribute (ASLA) advertisements defined in
      <xref target="RFC8919" format="default" sectionFormat="of" derivedContent="RFC8919"/> or  <xref target="RFC8920" format="default" sectionFormat="of" derivedContent="RFC8920"/> unless, in the case of IS-IS, 
      the L-flag is set in the ASLA advertisement. When the L-flag is set, then legacy 
      advertisements <bcp14>MUST</bcp14> be used, subject to the procedures and constraints defined in 
      <xref target="RFC8919" section="4.2" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8919#section-4.2" derivedContent="RFC8919"/> and <xref target="ISISFADLTLVS" format="default" sectionFormat="of" derivedContent="Section 6"/>.</t>
      <t indent="0" pn="section-12-3">The mandatory use of ASLA advertisements applies to link attributes
      specifically mentioned in this document (Min Unidirectional Link Delay,
      TE Default Metric, Administrative Group, Extended Administrative Group,
      and Shared Risk Link Group) and any other link attributes that may be
      used in support of Flex-Algorithm in the future.</t>
      <t indent="0" pn="section-12-4">A new Application Identifier Bit is defined to indicate that the ASLA
      advertisement is associated with the Flex-Algorithm application. This
      bit is set in the Standard Application Bit Mask (SABM) defined in <xref target="RFC8919" format="default" sectionFormat="of" derivedContent="RFC8919"/> or <xref target="RFC8920" format="default" sectionFormat="of" derivedContent="RFC8920"/>: </t>
      <dl newline="false" spacing="normal" indent="3" pn="section-12-5">
        <dt pn="section-12-5.1">Bit 3:</dt>
        <dd pn="section-12-5.2">Flexible Algorithm (X-bit)</dd>
      </dl>
      <t indent="0" pn="section-12-6">ASLA Admin Group Advertisements to be used by the Flexible Algorithm
      application <bcp14>MAY</bcp14> use either the Administrative Group or Extended
      Administrative Group encodings.</t>
      <t indent="0" pn="section-12-7">A receiver supporting this specification <bcp14>MUST</bcp14> accept both ASLA 
      Administrative Group and Extended Administrative Group TLVs, as defined in
      <xref target="RFC8919" format="default" sectionFormat="of" derivedContent="RFC8919"/> or <xref target="RFC8920" format="default" sectionFormat="of" derivedContent="RFC8920"/>. In the case of IS-IS, if 
      the L-flag is set in the ASLA advertisement, as defined in <xref target="RFC8919" section="4.2" sectionFormat="of" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8919#section-4.2" derivedContent="RFC8919"/>, then the receiver <bcp14>MUST</bcp14> be able to accept both the Administrative Group TLV, 
      as defined in <xref target="RFC5305" format="default" sectionFormat="of" derivedContent="RFC5305"/>, and the Extended Administrative Group TLV, as 
      defined in <xref target="RFC7308" format="default" sectionFormat="of" derivedContent="RFC7308"/>.</t>
    </section>
    <section anchor="FLEXALGPATHCALC" numbered="true" toc="include" removeInRFC="false" pn="section-13">
      <name slugifiedName="name-calculation-of-flexible-alg">Calculation of Flexible Algorithm Paths</name>
      <t indent="0" pn="section-13-1">A router <bcp14>MUST</bcp14> be configured to participate in a given Flex-Algorithm
      K and <bcp14>MUST</bcp14> select the FAD based on the rules defined in <xref target="COMMONLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.3"/> before it can compute any path for that
      Flex-Algorithm.</t>
      <t indent="0" pn="section-13-2">No specific two-way connectivity check is performed during the Flex-Algorithm  
      path computation. The result of the existing Flex-Algorithm-agnostic, two-way 
      connectivity check is used during the Flex-Algorithm path computation.</t>
      <t indent="0" pn="section-13-3">As described in <xref target="FLEXALGPART" format="default" sectionFormat="of" derivedContent="Section 11"/>, participation for any
      particular Flex-Algorithm <bcp14>MUST</bcp14> be advertised on a per data plane basis.
      Calculation of the paths for any particular Flex-Algorithm is
      data plane specific.</t>
      <t indent="0" pn="section-13-4">Multiple data planes <bcp14>MAY</bcp14> use the same Flex-Algorithm value at the
      same time and, as such, share the FAD for it. Traffic for each data plane
      will be forwarded based on the data-plane-specific forwarding entries.</t>
      <t indent="0" pn="section-13-5">The Flex-Algorithm Definition is data plane independent and is used by
      all Flex-Algorithm data planes.</t>
      <t indent="0" pn="section-13-6">The way various data planes handle nodes that do not participate in
      Flexible Algorithm is data plane specific. If the data plane only
      wants to consider participating nodes during the Flex-Algorithm
      calculation, then when computing paths for a given Flex-Algorithm, all
      nodes that do not advertise participation for that Flex-Algorithm in
      their data-plane-specific advertisements <bcp14>MUST</bcp14> be pruned from the
      topology. Segment Routing, including both SR-MPLS and SRv6, are
      data planes that <bcp14>MUST</bcp14> use such pruning when computing Flex-Algorithm
      paths.</t>
      <t indent="0" pn="section-13-7">When computing the path for a given Flex-Algorithm, the metric-type
      that is part of the Flex-Algorithm Definition (<xref target="FLEXALGDEF" format="default" sectionFormat="of" derivedContent="Section 5"/>) <bcp14>MUST</bcp14> be used.</t>
      <t indent="0" pn="section-13-8">When computing the path for a given Flex-Algorithm, the
      calculation-type that is part of the Flex-Algorithm Definition (<xref target="FLEXALGDEF" format="default" sectionFormat="of" derivedContent="Section 5"/>) <bcp14>MUST</bcp14> be used.</t>
      <t indent="0" pn="section-13-9">Various links that include or exclude rules can be part of the
      Flex-Algorithm Definition. To refer to a particular bit within an Admin Group or
      Extended Admin Group, we use the term "color".</t>
      <t indent="0" pn="section-13-10">Rules, in the order as specified below, <bcp14>MUST</bcp14> be used to prune links
      from the topology during the Flex-Algorithm computation.</t>
      <t indent="0" pn="section-13-11">For all links in the topology: </t>
      <ol spacing="normal" type="1" indent="adaptive" start="1" pn="section-13-12">
        <li pn="section-13-12.1" derivedCounter="1.">Check if any exclude Administrative Group rule is part of the Flex-Algorithm
        Definition. If such exclude rule exists, check if any color that is
        part of the exclude rule is also set on the link. If such a color is
        set, the link <bcp14>MUST</bcp14> be pruned from the computation.</li>
        <li pn="section-13-12.2" derivedCounter="2.">Check if any exclude SRLG rule is part of the Flex-Algorithm
        Definition. If such exclude rule exists, check if the link is part
        of any SRLG that is also part of the SRLG exclude rule. If the link
        is part of such SRLG, the link <bcp14>MUST</bcp14> be pruned from the
        computation.</li>
        <li pn="section-13-12.3" derivedCounter="3.">Check if any include-any Administrative Group rule is part of the Flex-Algorithm
        Definition. If such include-any rule exists, check if any color that
        is part of the include-any rule is also set on the link. If no such
        color is set, the link <bcp14>MUST</bcp14> be pruned from the computation.</li>
        <li pn="section-13-12.4" derivedCounter="4.">Check if any include-all Administrative Group rule is part of the Flex-Algorithm
        Definition. If such include-all rule exists, check if all colors
        that are part of the include-all rule are also set on the link. If
        all such colors are not set on the link, the link <bcp14>MUST</bcp14> be pruned
        from the computation.</li>
        <li pn="section-13-12.5" derivedCounter="5.">If the Flex-Algorithm Definition uses something other than the IGP metric
        (<xref target="FLEXALGDEF" format="default" sectionFormat="of" derivedContent="Section 5"/>), and such metric is not
	advertised for the particular link in a topology for which the computation is
	done, such link <bcp14>MUST</bcp14> be pruned from the computation. A metric of
	value 0 <bcp14>MUST NOT</bcp14> be assumed in such a case.</li>
      </ol>
      <section anchor="FLEXALGPATHCALCINTER" numbered="true" toc="include" removeInRFC="false" pn="section-13.1">
        <name slugifiedName="name-multi-area-and-multi-domain">Multi-area and Multi-domain Considerations</name>
        <t indent="0" pn="section-13.1-1">Any IGP Shortest Path Tree calculation is limited to a single area.
        This applies to Flex-Algorithm calculations as well. Given that the
        computing router does not have visibility of the topology of the next
        areas or domain, the Flex-Algorithm-specific path to an inter-area or
        inter-domain prefix will be computed for the local area only. The
        egress L1/L2 router (ABR in OSPF), or ASBR for an inter-domain case, will
        be selected based on the best path for the given Flex-Algorithm in the
        local area, and such egress ABR or ASBR router will be responsible to
        compute the best Flex-Algorithm-specific path over the next area or
        domain. This may produce an end-to-end path, which is suboptimal
        based on Flex-Algorithm constraints. In cases where the ABR or ASBR
        has no reachability to a prefix for a given Flex-Algorithm in the next
        area or domain, the traffic could be dropped by the ABR/ASBR.</t>
        <t indent="0" pn="section-13.1-2">To allow the optimal end-to-end path for an inter-area or
        inter-domain prefix for any Flex-Algorithm to be computed, the FAPM
        has been defined in Sections <xref target="ISISFLEXMETRIC" format="counter" sectionFormat="of" derivedContent="8"/>
	and <xref target="OSPFFLEXMETRIC" format="counter" sectionFormat="of" derivedContent="9"/>. For external route
	calculation for prefixes originated by ASBRs in remote areas in OSPF, the FAAM
	has been defined in <xref target="OSPFFAASBRMETRIC" format="default" sectionFormat="of" derivedContent="Section 10.2"/> for the
	ABR to indicate its ASBR reachability along with the metric for the specific
        Flex-Algorithm.</t>
        <t indent="0" pn="section-13.1-3">If the FAD selected based on the rules defined in <xref target="COMMONLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.3"/> includes the M-flag, an ABR or an ASBR
	<bcp14>MUST</bcp14> include the FAPM (see Sections <xref target="ISISFLEXMETRIC" format="counter" sectionFormat="of" derivedContent="8"/> and <xref target="OSPFFLEXMETRIC" format="counter" sectionFormat="of" derivedContent="9"/>) when
	advertising the prefix that is
        reachable in a given Flex-Algorithm between areas or domains. Such
        metric will be equal to the metric to reach the prefix for that
        Flex-Algorithm in its source area or domain. This is similar in nature
        to how the metric is set when prefixes are advertised between areas or
        domains for the default algorithm. When a prefix is unreachable in its
        source area or domain in a specific Flex-Algorithm, then an ABR or
        ASBR <bcp14>MUST NOT</bcp14> include the FAPM for that Flex-Algorithm when
        advertising the prefix between areas or domains.</t>
        <t indent="0" pn="section-13.1-4">If the FAD selected based on the rules defined in <xref target="COMMONLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.3"/> includes the M-flag, the FAPM <bcp14>MUST</bcp14> be used
        during the calculation of prefix reachability for the inter-area and
        external prefixes. If the FAPM for the Flex-Algorithm is not
        advertised with the inter-area or external prefix reachability
        advertisement, the prefix <bcp14>MUST</bcp14> be considered as unreachable for that
        Flex-Algorithm. Similarly, in the case of OSPF, for ASBRs in remote
        areas, if the FAAM is not advertised by the local ABR(s), the ASBR
        <bcp14>MUST</bcp14> be considered as unreachable for that Flex-Algorithm, and the
        external prefix advertisements from such an ASBR are not considered
        for that Flex-Algorithm.</t>
        <t indent="0" pn="section-13.1-5">The Flex-Algorithm prefix metrics and the OSPF Flex-Algorithm ASBR
        metrics <bcp14>MUST NOT</bcp14> be used during the Flex-Algorithm computation
	unless the FAD selected based on the rules defined in <xref target="COMMONLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.3"/> includes the M-flag, as described in
	Sections <xref target="ISISFLEXALGFLAG" format="counter" sectionFormat="of" derivedContent="6.4"/> or <xref target="OSPFFLEXALGFLAG" format="counter" sectionFormat="of" derivedContent="7.4"/>.</t>
        <t indent="0" pn="section-13.1-6">In the case of OSPF, when calculating external routes in a
        Flex-Algorithm, if the winning FAD includes the M-flag, and the
        advertising ASBR is in a remote area, the metric will be the sum of
        the following:</t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-13.1-7">
          <li pn="section-13.1-7.1">the FAPM for that Flex-Algorithm advertised with the external
            route by the ASBR</li>
          <li pn="section-13.1-7.2">the metric to reach the ASBR for that Flex-Algorithm from the
