rfc9855v1.txt		rfc9855.txt
	Internet Engineering Task Force (IETF) A. Bashandy		Internet Engineering Task Force (IETF) A. Bashandy
	Request for Comments: 9855 Individual		Request for Comments: 9855 HPE
	Category: Standards Track S. Litkowski		Category: Standards Track S. Litkowski
	ISSN: 2070-1721 C. Filsfils		ISSN: 2070-1721 C. Filsfils
	Cisco Systems		Cisco Systems
	P. Francois		P. Francois
	INSA Lyon		INSA Lyon
	B. Decraene		B. Decraene
	Orange		Orange
	D. Voyer		D. Voyer
	Bell Canada		Bell Canada
	September 2025		October 2025
	Topology Independent Fast Reroute Using Segment Routing		Topology Independent Fast Reroute Using Segment Routing
	Abstract		Abstract
	This document presents Topology Independent Loop-Free Alternate (TI-		This document presents Topology Independent Loop-Free Alternate (TI-
	LFA) Fast Reroute (FRR), which is aimed at providing protection of		LFA) Fast Reroute (FRR), which is aimed at providing protection of
	node and adjacency segments within the Segment Routing (SR)		node and Adjacency segments within the Segment Routing (SR)
	framework. This FRR behavior builds on proven IP FRR concepts being		framework. This FRR behavior builds on proven IP FRR concepts being
	LFAs, Remote LFAs (RLFAs), and remote LFAs with directed forwarding		LFAs, Remote LFAs (RLFAs), and Directed Loop-Free Alternates (DLFAs).
	(DLFAs). It extends these concepts to provide guaranteed coverage in		It extends these concepts to provide guaranteed coverage in any two-
	any two-connected networks using a link-state IGP. An important		connected networks using a link-state IGP. An important aspect of
	aspect of TI-LFA is the FRR path selection approach establishing		TI-LFA is the FRR path selection approach establishing protection
	protection over the expected post-convergence paths from the Point of		over the expected post-convergence paths from the Point of Local
	Local Repair (PLR), reducing the operational need to control the tie-		Repair (PLR), reducing the operational need to control the tie-breaks
	breaks among various FRR options.		among various FRR options.
	Status of This Memo		Status of This Memo
	This is an Internet Standards Track document.		This is an Internet Standards Track document.
	This document is a product of the Internet Engineering Task Force		This document is a product of the Internet Engineering Task Force
	(IETF). It represents the consensus of the IETF community. It has		(IETF). It represents the consensus of the IETF community. It has
	received public review and has been approved for publication by the		received public review and has been approved for publication by the
	Internet Engineering Steering Group (IESG). Further information on		Internet Engineering Steering Group (IESG). Further information on
	Internet Standards is available in Section 2 of RFC 7841.		Internet Standards is available in Section 2 of RFC 7841.
	skipping to change at line 67 ¶		skipping to change at line 67 ¶
	Trust Legal Provisions and are provided without warranty as described		Trust Legal Provisions and are provided without warranty as described
	in the Revised BSD License.		in the Revised BSD License.
	Table of Contents		Table of Contents
	1. Introduction		1. Introduction
	2. Terminology		2. Terminology
	2.1. Abbreviations and Notations		2.1. Abbreviations and Notations
	2.2. Conventions Used in This Document		2.2. Conventions Used in This Document
	3. Base Principle		3. Base Principle
	4. Intersecting P-Space and Q-Space with Post-Convergence Paths		4. Intersecting P-space and Q-space with Post-Convergence Paths
	4.1. Extended P-Space Property Computation for a Resource X over		4.1. Extended P-space Property Computation for a Resource X over
	Post-Convergence Paths		Post-Convergence Paths
	4.2. Q-Space Property Computation for a Resource X over		4.2. Q-space Property Computation for a Resource X over
	Post-Convergence Paths		Post-Convergence Paths
	4.3. Scaling Considerations When Computing Q-Space		4.3. Scaling Considerations When Computing Q-space
	5. TI-LFA Repair Path		5. TI-LFA Repair Path
	5.1. FRR Path Using a Direct Neighbor		5.1. FRR Path Using a Direct Neighbor
	5.2. FRR Path Using a PQ Node		5.2. FRR Path Using a PQ Node
	5.3. FRR Path Using a P Node and Q Node That Are Adjacent		5.3. FRR Path Using a P Node and Q Node That Are Adjacent
	5.4. Connecting Distant P and Q Nodes Along Post-Convergence		5.4. Connecting Distant P and Q Nodes Along Post-Convergence
	Paths		Paths
	6. Building TI-LFA Repair Lists for SR Segments		6. Building TI-LFA Repair Lists for SR Segments
