<?xml version="1.0" encoding="utf-8"?> <!DOCTYPE rfcSYSTEM "rfc2629.dtd"[ <!ENTITYRFC2119 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml">nbsp " "> <!ENTITYRFC5384 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5286.xml">zwsp "​"> <!ENTITYRFC5714 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5714.xml">nbhy "‑"> <!ENTITYRFC5715 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5715.xml"> <!ENTITY RFC6571 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6571.xml"> <!ENTITY RFC6976 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6976.xml"> <!ENTITY RFC7490 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7490.xml"> <!ENTITY RFC7916 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7916.xml"> <!ENTITY RFC8174 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"> <!ENTITY RFC8333 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8333.xml"> <!ENTITY RFC8402 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8402.xml"> <!ENTITY RFC8660 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8660.xml"> <!ENTITY RFC8665 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8665.xml"> <!ENTITY RFC8667 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8667.xml"> <!ENTITY RFC8754 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8754.xml"> <!ENTITY RFC8986 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8986.xml"> <!ENTITY RFC9256 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.9256.xml"> <!ENTITY I-D.bashandy-rtgwg-segment-routing-uloop SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml3/reference.I-D.bashandy-rtgwg-segment-routing-uloop.xml"> <!ENTITY FLEXALGO SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.9350.xml">wj "⁠"> ]> <rfc xmlns:xi="http://www.w3.org/2001/XInclude" category="std" consensus="true" docName="draft-ietf-rtgwg-segment-routing-ti-lfa-21" number="9855" ipr="trust200902"submissionType="IETF"> <!-- Generated by id2xml 1.4.4 on 2018-12-03T18:16:52Z --> <?rfc compact="yes"?> <?rfc text-list-symbols="o*+-"?> <?rfc subcompact="no"?> <?rfc sortrefs="yes"?> <?rfc symrefs="yes"?> <?rfc strict="yes"?> <?rfc toc="yes"?>submissionType="IETF" obsoletes="" updates="" xml:lang="en" sortRefs="true" symRefs="true" tocInclude="true" version="3"> <front> <title abbrev="SR TI-LFA">Topology Independent Fast RerouteusingUsing Segment Routing</title> <seriesInfo name="RFC" value="9855"/> <author fullname="Ahmed Bashandy" initials="A." surname="Bashandy"> <organization>Individual</organization> <address><postal> <street/> <city/> <country/> </postal><email>abashandy.ietf@gmail.com</email> </address> </author> <author fullname="Stephane Litkowski" initials="S." surname="Litkowski"> <organization>Cisco Systems</organization> <address> <postal><street/> <city/><country>France</country> </postal> <email>slitkows@cisco.com</email> </address> </author> <author fullname="Clarence Filsfils" initials="C." surname="Filsfils"> <organization>Cisco Systems</organization> <address> <postal><street/><city>Brussels</city> <country>Belgium</country> </postal> <email>cfilsfil@cisco.com</email> </address> </author> <author fullname="Pierre Francois" initials="P." surname="Francois"> <organization>INSA Lyon</organization> <address><postal> <street/> <city/> <country/> </postal><email>pierre.francois@insa-lyon.fr</email> </address> </author> <author fullname="Bruno Decraene" initials="B." surname="Decraene"> <organization>Orange</organization> <address> <postal><street/><city>Issy-les-Moulineaux</city> <country>France</country> </postal> <email>bruno.decraene@orange.com</email> </address> </author> <author fullname="Daniel Voyer" initials="D." surname="Voyer"> <organization>Bell Canada</organization> <address> <postal><street/> <city/><country>Canada</country> </postal> <email>daniel.voyer@bell.ca</email> </address> </author><abstract> <t>This<date year="2025" month="September"/> <area>RTG</area> <workgroup>rtgwg</workgroup> <!-- [rfced] Please insert any keywords (beyond those that appear in the title) for use on https://www.rfc-editor.org/search. --> <keyword>example</keyword> <!-- [rfced] FYI - We have made some adjustments to the abstract in order to clarify the expansions of some abbreviations. Please review and let us know if any further updates are necessary. Original: This document presents Topology Independent Loop-free Alternate Fast Reroute (TI-LFA), aimed at providing protection of node and adjacency segments within the Segment Routing (SR) framework. This Fast Reroute (FRR) behavior builds on proven IP Fast Reroute concepts being LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding (DLFA). Current: This document presents Topology Independent Loop-Free Alternate (TI- LFA) Fast Reroute (FRR), which is aimed at providing protection of node and adjacency segments within the Segment Routing (SR) framework. This FRR behavior builds on proven IP FRR concepts being LFAs, Remote LFAs (RLFAs), and remote LFAs with directed forwarding (DLFAs). --> <abstract> <t>This document presents Topology Independent Loop-Free Alternate (TI-LFA) Fast Reroute (FRR), which is aimed at providing protection of node and adjacency segments within the Segment Routing (SR) framework. This FRR behavior builds on proven IP FRR concepts being LFAs, Remote LFAs (RLFAs), and remote LFAs with directed forwarding (DLFAs). It extends these concepts to provide guaranteed coverage in any two-connected networks using a link-state IGP. An important aspect of TI-LFA is the FRR path selection approach establishing protection over the expected post-convergence paths from thepointPoint oflocal repair,Local Repair (PLR), reducing the operational need to control the tie-breaks among various FRR options.</t> </abstract> </front> <middle> <sectionanchor="acronyms" title="Acronyms"> <t><list style="symbols"> <t>DLFA: Remote LFA with Directed forwarding.</t> <t>FRR: Fast Re-route.</t> <t>IGP: Interior Gateway Protocol.</t> <t>LFA: Loop-Free Alternate.</t> <t>LSDB: Link State DataBase.</t> <t>PLR: Point of Local Repair.</t> <t>RL: Repair list.</t> <t>RLFA: Remote LFA.</t> <t>SID: Segment Identifier.</t> <t>SPF: Shortest Path First.</t> <t>SR: Segment Routing.</t> <t>SRLG: Shared Risk Link Group.</t> <t>TI-LFA: Topology Independent LFA.</t> </list></t> </section> <sectionanchor="introduction"title="Introduction">numbered="true" toc="default"> <name>Introduction</name> <t>This document outlines a local repair mechanism that leverages Segment Routing (SR) to restore end-to-end connectivity in the event of a failure involving a directly connected network component. This mechanism is designed for standard link-state Interior Gateway Protocol (IGP) shortest path scenarios. Non-SR mechanisms for local repair are beyond the scope of this document. Non-local failures are addressed in a separate document <xreftarget="I-D.bashandy-rtgwg-segment-routing-uloop"/>.</t>target="I-D.bashandy-rtgwg-segment-routing-uloop" format="default"/>.</t> <t>The termtopology independentTopology Independent (TI) describes the capability providing aloop freeloop-free backup path that is effectiveaccrossacross all network topologies. This provides a major improvement compared to LFA <xreftarget="RFC5286"/>target="RFC5286" format="default"/> andremote LFARLFA <xreftarget="RFC7490"/>target="RFC7490" format="default"/>, which cannot provide a complete protection coverage in some topologies as described in <xreftarget="RFC6571"/>.</t>target="RFC6571" format="default"/>.</t> <t>When the network reconverges after failure, micro-loops <xreftarget="RFC5715"/>target="RFC5715" format="default"/> can form due to transient inconsistencies in the forwarding tables of different routers. If it is determined that micro-loops are a significant issue in the deployment, then a suitable loop-free convergencemethod,method should be implemented, such as one of those described in <xreftarget="RFC5715"/>,target="RFC5715" format="default"/>, <xreftarget="RFC6976"/>,target="RFC6976" format="default"/>, <xreftarget="RFC8333"/>,target="RFC8333" format="default"/>, or <xreftarget="I-D.bashandy-rtgwg-segment-routing-uloop"/> should be implemented.</t>target="I-D.bashandy-rtgwg-segment-routing-uloop" format="default"/>.</t> <t>TI-LFA operates locally at the Point of Local Repair (PLR) upon detecting a failure in one of its direct links. Consequently, this local operation does not influence:<list style="symbols"></t> <ul spacing="normal"> <li> <t>Micro-loops that may or may not form during the distributedInterior Gateway Protocol (IGP)IGP convergence as delineated in <xreftarget="RFC5715"/>: <list style="symbols">target="RFC5715" format="default"/>: </t> <ul spacing="normal"> <li> <t>These micro-loops occur on routes directed towards the destination that do not traverseTI-LFA-configured paths.paths configured for TI-LFA. According to <xreftarget="RFC5714"/>,target="RFC5714" format="default"/>, the formation of such micro-loops can prevent traffic from reaching the PLR, thereby bypassing the TI-LFA paths established for rerouting.</t></list></t></li> </ul> </li> <li> <t>Micro-loops that may or may not develop when the previously failed link is restored to functionality.</t></list></t></li> </ul> <t>TI-LFA paths are activated from the instant the PLR detects a failure in a local link and remain in effect until theInterior Gateway Protocol (IGP)IGP convergence at the PLR is fully achieved. Consequently, they are not susceptible to micro-loops that may arise due to variations in the IGP convergence times across different nodes through which these paths traverse. This ensures a stable and predictable routing environment, minimizing disruptions typically associated with asynchronous network behavior. However, an early (relative to the other nodes) IGP convergence at the PLR and the consecutive”early”"early" release of TI-LFA paths may cause micro-loops, especially if these paths have been computed using the methods described inSectionSections <xreftarget="pq_backup"/>,target="pq_backup" format="counter"/>, <xreftarget="adj_pq_backup"/>,target="adj_pq_backup" format="counter"/>, or <xreftarget="remote_pq_backup"/>target="remote_pq_backup" format="counter"/> ofthethis document. One of the possible ways to prevent such micro-loops is local convergence delay(<xref target="RFC8333"/>).