            local ABR, i.e., the FAAM for that Flex-Algorithm advertised by the
            ABR in the local area for that ASBR</li>
          <li pn="section-13.1-7.3">the Flex-Algorithm-specific metric to reach the local ABR</li>
        </ul>
        <t indent="0" pn="section-13.1-8">This is similar in nature to how the metric is calculated for
        routes learned from remote ASBRs in the default algorithm using the
        OSPFv2 Type 4 ASBR summary-LSA and the OSPFv3 Inter-Area-Router-LSA.</t>
        <t indent="0" pn="section-13.1-9">If the FAD selected based on the rules defined in <xref target="COMMONLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.3"/> does not include the M-flag, then the IGP
        metrics associated with the prefix reachability advertisements used by
        the base IS-IS and OSPF protocol <bcp14>MUST</bcp14> be used for the Flex-Algorithm
        route computation. Similarly, in the case of external route
        calculations in OSPF, the ASBR reachability is determined based on the
        base OSPFv2 Type 4 summary-LSA and the OSFPv3 Inter-Area-Router-LSA.</t>
        <t indent="0" pn="section-13.1-10">It is <bcp14>NOT RECOMMENDED</bcp14> to use the Flex-Algorithm for inter-area or
        inter-domain prefix reachability without the M-flag set. The reason is
        that, without the explicit Flex-Algorithm prefix metric advertisement
        (and the Flex-Algorithm ASBR metric advertisement in the case of OSPF
        external route calculation), it is not possible to conclude whether
        the ABR or ASBR has reachability to the inter-area or inter-domain
        prefix for a given Flex-Algorithm in the next area or domain. Sending
        the Flex-Algorithm traffic for such a prefix towards the ABR or ASBR may
        result in traffic looping or persistent traffic drop.</t>
        <t indent="0" pn="section-13.1-11">During the route computation, it is possible for the Flex-Algorithm-specific
	metric to exceed the maximum value that can be stored in an
        unsigned 32-bit variable. In such scenarios, the value <bcp14>MUST</bcp14> be
        considered to be of value 0xFFFFFFFF during the computation and
        advertised as such.</t>
        <t indent="0" pn="section-13.1-12">The FAPM <bcp14>MUST NOT</bcp14> be advertised with IS-IS L1 or L2 intra-area,
        OSPFv2 intra-area, or OSPFv3 intra-area routes. If the FAPM is
        advertised for these route-types, it <bcp14>MUST</bcp14> be ignored during the prefix
        reachability calculation.</t>
        <t indent="0" pn="section-13.1-13">The M-flag in the FAD is not applicable to prefixes advertised as SRv6
        locators. The IS-IS SRv6 Locator TLV <xref target="RFC9352" format="default" sectionFormat="of" derivedContent="RFC9352"/> includes the Algorithm
        and Metric fields. When the SRv6 Locator is advertised between areas
        or domains, the Metric field in the Locator TLV of IS-IS <bcp14>MUST</bcp14> be used
        irrespective of the M-flag in the FAD advertisement.</t>
        <t indent="0" pn="section-13.1-14">OSPF external and NSSA external prefix advertisements <bcp14>MAY</bcp14> include a
        non-zero forwarding address in the prefix advertisements in the base
        protocol. In such a scenario, the Flex-Algorithm-specific reachability
        of the external prefix is determined by Flex-Algorithm-specific
        reachability of the forwarding address.</t>
        <t indent="0" pn="section-13.1-15">In OSPF, the procedures for translation of NSSA external prefix
        advertisements into external prefix advertisements performed by an
        NSSA ABR <xref target="RFC3101" format="default" sectionFormat="of" derivedContent="RFC3101"/> remain unchanged for Flex-Algorithm.
        An NSSA translator <bcp14>MUST</bcp14> include the OSPF FAPM sub-TLVs for all
        Flex-Algorithms that are in the original NSSA external prefix
        advertisement from the NSSA ASBR in the translated external prefix
        advertisement generated by it, regardless of its participation in those
        Flex-Algorithms or its having reachability to the NSSA ASBR in those
        Flex-Algorithms.</t>
        <t indent="0" pn="section-13.1-16">An area could become partitioned from the perspective of the Flex-Algorithm 
        due to the constraints and/or metric being used for it while maintaining the 
        continuity in the base algorithm. When that happens, some destinations inside that area
        could become unreachable in that Flex-Algorithm. These destinations will not be able
        to use an inter-area path. This is the consequence of the fact that the 
        inter-area prefix reachability advertisement would not be available for these 
        intra-area destinations within the area. It is <bcp14>RECOMMENDED</bcp14> to minimize the risk of 
        such partitioning by providing enough redundancy inside the area for each 
        Flex-Algorithm being used.</t>
      </section>
    </section>
    <section anchor="FLEXALFORW" numbered="true" toc="include" removeInRFC="false" pn="section-14">
      <name slugifiedName="name-flex-algorithm-and-forwardi">Flex-Algorithm and Forwarding Plane</name>
      <t indent="0" pn="section-14-1">This section describes how Flex-Algorithm paths are used in
      forwarding.</t>
      <section anchor="FLEXALGPSRFORW" numbered="true" toc="include" removeInRFC="false" pn="section-14.1">
        <name slugifiedName="name-segment-routing-mpls-forwar">Segment Routing MPLS Forwarding for Flex-Algorithm</name>
        <t indent="0" pn="section-14.1-1">This section describes how Flex-Algorithm paths are used with SR
        MPLS forwarding.</t>
        <t indent="0" pn="section-14.1-2">Prefix-SID advertisements include an SR-Algorithm value and, as
        such, are associated with the specified SR-Algorithm. Prefix-SIDs are
        also associated with a specific topology that is inherited from the
        associated prefix reachability advertisement. When the algorithm value
        advertised is a Flex-Algorithm value, the Prefix-SID is associated
        with paths calculated using that Flex-Algorithm in the associated
        topology.</t>
        <t indent="0" pn="section-14.1-3">A Flex-Algorithm path <bcp14>MUST</bcp14> be installed in the MPLS forwarding
        plane using the MPLS label that corresponds to the Prefix-SID that was
        advertised for that Flex-algorithm. If the Prefix-SID for a given
        Flex-Algorithm is not known, the Flex-Algorithm-specific path cannot
        be installed in the MPLS forwarding plane.</t>
        <t indent="0" pn="section-14.1-4">Traffic that is supposed to be routed via Flex-Algorithm-specific
        paths <bcp14>MUST</bcp14> be dropped when there are no such paths available.</t>
        <t indent="0" pn="section-14.1-5">Loop Free Alternate (LFA) paths (<xref target="RFC6571" format="default" sectionFormat="of" derivedContent="RFC6571"/> or its variants) 
        for a given Flex-Algorithm <bcp14>MUST</bcp14> be computed using the same constraints as the 
        calculation of the primary paths for that Flex-Algorithm. LFA paths <bcp14>MUST</bcp14> only 
        use Prefix-SIDs advertised specifically for the given algorithm. LFA paths <bcp14>MUST NOT</bcp14>
        use an Adjacency SID that belongs to a link that has been pruned from
        the Flex-Algorithm computation.</t>
        <t indent="0" pn="section-14.1-6">If LFA protection is being used to protect a given Flex-Algorithm
        path, all routers in the area participating in the given
        Flex-Algorithm <bcp14>SHOULD</bcp14> advertise at least one
	Flex-Algorithm-specific Node-SID. These Node-SIDs are used to steer traffic over
	the LFA-computed backup path.</t>
      </section>
      <section anchor="FLEXALGPSRV6FORW" numbered="true" toc="include" removeInRFC="false" pn="section-14.2">
        <name slugifiedName="name-srv6-forwarding-for-flex-al">SRv6 Forwarding for Flex-Algorithm</name>
        <t indent="0" pn="section-14.2-1">This section describes how Flex-Algorithm paths are used with SRv6
        forwarding.</t>
        <t indent="0" pn="section-14.2-2">In SRv6, a node is provisioned with a (topology, algorithm) specific
        locator for each of the topology/algorithm pairs supported by that
        node. Each locator is an aggregate prefix for all SIDs provisioned on
        that node that have the matching topology/algorithm.</t>
        <t indent="0" pn="section-14.2-3">The SRv6 locator advertisement in IS-IS <xref target="RFC9352" format="default" sectionFormat="of" derivedContent="RFC9352"/> includes the Multi-Topology Identifier (MTID) value
        that associates the locator with a specific topology. SRv6 locator
        advertisements also include an algorithm value that explicitly
        associates the locator with a specific algorithm. When the algorithm
        value advertised with a locator represents a Flex-Algorithm, the paths
        to the locator prefix <bcp14>MUST</bcp14> be calculated using the specified
        Flex-Algorithm in the associated topology.</t>
        <t indent="0" pn="section-14.2-4">Forwarding entries for the locator prefixes advertised in IS-IS <bcp14>MUST</bcp14>
        be installed in the forwarding plane of the receiving SRv6-capable
        routers when the associated topology/algorithm is participating in
        them. Forwarding entries for locators associated with Flex-Algorithms
        in which the node is not participating <bcp14>MUST NOT</bcp14> be installed in the
        forwarding plane.</t>
        <t indent="0" pn="section-14.2-5">When the locator is associated with a Flex-Algorithm, LFA paths to
        the locator prefix <bcp14>MUST</bcp14> be calculated using such Flex-Algorithm in the
        associated topology to guarantee that they follow the same
        constraints as the calculation of the primary paths. LFA paths <bcp14>MUST</bcp14>
        only use SRv6 SIDs advertised specifically for the given
        Flex-Algorithm.</t>
        <t indent="0" pn="section-14.2-6">If LFA protection is being used to protect locators associated with
        a given Flex-Algorithm, all routers in the area participating in the
        given Flex-Algorithm <bcp14>SHOULD</bcp14> advertise at least one
	Flex-Algorithm-specific locator and END SID per node and one END.X SID for every
	link that has not been pruned from such Flex-Algorithm computation. These
        locators and SIDs are used to steer traffic over the LFA-computed
        backup path.</t>
      </section>
      <section anchor="FLEXALGPSRFORWOTHER" numbered="true" toc="include" removeInRFC="false" pn="section-14.3">
        <name slugifiedName="name-other-data-planes-forwardin">Other Data Planes' Forwarding for Flex-Algorithm</name>
        <t indent="0" pn="section-14.3-1">Any data plane that wants to use Flex-Algorithm-specific
        forwarding needs to install some form of Flex-Algorithm-specific
        forwarding entries.</t>
        <t indent="0" pn="section-14.3-2">Data-plane-specific forwarding for Flex-Algorithms <bcp14>MUST</bcp14> be defined
        for each data plane and is outside the scope of this document.</t>
      </section>
    </section>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-15">
      <name slugifiedName="name-operational-considerations">Operational Considerations</name>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-15.1">
        <name slugifiedName="name-inter-area-considerations">Inter-area Considerations</name>
        <t indent="0" pn="section-15.1-1">The scope of the Flex-Algorithm computation and the scope of the
        FAD is an area. In IS-IS, the Router Capability TLV in which the FAD sub-TLV is
        advertised <bcp14>MUST</bcp14> have the S bit clear, which prevents it from being flooded
        outside the level in which it was originated. Even though in OSPF
        the FAD sub-TLV can be flooded in an RI LSA that has an AS flooding
        scope, the FAD selection is performed for each individual area in
        which it is being used.</t>
        <t indent="0" pn="section-15.1-2">There is no requirement for the FAD for a particular Flex-Algorithm
        to be identical in all areas in the network. For example, traffic for
        the same Flex-Algorithm may be optimized for minimal delay (e.g.,
        using delay metric) in one area or level while being optimized for
        available bandwidth (e.g., using IGP metric) in another area or
        level.</t>
        <t indent="0" pn="section-15.1-3">As described in <xref target="ISISFLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.1"/>, IS-IS allows the
        regeneration of the winning FAD from level 2, without any
        modification to it, into a level 1 area. This allows the operator to
        configure the FAD in one or multiple routers in level 2, without
        the need to repeat the same task in each level 1 area, if the intent
        is to have the same FAD for the particular Flex-Algorithm across all
        levels. This can similarly be achieved in OSPF by using the AS
        flooding scope of the RI LSA in which the FAD sub-TLV for the
        particular Flex-Algorithm is advertised.</t>
        <t indent="0" pn="section-15.1-4">Regeneration of the FAD from a level 1 area to the level 2 area is not
        supported in IS-IS, so if the intent is to regenerate the FAD between
        IS-IS levels, the FAD <bcp14>MUST</bcp14> be defined on a router(s) that is in level 2.