	6.1. The Active Segment Is a Node Segment		6.1. The Active Segment Is a Node Segment
	6.2. The Active Segment Is an Adjacency Segment		6.2. The Active Segment Is an Adjacency Segment
	6.2.1. Protecting [Adjacency, Adjacency] Segment Lists		6.2.1. Protecting [Adjacency, Adjacency] Segment Lists
	6.2.2. Protecting [Adjacency, Node] Segment Lists		6.2.2. Protecting [Adjacency, Node] Segment Lists
	7. Dataplane-Specific Considerations		7. Data Plane-Specific Considerations
	7.1. MPLS Dataplane Considerations		7.1. MPLS Data Plane Considerations
	7.2. SRv6 Dataplane Considerations		7.2. SRv6 Data Plane Considerations
	8. TI-LFA and SR Algorithms		8. TI-LFA and SR Algorithms
	9. Usage of Adjacency Segments in the Repair List		9. Usage of Adjacency Segments in the Repair List
	10. Security Considerations		10. Security Considerations
	11. IANA Considerations		11. IANA Considerations
	12. References		12. References
	12.1. Normative References		12.1. Normative References
	12.2. Informative References		12.2. Informative References
	Appendix A. Advantages of Using the Expected Post-Convergence Path		Appendix A. Advantages of Using the Expected Post-Convergence Path
	During FRR		During FRR
	Appendix B. Analysis Based on Real Network Topologies		Appendix B. Analysis Based on Real Network Topologies
	skipping to change at line 187 ¶		skipping to change at line 187 ¶
	transmission pipe).		transmission pipe).
	Protection techniques outlined in this document are limited to		Protection techniques outlined in this document are limited to
	protecting links, nodes, and SRLGs that are within a link-state IGP		protecting links, nodes, and SRLGs that are within a link-state IGP
	area. Protecting domain exit routers and/or links attached to		area. Protecting domain exit routers and/or links attached to
	another routing domain is beyond the scope of this document.		another routing domain is beyond the scope of this document.
	By utilizing SR, TI-LFA eliminates the need to establish Targeted		By utilizing SR, TI-LFA eliminates the need to establish Targeted
	Label Distribution Protocol sessions with remote nodes for leveraging		Label Distribution Protocol sessions with remote nodes for leveraging
	the benefits of Remote Loop-Free Alternates (RLFAs) [RFC7490]		the benefits of Remote Loop-Free Alternates (RLFAs) [RFC7490]
	[RFC7916] or Directed Loop-Free Alternates (DLFAs) [RFC5714]. All		[RFC7916] or Directed Loop-Free Alternates (DLFAs) [IPFRR-TUNNELS].
	the Segment Identifiers (SIDs) required are present within the Link		All the Segment Identifiers (SIDs) required are present within the
	State Database (LSDB) of the IGP. Consequently, there is no longer a		Link State Database (LSDB) of the IGP. Consequently, there is no
	necessity to prefer LFAs over RLFAs or DLFAs, nor is there a need to		longer a necessity to prefer LFAs over RLFAs or DLFAs, nor is there a
	minimize the number of RLFA or DLFA repair nodes.		need to minimize the number of RLFA or DLFA repair nodes.
	Utilizing SR makes the requirement unnecessary to establish an		Utilizing SR also eliminates the need to establish an additional
	additional state within the network for enforcing explicit Fast		state within the network for enforcing explicit Fast Reroute (FRR)
	Reroute (FRR) paths. This spares the nodes from maintaining a		paths. This spares the nodes from maintaining a supplementary state
	supplementary state and frees the operator from the necessity to		and frees the operator from the necessity to implement additional
	implement additional protocols or protocol sessions solely to augment		protocols or protocol sessions solely to augment protection coverage.
	protection coverage.
	TI-LFA also brings the benefit of the ability to provide a backup		TI-LFA also brings the benefit of the ability to provide a backup
	path that follows the expected post-convergence path considering a		path that follows the expected post-convergence path considering a
	particular failure, which reduces the need of locally configured		particular failure, which reduces the need of locally configured
	policies that influence the backup path selection [RFC7916]. The		policies that influence the backup path selection [RFC7916]. The
	easiest way to express the expected post-convergence path in a loop-		easiest way to express the expected post-convergence path in a loop-
	free manner is to encode it as a list of adjacency segments.		free manner is to encode it as a list of Adjacency segments.