</t><xref target="RFC8333" format="default"/>.</t> <t>TI-LFA procedures are complementary to the application of any micro-loop avoidance procedures in the case of link or nodefailure: <list style="symbols">failure:</t> <ul spacing="normal"> <li> <t>Link or node failure requires some urgent action to restore the traffic that passedthruthrough the failed resource. TI-LFA paths are pre-computed andpre-installed and thereforepre-installed; therefore, they are suitable for urgentrecovery</t>recovery.</t> </li> <li> <t>The paths used in the micro-loop avoidance procedures typically cannot be pre-computed.</t></list></t></li> </ul> <t>For each destination (as specified by the IGP) in the network, TI-LFA pre-installs a backup forwarding entry for each protected destination ready to be activated upon detection of the failure of a link used to reach the destination. TI-LFA provides protection in the event of any one of the following: single link failure, single node failure, or singleSRLGShared Risk Link Group (SRLG) failure. In link failure mode, the destination is protected assuming the failure of the link. In node protection mode, the destination is protected assuming that the neighbor connected to the primary link (see <xreftarget="terminology"/>target="terminology" format="default"/>) has failed. In SRLG protecting mode, the destination is protected assuming that a configured set of links sharing fate with the primary link has failed(e.g.(e.g., a linecard or a set of links sharing a common transmission pipe).</t> <t>Protection techniques outlined in this document are limited to protecting links, nodes, and SRLGs that are within a link-state IGP area. Protecting domain exit routers and/or links attached to another routingdomains aredomain is beyond the scope of thisdocument</t> <t>Bydocument.</t> <!-- [rfced] We were unable to find the term "Directed Loop-Free Alternates (DLFA)" mentioned in RFC 5714. Is there an alternative reference that could be used here? Original: By utilizing Segment Routing (SR), TI-LFA eliminates the need to establish Targeted Label Distribution Protocol sessions with remote nodes for leveraging the benefits of Remote Loop-Free Alternates (RLFA)<xref target="RFC7490"/><xref target="RFC7916"/>[RFC7490][RFC7916] or Directed Loop-Free Alternates (DLFA) [RFC5714]. --> <t>By utilizing SR, TI-LFA eliminates the need to establish Targeted Label Distribution Protocol sessions with remote nodes for leveraging the benefits of Remote Loop-Free Alternates (RLFAs) <xref target="RFC7490" format="default"/> <xref target="RFC7916" format="default"/> or Directed Loop-Free Alternates (DLFAs) <xreftarget="RFC5714"/>.target="RFC5714" format="default"/>. All the Segment Identifiers (SIDs) required are present within the Link State Database (LSDB) of theInterior Gateway Protocol (IGP).IGP. Consequently, there is no longer a necessity to prefer LFAs over RLFAs or DLFAs, nor is there a need to minimize the number of RLFA or DLFA repair nodes.</t> <!--[rfced] To improve readability, may we update "makes the requirement unnecessary" to "eliminates the need" in the sentence below? Original: Utilizing SR makes the requirement unnecessary to establish additional state within the network for enforcing explicit Fast Reroute (FRR) paths. Perhaps: Utilizing SR also eliminates the need to establish an additional state within the network for enforcing explicit Fast Reroute (FRR) paths. --> <t>Utilizing SR makes the requirement unnecessary to establish an additional state within the network for enforcing explicit Fast Reroute (FRR) paths. This spares the nodes from maintaining a supplementary state and frees the operator from the necessity to implement additional protocols or protocol sessions solely to augment protection coverage.</t> <t>TI-LFA also brings the benefit of the ability to provide a backup path that follows the expected post-convergence path considering a particularfailurefailure, which reduces the need of locally configured policies that influence the backup path selection(<xref target="RFC7916"/>).<xref target="RFC7916" format="default"/>. The easiest way to express the expected post-convergence path in a loop-free manner is to encode it as a list of adjacency segments. However, this may create a long segment list that some hardware may not be able to program. One of the challenges of TI-LFA is to encode the expected post-convergence path by combining adjacency segments and node segments. Each implementation may independently develop its own algorithm for optimizing the ordered segment list. This document provides an outline of the fundamental concepts applicable to constructing the SR backup path, along with the related dataplane procedures. <xreftarget="advantages-post-conv-path"/> describestarget="advantages-post-conv-path" format="default"/> contains a more detailed description of some of thepost-convergence path relatedaspects of TI-LFA related to post-convergence path.</t> <!-- [rfced] To improve readability, we have reformatted the text that appears at the end of the Introduction into a bulleted list. Please review. In addition, may we adjust these three items for consistency with the other list items (so that each list item begins with the section number it refers to)? Note: The section numbers inmore detail.</t> <t><xref target="terminology"/>this document have changed so they may appear differently in the "Perhaps" text. Original: Using the properties defined in Section 5, Section 6 describes how to compute protection lists that encode a loop-free post-convergence path towards the destination. ... Certain considerations are needed when adjacency segments are used in a repare list. Section 10 provides an overview of these considerations. ... By implementing the algorithms detailed in this document within actual service provider and large enterprise network environments, real-life measurements are presented regarding the number of SIDs utilized by repair paths. These measurements are summarized in Appendix B. Perhaps: * Section 5 describes how to compute protection lists that encode a loop-free post-convergence path towards the destination using the properties defined in Section 4. ... * Section 9 provides an overview of the certain considerations that are needed when adjacency segments are used in a repair list. ... * Appendix B summarizes the measurements from implementing the algorithms detailed in this document within actual service provider and large enterprise network environments. Real-life measurements are presented regarding the number of SIDs utilized by repair paths. --> <t>This document is structured as follows:</t> <ul> <li><xref target="terminology" format="default"/> defines the main notations used in the document. They are in line with <xreftarget="RFC5714"/>.</t> <t><xref target="base"/>target="RFC5714" format="default"/>.</li> <li><xref target="base" format="default"/> defines the main principles of TI-LFA backup pathcomputation.</t> <t><xref target="pq_space_intersect"/>computation.</li> <li><xref target="pq_space_intersect" format="default"/> suggests to compute the P-Space and Q-Space properties defined in <xreftarget="terminology"/>,target="terminology" format="default"/> for the specific case of nodes lying over the post-convergence paths towards the protecteddestinations.</t> <t>Usingdestinations.</li> <li>Using the properties defined in <xreftarget="pq_space_intersect"/>,target="pq_space_intersect" format="default"/>, <xreftarget="tilfa_repair_path"/>target="tilfa_repair_path" format="default"/> describes how to compute protection lists that encode a loop-free post-convergence path towards thedestination.</t> <t><xref target="repairlist"/>destination.</li> <li><xref target="repairlist" format="default"/> defines the segment operations to be applied by the PLR to ensure consistency with the forwarding state of the repairnode.</t> <t><xref target="dataplane"/>node.</li> <li><xref target="dataplane" format="default"/> discusses aspects that are specific to thedataplane.</t> <t><xref target="tilfa-sr-algo"/>dataplane.</li> <li><xref target="tilfa-sr-algo" format="default"/> discusses the relationship between TI-LFA and theSR-algorithm.</t> <t>CertainSR algorithm.</li> <li>Certain considerations are needed when adjacency segments are used in areparerepair list. <xreftarget="adj-sid-repair-list"/>target="adj-sid-repair-list" format="default"/> provides an overview of theseconsiderations.</t> <t><xref target="security"/>considerations.</li> <li><xref target="security" format="default"/> discusses securityconsiderations.</t> <t><xref target="advantages-post-conv-path"/>considerations.</li> <li><xref target="advantages-post-conv-path" format="default"/> highlights advantages of using the expected post-convergence path duringFRR.</t> <t>ByFRR.</li> <li>By implementing the algorithms detailed in this document within actual service provider and large enterprise network environments, real-life measurements are presented regarding the number of SIDs utilized by repair paths. These measurements are summarized in <xreftarget="analysis"/>.</t>target="analysis" format="default"/>.</li> </ul> </section> <section anchor="terminology"title="Terminology">numbered="true" toc="default"> <name>Terminology</name> <section anchor="acronyms" numbered="true" toc="default"> <name>Abbreviations and Notations</name> <dl spacing="normal" newline="false"> <dt>DLFA:</dt><dd>Directed Loop-Free Alternate</dd> <dt>FRR:</dt><dd>Fast Reroute</dd> <dt>IGP:</dt><dd>Interior Gateway Protocol</dd> <dt>LFA:</dt><dd>Loop-Free Alternate</dd> <dt>LSDB:</dt><dd>Link State Database</dd> <dt>PLR:</dt><dd>Point of Local Repair</dd> <dt>RL:</dt><dd>Repair List</dd> <dt>RLFA:</dt><dd>Remote Loop-Free Alternate</dd> <dt>SID:</dt><dd>Segment Identifier</dd> <dt>SPF:</dt><dd>Shortest Path First</dd> <dt>SPT:</dt><dd>Shortest Path Tree</dd> <dt>SR:</dt><dd>Segment Routing</dd> <dt>SRLG:</dt><dd>Shared Risk Link Group</dd> <dt>TI-LFA:</dt><dd>Topology Independent Loop-Free Alternate</dd> </dl> <!