        In OSPF, the FAD definition can be done in any area and propagated
        to all routers in the OSPF routing domain by using the AS flooding
        scope of the RI LSA.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-15.2">
        <name slugifiedName="name-usage-of-the-srlg-exclude-r">Usage of the SRLG Exclude Rule with Flex-Algorithm</name>
        <t indent="0" pn="section-15.2-1">There are two different ways in which SRLG information can be used
        with Flex-Algorithms: </t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-15.2-2">
          <li pn="section-15.2-2.1">In a context of a single Flex-Algorithm, it can be used for
          computation of backup paths, as described in <xref target="I-D.ietf-rtgwg-segment-routing-ti-lfa" format="default" sectionFormat="of" derivedContent="RTGWG-SEGMENT-ROUTING-TI-LFA"/>. This usage
	  does not require association of any specific SRLG constraint with the
          given Flex-Algorithm Definition.</li>
          <li pn="section-15.2-2.2">
            <t indent="0" pn="section-15.2-2.2.1">In the context of multiple Flex-Algorithms, it can be used for
            creating disjoint sets of paths by pruning the links belonging to
            a specific SRLG from the topology on which a specific
            Flex-Algorithm computes its paths. This usage: </t>
            <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-15.2-2.2.2">
              <li pn="section-15.2-2.2.2.1">facilitates the usage of already deployed SRLG
              configurations for the setup of disjoint paths between two or more
              Flex-Algorithms and</li>
              <li pn="section-15.2-2.2.2.2">requires explicit association of a given Flex-Algorithm
              with a specific set of SRLG constraints, as defined in Sections <xref target="ISISFLEXALGEXSRLGTLV" format="counter" sectionFormat="of" derivedContent="6.5"/> and <xref target="OSPFFLEXALGEXSRLGTLV" format="counter" sectionFormat="of" derivedContent="7.5"/>.</li>
            </ul>
          </li>
        </ul>
        <t indent="0" pn="section-15.2-3">The two usages mentioned above are orthogonal.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-15.3">
        <name slugifiedName="name-max-metric-consideration">Max-Metric Consideration</name>
        <t indent="0" pn="section-15.3-1">Both IS-IS and OSPF have a mechanism to set the IGP metric on a link
        to a value that would make the link either unreachable or serve
        as the link of last resort. Similar functionality would be needed for
        the Min Unidirectional Link Delay and TE metric, as these can be used
        to compute Flex-Algorithm paths.</t>
        <t indent="0" pn="section-15.3-2">The link can be made unreachable for all Flex-Algorithms that use the
        Min Unidirectional Link Delay as a metric, as described in <xref target="ISISFLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.1"/>, by removing the Flex-Algorithm ASLA Min
        Unidirectional Link Delay advertisement for the link. The link can be
        made the link of last resort by setting the delay value in the
        Flex-Algorithm ASLA delay advertisement for the link to the value of
        16,777,215 (2<sup>24</sup> - 1).</t>
        <t indent="0" pn="section-15.3-3">The link can be made unreachable for all Flex-Algorithms that use the
        TE metric, as described in <xref target="ISISFLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.1"/>, by
        removing the Flex-Algorithm ASLA TE metric advertisement for the link.
        The link can be made the link of last resort by setting the TE metric
        value in the Flex-Algorithm ASLA delay advertisement for the link to
        the value of (2<sup>24</sup> - 1) in IS-IS and (2<sup>32</sup> - 1) in OSPF.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-15.4">
        <name slugifiedName="name-flexible-algorithm-definitio">Flexible Algorithm Definition and Changes</name>
        <t indent="0" pn="section-15.4-1">When configuring a node to participate in a specific Flex-Algorithm,
      the components of the FAD (calculation-type, metric-type, and constraints) 
      should be considered carefully. The configuration of participation in a particular
      Flex-Algorithm doesn't guarantee that the node will actively participate in it,
      because it may not support the calculation-type, the metric-type, or some
      constraint advertised by the winning FAD (see <xref target="COMMONLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.3"/>). Changes in 
      the FAD configuration should also be considered in light of the capabilities of 
      the participating routers in the scope of the FAD advertisement.</t>
        <t indent="0" pn="section-15.4-2">As <xref target="COMMONLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.3"/> notes, a change in the Flex-Algorithm 
      Definition may require network-wide Shortest Path First (SPF) recomputation and network reconvergence. 
      This potential for disruption should be taken into consideration when planning 
      and making changes to the FAD.</t>
      </section>
      <section numbered="true" toc="include" removeInRFC="false" pn="section-15.5">
        <name slugifiedName="name-number-of-flex-algorithms">Number of Flex-Algorithms</name>
        <t indent="0" pn="section-15.5-1">The maximum number of Flex-Algorithms is determined by the algorithm range 
      128-255, as specified in <xref target="FLEXALG" format="default" sectionFormat="of" derivedContent="Section 4"/>. Although possible, 
      it is not expected that all of them will be used simultaneously. Typically, 
      only a limited subset of Flex-Algorithms is expected to be deployed in the network.</t>
      </section>
    </section>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-16">
      <name slugifiedName="name-backward-compatibility">Backward Compatibility</name>
      <t indent="0" pn="section-16-1">This extension brings no new backward-compatibility issues. IS-IS,
      OSPFv2, and OSPFv3 all have well-defined handling of unrecognized TLVs
      and sub-TLVs that allows the introduction of new extensions, similar
      to those defined here, without introducing any interoperability
      issues.</t>
    </section>
    <section numbered="true" toc="include" removeInRFC="false" pn="section-17">
      <name slugifiedName="name-security-considerations">Security Considerations</name>
      <t indent="0" pn="section-17-1">This document adds two new ways to disrupt IGP networks: </t>
      <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-17-2">
        <li pn="section-17-2.1">An attacker can hijack a particular Flex-Algorithm by advertising
      a FAD with a priority of 255 (or any priority higher than that of
      the legitimate nodes).</li>
        <li pn="section-17-2.2">An attacker could make it look like a router supports a
      particular Flex-Algorithm when it actually doesn't, or vice
      versa.</li>
      </ul>
      <t indent="0" pn="section-17-3">Both of these attacks can be addressed by the existing security
      extensions, as described in <xref target="RFC5304" format="default" sectionFormat="of" derivedContent="RFC5304"/> and <xref target="RFC5310" format="default" sectionFormat="of" derivedContent="RFC5310"/> for IS-IS, in <xref target="RFC2328" format="default" sectionFormat="of" derivedContent="RFC2328"/> and <xref target="RFC7474" format="default" sectionFormat="of" derivedContent="RFC7474"/> for OSPFv2, and in <xref target="RFC4552" format="default" sectionFormat="of" derivedContent="RFC4552"/> and <xref target="RFC5340" format="default" sectionFormat="of" derivedContent="RFC5340"/> for OSPFv3.</t>
      <t indent="0" pn="section-17-4">If the node that is authenticated is taken over by an attacker, such rogue
      node can advertise the FAD for any Flex-Algorithm. Doing so may result in 
      traffic for such Flex-Algorithm to be misrouted, or not delivered 
      at all, for example, by using an unsupported metric-type, calculation-type, 
      or constraint. Such attack is not preventable through authentication, and it is 
      not different from advertising any other incorrect information through IS-IS or 
      OSPF.</t>
    </section>
    <section anchor="IANA" numbered="true" toc="include" removeInRFC="false" pn="section-18">
      <name slugifiedName="name-iana-considerations">IANA Considerations</name>
      <section anchor="IANAIGP" numbered="true" toc="include" removeInRFC="false" pn="section-18.1">
        <name slugifiedName="name-igp-iana-considerations">IGP IANA Considerations</name>
        <section anchor="IANAIGPIGPALGTYPE" numbered="true" toc="include" removeInRFC="false" pn="section-18.1.1">
          <name slugifiedName="name-igp-algorithm-types-registr">IGP Algorithm Types Registry</name>
          <t indent="0" pn="section-18.1.1-1">This document makes the following registration in the "IGP
          Algorithm Types" registry: </t>
          <table align="center" pn="table-1">
            <name slugifiedName="name-igp-algorithm-types-registry">IGP Algorithm Types Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Value</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">128-255</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithms</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="FLEXALG" format="default" sectionFormat="of" derivedContent="Section 4"/></td>
              </tr>
            </tbody>
          </table>
        </section>
        <section anchor="IGPMETRICTYPE" numbered="true" toc="include" removeInRFC="false" pn="section-18.1.2">
          <name slugifiedName="name-igp-metric-type-registry">IGP Metric-Type Registry</name>
          <t indent="0" pn="section-18.1.2-1">IANA has created the "IGP Metric-Type" registry
          within the "Interior Gateway Protocol (IGP) Parameters" registry group. The registration policy is "Standards
          Action" <xref target="RFC8126" format="default" sectionFormat="of" derivedContent="RFC8126"/> <xref target="RFC7120" format="default" sectionFormat="of" derivedContent="RFC7120"/>. Values are assigned from the range 0-255 and have been registered as follows. </t>
          <table align="center" pn="table-2">
            <name slugifiedName="name-igp-metric-type-registry-2">IGP Metric-Type Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Type</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">0</td>
                <td align="left" colspan="1" rowspan="1">IGP Metric</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="ISISFLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.1"/></td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">1</td>
                <td align="left" colspan="1" rowspan="1">Min Unidirectional Link Delay as defined in <xref target="RFC8570" section="4.2" sectionFormat="comma" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8570#section-4.2" derivedContent="RFC8570"/> and <xref target="RFC7471" section="4.2" sectionFormat="comma" format="default" derivedLink="https://rfc-editor.org/rfc/rfc7471#section-4.2" derivedContent="RFC7471"/></td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="ISISFLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.1"/></td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">2</td>
                <td align="left" colspan="1" rowspan="1">Traffic Engineering Default Metric as defined in <xref target="RFC5305" section="3.7" sectionFormat="comma" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5305#section-3.7" derivedContent="RFC5305"/> and Traffic Engineering Metric as defined in <xref target="RFC3630" section="2.5.5" sectionFormat="comma" format="default" derivedLink="https://rfc-editor.org/rfc/rfc3630#section-2.5.5" derivedContent="RFC3630"/></td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="ISISFLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.1"/></td>
              </tr>
            </tbody>
          </table>
        </section>
      </section>
      <section anchor="IANAFADFLGAS" numbered="true" toc="include" removeInRFC="false" pn="section-18.2">
        <name slugifiedName="name-igp-flexible-algorithm-defi">IGP Flexible Algorithm Definition Flags Registry</name>
        <t indent="0" pn="section-18.2-1">IANA has created the "IGP Flexible
        Algorithm Definition Flags" registry within the "Interior Gateway
        Protocol (IGP) Parameters" registry group. The registration policy
        is "Standards Action". New registrations should be assigned in 
        ascending bit order (see <xref target="ISISFLEXALGFLAG" format="default" sectionFormat="of" derivedContent="Section 6.4"/>); the following single bit has been assigned as follows.</t>
        <table align="center" pn="table-3">
          <name slugifiedName="name-igp-flexible-algorithm-defin">IGP Flexible Algorithm Definition Flags Registry</name>
          <thead>
            <tr>
              <th align="left" colspan="1" rowspan="1">Bit</th>
              <th align="left" colspan="1" rowspan="1">Name</th>
              <th align="left" colspan="1" rowspan="1">Reference</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="left" colspan="1" rowspan="1">0</td>
              <td align="left" colspan="1" rowspan="1">Prefix Metric Flag (M-flag)</td>
              <td align="left" colspan="1" rowspan="1">RFC 9350, Sections <xref target="ISISFLEXALGFLAG" format="counter" sectionFormat="of" derivedContent="6.4"/> and  <xref target="OSPFFLEXALGFLAG" format="counter" sectionFormat="of" derivedContent="7.4"/></td>
            </tr>
          </tbody>
        </table>
      </section>
      <section anchor="IANAISIS" numbered="true" toc="include" removeInRFC="false" pn="section-18.3">
        <name slugifiedName="name-is-is-iana-considerations">IS-IS IANA Considerations</name>
        <section anchor="SUBTLVS" numbered="true" toc="include" removeInRFC="false" pn="section-18.3.1">