	However, this may create a long segment list that some hardware may		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		not be able to program. One of the challenges of TI-LFA is to encode
	the expected post-convergence path by combining adjacency segments		the expected post-convergence path by combining Adjacency segments
	and node segments. Each implementation may independently develop its		and node segments. Each implementation may independently develop its
	own algorithm for optimizing the ordered segment list. This document		own algorithm for optimizing the ordered segment list. This document
	provides an outline of the fundamental concepts applicable to		provides an outline of the fundamental concepts applicable to
	constructing the SR backup path, along with the related dataplane		constructing the SR backup path, along with the related data plane
	procedures. Appendix A contains a more detailed description of some		procedures. Appendix A contains a more detailed description of some
	of the aspects of TI-LFA related to post-convergence path.		of the aspects of TI-LFA related to post-convergence path.
	This document is structured as follows:		This document is structured as follows:
	* Section 2 defines the main notations used in the document. They		* Section 2 defines the main notations used in the document. They
	are in line with [RFC5714].		are in line with [RFC5714].
	* Section 3 defines the main principles of TI-LFA backup path		* Section 3 defines the main principles of TI-LFA backup path
	computation.		computation.
	* Section 4 suggests to compute the P-Space and Q-Space properties		* Section 4 suggests to compute the P-space and Q-space properties
	defined in Section 2 for the specific case of nodes lying over the		defined in Section 2 for the specific case of nodes lying over the
	post-convergence paths towards the protected destinations.		post-convergence paths towards the protected destinations.
	* Using the properties defined in Section 4, Section 5 describes how		* Section 5 describes how to compute protection lists that encode a
	to compute protection lists that encode a loop-free post-		loop-free post-convergence path towards the destination using the
	convergence path towards the destination.		properties defined in Section 4.
	* Section 6 defines the segment operations to be applied by the PLR		* Section 6 defines the segment operations to be applied by the PLR
	to ensure consistency with the forwarding state of the repair		to ensure consistency with the forwarding state of the repair
	node.		node.
	* Section 7 discusses aspects that are specific to the dataplane.		* Section 7 discusses aspects that are specific to the data plane.
	* Section 8 discusses the relationship between TI-LFA and the SR		* Section 8 discusses the relationship between TI-LFA and the SR
	algorithm.		algorithm.
	* Certain considerations are needed when adjacency segments are used		* Section 9 provides an overview of the certain considerations that
	in a repair list. Section 9 provides an overview of these		are needed when Adjacency segments are used in a Repair List (RL).
	considerations.
	* Section 10 discusses security considerations.		* Section 10 discusses security considerations.
	* Appendix A highlights advantages of using the expected post-		* Appendix A highlights advantages of using the expected post-
	convergence path during FRR.		convergence path during FRR.
	* By implementing the algorithms detailed in this document within		* Appendix B summarizes the measurements from implementing the
	actual service provider and large enterprise network environments,		algorithms detailed in this document within actual service
	real-life measurements are presented regarding the number of SIDs		provider and large enterprise network environments. Real-life
	utilized by repair paths. These measurements are summarized in		measurements are presented regarding the number of SIDs utilized
	Appendix B.		by repair paths.
	2. Terminology		2. Terminology
	2.1. Abbreviations and Notations		2.1. Abbreviations and Notations
	DLFA: Directed Loop-Free Alternate		DLFA: Directed Loop-Free Alternate
	FRR: Fast Reroute		FRR: Fast Reroute
	IGP: Interior Gateway Protocol		IGP: Interior Gateway Protocol
	skipping to change at line 299 ¶		skipping to change at line 297 ¶
	* The terms "old" and "new" topologies refer to the LSDB state		* The terms "old" and "new" topologies refer to the LSDB state
	before and after the considered failure, respectively.		before and after the considered failure, respectively.
	* SPT_old(R) is the SPT rooted at node R in the initial state of the		* SPT_old(R) is the SPT rooted at node R in the initial state of the
	network.		network.
	* SPT_new(R, X) is the SPT rooted at node R in the state of the		* SPT_new(R, X) is the SPT rooted at node R in the state of the
	network after the resource X has failed.		network after the resource X has failed.
	* The Point of Local Repair (PLR) is the router that applies fast		* The PLR is the router that applies fast traffic restoration after
	traffic restoration after detecting failure in a directly attached		detecting failure in a directly attached link, set of links, and/
	link, set of links, and/or node.		or node.
	* Similar to [RFC7490], the concept of P-Space and Q-Space is used		* Similar to [RFC7490], the concept of P-space and Q-space is used
	for TI-LFA.		for TI-LFA.