-- [rfced] FYI - The main notations in the Terminology section were formatted inconsistently, so we have reformatted those items into a bulleted list. Please review the changes to the following items in particular: Original: Primary Interface: Primary Outgoing Interface: One of the outgoing interfaces towards a destination according to the IGP link-state protocol Primary Link: A link connected to the primary interface adj-sid(S-F): Adjacency Segment from node S to node F Current: * The primary interface and the primary outgoing interface are one of the outgoing interfaces towards a destination according to the IGP link-state protocol. * The primary link is a link connected to the primary interface. * The adj-sid(S-F) is the adjacency segment from node S to node F. --> <t>The main notations used in this document are defined asfollows.</t> <t>Thefollows:</t> <ul> <li>The terms "old" and "new" topologies refer to theLink State Database (LSDB)LSDB state before and after the considered failure,respectively.</t> <t>SPT_old(R)respectively.</li> <li>SPT_old(R) is theShortest Path TreeSPT rooted at node R in the initial state of thenetwork.</t> <t>SPT_new(R,network.</li> <li>SPT_new(R, X) is theShortest Path TreeSPT rooted at node R in the state of the network after the resource X hasfailed.</t> <t>PLR stands for "Pointfailed.</li> <li>The Point of LocalRepair". ItRepair (PLR) is the router that applies fast traffic restoration after detecting failure in a directly attached link, set of links, and/ornode.</t> <t>Similarnode.</li> <li>Similar to <xreftarget="RFC7490"/>,target="RFC7490" format="default"/>, the concept of P-Space and Q-Space is used forTI-LFA.</t> <t>TheTI-LFA.</li> <li>The P-space P(R,X) of a router R with regard to a resource X(e.g.(e.g., a link S-F, a node F, oraan SRLG) is the set of routers reachable from R using the pre-convergence shortest paths without any of those paths (including equal-cost path splits) transiting through X. A P node is a node that belongs to theP-space.</t> <t>ConsiderP-space.</li> <li>Consider the set of neighbors of a router R and a resource X. Exclude from thatset,set the neighbors that are reachable from R using X. TheExtendedextended P-Space P'(R,X) of a node R with regard to a resource X is the union of the P-spaces of the neighbors in that reduced set of neighbors with regard to the resourceX.</t> <t>TheX.</li> <li>The Q-space Q(R,X) of a router R with regard to a resource X is the set of routers from which R can be reached without any path (including equal-cost path splits) transiting through X. A Q node is a node that belongs to theQ-space </t> <t>EP(P,Q-space.</li> <li>EP(P, Q) is an explicit SR path from a node P to a nodeQ.</t> <t>Primary Interface: Primary Outgoing Interface: OneQ.</li> <li>The primary interface and primary outgoing interface are one of the outgoing interfaces towards a destination according to the IGP link-stateprotocol</t> <t>Primary Link: Aprotocol.</li> <li>The primary link is a link connected to the primaryinterface</t> <t>adj-sid(S-F): Adjacency Segmentinterface.</li> <li>The adj-sid(S-F) is the adjacency segment from node S to nodeF</t>F.</li> </ul> </section> <section anchor="conventions"title="Conventions usednumbered="true" toc="default"> <name>Conventions Used inthis document"> <t>TheThis Document</name> <t> The key words"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY","<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>", "<bcp14>MAY</bcp14>", and"OPTIONAL""<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as described inBCP 14BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only when, they appear in all capitals, as shownhere.</t>here. </t> </section> </section> <section anchor="base"title="Base principle">numbered="true" toc="default"> <name>Base Principle</name> <t>The basic algorithm to compute the repair path is to pre-compute SPT_new(R,X)andand, for each destination, encode the repair path as a loop-free segment list. One way to provide a loop-free segment list is to use adjacency SIDs only. However, this approach may create very long SID lists that hardware may not be able to handle due toMSD (MaximumMaximum SIDDepth)Depth (MSD) limitations.</t> <t>An implementation is free to use any local optimization to provide smaller segment lists by combining Node SIDs and Adjacency SIDs. In addition, the usage of Node-SIDs allowto maximizefor maximizing ECMPs over the backup path. These optimizations are out of scope of thisdocument, howeverdocument; however, the subsequent sections provide some guidance on how to leverage P-Spaces and Q-Spaces to optimize the size of the segment list.</t> </section> <section anchor="pq_space_intersect"title="Intersectingnumbered="true" toc="default"> <name>Intersecting P-Space and Q-Space withpost-convergence paths">Post-Convergence Paths</name> <t>One of the challenges of defining an SR path following the expected post-convergence path is to reduce the size of the segment list. In order to reduce this segment list, an implementationMAY<bcp14>MAY</bcp14> determine theP-Space/ExtendedP-Space / extended P-Space and Q-Space properties (defined in <xreftarget="RFC7490"/>)target="RFC7490" format="default"/>) of the nodes along the expected post-convergence path from the PLR to the protected destination and compute an SR explicit path from P to Q when they are not adjacent. Such properties will be used in <xreftarget="tilfa_repair_path"/>target="tilfa_repair_path" format="default"/> to compute the TI-LFA repair list.</t> <section anchor="extp_space"title="Extendednumbered="true" toc="default"> <name>Extended P-Spaceproperty computationProperty Computation for aresource X,Resource X overpost-convergence paths">Post-Convergence Paths</name> <t>The objective is to determine which nodes on the post-convergence path from the PLR R to the destination D are in the extended P-space of R with regard to resource X (where X can be a link or a set of links adjacent to thePLR,PLR or a neighbor node of the PLR).</t> <t>This can be foundby: <list style="symbols"> <t>Excludingby:</t> <ul spacing="normal"> <li> <t>excluding neighborswhichthat are not on the post-convergence path when computingP'(R,X)</t> <t>Then, intersectingP'(R,X), then</t> </li> <li> <t>intersecting the set of nodes belonging to the post-convergence path from R to D, assuming the failure of X, with P'(R, X).</t></list></t></li> </ul> </section> <section anchor="q_space"title="Q-Space property computationnumbered="true" toc="default"> <name>Q-Space Property Computation for aresource X,Resource X overpost-convergence paths">Post-Convergence Paths</name> <t>The goal is to determine which nodes on the post-convergence path from the Point of Local Repair (PLR) R to the destination D are in the Q-Space of destination D with regard to resource X (where X can be a link or a set of links adjacent to the PLR, or a neighbor node of the PLR).</t> <t>This can be found by intersecting the set of nodes belonging to the post-convergence path from R to D, assuming the failure of X, with Q(D, X).</t> </section> <section anchor="q_space_scaling"title="Scaling considerations when computing Q-Space">numbered="true" toc="default"> <name>Scaling Considerations When Computing Q-Space</name> <t><xreftarget="RFC7490"/>target="RFC7490" format="default"/> raises scaling concerns about computing a Q-Space per destination. Similar concerns may affect TI-LFA computation if an implementation tries to compute a reverse Shortest Path Tree(<xref target="RFC7490"/>)(SPT) <xref target="RFC7490" format="default"/> for every destination in the network to determine the Q-Space. It will be up to each implementation to determine the good tradeoff between scaling and accuracy of the optimization.</t> </section> </section> <!-- [rfced] To improve readability, may we break up this sentence into two sentences? If yes, would "the path" be the correct subject for the second sentence? Original: The repair list encodes the explicit, and possibly post-convergence, path to the destination, which avoids the protected resource X and, at the same time, is guaranteed to be loop-free irrespective of the state of FIBs along the nodes belonging to the explicit path as long as the states of the FIBs are programmed according to a link-state IGP. Perhaps: The repair list encodes the explicit (and possibly post-convergence) path to the destination, which avoids the protected resource X. At the same time, the path is guaranteed to be loop-free, irrespective of the state of FIBs along the nodes belonging to the explicit path, as long as the states of the FIBs are programmed according to a link-state IGP. --> <section anchor="tilfa_repair_path"title="TI-LFAnumbered="true" toc="default"> <name>TI-LFA Repairpath">Path</name> <t>The TI-LFA repair path consists of an outgoing interface and a list of segments(repair list(a Repair List (RL)) to insert on the SR header in accordance with the dataplane used. The repair list encodes the explicit, and possibly post-convergence, path to the destination, which avoids the protected resource X and, at the same time, is guaranteed to be loop-free irrespective of the state of FIBs along the nodes belonging to the explicit path as long as the states of the FIBs are programmed according to a link-state IGP. Thus, there is no need for anyco-ordinationcoordination or message exchange between the PLR and any other router in the network.</t> <t>The TI-LFA repair path is found by intersecting P(S,X) and Q(D,X) with the post-convergence path to D and computing the explicitSR- basedSR-based path EP(P, Q) from a node P in P(S,X) to a node Q in Q(D,X) when these nodes are not adjacent along thepost convergencepost-convergence path. The TI-LFA repair list is expressed generally as (Node-SID(P), EP(P, Q)).