          <name slugifiedName="name-is-is-sub-tlvs-for-is-is-ro">IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV Registry</name>
          <t indent="0" pn="section-18.3.1-1">This document makes the following registration in the 
          "IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV" registry. </t>
          <table align="center" pn="table-4">
            <name slugifiedName="name-is-is-sub-tlvs-for-is-is-rou">IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Value</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">26</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Definition (FAD)</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="ISISFLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.1"/></td>
              </tr>
            </tbody>
          </table>
        </section>
        <section anchor="SUBTLVS135" numbered="true" toc="include" removeInRFC="false" pn="section-18.3.2">
          <name slugifiedName="name-is-is-sub-tlvs-for-tlvs-adv">IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability Registry</name>
          <t indent="0" pn="section-18.3.2-1">This document makes the following registration in the 
          "IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability" registry. </t>
          <table align="center" pn="table-5">
            <name slugifiedName="name-is-is-sub-tlvs-for-tlvs-adve">IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Type</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">27</th>
                <th align="left" colspan="1" rowspan="1">135</th>
                <th align="left" colspan="1" rowspan="1">235</th>
                <th align="left" colspan="1" rowspan="1">236</th>
                <th align="left" colspan="1" rowspan="1">237</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">6</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Prefix Metric (FAPM)</td>
                <td align="left" colspan="1" rowspan="1">n</td>
                <td align="left" colspan="1" rowspan="1">y</td>
                <td align="left" colspan="1" rowspan="1">y</td>
                <td align="left" colspan="1" rowspan="1">y</td>
                <td align="left" colspan="1" rowspan="1">y</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="ISISFLEXMETRIC" format="default" sectionFormat="of" derivedContent="Section 8"/></td>
              </tr>
            </tbody>
          </table>
        </section>
        <section anchor="SUBTLVREGISTRY" numbered="true" toc="include" removeInRFC="false" pn="section-18.3.3">
          <name slugifiedName="name-is-is-sub-sub-tlvs-for-flex">IS-IS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV Registry</name>
          <t indent="0" pn="section-18.3.3-1">IANA has created the "IS-IS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV" registry within the
  	  "IS-IS TLV Codepoints" registry group. The registration procedure is "Expert Review" (note that the "IS-IS TLV
      Codepoints" registry group includes Expert Review guidance that applies
	  to all registries thereunder).</t>
          <t indent="0" pn="section-18.3.3-2">The sub-sub-TLVs defined in this document have been assigned as follows.
          </t>
          <table align="center" pn="table-6">
            <name slugifiedName="name-is-is-sub-sub-tlvs-for-flexi">IS-IS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Type</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">0</td>
                <td align="left" colspan="1" rowspan="1">Reserved</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350</td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">1</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Exclude Admin Group</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="ISISFLEXALGEXLTLV" format="default" sectionFormat="of" derivedContent="Section 6.1"/></td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">2</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Include-Any Admin Group</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="ISISFLEXALGINCANYTLV" format="default" sectionFormat="of" derivedContent="Section 6.2"/></td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">3</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Include-All Admin Group</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="ISISFLEXALGINCALLTLV" format="default" sectionFormat="of" derivedContent="Section 6.3"/></td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">4</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Definition Flags</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="ISISFLEXALGFLAG" format="default" sectionFormat="of" derivedContent="Section 6.4"/></td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">5</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Exclude SRLG</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="ISISFLEXALGEXSRLGTLV" format="default" sectionFormat="of" derivedContent="Section 6.5"/></td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">6-255</td>
                <td align="left" colspan="1" rowspan="1">Unassigned</td>
                <td align="left" colspan="1" rowspan="1"/>
              </tr>
            </tbody>
          </table>
        </section>
      </section>
      <section anchor="IANAOSPF" numbered="true" toc="include" removeInRFC="false" pn="section-18.4">
        <name slugifiedName="name-ospf-iana-considerations">OSPF IANA Considerations</name>
        <section anchor="RITLVREG" numbered="true" toc="include" removeInRFC="false" pn="section-18.4.1">
          <name slugifiedName="name-ospf-router-information-ri-">OSPF Router Information (RI) TLVs Registry</name>
          <t indent="0" pn="section-18.4.1-1">This document makes the following registration in the "OSPF 
          Router Information (RI) TLVs" registry. </t>
          <table align="center" pn="table-7">
            <name slugifiedName="name-ospf-router-information-ri-t">OSPF Router Information (RI) TLVs Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Value</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">16</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Definition (FAD) TLV</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="OSPFFLEXALGTLV" format="default" sectionFormat="of" derivedContent="Section 5.2"/></td>
              </tr>
            </tbody>
          </table>
        </section>
        <section anchor="SUBTLVEXTPFX" numbered="true" toc="include" removeInRFC="false" pn="section-18.4.2">
          <name slugifiedName="name-ospfv2-extended-prefix-tlv-">OSPFv2 Extended Prefix TLV Sub-TLVs Registry</name>
          <t indent="0" pn="section-18.4.2-1">This document makes the following registration in the "OSPFv2
          Extended Prefix TLV Sub-TLVs" registry. </t>
          <table align="center" pn="table-8">
            <name slugifiedName="name-ospfv2-extended-prefix-tlv-s">OSPFv2 Extended Prefix TLV Sub-TLVs Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Value</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">3</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Prefix Metric (FAPM)</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="OSPFFLEXMETRIC" format="default" sectionFormat="of" derivedContent="Section 9"/></td>
              </tr>
            </tbody>
          </table>
        </section>
        <section anchor="SUBTLVV3EXTLSA" numbered="true" toc="include" removeInRFC="false" pn="section-18.4.3">
          <name slugifiedName="name-ospfv3-extended-lsa-sub-tlv">OSPFv3 Extended-LSA Sub-TLVs Registry</name>
          <t indent="0" pn="section-18.4.3-1">This document makes the following registrations in the "OSPFv3
          Extended-LSA Sub-TLVs" registry. </t>
          <table align="center" pn="table-9">
            <name slugifiedName="name-ospfv3-extended-lsa-sub-tlvs">OSPFv3 Extended-LSA Sub-TLVs Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Value</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">26</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Prefix Metric (FAPM)</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="OSPFFLEXMETRIC" format="default" sectionFormat="of" derivedContent="Section 9"/></td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">33</td>
                <td align="left" colspan="1" rowspan="1">OSPF Flexible Algorithm ASBR Metric</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="OSPFFAASBRMETRIC" format="default" sectionFormat="of" derivedContent="Section 10.2"/></td>
              </tr>
            </tbody>
          </table>
        </section>
        <section anchor="FAPMFLAGS" numbered="true" toc="include" removeInRFC="false" pn="section-18.4.4">
          <name slugifiedName="name-ospf-flex-algorithm-prefix-">OSPF Flex-Algorithm Prefix Metric Bits Registry</name>
          <t indent="0" pn="section-18.4.4-1">IANA has created the "OSPF Flex-Algorithm
          Prefix Metric Bits" registry under the "Open Shortest Path First 
          (OSPF) Parameters" registry. The registration procedure
          is "IETF Review". Bits 1-7 are unassigned, and the initial value has been assigned as follows.</t>
          <table align="center" pn="table-10">
            <name slugifiedName="name-ospf-flex-algorithm-prefix-m">OSPF Flex-Algorithm Prefix Metric Bits Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Bit Number</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">0</td>
                <td align="left" colspan="1" rowspan="1">E bit - External Type</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="OSPFFLEXMETRIC" format="default" sectionFormat="of" derivedContent="Section 9"/></td>
              </tr>
            </tbody>
          </table>
        </section>
        <section anchor="OPQLSAOPTS" numbered="true" toc="include" removeInRFC="false" pn="section-18.4.5">
          <name slugifiedName="name-opaque-link-state-advertise">Opaque Link-State Advertisements (LSA) Option Types Registry</name>
          <t indent="0" pn="section-18.4.5-1">This document makes the following registration in the "Opaque Link-State 
          Advertisements (LSA) Option Types" registry within the "Open Shortest Path First 
          (OSPF) Opaque Link-State Advertisements (LSA) Option Types" registry group. </t>
          <table align="center" pn="table-11">
            <name slugifiedName="name-opaque-link-state-advertisem">Opaque Link-State Advertisements (LSA) Option Types Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Value</th>
                <th align="left" colspan="1" rowspan="1">Opaque Type</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">11</td>
                <td align="left" colspan="1" rowspan="1">OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) LSA</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="OSPFEXTASBRLSA" format="default" sectionFormat="of" derivedContent="Section 10.1"/></td>
              </tr>
            </tbody>
          </table>
        </section>
        <section anchor="IAASBRTLV" numbered="true" toc="include" removeInRFC="false" pn="section-18.4.6">
          <name slugifiedName="name-ospfv2-extended-inter-area-as">OSPFv2 Extended Inter-Area ASBR TLVs Registry</name>
          <t indent="0" pn="section-18.4.6-1">IANA has created the "OSPFv2 Extended
          Inter-Area ASBR TLVs" registry within the "Open Shortest Path First v2 (OSPFv2) Parameters" registry group. The registration procedure is "IETF Review" or "IESG Approval". The initial value has been assigned as follows.</t>
          <table align="center" pn="table-12">
            <name slugifiedName="name-ospfv2-extended-inter-area-asb">OSPFv2 Extended Inter-Area ASBR TLVs Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Value</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">1</td>
                <td align="left" colspan="1" rowspan="1">Extended Inter-Area ASBR</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350</td>
              </tr>
            </tbody>
          </table>
          <t indent="0" pn="section-18.4.6-3">The values 2-32767 are unassigned, the values 32768-33023
          are reserved for Experimental Use, and the values 0 and 33024-65535 are
	  reserved.</t>
        </section>
        <section anchor="IAASBRSTLV" numbered="true" toc="include" removeInRFC="false" pn="section-18.4.7">
          <name slugifiedName="name-ospfv2-extended-inter-area-asbr">OSPFv2 Extended Inter-Area ASBR Sub-TLVs Registry</name>
          <t indent="0" pn="section-18.4.7-1">IANA has created the "OSPFv2 Extended
          Inter-Area ASBR Sub-TLVs" registry under the "Open Shortest Path 
          First v2 (OSPFv2) Parameters" registry. The registration procedure is "IETF Review" or "IESG Approval". The initial value has been assigned as follows.</t>
          <table align="center" pn="table-13">
            <name slugifiedName="name-ospfv2-extended-inter-area-asbr-">OSPFv2 Extended Inter-Area ASBR Sub-TLVs Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Value</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">1</td>
                <td align="left" colspan="1" rowspan="1">OSPF Flexible Algorithm ASBR Metric</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350</td>
              </tr>
            </tbody>
          </table>
          <t indent="0" pn="section-18.4.7-3">The values 2-32767 are unassigned, the values 32768-33023
          are reserved for Experimental Use, and the values 0 and 33024-65535 are
	  reserved.</t>
        </section>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-18.4.8">
          <name slugifiedName="name-ospf-flexible-algorithm-defin">OSPF Flexible Algorithm Definition TLV Sub-TLVs Registry</name>
          <t indent="0" pn="section-18.4.8-1">IANA has created the "OSPF Flexible Algorithm Definition TLV Sub-TLVs" registry within the "Open Shortest Path First (OSPF) Parameters" registry group. The registration procedure is "IETF Review" or "IESG Approval". 