	* The P-space P(R,X) of a router R with regard to a resource X		* The P-space P(R, X) of a router R with regard to a resource X
	(e.g., a link S-F, a node F, or an SRLG) is the set of routers		(e.g., a link S-F, a node F, or an SRLG) is the set of routers
	reachable from R using the pre-convergence shortest paths without		reachable from R using the pre-convergence shortest paths without
	any of those paths (including equal-cost path splits) transiting		any of those paths (including equal-cost path splits) transiting
	through X. A P node is a node that belongs to the P-space.		through X. A P node is a node that belongs to the P-space.
	* Consider the set of neighbors of a router R and a resource X.		* Consider the set of neighbors of a router R and a resource X.
	Exclude from that set the neighbors that are reachable from R		Exclude from that set the neighbors that are reachable from R
	using X. The extended P-Space P'(R,X) of a node R with regard to		using X. The extended 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		a resource X is the union of the P-spaces of the neighbors in that
	reduced set of neighbors with regard to the resource X.		reduced set of neighbors with regard to the resource X.
	* The Q-space Q(R,X) of a router R with regard to a resource X is		* 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		the set of routers from which R can be reached without any path
	(including equal-cost path splits) transiting through X. A Q node		(including equal-cost path splits) transiting through X. A Q node
	is a node that belongs to the Q-space.		is a node that belongs to the Q-space.
	* EP(P, Q) is an explicit SR path from a node P to a node Q.		* EP(P, Q) is an explicit SR path from a node P to a node Q.
	* The primary interface and primary outgoing interface are one of		* The primary interface and primary outgoing interface are one of
	the outgoing interfaces towards a destination according to the IGP		the outgoing interfaces towards a destination according to the IGP
	link-state protocol.		link-state protocol.
	* The primary link is a link connected to the primary interface.		* 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.		* The Adj-SID(S-F) is the Adjacency segment from node S to node F.
	2.2. Conventions Used in This Document		2.2. Conventions Used in This Document
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in		"OPTIONAL" in this document are to be interpreted as described in
	BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all		BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
	capitals, as shown here.		capitals, as shown here.
	3. Base Principle		3. Base Principle
	The basic algorithm to compute the repair path is to pre-compute		The basic algorithm to compute the repair path is to pre-compute
	SPT_new(R,X) and, for each destination, encode the repair path as a		SPT_new(R, X) and, for each destination, encode the repair path as a
	loop-free segment list. One way to provide a loop-free segment list		loop-free segment list. One way to provide a loop-free segment list
	is to use adjacency SIDs only. However, this approach may create		is to use Adjacency SIDs only. However, this approach may create
	very long SID lists that hardware may not be able to handle due to		very long SID lists that hardware may not be able to handle due to
	Maximum SID Depth (MSD) limitations.		Maximum SID Depth (MSD) limitations.
	An implementation is free to use any local optimization to provide		An implementation is free to use any local optimization to provide
	smaller segment lists by combining Node SIDs and Adjacency SIDs. In		smaller segment lists by combining Node-SIDs and Adjacency SIDs. In
	addition, the usage of Node-SIDs allow for maximizing ECMPs over the		addition, the usage of Node-SIDs allow for maximizing ECMPs over the
	backup path. These optimizations are out of scope of this document;		backup path. These optimizations are out of scope of this document;
	however, the subsequent sections provide some guidance on how to		however, the subsequent sections provide some guidance on how to
	leverage P-Spaces and Q-Spaces to optimize the size of the segment		leverage P-spaces and Q-spaces to optimize the size of the segment
	list.		list.
	4. Intersecting P-Space and Q-Space with Post-Convergence Paths		4. Intersecting P-space and Q-space with Post-Convergence Paths
	One of the challenges of defining an SR path following the expected		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		post-convergence path is to reduce the size of the segment list. In
	order to reduce this segment list, an implementation MAY determine		order to reduce this segment list, an implementation MAY determine
	the P-Space / extended P-Space and Q-Space properties (defined in		the P-space / extended P-space and Q-space properties (defined in
	[RFC7490]) of the nodes along the expected post-convergence path from		[RFC7490]) of the nodes along the expected post-convergence path from
	the PLR to the protected destination and compute an SR explicit path		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		from P to Q when they are not adjacent. Such properties will be used
	in Section 5 to compute the TI-LFA repair list.		in Section 5 to compute the TI-LFA RL.
	4.1. Extended P-Space Property Computation for a Resource X over Post-		4.1. Extended P-space Property Computation for a Resource X over Post-
	Convergence Paths		Convergence Paths
	The objective is to determine which nodes on the post-convergence		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		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		of R 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).		links adjacent to the PLR or a neighbor node of the PLR).