</t> <figureanchor="sample-topo1" title="Sample topologyanchor="sample-topo1"> <name>Sample Topology withTI-LFA"> <artwork>TI-LFA</name> <artwork name="" type="" align="left" alt=""><![CDATA[ S ------- N1 ----------- D *\ | \ | * \ | \ | * \ | \ | * N2-----R1****R2 *** R3 * * N3 ********* ***** : link with high metric (1k) ----- : link with metric 1</artwork>]]></artwork> </figure> <t>As an example, in <xreftarget="sample-topo1"/>,target="sample-topo1" format="default"/>, the focus is on the TI-LFA backup from S to D, considering the failure of node N1.</t><t><list style="symbols"><ul spacing="normal"> <li> <t>First, P(S, N1) is computed and results in [N3, N2, R1].</t> </li> <li> <t>Then, Q(D, N1) is computed and results in [R3].</t> </li> <li> <t>The expected post-convergence path from S to D considering the failure of N1 is <N2 -> R1 -> R2 -> R3 -> D> (we are naming itPCPath"PCPath" in this example).</t> </li> <li> <t>P(S, N1) intersection with PCPath is [N2,R1],R1]. With R1 being the deeper downstream node in PCPath, it can be assumed to be used as a P node (this is anexampleexample, and an implementation could use a different strategy to choose the P node).</t> </li> <li> <t>Q(D, N1) intersection with PCPath is [R3], so R3 is picked as a Q node. AnSR explicitSR-explicit path is then computed from R1 (P node) to R3 (Q node) following PCPath (R1 -> R2 -> R3): <Adj-Sid(R1-R2), Adj-Sid(R2-R3)>.</t></list></li> </ul> <!-- [rfced] FYI - We have updated the "0" in "Adj-Sid(R20R3)" to "-". Please review and let us know if further updates are needed. Original: As a result, the TI-LFA repair list of S for destination D considering the failure of node N1 is: <Node-SID(R1), Adj-Sid(R1-R2), Adj-Sid(R20R3)> Current: As a result, the TI-LFA repair list of S for destination D considering the failure of node N1 is: <Node-SID(R1), Adj-Sid(R1-R2), Adj-Sid(R2-R3)>. --> <t> As a result, the TI-LFA repair list of S for destination D considering the failure of node N1 is: <Node-SID(R1), Adj-Sid(R1-R2),Adj-Sid(R20R3)>.</t>Adj-Sid(R2-R3)>.</t> <t>Most often, the TI-LFA repair list has a simpler form, as described in the following sections. <xreftarget="analysis"/>target="analysis" format="default"/> provides statistics for the number of SIDs in the explicit path to protect against various failures.</t> <section anchor="direct_backup"title="FRR path usingnumbered="true" toc="default"> <name>FRR Path Using adirect neighbor">Direct Neighbor</name> <t>When a direct neighbor is in P(S,X) andQ(D,x)Q(D,x), and the link to that direct neighbor is on the post-convergence path, the outgoing interface is set to that neighbor and the repair segment list is empty.</t> <t>This is comparable to a post-convergence LFA FRR repair.</t> </section> <section anchor="pq_backup"title="FRR path usingnumbered="true" toc="default"> <name>FRR Path Using a PQnode">Node</name> <t>When a remote node R is in P(S,X) and Q(D,x) and on the post-convergence path, the repair list is made of a single node segment toRR, and the outgoing interface is set to the outgoing interface used to reach R.</t> <t>This is comparable to a post-convergence RLFA repair tunnel.</t> </section> <section anchor="adj_pq_backup"title="FRR path usingnumbered="true" toc="default"> <name>FRR Path Using a PnodeNode and Qnode that are adjacent">Node That Are Adjacent</name> <t>When a node P is in P(S,X) and a node Q is inQ(D,x)Q(D,x), and both are on the post-convergence path andbothare adjacent to each other, the repair list is made of two segments:Aa node segment to P (to be processed first), followed by an adjacency segment from P to Q.</t> <t>This is comparable to a post-convergence DLFA (LFA with directed forwarding) repair tunnel.</t> </section> <section anchor="remote_pq_backup"title="Connecting distantnumbered="true" toc="default"> <name>Connecting Distant P and Qnodes along post-convergence paths">Nodes Along Post-Convergence Paths</name> <t>In some cases, there is no adjacent P and Q node along the post- convergence path. As mentioned in <xreftarget="base"/>,target="base" format="default"/>, a list of adjacency SIDs can be used to encode the path between P and Q. However, the PLR can perform additional computations to compute a list of segments that represent a loop-free path from P to Q. How these computations are done is out of scope of this document and is left to implementation.</t> </section> </section> <section anchor="repairlist"title="Buildingnumbered="true" toc="default"> <name>Building TI-LFArepair listsRepair Lists for SRSegments">Segments</name> <t>The following sections describe how to build the repair lists using the terminology defined in <xreftarget="RFC8402"/>.target="RFC8402" format="default"/>. The procedures described in this section are equally applicable to bothSR-MPLSthe Segment Routing over MPLS (SR-MPLS) andSRv6the Segment Routing over IPv6 (SRv6) dataplane, while the dataplane-specific considerations are described in <xreftarget="dataplane"/>.</t> <t>In this section,target="dataplane" format="default"/>.</t> <t>This section explains the process by which a protecting router S handles the active segment of a packet upon the failure of its primary outgoing interface for thepacket, S-F, is explained.packet S-F. The failure of the primary outgoing interface may occur due to various triggers, such as link failure, neighbor node failure, and others.</t> <section anchor="link-protect-node-sid"title="The active segment isnumbered="true" toc="default"> <name>The Active Segment Is anode segment">Node Segment</name> <t>The active segmentMUST<bcp14>MUST</bcp14> be kept on the SR header unchanged and the repair listMUST<bcp14>MUST</bcp14> be added. The active segment becomes the first segment after the repair list. The way the repair list is added depends on the dataplane used (see <xreftarget="dataplane"/>).</t>target="dataplane" format="default"/>).</t> </section> <section anchor="link-protect-adj-sid"title="The active segment isnumbered="true" toc="default"> <name>The Active Segment Is anadjacency segment"> <t>TheAdjacency Segment</name> <t>This section defines the FRR behavior applied by S for any packet received with an active adjacency segmentS-F,S-F for which protection wasenabled, is defined here.enabled. Since protection has been enabled for the segment S-F and signaled in the IGP (for instance, using protocol extensions from <xreftarget="RFC8667"/>target="RFC8667" format="default"/> and <xreftarget="RFC8665"/>),target="RFC8665" format="default"/>), a calculator of any SR policy utilizing this segment is aware that it may be transiently rerouted out of S-F in the event of an S-F failure.</t> <t>The simplest approach for link protection of an adjacency segment S-F is to create a repair list that will carry the traffic to F. To do so, one or more“PUSH”"PUSH" operations are performed. If the repair list, while avoiding S-F, terminates on F, S only pushes segments of the repair list. Otherwise, S pushes a node segment of F, followed by the segments of the repair list. For details on the "NEXT" and "PUSH" operations, refer to <xreftarget="RFC8402"/>.</t>target="RFC8402" format="default"/>.</t> <t>This method, which merges back the traffic at the remote end of the adjacency segment, has the advantage of keeping as much traffic as possiblethe trafficon the pre-failure path. When SR policies are involved and strict compliance with the policy is required, an end-to-end protection (beyond the scope of this document) should be preferred over the local repair mechanism described above.</t> <t> Note, however, that when the SR source node is usingtraffic engineeringTraffic Engineering (TE), it will generally not be possible for the PLR to know what post-convergence path will be selected by the source node once it detects the failure, since computation of the TE path is a local matter that depends on constraints that may not be known at the PLR. Therefore, no method applied at the PLR can guarantee protection will follow the post-convergence path.</t> <t>The case where the active segment is followed by another adjacency segment is distinguished from the case where it is followed by a node segment. Repair techniques for the respective cases are provided in the following subsections.</t> <section anchor="link-protect-adj-sid-adj-sid"title="Protectingnumbered="true" toc="default"> <name>Protecting [Adjacency, Adjacency]segment lists">Segment Lists</name> <t>If the next segment in the list is an Adjacency segment, then the packet has to be conveyed to F.</t> <t>To do so, SMUST<bcp14>MUST</bcp14> apply a "NEXT" operation on Adj-Sid(S-F) and then one or more“PUSH”"PUSH" operations. If the repair list, while avoiding S-F, terminates on F, S only pushes the segments of the repair list. Otherwise, S pushes a node segment of F, followed by the segments of the repair list. For details on the "NEXT" and "PUSH" operations, refer to <xreftarget="RFC8402"/>.</t>target="RFC8402" format="default"/>.</t> <t>Upon failure of S-F, a packet reaching S with a segment list matching [adj-sid(S-F),adj-sid(F-M),...] will thus leave S with a segment list matching [RL(F),node(F),adj-sid(F-M),...], where RL(F) is the repair list for destination F.</t> </section> <section anchor="link-protect-adj-sid-node-sid"title="Protectingnumbered="true" toc="default"> <name>Protecting [Adjacency, Node]segment lists">Segment Lists</name> <t>If the next segment in the stack is a node segment, say for node T, the segment list on the packet matches [adj-sid(S-F),node(T),...].</t> <t>In this case, SMUST<bcp14>MUST</bcp14> apply a "NEXT" operation on the Adjacency segment related to S-F, followed by a "PUSH" of a repair list redirecting the traffic to a node Q, whose path to node segment T is not affected by the failure.</t> <t>Upon failure of S-F, packets reaching S with a segment list matching [adj-sid(S-F), node(T),...],...] would leave S with a segment list matching [RL(Q),node(T), ...].</t> </section> </section> </section> <section anchor="dataplane"title="Dataplane specific considerations">numbered="true" toc="default"> <name>Dataplane-Specific Considerations</name> <section anchor="mpls-dataplane"title="MPLS dataplane considerations"> <t>MPLSnumbered="true" toc="default"> <name>MPLS Dataplane Considerations</name> <t>The MPLS dataplane for Segment Routing (SR) is described in <xreftarget="RFC8660"/>.