          </t>
          <t indent="0" pn="section-18.4.8-2">The "OSPF Flexible Algorithm Definition TLV Sub-TLVs" registry
          will define sub-TLVs at any level of nesting for the Flexible
          Algorithm TLV, and new values can be allocated via the registration procedure.</t>
          <t indent="0" pn="section-18.4.8-3">This document registers the following sub-TLVs. </t>
          <table align="center" pn="table-14">
            <name slugifiedName="name-ospf-flexible-algorithm-defini">OSPF Flexible Algorithm Definition TLV Sub-TLVs Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Bit Number</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">0</td>
                <td align="left" colspan="1" rowspan="1">Reserved</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350</td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">1</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Exclude Admin Group</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="OSPFFLEXALGEXLTLV" format="default" sectionFormat="of" derivedContent="Section 7.1"/></td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">2</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Include-Any Admin Group</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="OSPFFLEXALGINCANYTLV" format="default" sectionFormat="of" derivedContent="Section 7.2"/></td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">3</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Include-All Admin Group</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="OSPFFLEXALGINCALLTLV" format="default" sectionFormat="of" derivedContent="Section 7.3"/></td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">4</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Definition Flags</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="OSPFFLEXALGFLAG" format="default" sectionFormat="of" derivedContent="Section 7.4"/></td>
              </tr>
              <tr>
                <td align="left" colspan="1" rowspan="1">5</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm Exclude SRLG</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="OSPFFLEXALGEXSRLGTLV" format="default" sectionFormat="of" derivedContent="Section 7.5"/></td>
              </tr>
            </tbody>
          </table>
          <t indent="0" pn="section-18.4.8-5">The values 6-32767 are unassigned, and values 32768-33023 are for 
          Experimental Use; these will not be registered with IANA.</t>
          <t indent="0" pn="section-18.4.8-6">Types in the range 33024-65535 are not to be assigned at this
          time. Before any assignments can be made in the 33024-65535 range,
          there <bcp14>MUST</bcp14> be an IETF specification that specifies IANA
          considerations that cover the range being assigned.</t>
        </section>
        <section numbered="true" toc="include" removeInRFC="false" pn="section-18.4.9">
          <name slugifiedName="name-link-attribute-application-">Link Attribute Application Identifiers Registry</name>
          <t indent="0" pn="section-18.4.9-1">This document registers the following bit in the "Link Attribute
          Application Identifiers" registry.</t>
          <table align="center" pn="table-15">
            <name slugifiedName="name-link-attribute-application-i">Link Attribute Application Identifiers Registry</name>
            <thead>
              <tr>
                <th align="left" colspan="1" rowspan="1">Bit</th>
                <th align="left" colspan="1" rowspan="1">Description</th>
                <th align="left" colspan="1" rowspan="1">Reference</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left" colspan="1" rowspan="1">3</td>
                <td align="left" colspan="1" rowspan="1">Flexible Algorithm (X-bit)</td>
                <td align="left" colspan="1" rowspan="1">RFC 9350, <xref target="FLEXALGLINKATTR" format="default" sectionFormat="of" derivedContent="Section 12"/></td>
              </tr>
            </tbody>
          </table>
        </section>
      </section>
    </section>
  </middle>
  <back>
    <displayreference target="I-D.gulkohegde-routing-planes-using-sr" to="ROUTING-PLANES-USING-SR"/>
    <displayreference target="I-D.ietf-rtgwg-segment-routing-ti-lfa" to="RTGWG-SEGMENT-ROUTING-TI-LFA"/>
    <references pn="section-19">
      <name slugifiedName="name-references">References</name>
      <references pn="section-19.1">
        <name slugifiedName="name-normative-references">Normative References</name>
        <reference anchor="ISO10589" quoteTitle="true" derivedAnchor="ISO10589">
          <front>
            <title>Information technology - Telecommunications and information exchange between systems - Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)</title>
            <author>
              <organization abbrev="ISO" showOnFrontPage="true">International Organization for Standardization</organization>
            </author>
            <date month="November" year="2002"/>
          </front>
          <seriesInfo name="ISO/IEC" value="10589:2002"/>
          <refcontent>Second Edition</refcontent>
        </reference>
        <reference anchor="RFC2119" target="https://www.rfc-editor.org/info/rfc2119" quoteTitle="true" derivedAnchor="RFC2119">
          <front>
            <title>Key words for use in RFCs to Indicate Requirement Levels</title>
            <author fullname="S. Bradner" initials="S." surname="Bradner"/>
            <date month="March" year="1997"/>
            <abstract>
              <t indent="0">In many standards track documents several words are used to signify the requirements in the specification.  These words are often capitalized.  This document defines these words as they should be interpreted in IETF documents.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="14"/>
          <seriesInfo name="RFC" value="2119"/>
          <seriesInfo name="DOI" value="10.17487/RFC2119"/>
        </reference>
        <reference anchor="RFC4203" target="https://www.rfc-editor.org/info/rfc4203" quoteTitle="true" derivedAnchor="RFC4203">
          <front>
            <title>OSPF Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)</title>
            <author fullname="K. Kompella" initials="K." role="editor" surname="Kompella"/>
            <author fullname="Y. Rekhter" initials="Y." role="editor" surname="Rekhter"/>
            <date month="October" year="2005"/>
            <abstract>
              <t indent="0">This document specifies encoding of extensions to the OSPF routing protocol in support of Generalized Multi-Protocol Label Switching (GMPLS). [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="4203"/>
          <seriesInfo name="DOI" value="10.17487/RFC4203"/>
        </reference>
        <reference anchor="RFC5250" target="https://www.rfc-editor.org/info/rfc5250" quoteTitle="true" derivedAnchor="RFC5250">
          <front>
            <title>The OSPF Opaque LSA Option</title>
            <author fullname="L. Berger" initials="L." surname="Berger"/>
            <author fullname="I. Bryskin" initials="I." surname="Bryskin"/>
            <author fullname="A. Zinin" initials="A." surname="Zinin"/>
            <author fullname="R. Coltun" initials="R." surname="Coltun"/>
            <date month="July" year="2008"/>
            <abstract>
              <t indent="0">This document defines enhancements to the OSPF protocol to support a new class of link state advertisements (LSAs) called Opaque LSAs. Opaque LSAs provide a generalized mechanism to allow for the future extensibility of OSPF. Opaque LSAs consist of a standard LSA header followed by application-specific information. The information field may be used directly by OSPF or by other applications. Standard OSPF link-state database flooding mechanisms are used to distribute Opaque LSAs to all or some limited portion of the OSPF topology.</t>
              <t indent="0">This document replaces RFC 2370 and adds to it a mechanism to enable an OSPF router to validate Autonomous System (AS)-scope Opaque LSAs originated outside of the router's OSPF area. [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5250"/>
          <seriesInfo name="DOI" value="10.17487/RFC5250"/>
        </reference>
        <reference anchor="RFC5307" target="https://www.rfc-editor.org/info/rfc5307" quoteTitle="true" derivedAnchor="RFC5307">
          <front>
            <title>IS-IS Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)</title>
            <author fullname="K. Kompella" initials="K." role="editor" surname="Kompella"/>
            <author fullname="Y. Rekhter" initials="Y." role="editor" surname="Rekhter"/>
            <date month="October" year="2008"/>
            <abstract>
              <t indent="0">This document specifies encoding of extensions to the IS-IS routing protocol in support of Generalized Multi-Protocol Label Switching (GMPLS). [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5307"/>
          <seriesInfo name="DOI" value="10.17487/RFC5307"/>
        </reference>
        <reference anchor="RFC7308" target="https://www.rfc-editor.org/info/rfc7308" quoteTitle="true" derivedAnchor="RFC7308">
          <front>
            <title>Extended Administrative Groups in MPLS Traffic Engineering (MPLS-TE)</title>
            <author fullname="E. Osborne" initials="E." surname="Osborne"/>
            <date month="July" year="2014"/>
            <abstract>
              <t indent="0">MPLS Traffic Engineering (MPLS-TE) advertises 32 administrative groups (commonly referred to as "colors" or "link colors") using the Administrative Group sub-TLV. This is defined for OSPFv2 (RFC 3630), OSPFv3 (RFC 5329) and IS-IS (RFC 5305).</t>
              <t indent="0">This document adds a sub-TLV to the IGP TE extensions, "Extended Administrative Group". This sub-TLV provides for additional administrative groups (link colors) beyond the current limit of 32.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7308"/>
          <seriesInfo name="DOI" value="10.17487/RFC7308"/>
        </reference>
        <reference anchor="RFC7684" target="https://www.rfc-editor.org/info/rfc7684" quoteTitle="true" derivedAnchor="RFC7684">
          <front>
            <title>OSPFv2 Prefix/Link Attribute Advertisement</title>
            <author fullname="P. Psenak" initials="P." surname="Psenak"/>
            <author fullname="H. Gredler" initials="H." surname="Gredler"/>
            <author fullname="R. Shakir" initials="R." surname="Shakir"/>
            <author fullname="W. Henderickx" initials="W." surname="Henderickx"/>
            <author fullname="J. Tantsura" initials="J." surname="Tantsura"/>
            <author fullname="A. Lindem" initials="A." surname="Lindem"/>
            <date month="November" year="2015"/>
            <abstract>
              <t indent="0">OSPFv2 requires functional extension beyond what can readily be done with the fixed-format Link State Advertisements (LSAs) as described in RFC 2328.  This document defines OSPFv2 Opaque LSAs based on Type-Length-Value (TLV) tuples that can be used to associate additional attributes with prefixes or links.  Depending on the application, these prefixes and links may or may not be advertised in the fixed-format LSAs.  The OSPFv2 Opaque LSAs are optional and fully backward compatible.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7684"/>
          <seriesInfo name="DOI" value="10.17487/RFC7684"/>
        </reference>
        <reference anchor="RFC7770" target="https://www.rfc-editor.org/info/rfc7770" quoteTitle="true" derivedAnchor="RFC7770">
          <front>
            <title>Extensions to OSPF for Advertising Optional Router Capabilities</title>
            <author fullname="A. Lindem" initials="A." role="editor" surname="Lindem"/>
            <author fullname="N. Shen" initials="N." surname="Shen"/>
            <author fullname="JP. Vasseur" initials="JP." surname="Vasseur"/>
            <author fullname="R. Aggarwal" initials="R." surname="Aggarwal"/>
            <author fullname="S. Shaffer" initials="S." surname="Shaffer"/>
            <date month="February" year="2016"/>
            <abstract>
              <t indent="0">It is useful for routers in an OSPFv2 or OSPFv3 routing domain to know the capabilities of their neighbors and other routers in the routing domain.  This document proposes extensions to OSPFv2 and OSPFv3 for advertising optional router capabilities.  The Router Information (RI) Link State Advertisement (LSA) is defined for this purpose.  In OSPFv2, the RI LSA will be implemented with an Opaque LSA type ID.  In OSPFv3, the RI LSA will be implemented with a unique LSA type function code.  In both protocols, the RI LSA can be advertised at any of the defined flooding scopes (link, area, or autonomous system (AS)).  This document obsoletes RFC 4970 by providing a revised specification that includes support for advertisement of multiple instances of the RI LSA and a TLV for functional capabilities.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7770"/>
          <seriesInfo name="DOI" value="10.17487/RFC7770"/>
        </reference>
        <reference anchor="RFC7981" target="https://www.rfc-editor.org/info/rfc7981" quoteTitle="true" derivedAnchor="RFC7981">
          <front>
            <title>IS-IS Extensions for Advertising Router Information</title>
            <author fullname="L. Ginsberg" initials="L." surname="Ginsberg"/>
            <author fullname="S. Previdi" initials="S." surname="Previdi"/>
            <author fullname="M. Chen" initials="M." surname="Chen"/>
            <date month="October" year="2016"/>