	This can be found by:		This can be found by:
	* excluding neighbors that are not on the post-convergence path when		* excluding neighbors that are not on the post-convergence path when
	computing P'(R,X), then		computing P'(R, X), then
	* intersecting the set of nodes belonging to the post-convergence		* intersecting the set of nodes belonging to the post-convergence
	path from R to D, assuming the failure of X, with P'(R, X).		path from R to D, assuming the failure of X, with P'(R, X).
	4.2. Q-Space Property Computation for a Resource X over Post-		4.2. Q-space Property Computation for a Resource X over Post-
	Convergence Paths		Convergence Paths
	The goal is to determine which nodes on the post-convergence path		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		from the PLR R to the destination D are in the Q-space of destination
	the Q-Space of destination D with regard to resource X (where X can		D with regard to resource X (where X can be a link or a set of links
	be a link or a set of links adjacent to the PLR, or a neighbor node		adjacent to the PLR, or a neighbor node of the PLR).
	of the PLR).
	This can be found by intersecting the set of nodes belonging to the		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		post-convergence path from R to D, assuming the failure of X, with
	Q(D, X).		Q(D, X).
	4.3. Scaling Considerations When Computing Q-Space		4.3. Scaling Considerations When Computing Q-space
	[RFC7490] raises scaling concerns about computing a Q-Space per		[RFC7490] raises scaling concerns about computing a Q-space per
	destination. Similar concerns may affect TI-LFA computation if an		destination. Similar concerns may affect TI-LFA computation if an
	implementation tries to compute a reverse Shortest Path Tree (SPT)		implementation tries to compute a reverse Shortest Path Tree (SPT)
	[RFC7490] for every destination in the network to determine the		[RFC7490] for every destination in the network to determine the
	Q-Space. It will be up to each implementation to determine the good		Q-space. It will be up to each implementation to determine the good
	tradeoff between scaling and accuracy of the optimization.		tradeoff between scaling and accuracy of the optimization.
	5. TI-LFA Repair Path		5. TI-LFA Repair Path
	The TI-LFA repair path consists of an outgoing interface and a list		The TI-LFA repair path consists of an outgoing interface and a list
	of segments (a Repair List (RL)) to insert on the SR header in		of segments (a Repair List (RL)) to insert on the SR header in
	accordance with the dataplane used. The repair list encodes the		accordance with the data plane used. The RL encodes the explicit
	explicit, and possibly post-convergence, path to the destination,		(and possibly post-convergence) path to the destination, which avoids
	which avoids the protected resource X and, at the same time, is		the protected resource X. At the same time, the RL is guaranteed to
	guaranteed to be loop-free irrespective of the state of FIBs along		be loop-free, irrespective of the state of FIBs along the nodes
	the nodes belonging to the explicit path as long as the states of the		belonging to the explicit path, as long as the states of the FIBs are
	FIBs are programmed according to a link-state IGP. Thus, there is no		programmed according to a link-state IGP. Thus, there is no need for
	need for any coordination or message exchange between the PLR and any		any coordination or message exchange between the PLR and any other
	other router in the network.		router in the network.
	The TI-LFA repair path is found by intersecting P(S,X) and Q(D,X)		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 explicit SR-		with the post-convergence path to D and computing the explicit SR-
	based path EP(P, Q) from a node P in P(S,X) to a node Q in Q(D,X)		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 the post-convergence path.		when these nodes are not adjacent along the post-convergence path.
	The TI-LFA repair list is expressed generally as (Node-SID(P), EP(P,		The TI-LFA RL is expressed generally as (Node-SID(P), EP(P, Q)).
	Q)).
	S ------- N1 ----------- D		S --------- N1 --------- D
	*\ | \ |		*\ | \ |
	* \ | \ |		* \ | \ |
	* \ | \ |		* \ | \ |
	* N2-----R1****R2 *** R3		* N2----- R1***R2 *** R3
	* *		* *
	N3 *********		N3 **********
	***** : link with high metric (1k)		***** : link with high metric (1k)
	----- : link with metric 1		----- : link with metric 1
	Figure 1: Sample Topology with TI-LFA		Figure 1: Sample Topology with TI-LFA
	As an example, in Figure 1, the focus is on the TI-LFA backup from S		As an example, in Figure 1, the focus is on the TI-LFA backup from S
	to D, considering the failure of node N1.		to D, considering the failure of node N1.
	* First, P(S, N1) is computed and results in [N3, N2, R1].		* First, P(S, N1) is computed and results in [N3, N2, R1].
	skipping to change at line 458 ¶		skipping to change at line 454 ¶
	failure of N1 is <N2 -> R1 -> R2 -> R3 -> D> (we are naming it		failure of N1 is <N2 -> R1 -> R2 -> R3 -> D> (we are naming it
	"PCPath" in this example).		"PCPath" in this example).