</t>target="RFC8660" format="default"/>.</t> <t>The following dataplane behaviors apply when creating a repair list using an MPLSdataplane: <list style="numbers">dataplane:</t> <ol spacing="normal" type="1"><li> <t>If the active segment is a node segment that has been signaled with penultimate hoppoppingpopping, and the repair list ends with an adjacency segment terminating on a node that advertisedNEXTthe "NEXT" operation <xreftarget="RFC8402"/>target="RFC8402" format="default"/> of the active segment, then the active segmentMUST<bcp14>MUST</bcp14> be popped before pushing the repair list.</t> </li> <li> <t>If the active segment is a nodesegmentsegment, but the other conditions in 1. are not met, the active segmentMUST<bcp14>MUST</bcp14> be popped and then pushed again with a label value computed according to the Segment Routing Global Block (SRGB) of Q, where Q is the endpoint of the repair list. Finally, the repair listMUST<bcp14>MUST</bcp14> be pushed.</t></list></t></li> </ol> </section> <section anchor="srv6-dataplane"title="SRv6 dataplane considerations">numbered="true" toc="default"> <name>SRv6 Dataplane Considerations</name> <t>SRv6 dataplane and programming instructions are described respectively in <xreftarget="RFC8754"/>target="RFC8754" format="default"/> and <xreftarget="RFC8986"/>.</t>target="RFC8986" format="default"/>.</t> <t>The TI-LFA path computation algorithm is the same as in the SR-MPLS dataplane.Note howeverNote, however, that the Adjacency SIDs are typically globally routed. In such a case, there is no need for preceding an adjacency SID with a Prefix-SID <xreftarget="RFC8402"/>target="RFC8402" format="default"/>, and the resulting repair list is likely shorter.</t> <t>If the traffic is protected at a Transit Node, then an SRv6 SID list is added on the packet to apply the repair list. The addition of the repair list follows theheadendhead-end behaviors as specified insection 5 of<xreftarget="RFC8986"/>.</t>target="RFC8986" sectionFormat="of" section="5"/>.</t> <t>If the traffic is protected at an SR Segment Endpoint Node, first the Segment Endpoint packet processing is executed.ThenThen, the packet is protected as ifitsit were a transit packet.</t> </section> </section> <section anchor="tilfa-sr-algo"title="TI-LFAnumbered="true" toc="default"> <name>TI-LFA and SRalgorithms">Algorithms</name> <t>SR allows an operator to bind an algorithm to aprefix-SIDPrefix-SID (as defined in <xreftarget="RFC8402"/>.target="RFC8402" format="default"/>). The algorithm value dictates how the path to the prefix is computed. The SR default algorithm is knownhasas the "Shortest Path" algorithm. The SR default algorithm allows an operator to override the IGP shortest path by using local policies. When TI-LFA uses Node-SIDs associated with the default algorithm, there is no guarantee that the path will beloop-freeloop-free, as a local policy may haveoverridenoverridden the expected IGP path. As the local policies are defined by the operator, it becomes the responsibility of this operator to ensure that the deployed policies do not affect the TI-LFA deployment. It should be noted that such a situation can already happen today with existing mechanisms such asremote LFA.</t>RLFA.</t> <t><xreftarget="RFC9350"/>target="RFC9350" format="default"/> defines aflexible algorithm (FlexAlgo)Flexible Algorithm framework to be associated with Prefix-SIDs.FlexAlgoA Flexible Algorithm allows a user to associate a constrained path to a Prefix-SID rather than using the regular IGP shortest path. An implementationMAY<bcp14>MAY</bcp14> support TI-LFA to protect Node-SIDs associated with aFlex Algo.Flexible Algorithm. In such a case, rather than computing the expected post-convergence path based on the regular SPF, an implementationSHOULD<bcp14>SHOULD</bcp14> use the constrained SPF algorithm bound to theFlex AlgoFlexible Algorithm (using theFlex AlgoFlexible Algorithm Definition) instead of the regular Dijkstra in all the SPF/rSPF computations that are occurring during the TI-LFA computation. This includes the computation of the P-Space and Q-Space as well as the post-convergence path. Furthermore, the implementationSHOULD<bcp14>SHOULD</bcp14> only use Node-SIDs/Adj-SIDs bound to theFlex AlgoFlexible Algorithm and/or unprotected Adj-SIDs of the regular SPF to build the repair list. The use of regular Dijkstra for the TI-LFA computation or for buildingofthe repair path using SIDs other than those recommended does not ensure that the traffic going over the TI-LFA repair path during thefast-rerouteFRR period is honoring theFlex AlgoFlexible Algorithm constraints.</t> </section> <section anchor="adj-sid-repair-list"title="Usagenumbered="true" toc="default"> <name>Usage of AdjacencysegmentsSegments in therepair list">Repair List</name> <t>The repair list of segments computed by TI-LFA may contain one or more adjacency segments. An adjacency segment may be protected or not protected.</t> <figure anchor="cascaded-frr"><artwork><artwork name="" type="" align="left" alt=""><![CDATA[ S --- R2 --- R3 ---- R4 --- R5 --- D * | \ * * | \ * R7 ** R8 * | * | R9 -- R10</artwork>]]></artwork> </figure> <t>In <xreftarget="cascaded-frr"/>,target="cascaded-frr" format="default"/>, all the metrics are equal to 1 exceptR2-R7,R7-R8,R8-R4,R7-R9R2-R7,R7-R8,R8-R4,R7-R9, which have a metric of 1000. Considering R2 as a PLR to protect against the failure of node R3 for the traffic S->D, the repair list computed by R2 will be[adj-sid(R7-R8),adj-sid(R8-R4)][adj-sid(R7-R8),adj-sid(R8-R4)], and the outgoing interface will be to R7. If R3 fails, R2 pushes the repair list onto the incoming packet to D. During the FRR, if R7-R8 fails and if TI-LFA has picked a protected adjacency segment for adj-sid(R7-R8), R7 will push an additional repair list onto the packet following the procedures defined in <xreftarget="repairlist"/>.</t> <t>Totarget="repairlist" format="default"/>.</t> <!--[rfced] May we update "non protected" to "unprotected" in the sentence below? Original: To avoid the possibility of this double FRR activation, an implementation of TI-LFA MAY pick only non protected adjacency segments when building the repair list.However,Perhaps: To avoid the possibility of this double FRR activation, an implementation of TI-LFA MAY pick only unprotected adjacency segments when building the repair list. --> <t>To avoid the possibility of this double FRR activation, an implementation of TI-LFA <bcp14>MAY</bcp14> pick only non-protected adjacency segments when building the repair list. However, it is important to note that FRR in general is intended to protect for a single pre-planned failure. If the failure that happens is worse than expected or multiple failures happen, FRR is not guaranteed to work. In such a case, fast IGP convergence remains important to restore traffic as quickly as possible.</t> </section> <section anchor="security"title="Security Considerations">numbered="true" toc="default"> <name>Security Considerations</name> <t>The techniques described in this document are internal functionalities to a router that can guarantee an upper bound on the time taken to restore traffic flow upon the failure of a directly connected link or node. As these techniques steer traffic to the post-convergence path as quickly as possible, this serves to minimize the disruption associated with a localfailurefailure, which can be seen as a modest security enhancement. The protectionmechanismsmechanism does not protect external destinations, but rather provides quick restoration fordestinationdestinations that are internal to a routing domain.</t><t>Security<t>The security considerations described in <xreftarget="RFC5286"/>target="RFC5286" format="default"/> and <xreftarget="RFC7490"/>target="RFC7490" format="default"/> apply to this document. Similarly, as the solution described inthethis document is based onSegment RoutingSR technology, the reader should be aware of the security considerations related to this technology(<xref target="RFC8402"/>)(see <xref target="RFC8402" format="default"/>) and its dataplane instantiations(<xref target="RFC8660"/>,(see <xreftarget="RFC8754"/>target="RFC8660" format="default"/>, <xref target="RFC8754" format="default"/>, and <xreftarget="RFC8986"/>).target="RFC8986" format="default"/>). However, this document does not introduce additional securityconcern.</t>concerns.</t> </section> <section anchor="iana"title="IANA Considerations"> <t>No requirements for IANA</t> </section> <section anchor="contributors" title="Contributors"> <t>In addition to the authors listed on the front page, the following co-authors have also contributed to this document: <list style="symbol"> <t>Francois Clad, Cisco Systems</t> <t>Pablo Camarillo, Cisco Systems</t> </list></t> </section> <section anchor="ack" title="Acknowledgments"> <t>The authors would like to thank Les Ginsberg, Stewart Bryant, Alexander Vainsthein, Chris Bowers, Shraddha Hedge, Wes Hardaker, Gunter Van de Velde and John Scudder for their valuable comments.</t>numbered="true" toc="default"> <name>IANA Considerations</name> <t>This document has no IANA actions.</t> </section> </middle> <back><references title="Normative References"> &RFC2119; &RFC7916; &RFC8174; &RFC8402; &RFC8660; &RFC8754; &RFC8986;<displayreference target="I-D.bashandy-rtgwg-segment-routing-uloop" to="SR-LOOP"/> <references> <name>References</name> <references> <name>Normative References</name> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7916.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8402.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8660.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8754.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8986.xml"/> </references> <references> <name>Informative References</name> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5714.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5715.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5286.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6976.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7490.