            <abstract>
              <t indent="0">This document defines a new optional Intermediate System to Intermediate System (IS-IS) TLV named CAPABILITY, formed of multiple sub-TLVs, which allows a router to announce its capabilities within an IS-IS level or the entire routing domain.  This document obsoletes RFC 4971.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7981"/>
          <seriesInfo name="DOI" value="10.17487/RFC7981"/>
        </reference>
        <reference anchor="RFC8174" target="https://www.rfc-editor.org/info/rfc8174" quoteTitle="true" derivedAnchor="RFC8174">
          <front>
            <title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>
            <author fullname="B. Leiba" initials="B." surname="Leiba"/>
            <date month="May" year="2017"/>
            <abstract>
              <t indent="0">RFC 2119 specifies common key words that may be used in protocol specifications.  This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="14"/>
          <seriesInfo name="RFC" value="8174"/>
          <seriesInfo name="DOI" value="10.17487/RFC8174"/>
        </reference>
        <reference anchor="RFC8362" target="https://www.rfc-editor.org/info/rfc8362" quoteTitle="true" derivedAnchor="RFC8362">
          <front>
            <title>OSPFv3 Link State Advertisement (LSA) Extensibility</title>
            <author fullname="A. Lindem" initials="A." surname="Lindem"/>
            <author fullname="A. Roy" initials="A." surname="Roy"/>
            <author fullname="D. Goethals" initials="D." surname="Goethals"/>
            <author fullname="V. Reddy Vallem" initials="V." surname="Reddy Vallem"/>
            <author fullname="F. Baker" initials="F." surname="Baker"/>
            <date month="April" year="2018"/>
            <abstract>
              <t indent="0">OSPFv3 requires functional extension beyond what can readily be done with the fixed-format Link State Advertisement (LSA) as described in RFC 5340. Without LSA extension, attributes associated with OSPFv3 links and advertised IPv6 prefixes must be advertised in separate LSAs and correlated to the fixed-format LSAs. This document extends the LSA format by encoding the existing OSPFv3 LSA information in Type-Length-Value (TLV) tuples and allowing advertisement of additional information with additional TLVs. Backward-compatibility mechanisms are also described.</t>
              <t indent="0">This document updates RFC 5340, "OSPF for IPv6", and RFC 5838, "Support of Address Families in OSPFv3", by providing TLV-based encodings for the base OSPFv3 unicast support and OSPFv3 address family support.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8362"/>
          <seriesInfo name="DOI" value="10.17487/RFC8362"/>
        </reference>
        <reference anchor="RFC8660" target="https://www.rfc-editor.org/info/rfc8660" quoteTitle="true" derivedAnchor="RFC8660">
          <front>
            <title>Segment Routing with the MPLS Data Plane</title>
            <author fullname="A. Bashandy" initials="A." role="editor" surname="Bashandy"/>
            <author fullname="C. Filsfils" initials="C." role="editor" surname="Filsfils"/>
            <author fullname="S. Previdi" initials="S." surname="Previdi"/>
            <author fullname="B. Decraene" initials="B." surname="Decraene"/>
            <author fullname="S. Litkowski" initials="S." surname="Litkowski"/>
            <author fullname="R. Shakir" initials="R." surname="Shakir"/>
            <date month="December" year="2019"/>
            <abstract>
              <t indent="0">Segment Routing (SR) leverages the source-routing paradigm.  A node steers a packet through a controlled set of instructions, called segments, by prepending the packet with an SR header.  In the MPLS data plane, the SR header is instantiated through a label stack.  This document specifies the forwarding behavior to allow instantiating SR over the MPLS data plane (SR-MPLS).</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8660"/>
          <seriesInfo name="DOI" value="10.17487/RFC8660"/>
        </reference>
        <reference anchor="RFC8665" target="https://www.rfc-editor.org/info/rfc8665" quoteTitle="true" derivedAnchor="RFC8665">
          <front>
            <title>OSPF Extensions for Segment Routing</title>
            <author fullname="P. Psenak" initials="P." role="editor" surname="Psenak"/>
            <author fullname="S. Previdi" initials="S." role="editor" surname="Previdi"/>
            <author fullname="C. Filsfils" initials="C." surname="Filsfils"/>
            <author fullname="H. Gredler" initials="H." surname="Gredler"/>
            <author fullname="R. Shakir" initials="R." surname="Shakir"/>
            <author fullname="W. Henderickx" initials="W." surname="Henderickx"/>
            <author fullname="J. Tantsura" initials="J." surname="Tantsura"/>
            <date month="December" year="2019"/>
            <abstract>
              <t indent="0">Segment Routing (SR) allows a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological subpaths called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF).</t>
              <t indent="0">This document describes the OSPFv2 extensions required for Segment Routing.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8665"/>
          <seriesInfo name="DOI" value="10.17487/RFC8665"/>
        </reference>
        <reference anchor="RFC8666" target="https://www.rfc-editor.org/info/rfc8666" quoteTitle="true" derivedAnchor="RFC8666">
          <front>
            <title>OSPFv3 Extensions for Segment Routing</title>
            <author fullname="P. Psenak" initials="P." role="editor" surname="Psenak"/>
            <author fullname="S. Previdi" initials="S." role="editor" surname="Previdi"/>
            <date month="December" year="2019"/>
            <abstract>
              <t indent="0">Segment Routing (SR) allows a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological subpaths called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF).</t>
              <t indent="0">This document describes the OSPFv3 extensions required for Segment Routing with the MPLS data plane.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8666"/>
          <seriesInfo name="DOI" value="10.17487/RFC8666"/>
        </reference>
        <reference anchor="RFC8667" target="https://www.rfc-editor.org/info/rfc8667" quoteTitle="true" derivedAnchor="RFC8667">
          <front>
            <title>IS-IS Extensions for Segment Routing</title>
            <author fullname="S. Previdi" initials="S." role="editor" surname="Previdi"/>
            <author fullname="L. Ginsberg" initials="L." role="editor" surname="Ginsberg"/>
            <author fullname="C. Filsfils" initials="C." surname="Filsfils"/>
            <author fullname="A. Bashandy" initials="A." surname="Bashandy"/>
            <author fullname="H. Gredler" initials="H." surname="Gredler"/>
            <author fullname="B. Decraene" initials="B." surname="Decraene"/>
            <date month="December" year="2019"/>
            <abstract>
              <t indent="0">Segment Routing (SR) allows for a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological sub-paths, called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF).</t>
              <t indent="0">This document describes the IS-IS extensions that need to be introduced for Segment Routing operating on an MPLS data plane.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8667"/>
          <seriesInfo name="DOI" value="10.17487/RFC8667"/>
        </reference>
        <reference anchor="RFC8919" target="https://www.rfc-editor.org/info/rfc8919" quoteTitle="true" derivedAnchor="RFC8919">
          <front>
            <title>IS-IS Application-Specific Link Attributes</title>
            <author fullname="L. Ginsberg" initials="L." surname="Ginsberg"/>
            <author fullname="P. Psenak" initials="P." surname="Psenak"/>
            <author fullname="S. Previdi" initials="S." surname="Previdi"/>
            <author fullname="W. Henderickx" initials="W." surname="Henderickx"/>
            <author fullname="J. Drake" initials="J." surname="Drake"/>
            <date month="October" year="2020"/>
            <abstract>
              <t indent="0">Existing traffic-engineering-related link attribute advertisements have been defined and are used in RSVP-TE deployments.  Since the original RSVP-TE use case was defined, additional applications (e.g., Segment Routing Policy and Loop-Free Alternates) that also make use of the link attribute advertisements have been defined.  In cases where multiple applications wish to make use of these link attributes, the current advertisements do not support application-specific values for a given attribute, nor do they support indication of which applications are using the advertised value for a given link.  This document introduces new link attribute advertisements that address both of these shortcomings.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8919"/>
          <seriesInfo name="DOI" value="10.17487/RFC8919"/>
        </reference>
        <reference anchor="RFC8920" target="https://www.rfc-editor.org/info/rfc8920" quoteTitle="true" derivedAnchor="RFC8920">
          <front>
            <title>OSPF Application-Specific Link Attributes</title>
            <author fullname="P. Psenak" initials="P." role="editor" surname="Psenak"/>
            <author fullname="L. Ginsberg" initials="L." surname="Ginsberg"/>
            <author fullname="W. Henderickx" initials="W." surname="Henderickx"/>
            <author fullname="J. Tantsura" initials="J." surname="Tantsura"/>
            <author fullname="J. Drake" initials="J." surname="Drake"/>
            <date month="October" year="2020"/>
            <abstract>
              <t indent="0">Existing traffic-engineering-related link attribute advertisements have been defined and are used in RSVP-TE deployments.  Since the original RSVP-TE use case was defined, additional applications (e.g., Segment Routing Policy and Loop-Free Alternates) that also make use of the link attribute advertisements have been defined.  In cases where multiple applications wish to make use of these link attributes, the current advertisements do not support application-specific values for a given attribute, nor do they support indication of which applications are using the advertised value for a given link.  This document introduces new link attribute advertisements in OSPFv2 and OSPFv3 that address both of these shortcomings.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8920"/>
          <seriesInfo name="DOI" value="10.17487/RFC8920"/>
        </reference>
        <reference anchor="RFC9352" target="https://www.rfc-editor.org/info/rfc9352" quoteTitle="true" derivedAnchor="RFC9352">
          <front>
            <title>IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane</title>
            <author initials="P" surname="Psenak" fullname="Peter Psenak" role="editor">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C" surname="Filsfils" fullname="Clarence Filsfils">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="A" surname="Bashandy" fullname="Ahmed Bashandy">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="B" surname="Decraene" fullname="Bruno Decraene">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="Z" surname="Hu" fullname="Zhibo Hu">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2023" month="February"/>
          </front>
          <seriesInfo name="RFC" value="9352"/>
          <seriesInfo name="DOI" value="10.17487/RFC9352"/>
        </reference>
      </references>
      <references pn="section-19.2">
        <name slugifiedName="name-informative-references">Informative References</name>
        <reference anchor="RFC2328" target="https://www.rfc-editor.org/info/rfc2328" quoteTitle="true" derivedAnchor="RFC2328">
          <front>
            <title>OSPF Version 2</title>
            <author fullname="J. Moy" initials="J." surname="Moy"/>
            <date month="April" year="1998"/>
            <abstract>
              <t indent="0">This memo documents version 2 of the OSPF protocol.  OSPF is a link- state routing protocol. [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="STD" value="54"/>
          <seriesInfo name="RFC" value="2328"/>
          <seriesInfo name="DOI" value="10.17487/RFC2328"/>
        </reference>
        <reference anchor="RFC3101" target="https://www.rfc-editor.org/info/rfc3101" quoteTitle="true" derivedAnchor="RFC3101">
          <front>
            <title>The OSPF Not-So-Stubby Area (NSSA) Option</title>
            <author fullname="P. Murphy" initials="P." surname="Murphy"/>
            <date month="January" year="2003"/>
            <abstract>
              <t indent="0">This memo documents an optional type of Open Shortest Path First (OSPF) area that is somewhat humorously referred to as a "not-so-stubby" area (or NSSA).  NSSAs are similar to the existing OSPF stub area configuration option but have the additional capability of importing AS external routes in a limited fashion.  The OSPF NSSA Option was originally defined in RFC 1587.  The functional differences between this memo and RFC 1587 are explained in Appendix F.  All differences, while expanding capability, are backward-compatible in nature.  Implementations of this memo and of RFC 1587 will interoperate. [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3101"/>
          <seriesInfo name="DOI" value="10.17487/RFC3101"/>
        </reference>