	* P(S, N1) intersection with PCPath is [N2, R1]. With R1 being the		* P(S, N1) intersection with PCPath is [N2, R1]. With R1 being the
	deeper downstream node in PCPath, it can be assumed to be used as		deeper downstream node in PCPath, it can be assumed to be used as
	a P node (this is an example, and an implementation could use a		a P node (this is an example, and an implementation could use a
	different strategy to choose the P node).		different strategy to choose the P node).
	* Q(D, N1) intersection with PCPath is [R3], so R3 is picked as a Q		* Q(D, N1) intersection with PCPath is [R3], so R3 is picked as a Q
	node. An SR-explicit path is then computed from R1 (P node) to R3		node. An SR-explicit path is then computed from R1 (P node) to R3
	(Q node) following PCPath (R1 -> R2 -> R3): <Adj-Sid(R1-R2), Adj-		(Q node) following PCPath (R1 -> R2 -> R3): <Adj-SID(R1-R2), Adj-
	Sid(R2-R3)>.		SID(R2-R3)>.
	As a result, the TI-LFA repair list of S for destination D		As a result, the TI-LFA RL of S for destination D considering the
	considering the failure of node N1 is: <Node-SID(R1), Adj-Sid(R1-R2),		failure of node N1 is: <Node-SID(R1), Adj-SID(R1-R2), Adj-
	Adj-Sid(R2-R3)>.		SID(R2-R3)>.
	Most often, the TI-LFA repair list has a simpler form, as described		Most often, the TI-LFA RL has a simpler form, as described in the
	in the following sections. Appendix B provides statistics for the		following sections. Appendix B provides statistics for the number of
	number of SIDs in the explicit path to protect against various		SIDs in the explicit path to protect against various failures.
	failures.
	5.1. FRR Path Using a Direct Neighbor		5.1. FRR Path Using a Direct Neighbor
	When a direct neighbor is in P(S,X) and Q(D,x), and the link to that		When a direct neighbor is in P(S, X) and Q(D, x), and the link to
	direct neighbor is on the post-convergence path, the outgoing		that direct neighbor is on the post-convergence path, the outgoing
	interface is set to that neighbor and the repair segment list is		interface is set to that neighbor and the repair segment list is
	empty.		empty.
	This is comparable to a post-convergence LFA FRR repair.		This is comparable to a post-convergence LFA FRR repair.
	5.2. FRR Path Using a PQ Node		5.2. FRR Path Using a PQ Node
	When a remote node R is in P(S,X) and Q(D,x) and on the post-		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 to		convergence path, the RL is made of a single node segment to R, and
	R, and the outgoing interface is set to the outgoing interface used		the outgoing interface is set to the outgoing interface used to reach
	to reach R.		R.
	This is comparable to a post-convergence RLFA repair tunnel.		This is comparable to a post-convergence RLFA repair tunnel.
	5.3. FRR Path Using a P Node and Q Node That Are Adjacent		5.3. FRR Path Using a P Node and Q Node That Are Adjacent
	When a node P is in P(S,X) and a node Q is in Q(D,x), and both are on		When a node P is in P(S, X) and a node Q is in Q(D, x), and both are
	the post-convergence path and are adjacent to each other, the repair		on the post-convergence path and are adjacent to each other, the RL
	list is made of two segments: a node segment to P (to be processed		is made of two segments: a node segment to P (to be processed first),
	first), followed by an adjacency segment from P to Q.		followed by an Adjacency segment from P to Q.
	This is comparable to a post-convergence DLFA (LFA with directed		This is comparable to a post-convergence Directed Loop-Free Alternate
	forwarding) repair tunnel.		(DLFA) repair tunnel.
	5.4. Connecting Distant P and Q Nodes Along Post-Convergence Paths		5.4. Connecting Distant P and Q Nodes Along Post-Convergence Paths
	In some cases, there is no adjacent P and Q node along the post-		In some cases, there is no adjacent P and Q node along the
	convergence path. As mentioned in Section 3, a list of adjacency		post-convergence path. As mentioned in Section 3, a list of
	SIDs can be used to encode the path between P and Q. However, the		Adjacency SIDs can be used to encode the path between P and Q.
	PLR can perform additional computations to compute a list of segments		However, the PLR can perform additional computations to compute a
	that represent a loop-free path from P to Q. How these computations		shorter list of segments that represent a loop-free path from P to Q.
	are done is out of scope of this document and is left to		How these computations are done is out of scope of this document and
	implementation.		is left to implementation.