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8333.xml"/> <!-- [I-D.bashandy-rtgwg-segment-routing-uloop] draft-bashandy-rtgwg-segment-routing-uloop-17 IESG State: Expired as of 03/19/25. --> <xi:include href="https://bib.ietf.org/public/rfc/bibxml3/reference.I-D.bashandy-rtgwg-segment-routing-uloop.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9350.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9256.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6571.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8665.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8667.xml"/> </references><references title="Informative References"> <?rfc include="reference.RFC.5714" ?> <?rfc include="reference.RFC.5715" ?> <?rfc include="reference.RFC.5286" ?> <?rfc include="reference.RFC.6976" ?> <?rfc include="reference.RFC.7490" ?> <?rfc include="reference.RFC.8333" ?> <?rfc include="reference.I-D.bashandy-rtgwg-segment-routing-uloop"?> &FLEXALGO; &RFC9256; &RFC6571; &RFC8665; &RFC8667;</references> <section anchor="advantages-post-conv-path"title="Advantagesnumbered="true" toc="default"> <name>Advantages ofusingUsing theexpected post-convergence path during FRR">Expected Post-Convergence Path During FRR</name> <t><xreftarget="RFC7916"/> raisedtarget="RFC7916" format="default"/> raises several operational considerations when using LFA orremote LFA.RLFA. <xreftarget="RFC7916"/> Section 3target="RFC7916" sectionFormat="of" section="3"/> presents a case where a high bandwidth link between two core routers is protected through aPEProvider Edge (PE) router connected with low bandwidth links. In such a case, congestion may happen when the FRR backup path is activated. <xreftarget="RFC7916"/>target="RFC7916" format="default"/> introduces a local policy framework to let the operator tuning manually the best alternate election based on its own requirements.</t> <t>From a network capacity planning point of view, it is often assumed for simplicity that if a link L fails on a particular node X, the bandwidth consumed on L will be spread over some of the remaining links of X. The remaining links to be used are determined by the IGP routing considering that the link L has failed (we assume that the traffic uses the post-convergence path starting from the node X). In <xreftarget="figure1"/>,target="figure1" format="default"/>, we consider a network with all metrics equal to 1 except the metrics on links used by PE1,PE2PE2, andPE3PE3, which are 1000. An easy network capacity planning method is to consider that if the link L (X-B) fails, the traffic actually flowing through L will be spread over the remaining links of X (X-H, X-D, X-A). Considering the IGP metrics, only X-H and X-D can be used in reality to carry the traffic flowing through the link L. As a consequence, the bandwidth of links X-H and X-D is sized according to this rule. We should observe that this capacity planning policyworks, howeverworks; however, it is not fully accurate.</t> <t>In <xreftarget="figure1"/>,target="figure1" format="default"/>, considering that the source of traffic is only from PE1 and PE4, when the link L fails, depending on the convergence speed of the nodes, X may reroute its forwarding entries to the remote PEs onto X-H or X-D;howeverhowever, in a similar timeframe, PE1 will also reroute a subset of its traffic (the subset destined to PE2) out of its nominalpathpath, reducing the quantity of traffic received by X. The capacity planning rule presented previously has the drawback of oversizing thenetwork, howevernetwork; however, it allowsto preventfor preventing any transient congestion(when for example(for example, when X reroutes traffic before PE1 does).</t> <figure anchor="figure1"><artwork><artwork name="" type="" align="left" alt=""><![CDATA[ H --- I --- J | | \ PE4 | | PE3 \ | (L) | / A --- X --- B --- G / | | \ PE1 | | PE2 \ | | / C --- D --- E --- F</artwork>]]></artwork> </figure> <t>Based on this assumption, in order to facilitate the operation ofFRR,FRR and limit the implementation of local FRR policies, traffic can be steered by the PLR onto its expected post-convergence path during the FRR phase. In our example, when link L fails, X switches the traffic destined to PE3 and PE2 on the post-convergence paths. This is perfectlyinlinein line with the capacity planning rule that was presented before and alsoinlinein line with the fact that X may converge before PE1 (or any other upstream router) and may spread the X-B traffic onto the post-convergence paths rooted at X.</t> <t>It should benoted,noted that some networks may have a different capacity planning rule, leading to an allocation of less bandwidth on X-H and X-D links. In such a case, using the post-convergence paths rooted at X during FRR may introduce some congestion on X-H and X-D links.HoweverHowever, it is important tonote,note that a transient congestion may possiblyhappen,happen even without FRR activated, forinstanceinstance, when X converges before the upstream routers. Operators are still free to use the policy framework defined in <xreftarget="RFC7916"/>target="RFC7916" format="default"/> if the usage of the post-convergence paths rooted at the PLR is not suitable.</t> <t>Readers should be aware that FRR protection is pre-computing a backup path to protect against a particular type of failure (link, node, or SRLG). When using the post-convergence path as an FRR backup path, the computed post-convergence path is the one considering the failure we are protecting against. This means that FRR is using an expected post-convergence path, and this expected post-convergence path may be actually different from the post-convergence path used if the failure that happened is different from the failure FRR was protecting against. As an example, if the operator has implemented a protection against a node failure, the expected post-convergence path used during FRR will be the one considering that the node has failed. However, even if a single link is failing or a set of links is failing (instead of the full node), the node-protecting post-convergence path will be used. The consequence is that the path used during FRR is not optimal with respect to the failure that has actually occurred.</t> <t>Another consideration to take into accountis: whileis as follows: While using the expected post-convergence path for SR traffic using node segments only (for instance, PE to PE traffic using the shortest path) has some advantages, these advantages reduce when SR policies(<xref target="RFC9256"/>)<xref target="RFC9256" format="default"/> are involved. Asegment-listsegment list used in an SR policy is computed to obey a set of path constraints defined locally at the head-end or centrally in a controller. TI-LFA cannot be aware of such pathconstraintsconstraints, and there is no reason to expect the TI-LFA backup path protecting onesegmentssegment in that segment list to obey those constraints. When SR policies are used and the operator wants to have a backup pathwhichthat still follows the policy requirements, this backup path should be computed as part of the SR policy in the ingress node (or centralcontroller)controller), and the SR policy should not rely on local protection. Another option could be to useFlexAlgo (<xref target="RFC9350"/>)a Flexible Algorithm <xref target="RFC9350" format="default"/> to express the set of constraints and use a single node segment associated with aFlexAlgoFlexible Algorithm to reach the destination. When using a node segment associated with aFlexAlgo,Flexible Algorithm, TI-LFA keeps providing an optimal backup by applying the appropriate set of constraints. The relationship between TI-LFA and theSR-algorithmSR algorithm is detailed in <xreftarget="tilfa-sr-algo"/>.</t>target="tilfa-sr-algo" format="default"/>.</t> </section> <section anchor="analysis"title="Analysis basednumbered="true" toc="default"> <name>Analysis Based onreal network topologies">Real Network Topologies</name> <t>This section presents an analysis performed on real service provider and large enterprise network topologies. The objective of the analysis is to assess the number of SIDs required in an explicit path when the mechanisms described in this document are used to protect against the failure scenarios within the scope of this document. The number of segments described in this section are applicable to instantiatingsegment routingSR over the MPLS forwarding plane.</t> <t>The measurement belowindicate thatindicates that, for link and local SRLG protection, a1 SID1-SID repair path delivers more than 99% coverage. For nodeprotectionprotection, a2 SIDs2-SID repair path yields 99% coverage.</t><t>Table 1<t><xref target="t-1"/> below lists the characteristics of the networks used in our measurements. The number of links refers to the number of "bidirectional" links (not directed edges of the graph). The measurements are carried out as follows:</t><t><list style="symbols"><ul spacing="normal"> <li> <t>For each network, the algorithms described in this document are applied to protect all prefixes against link, node, and local SRLGfailure</t>failure.</t> </li> <li> <t>For each prefix, the number of SIDs used by the repair path isrecorded</t>recorded.</t> </li> <li> <t>The percentage of number of SIDs are listed in Tables2A/B, 3A/B,<xref target="t-2" format="counter"/>, <xref target="t-3" format="counter"/>, <xref target="t-4" format="counter"/>, <xref target="t-5" format="counter"/>, <xref target="t-6" format="counter"/>, and4A/B</t> </list></t><xref target="t-7" format="counter"/>.</t> </li> </ul> <t>The measurements listed in the tables indicate that for link and local SRLG protection,1 SIDa 1-SID repair path is sufficient to protect more than 99% of the prefix in almost all cases. For nodeprotection 2 SIDsprotection, 2-SID repair paths yield 99% coverage.