        <reference anchor="RFC3630" target="https://www.rfc-editor.org/info/rfc3630" quoteTitle="true" derivedAnchor="RFC3630">
          <front>
            <title>Traffic Engineering (TE) Extensions to OSPF Version 2</title>
            <author fullname="D. Katz" initials="D." surname="Katz"/>
            <author fullname="K. Kompella" initials="K." surname="Kompella"/>
            <author fullname="D. Yeung" initials="D." surname="Yeung"/>
            <date month="September" year="2003"/>
            <abstract>
              <t indent="0">This document describes extensions to the OSPF protocol version 2 to support intra-area Traffic Engineering (TE), using Opaque Link State Advertisements.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3630"/>
          <seriesInfo name="DOI" value="10.17487/RFC3630"/>
        </reference>
        <reference anchor="RFC3906" target="https://www.rfc-editor.org/info/rfc3906" quoteTitle="true" derivedAnchor="RFC3906">
          <front>
            <title>Calculating Interior Gateway Protocol (IGP) Routes Over Traffic Engineering Tunnels</title>
            <author fullname="N. Shen" initials="N." surname="Shen"/>
            <author fullname="H. Smit" initials="H." surname="Smit"/>
            <date month="October" year="2004"/>
            <abstract>
              <t indent="0">This document describes how conventional hop-by-hop link-state routing protocols interact with new Traffic Engineering capabilities to create Interior Gateway Protocol (IGP) shortcuts.  In particular, this document describes how Dijkstra's Shortest Path First (SPF) algorithm can be adapted so that link-state IGPs will calculate IP routes to forward traffic over tunnels that are set up by Traffic Engineering.  This memo provides information for the Internet community.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="3906"/>
          <seriesInfo name="DOI" value="10.17487/RFC3906"/>
        </reference>
        <reference anchor="RFC4552" target="https://www.rfc-editor.org/info/rfc4552" quoteTitle="true" derivedAnchor="RFC4552">
          <front>
            <title>Authentication/Confidentiality for OSPFv3</title>
            <author fullname="M. Gupta" initials="M." surname="Gupta"/>
            <author fullname="N. Melam" initials="N." surname="Melam"/>
            <date month="June" year="2006"/>
            <abstract>
              <t indent="0">This document describes means and mechanisms to provide authentication/confidentiality to OSPFv3 using an IPv6 Authentication Header/Encapsulating Security Payload (AH/ESP) extension header. [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="4552"/>
          <seriesInfo name="DOI" value="10.17487/RFC4552"/>
        </reference>
        <reference anchor="RFC5304" target="https://www.rfc-editor.org/info/rfc5304" quoteTitle="true" derivedAnchor="RFC5304">
          <front>
            <title>IS-IS Cryptographic Authentication</title>
            <author fullname="T. Li" initials="T." surname="Li"/>
            <author fullname="R. Atkinson" initials="R." surname="Atkinson"/>
            <date month="October" year="2008"/>
            <abstract>
              <t indent="0">This document describes the authentication of Intermediate System to Intermediate System (IS-IS) Protocol Data Units (PDUs) using the Hashed Message Authentication Codes - Message Digest 5 (HMAC-MD5) algorithm as found in RFC 2104. IS-IS is specified in International Standards Organization (ISO) 10589, with extensions to support Internet Protocol version 4 (IPv4) described in RFC 1195. The base specification includes an authentication mechanism that allows for multiple authentication algorithms. The base specification only specifies the algorithm for cleartext passwords. This document replaces RFC 3567.</t>
              <t indent="0">This document proposes an extension to that specification that allows the use of the HMAC-MD5 authentication algorithm to be used in conjunction with the existing authentication mechanisms. [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5304"/>
          <seriesInfo name="DOI" value="10.17487/RFC5304"/>
        </reference>
        <reference anchor="RFC5305" target="https://www.rfc-editor.org/info/rfc5305" quoteTitle="true" derivedAnchor="RFC5305">
          <front>
            <title>IS-IS Extensions for Traffic Engineering</title>
            <author fullname="T. Li" initials="T." surname="Li"/>
            <author fullname="H. Smit" initials="H." surname="Smit"/>
            <date month="October" year="2008"/>
            <abstract>
              <t indent="0">This document describes extensions to the Intermediate System to Intermediate System (IS-IS) protocol to support Traffic Engineering (TE).  This document extends the IS-IS protocol by specifying new information that an Intermediate System (router) can place in Link State Protocol Data Units (LSP).  This information describes additional details regarding the state of the network that are useful for traffic engineering computations. [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5305"/>
          <seriesInfo name="DOI" value="10.17487/RFC5305"/>
        </reference>
        <reference anchor="RFC5310" target="https://www.rfc-editor.org/info/rfc5310" quoteTitle="true" derivedAnchor="RFC5310">
          <front>
            <title>IS-IS Generic Cryptographic Authentication</title>
            <author fullname="M. Bhatia" initials="M." surname="Bhatia"/>
            <author fullname="V. Manral" initials="V." surname="Manral"/>
            <author fullname="T. Li" initials="T." surname="Li"/>
            <author fullname="R. Atkinson" initials="R." surname="Atkinson"/>
            <author fullname="R. White" initials="R." surname="White"/>
            <author fullname="M. Fanto" initials="M." surname="Fanto"/>
            <date month="February" year="2009"/>
            <abstract>
              <t indent="0">This document proposes an extension to Intermediate System to Intermediate System (IS-IS) to allow the use of any cryptographic authentication algorithm in addition to the already-documented authentication schemes, described in the base specification and RFC 5304. IS-IS is specified in International Standards Organization (ISO) 10589, with extensions to support Internet Protocol version 4 (IPv4) described in RFC 1195.</t>
              <t indent="0">Although this document has been written specifically for using the Hashed Message Authentication Code (HMAC) construct along with the Secure Hash Algorithm (SHA) family of cryptographic hash functions, the method described in this document is generic and can be used to extend IS-IS to support any cryptographic hash function in the future. [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5310"/>
          <seriesInfo name="DOI" value="10.17487/RFC5310"/>
        </reference>
        <reference anchor="RFC5340" target="https://www.rfc-editor.org/info/rfc5340" quoteTitle="true" derivedAnchor="RFC5340">
          <front>
            <title>OSPF for IPv6</title>
            <author fullname="R. Coltun" initials="R." surname="Coltun"/>
            <author fullname="D. Ferguson" initials="D." surname="Ferguson"/>
            <author fullname="J. Moy" initials="J." surname="Moy"/>
            <author fullname="A. Lindem" initials="A." surname="Lindem"/>
            <date month="July" year="2008"/>
            <abstract>
              <t indent="0">This document describes the modifications to OSPF to support version 6 of the Internet Protocol (IPv6). The fundamental mechanisms of OSPF (flooding, Designated Router (DR) election, area support, Short Path First (SPF) calculations, etc.) remain unchanged. However, some changes have been necessary, either due to changes in protocol semantics between IPv4 and IPv6, or simply to handle the increased address size of IPv6. These modifications will necessitate incrementing the protocol version from version 2 to version 3. OSPF for IPv6 is also referred to as OSPF version 3 (OSPFv3).</t>
              <t indent="0">Changes between OSPF for IPv4, OSPF Version 2, and OSPF for IPv6 as described herein include the following. Addressing semantics have been removed from OSPF packets and the basic Link State Advertisements (LSAs). New LSAs have been created to carry IPv6 addresses and prefixes. OSPF now runs on a per-link basis rather than on a per-IP-subnet basis. Flooding scope for LSAs has been generalized. Authentication has been removed from the OSPF protocol and instead relies on IPv6's Authentication Header and Encapsulating Security Payload (ESP).</t>
              <t indent="0">Even with larger IPv6 addresses, most packets in OSPF for IPv6 are almost as compact as those in OSPF for IPv4. Most fields and packet- size limitations present in OSPF for IPv4 have been relaxed. In addition, option handling has been made more flexible.</t>
              <t indent="0">All of OSPF for IPv4's optional capabilities, including demand circuit support and Not-So-Stubby Areas (NSSAs), are also supported in OSPF for IPv6. [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5340"/>
          <seriesInfo name="DOI" value="10.17487/RFC5340"/>
        </reference>
        <reference anchor="RFC6571" target="https://www.rfc-editor.org/info/rfc6571" quoteTitle="true" derivedAnchor="RFC6571">
          <front>
            <title>Loop-Free Alternate (LFA) Applicability in Service Provider (SP) Networks</title>
            <author fullname="C. Filsfils" initials="C." role="editor" surname="Filsfils"/>
            <author fullname="P. Francois" initials="P." role="editor" surname="Francois"/>
            <author fullname="M. Shand" initials="M." surname="Shand"/>
            <author fullname="B. Decraene" initials="B." surname="Decraene"/>
            <author fullname="J. Uttaro" initials="J." surname="Uttaro"/>
            <author fullname="N. Leymann" initials="N." surname="Leymann"/>
            <author fullname="M. Horneffer" initials="M." surname="Horneffer"/>
            <date month="June" year="2012"/>
            <abstract>
              <t indent="0">In this document, we analyze the applicability of the Loop-Free Alternate (LFA) method of providing IP fast reroute in both the core and access parts of Service Provider networks.  We consider both the link and node failure cases, and provide guidance on the applicability of LFAs to different network topologies, with special emphasis on the access parts of the network.  This document is not an Internet Standards Track specification; it is published for informational purposes.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="6571"/>
          <seriesInfo name="DOI" value="10.17487/RFC6571"/>
        </reference>
        <reference anchor="RFC7120" target="https://www.rfc-editor.org/info/rfc7120" quoteTitle="true" derivedAnchor="RFC7120">
          <front>
            <title>Early IANA Allocation of Standards Track Code Points</title>
            <author fullname="M. Cotton" initials="M." surname="Cotton"/>
            <date month="January" year="2014"/>
            <abstract>
              <t indent="0">This memo describes the process for early allocation of code points by IANA from registries for which "Specification Required", "RFC Required", "IETF Review", or "Standards Action" policies apply.  This process can be used to alleviate the problem where code point allocation is needed to facilitate desired or required implementation and deployment experience prior to publication of an RFC, which would normally trigger code point allocation.  The procedures in this document are intended to apply only to IETF Stream documents.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="100"/>
          <seriesInfo name="RFC" value="7120"/>
          <seriesInfo name="DOI" value="10.17487/RFC7120"/>
        </reference>
        <reference anchor="RFC7471" target="https://www.rfc-editor.org/info/rfc7471" quoteTitle="true" derivedAnchor="RFC7471">
          <front>
            <title>OSPF Traffic Engineering (TE) Metric Extensions</title>
            <author fullname="S. Giacalone" initials="S." surname="Giacalone"/>
            <author fullname="D. Ward" initials="D." surname="Ward"/>
            <author fullname="J. Drake" initials="J." surname="Drake"/>
            <author fullname="A. Atlas" initials="A." surname="Atlas"/>
            <author fullname="S. Previdi" initials="S." surname="Previdi"/>
            <date month="March" year="2015"/>
            <abstract>
              <t indent="0">In certain networks, such as, but not limited to, financial information networks (e.g., stock market data providers), network performance information (e.g., link propagation delay) is becoming critical to data path selection.</t>
              <t indent="0">This document describes common extensions to RFC 3630 "Traffic Engineering (TE) Extensions to OSPF Version 2" and RFC 5329 "Traffic Engineering Extensions to OSPF Version 3" to enable network performance information to be distributed in a scalable fashion. The information distributed using OSPF TE Metric Extensions can then be used to make path selection decisions based on network performance.</t>
              <t indent="0">Note that this document only covers the mechanisms by which network performance information is distributed. The mechanisms for measuring network performance information or using that information, once distributed, are outside the scope of this document.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7471"/>
          <seriesInfo name="DOI" value="10.17487/RFC7471"/>
        </reference>