	6. Building TI-LFA Repair Lists for SR Segments		6. Building TI-LFA Repair Lists for SR Segments
	The following sections describe how to build the repair lists using		The following sections describe how to build the RLs using the
	the terminology defined in [RFC8402]. The procedures described in		terminology defined in [RFC8402]. The procedures described in this
	this section are equally applicable to both the Segment Routing over		section are equally applicable to both the Segment Routing over MPLS
	MPLS (SR-MPLS) and the Segment Routing over IPv6 (SRv6) dataplane,		(SR-MPLS) and the Segment Routing over IPv6 (SRv6) data plane, while
	while the dataplane-specific considerations are described in		the data plane-specific considerations are described in Section 7.
	Section 7.
	This section explains the process by which a protecting router S		This section explains the process by which a protecting router S
	handles the active segment of a packet upon the failure of its		handles the active segment of a packet upon the failure of its
	primary outgoing interface for the packet S-F. The failure of the		primary outgoing interface for the packet S-F. The failure of the
	primary outgoing interface may occur due to various triggers, such as		primary outgoing interface may occur due to various triggers, such as
	link failure, neighbor node failure, and others.		link failure, neighbor node failure, and others.
	6.1. The Active Segment Is a Node Segment		6.1. The Active Segment Is a Node Segment
	The active segment MUST be kept on the SR header unchanged and the		The active segment MUST be kept on the SR header unchanged and the RL
	repair list MUST be added. The active segment becomes the first		MUST be added. The active segment becomes the first segment after
	segment after the repair list. The way the repair list is added		the RL. The way the RL is added depends on the data plane used (see
	depends on the dataplane used (see Section 7).		Section 7).
	6.2. The Active Segment Is an Adjacency Segment		6.2. The Active Segment Is an Adjacency Segment
	This section defines the FRR behavior applied by S for any packet		This section defines the FRR behavior applied by S for any packet
	received with an active adjacency segment S-F for which protection		received with an active Adjacency segment S-F for which protection
	was enabled. Since protection has been enabled for the segment S-F		was enabled. Since protection has been enabled for the segment S-F
	and signaled in the IGP (for instance, using protocol extensions from		and signaled in the IGP (for instance, using protocol extensions from
	[RFC8667] and [RFC8665]), a calculator of any SR policy utilizing		[RFC8667] and [RFC8665]), a calculator of any SR policy utilizing
	this segment is aware that it may be transiently rerouted out of S-F		this segment is aware that it may be transiently rerouted out of S-F
	in the event of an S-F failure.		in the event of an S-F failure.
	The simplest approach for link protection of an adjacency segment S-F		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		is to create an RL that will carry the traffic to F. To do so, one
	so, one or more "PUSH" operations are performed. If the repair list,		or more "PUSH" operations are performed. If the RL, while avoiding
	while avoiding S-F, terminates on F, S only pushes segments of the		S-F, terminates on F, S only pushes segments of the RL. Otherwise, S
	repair list. Otherwise, S pushes a node segment of F, followed by		pushes a node segment of F, followed by the segments of the RL. For
	the segments of the repair list. For details on the "NEXT" and		details on the "NEXT" and "PUSH" operations, refer to [RFC8402].
	"PUSH" operations, refer to [RFC8402].
	This method, which merges back the traffic at the remote end of the		This method, which merges back the traffic at the remote end of the
	adjacency segment, has the advantage of keeping as much traffic as		Adjacency segment, has the advantage of keeping as much traffic as
	possible on the pre-failure path. When SR policies are involved and		possible on the pre-failure path. When SR policies are involved and
	strict compliance with the policy is required, an end-to-end		strict compliance with the policy is required, an end-to-end
	protection (beyond the scope of this document) should be preferred		protection (beyond the scope of this document) should be preferred
	over the local repair mechanism described above.		over the local repair mechanism described above.
	Note, however, that when the SR source node is using Traffic		Note, however, that when the SR source node is using Traffic
	Engineering (TE), it will generally not be possible for the PLR to		Engineering (TE), it will generally not be possible for the PLR to
	know what post-convergence path will be selected by the source node		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		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		local matter that depends on constraints that may not be known at the
	PLR. Therefore, no method applied at the PLR can guarantee		PLR. Therefore, no method applied at the PLR can guarantee
	protection will follow the post-convergence path.		protection will follow the post-convergence path.
	The case where the active segment is followed by another adjacency		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 is distinguished from the case where it is followed by a node
	segment. Repair techniques for the respective cases are provided in		segment. Repair techniques for the respective cases are provided in
	the following subsections.		the following subsections.