</t><figure> <artwork> +-------------+------------+------------+------------+------------+ | Network | Nodes | Links |Node-to-Link| SRLG info? | | | | | Ratio | | +-------------+------------+------------+------------+------------+ | T1 | 408 | 665 | 1.63 | Yes | +-------------+------------+------------+------------+------------+ | T2 | 587 | 1083 | 1.84 | No | +-------------+------------+------------+------------+------------+ | T3 | 93 | 401 | 4.31 | Yes | +-------------+------------+------------+------------+------------+ | T4 | 247 | 393 | 1.59 | Yes | +-------------+------------+------------+------------+------------+ | T5 | 34 | 96 | 2.82 | Yes | +-------------+------------+------------+------------+------------+ | T6 | 50 | 78 | 1.56 | No | +-------------+------------+------------+------------+------------+ | T7 | 82 | 293 | 3.57 | No | +-------------+------------+------------+------------+------------+ | T8 | 35 | 41 | 1.17 | Yes | +-------------+------------+------------+------------+------------+ | T9 | 177 | 1371 | 7.74 | Yes | +-------------+------------+------------+------------+------------+ Table 1: Data<table anchor="t-1"> <name>Data SetDefinition </artwork> </figure>Definition</name> <thead> <tr> <th>Network</th> <th>Nodes</th> <th>Links</th> <th>Node-to-Link Ratio</th> <th>SRLG Info?</th> </tr> </thead> <tbody> <tr> <td>T1</td> <td>408</td> <td>665</td> <td>1.63</td> <td>Yes</td> </tr> <tr> <td>T2</td> <td>587</td> <td>1083</td> <td>1.84</td> <td>No</td> </tr> <tr> <td>T3</td> <td>93</td> <td>401</td> <td>4.31</td> <td>Yes</td> </tr> <tr> <td>T4</td> <td>247</td> <td>393</td> <td>1.59</td> <td>Yes</td> </tr> <tr> <td>T5</td> <td>34</td> <td>96</td> <td>2.82</td> <td>Yes</td> </tr> <tr> <td>T6</td> <td>50</td> <td>78</td> <td>1.56</td> <td>No</td> </tr> <tr> <td>T7</td> <td>82</td> <td>293</td> <td>3.57</td> <td>No</td> </tr> <tr> <td>T8</td> <td>35</td> <td>41</td> <td>1.17</td> <td>Yes</td> </tr> <tr> <td>T9</td> <td>177</td> <td>1371</td> <td>7.74</td> <td>Yes</td> </tr> </tbody> </table> <t>The rest of this section presents the measurements done on the actual topologies. Theconventionconventions that we useisare asfollows</t> <t><list style="symbols">follows:</t> <ul spacing="normal"> <li> <t>0 SIDs:theThe calculated repair path starts with a directly connected neighbor that is also aloop free alternate,loop-free alternate; in whichcasecase, there is no need to explicitly route the traffic using additional SIDs. This scenario is described in <xreftarget="direct_backup"/>.</t>target="direct_backup" format="default"/>.</t> </li> <li> <t>1SIDs: theSID: The repair node is a PQnode,node; in whichcasecase, only 1 SID is needed to guarantee a loop-free path. This scenario is covered in <xreftarget="pq_backup"/>.</t>target="pq_backup" format="default"/>.</t> </li> <li> <t>2 or more SIDs: The repair path consists of 2 or more SIDs as described in Sections <xreftarget="adj_pq_backup"/>target="adj_pq_backup" format="counter"/> and <xreftarget="remote_pq_backup"/>.target="remote_pq_backup" format="counter"/>. We do not cover the case for 2 SIDs (<xreftarget="adj_pq_backup"/>)target="adj_pq_backup" format="default"/>) separately because there was no granularity in the result.AlsoAlso, we treat thenode-SID+adj-SIDnode-SID + adj-SID and node-SID + node-SID the same because they do not differ from the data plane point of view.</t></list></t> <t>Table 2A</li> </ul> <t>Tables <xref target="t-2" format="counter"/> and2B<xref target="t-3" format="counter"/> below summarize the measurements on the number of SIDs needed for linkprotection</t> <figure> <artwork> +-------------+------------+------------+------------+------------+ | Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | +-------------+------------+------------+------------+------------+ | T1 | 74.3% | 25.3% | 0.5% | 0.0% | +-------------+------------+------------+------------+------------+ | T2 | 81.1% | 18.7% | 0.2% | 0.0% | +-------------+------------+------------+------------+------------+ | T3 | 95.9% | 4.1% | 0.1% | 0.0% | +-------------+------------+------------+------------+------------+ | T4 | 62.5% | 35.7% | 1.8% | 0.0% | +-------------+------------+------------+------------+------------+ | T5 | 85.7% | 14.3% | 0.0% | 0.0% | +-------------+------------+------------+------------+------------+ | T6 | 81.2% | 18.7% | 0.0% | 0.0% | +-------------+------------+------------+------------+------------+ | T7 | 98.9% | 1.1% | 0.0% | 0.0% | +-------------+------------+------------+------------+------------+ | T8 | 94.1% | 5.9% | 0.0% | 0.0% | +-------------+------------+------------+------------+------------+ | T9 | 98.9% | 1.0% | 0.0% | 0.0% | +-------------+------------+------------+------------+------------+ Table 2A: Link protection (repair size distribution) +-------------+------------+------------+------------+------------+ | Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | +-------------+------------+------------+------------+------------+ | T1 | 74.2% | 99.5% | 99.9% | 100.0% | +-------------+------------+------------+------------+------------+ | T2 | 81.1% | 99.8% | 100.0% | 100.0% | +-------------+------------+------------+------------+------------+ | T3 | 95.9% | 99.9% | 100.0% | 100.0% | +-------------+------------+------------+------------+------------+ | T4 | 62.5% | 98.2% | 100.0% | 100.0% | +-------------+------------+------------+------------+------------+ | T5 | 85.7% | 100.0% | 100.0% | 100.0% | +-------------+------------+------------+------------+------------+ | T6 | 81.2% | 99.9% | 100.0% | 100.0% | +-------------+------------+------------+------------+------------+ | T7 | 98,8% | 100.0% | 100.0% | 100.0% | +-------------+------------+------------+------------+------------+ | T8 | 94,1% | 100.0% | 100.0% | 100.0% | +-------------+------------+------------+------------+------------+ | T9 | 98,9% | 100.0% | 100.0% | 100.0% | +-------------+------------+------------+------------+------------+ Table 2B: Link protection repair size cumulative distribution Table 3Aprotection.</t> <table anchor="t-2"> <name>Link Protection (Repair Size Distribution)</name> <thead> <tr> <th>Network</th> <th>0 SIDs</th> <th>1 SID</th> <th>2 SIDs</th> <th>3 SIDs</th> </tr> </thead> <tbody> <tr> <td>T1</td> <td>74.3%</td> <td>25.3%</td> <td>0.5%</td> <td>0.0%</td> </tr> <tr> <td>T2</td> <td>81.1%</td> <td>18.7%</td> <td>0.2%</td> <td>0.0%</td> </tr> <tr> <td>T3</td> <td>95.9%</td> <td>4.1%</td> <td>0.1%</td> <td>0.0%</td> </tr> <tr> <td>T4</td> <td>62.5%</td> <td>35.7%</td> <td>1.8%</td> <td>0.0%</td> </tr> <tr> <td>T5</td> <td>85.7%</td> <td>14.3%</td> <td>0.0%</td> <td>0.0%</td> </tr> <tr> <td>T6</td> <td>81.2%</td> <td>18.7%</td> <td>0.0%</td> <td>0.0%</td> </tr> <tr> <td>T7</td> <td>98.9%</td> <td>1.1%</td> <td>0.0%</td> <td>0.0%</td> </tr> <tr> <td>T8</td> <td>94.1%</td> <td>5.9%</td> <td>0.0%</td> <td>0.0%</td> </tr> <tr> <td>T9</td> <td>98.9%</td> <td>1.0%</td> <td>0.0%</td> <td>0.0%</td> </tr> </tbody> </table> <table anchor="t-3"> <name>Link Protection (Repair Size Cumulative Distribution)</name> <thead> <tr> <th>Network</th> <th>0 SIDs</th> <th>1 SID</th> <th>2 SIDs</th> <th>3 SIDs</th> </tr> </thead> <tbody> <tr> <td>T1</td> <td>74.2%</td> <td>99.5%</td> <td>99.9%</td> <td>100.0%</td> </tr> <tr> <td>T2</td> <td>81.1%</td> <td>99.8%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T3</td> <td>95.9%</td> <td>99.9%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T4</td> <td>62.5%</td> <td>98.2%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T5</td> <td>85.7%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T6</td> <td>81.2%</td> <td>99.9%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T7</td> <td>98.8%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T8</td> <td>94.1%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T9</td> <td>98.9%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> </tr> </tbody> </table> <t>Tables <xref target="t-4" format="counter"/> and3B<xref target="t-5" format="counter"/> summarize the measurements on the number of SIDs needed for local SRLGprotection. +-------------+------------+------------+------------+------------+ | Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | +-------------+------------+------------+------------+------------+ | T1 | 74.2% | 25.3% | 0.5% | 0.0% | +-------------+------------+------------+------------+------------+ | T2 | Noprotection.</t> <table anchor="t-4"> <name>Local SRLGInformation | +-------------+------------+------------+------------+------------+ | T3 | 93.6% | 6.3% | 0.0% | 0.0% | +-------------+------------+------------+------------+------------+ | T4 | 62.5% | 35.6% | 1.8% | 0.0% | +-------------+------------+------------+------------+------------+ | T5 | 83.1% | 16.8% | 0.0% | 0.0% | +-------------+------------+------------+------------+------------+ | T6 | NoProtection (Repair Size Distribution)</name> <thead> <tr> <th>Network</th> <th>0 SIDs</th> <th>1 SID</th> <th>2 SIDs</th> <th>3 SIDs</th> </tr> </thead> <tbody> <tr> <td>T1</td> <td>74.2%</td> <td>25.3%</td> <td>0.5%</td> <td>0.0%</td> </tr> <tr> <td>T2</td> <td colspan="4">No SRLGInformation | +-------------+---------------------------------------------------+ | T7 | Noinformation</td> </tr> <tr> <td>T3</td> <td>93.6%</td> <td>6.3%</td> <td>0.0%</td> <td>0.0%</td> </tr> <tr> <td>T4</td> <td>62.5%</td> <td>35.6%</td> <td>1.8%</td> <td>0.0%</td> </tr> <tr> <td>T5</td> <td>83.1%</td> <td>16.8%</td> <td>0.0%</td> <td>0.0%</td> </tr> <tr> <td>T6</td> <td colspan="4">No SRLGInformation | +-------------+------------+------------+------------+------------+ | T8 | 85.2% | 14.8% | 0.0% | 0.0% | +-------------+------------+------------+------------+------------+ | T9 | 98,9% | 1.1% | 0.0% | 0.0% | +-------------+------------+------------+------------+------------+ Table 3A: Localinformation</td> </tr> <tr> <td>T7</td> <td colspan="4">No SRLGprotection repair size distribution +-------------+------------+------------+------------+------------+ | Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | +-------------+------------+------------+------------+------------+ | T1 | 74.2% | 99.5% | 99.9% | 100.0% | +-------------+------------+------------+------------+------------+ | T2 | Noinformation</td> </tr> <tr> <td>T8</td> <td>85.2%</td> <td>14.8%</td> <td>0.0%</td> <td>0.0%</td> </tr> <tr> <td>T9</td> <td>98.9%</td> <td>1.1%</td> <td>0.0%</td> <td>0.0%</td> </tr> </tbody> </table> <table