        <reference anchor="RFC7474" target="https://www.rfc-editor.org/info/rfc7474" quoteTitle="true" derivedAnchor="RFC7474">
          <front>
            <title>Security Extension for OSPFv2 When Using Manual Key Management</title>
            <author fullname="M. Bhatia" initials="M." surname="Bhatia"/>
            <author fullname="S. Hartman" initials="S." surname="Hartman"/>
            <author fullname="D. Zhang" initials="D." surname="Zhang"/>
            <author fullname="A. Lindem" initials="A." role="editor" surname="Lindem"/>
            <date month="April" year="2015"/>
            <abstract>
              <t indent="0">The current OSPFv2 cryptographic authentication mechanism as defined in RFCs 2328 and 5709 is vulnerable to both inter-session and intra- session replay attacks when using manual keying. Additionally, the existing cryptographic authentication mechanism does not cover the IP header. This omission can be exploited to carry out various types of attacks.</t>
              <t indent="0">This document defines changes to the authentication sequence number mechanism that will protect OSPFv2 from both inter-session and intra- session replay attacks when using manual keys for securing OSPFv2 protocol packets. Additionally, we also describe some changes in the cryptographic hash computation that will eliminate attacks resulting from OSPFv2 not protecting the IP header.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7474"/>
          <seriesInfo name="DOI" value="10.17487/RFC7474"/>
        </reference>
        <reference anchor="RFC8126" target="https://www.rfc-editor.org/info/rfc8126" quoteTitle="true" derivedAnchor="RFC8126">
          <front>
            <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
            <author fullname="M. Cotton" initials="M." surname="Cotton"/>
            <author fullname="B. Leiba" initials="B." surname="Leiba"/>
            <author fullname="T. Narten" initials="T." surname="Narten"/>
            <date month="June" year="2017"/>
            <abstract>
              <t indent="0">Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).</t>
              <t indent="0">To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.</t>
              <t indent="0">This is the third edition of this document; it obsoletes RFC 5226.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="26"/>
          <seriesInfo name="RFC" value="8126"/>
          <seriesInfo name="DOI" value="10.17487/RFC8126"/>
        </reference>
        <reference anchor="RFC8570" target="https://www.rfc-editor.org/info/rfc8570" quoteTitle="true" derivedAnchor="RFC8570">
          <front>
            <title>IS-IS Traffic Engineering (TE) Metric Extensions</title>
            <author fullname="L. Ginsberg" initials="L." role="editor" surname="Ginsberg"/>
            <author fullname="S. Previdi" initials="S." role="editor" surname="Previdi"/>
            <author fullname="S. Giacalone" initials="S." surname="Giacalone"/>
            <author fullname="D. Ward" initials="D." surname="Ward"/>
            <author fullname="J. Drake" initials="J." surname="Drake"/>
            <author fullname="Q. Wu" initials="Q." surname="Wu"/>
            <date month="March" year="2019"/>
            <abstract>
              <t indent="0">In certain networks, such as, but not limited to, financial information networks (e.g., stock market data providers), network-performance criteria (e.g., latency) are becoming as critical to data-path selection as other metrics.</t>
              <t indent="0">This document describes extensions to IS-IS Traffic Engineering Extensions (RFC 5305). These extensions provide a way to distribute and collect network-performance information in a scalable fashion. The information distributed using IS-IS TE Metric Extensions can then be used to make path-selection decisions based on network performance.</t>
              <t indent="0">Note that this document only covers the mechanisms with which network-performance information is distributed. The mechanisms for measuring network performance or acting on that information, once distributed, are outside the scope of this document.</t>
              <t indent="0">This document obsoletes RFC 7810.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8570"/>
          <seriesInfo name="DOI" value="10.17487/RFC8570"/>
        </reference>
        <reference anchor="RFC8986" target="https://www.rfc-editor.org/info/rfc8986" quoteTitle="true" derivedAnchor="RFC8986">
          <front>
            <title>Segment Routing over IPv6 (SRv6) Network Programming</title>
            <author fullname="C. Filsfils" initials="C." role="editor" surname="Filsfils"/>
            <author fullname="P. Camarillo" initials="P." role="editor" surname="Camarillo"/>
            <author fullname="J. Leddy" initials="J." surname="Leddy"/>
            <author fullname="D. Voyer" initials="D." surname="Voyer"/>
            <author fullname="S. Matsushima" initials="S." surname="Matsushima"/>
            <author fullname="Z. Li" initials="Z." surname="Li"/>
            <date month="February" year="2021"/>
            <abstract>
              <t indent="0">The Segment Routing over IPv6 (SRv6) Network Programming framework enables a network operator or an application to specify a packet processing program by encoding a sequence of instructions in the IPv6 packet header.</t>
              <t indent="0">Each instruction is implemented on one or several nodes in the network and identified by an SRv6 Segment Identifier in the packet.</t>
              <t indent="0">This document defines the SRv6 Network Programming concept and specifies the base set of SRv6 behaviors that enables the creation of interoperable overlays with underlay optimization.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8986"/>
          <seriesInfo name="DOI" value="10.17487/RFC8986"/>
        </reference>
        <reference anchor="I-D.gulkohegde-routing-planes-using-sr" target="https://datatracker.ietf.org/doc/html/draft-gulkohegde-routing-planes-using-sr-00" quoteTitle="true" derivedAnchor="ROUTING-PLANES-USING-SR">
          <front>
            <title>Separating Routing Planes using Segment Routing</title>
            <author initials="S." surname="Hegde" fullname="Shraddha Hegde">
              <organization showOnFrontPage="true">Juniper Networks</organization>
            </author>
            <author initials="A." surname="Gulko" fullname="Arkadiy Gulko">
              <organization showOnFrontPage="true">Thomson Reuters</organization>
            </author>
            <date month="March" day="13" year="2017"/>
            <abstract>
              <t indent="0">   Many network deployments arrange the network topologies in two or
   more planes.  The traffic generally uses one of the planes and fails
   over to the other plane when there are link or node failure.  Certain
   applications require the traffic to be strictly restricted to a
   particular plane and should not failover to the other plane.  This
   document proposes a solution for the strict planar routing using
   Segment Routing.


              </t>
            </abstract>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-gulkohegde-routing-planes-using-sr-00"/>
          <refcontent>Work in Progress</refcontent>
        </reference>
        <reference anchor="I-D.ietf-rtgwg-segment-routing-ti-lfa" target="https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-segment-routing-ti-lfa-09" quoteTitle="true" derivedAnchor="RTGWG-SEGMENT-ROUTING-TI-LFA">
          <front>
            <title>Topology Independent Fast Reroute using Segment Routing</title>
            <author initials="S." surname="Litkowski" fullname="Stephane Litkowski">
              <organization showOnFrontPage="true">Cisco Systems</organization>
            </author>
            <author initials="A." surname="Bashandy" fullname="Ahmed Bashandy">
              <organization showOnFrontPage="true">Individual</organization>
            </author>
            <author initials="C." surname="Filsfils" fullname="Clarence Filsfils">
              <organization showOnFrontPage="true">Cisco Systems</organization>
            </author>
            <author initials="P." surname="Francois" fullname="Pierre Francois">
              <organization showOnFrontPage="true">INSA Lyon</organization>
            </author>
            <author initials="B." surname="Decraene" fullname="Bruno Decraene">
              <organization showOnFrontPage="true">Orange</organization>
            </author>
            <author initials="D." surname="Voyer" fullname="Daniel Voyer">
              <organization showOnFrontPage="true">Bell Canada</organization>
            </author>
            <date month="December" day="23" year="2022"/>
            <abstract>
              <t indent="0">   This document presents Topology Independent Loop-free Alternate Fast
   Re-route (TI-LFA), aimed at providing protection of node and
   adjacency segments within the Segment Routing (SR) framework.  This
   Fast Re-route (FRR) behavior builds on proven IP-FRR concepts being
   LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding
   (DLFA).  It extends these concepts to provide guaranteed coverage in
   any two connected network using a link-state IGP.  A key aspect of
   TI-LFA is the FRR path selection approach establishing protection
   over the expected post-convergence paths from the point of local
   repair, reducing the operational need to control the tie-breaks among
   various FRR options.

              </t>
            </abstract>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-ietf-rtgwg-segment-routing-ti-lfa-09"/>
          <refcontent>Work in Progress</refcontent>
        </reference>
      </references>
    </references>
    <section anchor="ACK" numbered="false" toc="include" removeInRFC="false" pn="section-appendix.a">
      <name slugifiedName="name-acknowledgements">Acknowledgements</name>
      <t indent="0" pn="section-appendix.a-1">This document, among other things, addresses the problem that
      <xref target="I-D.gulkohegde-routing-planes-using-sr" format="default" sectionFormat="of" derivedContent="ROUTING-PLANES-USING-SR"/> was trying
      to solve. All authors of that document agreed to join this document.</t>
      <t indent="0" pn="section-appendix.a-2">Thanks to <contact fullname="Eric Rosen"/>, <contact fullname="Tony Przygienda"/>, <contact fullname="William Britto A. J."/>, <contact fullname="Gunter Van       de Velde"/>, <contact fullname="Dirk Goethals"/>, <contact fullname="Manju Sivaji"/>, and <contact fullname="Baalajee S."/> for their detailed
      review and excellent comments.</t>
      <t indent="0" pn="section-appendix.a-3">Thanks to <contact fullname="Cengiz Halit"/> for his review and feedback during
      the initial phase of the solution definition.</t>
      <t indent="0" pn="section-appendix.a-4">Thanks to <contact fullname="Kenji Kumaki"/> for his comments.</t>
      <t indent="0" pn="section-appendix.a-5">Thanks to <contact fullname="Acee Lindem"/> for editorial comments.</t>
    </section>
    <section anchor="authors-addresses" numbered="false" removeInRFC="false" toc="include" pn="section-appendix.b">
      <name slugifiedName="name-authors-addresses">Authors' Addresses</name>
      <author fullname="Peter Psenak" initials="P." role="editor" surname="Psenak">
        <organization showOnFrontPage="true">Cisco Systems, Inc.</organization>
        <address>
          <postal>
            <extaddr>Apollo Business Center</extaddr>
            <street>Mlynske nivy 43</street>
            <city>Bratislava</city>
            <code>82109</code>
            <country>Slovakia</country>
            <code/>
          </postal>
          <email>ppsenak@cisco.com</email>
        </address>
      </author>
      <author fullname="Shraddha Hegde" initials="S" surname="Hegde">
        <organization showOnFrontPage="true">Juniper Networks, Inc.</organization>
        <address>
          <postal>
            <extaddr>Embassy Business Park</extaddr>
            <street/>
            <city>Bangalore</city>
            <region>KA</region>
            <code>560093</code>
            <country>India</country>
            <code/>
          </postal>
          <email>shraddha@juniper.net</email>
        </address>
      </author>
      <author fullname="Clarence Filsfils" initials="C." surname="Filsfils">
        <organization showOnFrontPage="true">Cisco Systems, Inc.</organization>
        <address>
          <postal>
            <street/>
            <city>Brussels</city>
            <region/>
            <code/>
            <country>Belgium</country>
          </postal>
          <email>cfilsfil@cisco.com</email>
        </address>
      </author>
      <author fullname="Ketan Talaulikar" initials="K." surname="Talaulikar">
        <organization showOnFrontPage="true">Cisco Systems, Inc</organization>
        <address>
          <postal>
            <street/>
            <city/>
            <code/>
            <country>India</country>
          </postal>
          <email>ketant.ietf@gmail.com</email>
        </address>
      </author>
      <author fullname="Arkadiy Gulko" initials="A." surname="Gulko">
        <organization showOnFrontPage="true">Edward Jones</organization>
        <address>
          <postal>
            <street/>
            <city/>
            <region/>
            <code/>
            <country/>
          </postal>
          <email>arkadiy.gulko@edwardjones.com</email>
        </address>
      </author>
    </section>
  </back>
</rfc>