anchor="t-5"> <name>Local SRLGInformation | +-------------+------------+------------+------------+------------+ | T3 | 93.6% | 99.9% | 100.0% | 0.0% | +-------------+------------+------------+------------+------------+ | T4 | 62.5% | 98.2% | 100.0% | 100.0% | +-------------+------------+------------+------------+------------+ | T5 | 83.1% | 100.0% | 100.0% | 100.0% | +-------------+------------+------------+------------+------------+ | T6 | NoProtection (Repair Size Cumulative Distribution)</name> <thead> <tr> <th>Network</th> <th>0 SIDs</th> <th>1 SID</th> <th>2 SIDs</th> <th>3 SIDs</th> </tr> </thead> <tbody> <tr> <td>T1</td> <td>74.2%</td> <td>99.5%</td> <td>99.9%</td> <td>100.0%</td> </tr> <tr> <td>T2</td> <td colspan="4">No SRLGInformation | +-------------+---------------------------------------------------+ | T7 | Noinformation</td> </tr> <tr> <td>T3</td> <td>93.6%</td> <td>99.9%</td> <td>100.0%</td> <td>0.0%</td> </tr> <tr> <td>T4</td> <td>62.5%</td> <td>98.2%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T5</td> <td>83.1%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T6</td> <td colspan="4">No SRLGInformation | +-------------+------------+------------+------------+------------+ | T8 | 85.2% | 100.0% | 100.0% | 100.0% | +-------------+------------+------------+------------+------------+ | T9 | 98.9% | 100.0% | 100.0% | 100.0% | +-------------+------------+------------+------------+------------+ Table 3B: Localinformation</td> </tr> <tr> <td>T7</td> <td colspan="4">No SRLGprotection repair size Cumulative distribution Theinformation</td> </tr> <tr> <td>T8</td> <td>85.2%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T9</td> <td>98.9%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> </tr> </tbody> </table> <t>The remaining two tables summarize the measurements on the number of SIDs needed for nodeprotection. +---------+----------+----------+----------+----------+----------+ | Network | 0 SIDs | 1 SID | 2 SIDs | 3protection.</t> <table anchor="t-6"> <name>Node Protection (Repair Size Distribution)</name> <thead> <tr> <th>Network</th> <th>0 SIDs</th> <th>1 SID</th> <th>2 SIDs</th> <th>3 SIDs</th> <th>4 SIDs</th> </tr> </thead> <tbody> <tr> <td>T1</td> <td>49.8%</td> <td>47.9%</td> <td>2.1%</td> <td>0.1%</td> <td>0.0%</td> </tr> <tr> <td>T2</td> <td>36.5%</td> <td>59.6%</td> <td>3.6%</td> <td>0.2%</td> <td>0.0%</td> </tr> <tr> <td>T3</td> <td>73.3%</td> <td>25.6%</td> <td>1.1%</td> <td>0.0%</td> <td>0.0%</td> </tr> <tr> <td>T4</td> <td>36.1%</td> <td>57.3%</td> <td>6.3%</td> <td>0.2%</td> <td>0.0%</td> </tr> <tr> <td>T5</td> <td>73.2%</td> <td>26.8%</td> <td>0.0%</td> <td>0.0%</td> <td>0.0%</td> </tr> <tr> <td>T6</td> <td>78.3%</td> <td>21.3%</td> <td>0.3%</td> <td>0.0%</td> <td>0.0%</td> </tr> <tr> <td>T7</td> <td>66.1%</td> <td>32.8%</td> <td>1.1%</td> <td>0.0%</td> <td>0.0%</td> </tr> <tr> <td>T8</td> <td>59.7%</td> <td>40.2%</td> <td>0.0%</td> <td>0.0%</td> <td>0.0%</td> </tr> <tr> <td>T9</td> <td>98.9%</td> <td>1.0%</td> <td>0.0%</td> <td>0.0%</td> <td>0.0%</td> </tr> </tbody> </table> <table anchor="t-7"> <name>Node Protection (Repair Size Cumulative Distribution)</name> <thead> <tr> <th>Network</th> <th>0 SIDs</th> <th>1 SID</th> <th>2 SIDs</th> <th>3 SIDs</th> <th>4 SIDs</th> </tr> </thead> <tbody> <tr> <td>T1</td> <td>49.7%</td> <td>97.6%</td> <td>99.8%</td> <td>99.9%</td> <td>100.0%</td> </tr> <tr> <td>T2</td> <td>36.5%</td> <td>96.1%</td> <td>99.7%</td> <td>99.9%</td> <td>100.0%</td> </tr> <tr> <td>T3</td> <td>73.3%</td> <td>98.9%</td> <td>99.9%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T4</td> <td>36.1%</td> <td>93.4%</td> <td>99.8%</td> <td>99.9%</td> <td>100.0%</td> </tr> <tr> <td>T5</td> <td>73.2%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T6</td> <td>78.4%</td> <td>99.7%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T7</td> <td>66.1%</td> <td>98.9%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T8</td> <td>59.7%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> </tr> <tr> <td>T9</td> <td>98.9%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> <td>100.0%</td> </tr> </tbody> </table> </section> <section anchor="ack" numbered="false" toc="default"> <name>Acknowledgments</name> <t>The authors would like to thank <contact fullname="Les Ginsberg"/>, <contact fullname="Stewart Bryant"/>, <contact fullname="Alexander Vainsthein"/>, <contact fullname="Chris Bowers"/>, <contact fullname="Shraddha Hedge"/>, <contact fullname="Wes Hardaker"/>, <contact fullname="Gunter Van de Velde"/>, and <contact fullname="John Scudder"/> for their valuable comments.</t> </section> <section anchor="contributors" numbered="false" toc="default"> <name>Contributors</name> <t>In addition to the authors listed on the front page, the following co-authors have also contributed to this document:</t> <contact fullname="Francois Clad"> <organization>Cisco Systems</organization> </contact> <contact fullname="Pablo Camarillo"> <organization>Cisco Systems</organization> </contact> </section> </back> </rfc> <!-- [rfced] Terminology: a) We note different formatting and spacing for the following items throughout this document (some examples below). Please review and let us know if/how these items should be made consistent. spacing and apostrophe: P'(R,X) P'(R, X) P(R,X) spacing: [adj-sid(S-F),node(T),...] [adj-sid(S-F), node(T), ...] b) We note different capitalization and hyphenation for the following terms throughout this document (see some examples below). How should these be updated for consistency? Adjacency segment vs. adjacency segment Adjacency SIDs| 4vs. adjacency SIDs| +---------+----------+----------+----------+----------+----------+ | T1 | 49.8% | 47.9% | 2.1% | 0.1% | 0.0% | +---------+----------+----------+----------+----------+----------+ | T2 | 36,5% | 59.6% | 3.6% | 0.2% | 0.0% | +---------+----------+----------+----------+----------+----------+ | T3 | 73.3% | 25.6% | 1.1% | 0.0% | 0.0% | +---------+----------+----------+----------+----------+----------+ | T4 | 36.1% | 57.3% | 6.3% | 0.2% | 0.0% | +---------+----------+----------+----------+----------+----------+ | T5 | 73.2% | 26.8% | 0% | 0% | 0% | +---------+----------+----------+----------+----------+----------+ | T6 | 78.3% | 21.3% | 0.3% | 0% | 0% | +---------+----------+----------+----------+----------+----------+ | T7 | 66.1% | 32.8% | 1.1% | 0% | 0% | +---------+----------+----------+----------+----------+----------+ | T8 | 59.7% | 40.2% | 0% | 0% | 0% | +---------+----------+----------+----------+----------+----------+ | T9 | 98.9% | 1.0% | 0% | 0% | 0% | +---------+----------+----------+----------+----------+----------+ Table 4A:Adj-SID vs. Adj-Sid vs. adj-SID vs. adj-sid Nodeprotection (repair size distribution) +---------+----------+----------+----------+----------+----------+ | Network | 0 SIDs | 1SID| 2 SIDs | 3 SIDs | 4 SIDs | +---------+----------+----------+----------+----------+----------+ | T1 | 49.7% | 97.6% | 99.8% | 99.9% | 100% | +---------+----------+----------+----------+----------+----------+ | T2 | 36.5% | 96.1% | 99.7% | 99.9% | 100% | +---------+----------+----------+----------+----------+----------+ | T3 | 73.3% | 98.9% | 99.9% | 100.0% | 100% | +---------+----------+----------+----------+----------+----------+ | T4 | 36.1% | 93.4% | 99.8% | 99.9% | 100% | +---------+----------+----------+----------+----------+----------+ | T5 | 73.2% | 100.0% | 100.0% | 100.0% | 100% | +---------+----------+----------+----------+----------+----------+ | T6 | 78.4% | 99.7% | 100.0% | 100.0% | 100% | +---------+----------+----------+----------+----------+----------+ | T7 | 66.1% | 98.9% | 100.0% | 100.0% | 100% | +---------+----------+----------+----------+----------+----------+ | T8 | 59.7% | 100.0% | 100.0% | 100.0% | 100% | +---------+----------+----------+----------+----------+----------+ | T9 | 98.9% | 100.0% | 100.0% | 100.0% | 100% | +---------+----------+----------+----------+----------+----------+ Table 4B: Node protection (repair size cumulative distribution) </artwork> </figure> </section> </back> </rfc>vs. Node-SID vs. node-SID P-Space vs. P-space Q-Space vs. Q-space c) May we update all instances of "dataplane" to "data plane" for consistency with RFC 8660? d) FYI - For consistency with RFC 9350, we have updated the terms below as follows: OLD -> NEW FlexAlgo / Flex Algo -> Flexible Algorithm Flex Algo Definition -> Flexible Algorithm Definition --> <!-- [rfced] Abbreviations: a) We note that "DLFA" has been expanded inconsistently throughout the document. For consistency, may we update all of these expansions to be "Directed Loop-Free Alternates"? Original: remote LFAs with directed forwarding (DLFA) DLFA: Remote LFA with Directed forwarding DLFA (LFA with directed forwarding) Directed Loop-Free Alternates (DLFA) Perhaps: Directed Loop-Free Alternates (DLFA) b) Per Section 3.6 of RFC 7322 ("RFC Style Guide"), abbreviations should be expanded upon first use. How may we expand "rSPF" in the text below? Original: ...in all the SPF/rSPF computations that are occurring during the TI-LFA computation. c) Both the expansion and the acronym for the following terms are used throughout the document. Would you like to update to using the expansion upon first usage and the acronym for the rest of the document for consistency? Point of Local Repair (PLR) Repair List (RL) Segment Routing (SR) d) FYI - We have added expansions for the following abbreviations per Section 3.6 of RFC 7322 ("RFC Style Guide"). Please review each expansion in the document carefully to ensure correctness. Segment Routing over MPLS (SR-MPLS) Segment Routing over IPv6 (SRv6) Provider Edge (PE) --> <!-- [rfced] Please review the "Inclusive Language" portion of the online Style Guide <https://www.rfc-editor.org/styleguide/part2/#inclusive_language> and let us know if any changes are needed. Updates of this nature typically result in more precise language, which is helpful for readers. Note that our script did not flag any words in particular, but this should still be reviewed as a best practice. -->