	6.2.1. Protecting [Adjacency, Adjacency] Segment Lists		6.2.1. Protecting [Adjacency, Adjacency] Segment Lists
	If the next segment in the list is an Adjacency segment, then the		If the next segment in the list is an Adjacency segment, then the
	packet has to be conveyed to F.		packet has to be conveyed to F.
	To do so, S MUST apply a "NEXT" operation on Adj-Sid(S-F) and then		To do so, S MUST apply a "NEXT" operation on Adj-SID(S-F) and then
	one or more "PUSH" operations. If the repair list, while avoiding		one or more "PUSH" operations. If the RL, while avoiding S-F,
	S-F, terminates on F, S only pushes the segments of the repair list.		terminates on F, S only pushes the segments of the RL. Otherwise, S
	Otherwise, S pushes a node segment of F, followed by the segments of		pushes a node segment of F, followed by the segments of the RL. For
	the repair list. For details on the "NEXT" and "PUSH" operations,		details on the "NEXT" and "PUSH" operations, refer to [RFC8402].
	refer to [RFC8402].
	Upon failure of S-F, a packet reaching S with a segment list matching		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		[adj-sid(S-F), adj-sid(F-M), ...] will thus leave S with a segment
	matching [RL(F),node(F),adj-sid(F-M),...], where RL(F) is the repair		list matching [RL(F), node(F), adj-sid(F-M), ...], where RL(F) is the
	list for destination F.		RL for destination F.
	6.2.2. Protecting [Adjacency, Node] Segment Lists		6.2.2. Protecting [Adjacency, Node] Segment Lists
	If the next segment in the stack is a node segment, say for node 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),...].		the segment list on the packet matches [adj-sid(S-F), node(T), ...].
	In this case, S MUST apply a "NEXT" operation on the Adjacency		In this case, S MUST apply a "NEXT" operation on the Adjacency
	segment related to S-F, followed by a "PUSH" of a repair list		segment related to S-F, followed by a "PUSH" of an RL redirecting the
	redirecting the traffic to a node Q, whose path to node segment T is		traffic to a node Q, whose path to node segment T is not affected by
	not affected by the failure.		the failure.
	Upon failure of S-F, packets reaching S with a segment list matching		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		[adj-sid(S-F), node(T), ...] would leave S with a segment list
	matching [RL(Q),node(T), ...].		matching [RL(Q), node(T), ...].
	7. Dataplane-Specific Considerations		7. Data Plane-Specific Considerations
	7.1. MPLS Dataplane Considerations		7.1. MPLS Data Plane Considerations
	The MPLS dataplane for Segment Routing (SR) is described in		The MPLS data plane for SR is described in [RFC8660].
	[RFC8660].
	The following dataplane behaviors apply when creating a repair list		The following data plane behaviors apply when creating an RL using an
	using an MPLS dataplane:		MPLS data plane:
	1. If the active segment is a node segment that has been signaled		1. If the active segment is a node segment that has been signaled
	with penultimate hop popping, and the repair list ends with an		with penultimate hop popping, and the RL ends with an Adjacency
	adjacency segment terminating on a node that advertised the		segment terminating on the penultimate node of the active
	"NEXT" operation [RFC8402] of the active segment, then the active		segment, then the active segment MUST be popped before pushing
	segment MUST be popped before pushing the repair list.		the RL.
	2. If the active segment is a node segment, but the other conditions		2. If the active segment is a node segment, but the other conditions
	in 1. are not met, the active segment MUST be popped and then		in 1. are not met, the active segment MUST be popped and then
	pushed again with a label value computed according to the Segment		pushed again with a label value computed according to the Segment
	Routing Global Block (SRGB) of Q, where Q is the endpoint of the		Routing Global Block (SRGB) of Q, where Q is the endpoint of the
	repair list. Finally, the repair list MUST be pushed.		RL. Finally, the RL MUST be pushed.
	7.2. SRv6 Dataplane Considerations		7.2. SRv6 Data Plane Considerations
	SRv6 dataplane and programming instructions are described		SRv6 data plane and programming instructions are described
	respectively in [RFC8754] and [RFC8986].		respectively in [RFC8754] and [RFC8986].
	The TI-LFA path computation algorithm is the same as in the SR-MPLS		The TI-LFA path computation algorithm is the same as in the SR-MPLS
	dataplane. Note, however, that the Adjacency SIDs are typically		data plane. Note, however, that the Adjacency SIDs are typically
	globally routed. In such a case, there is no need for preceding an		globally routed. In such a case, there is no need for preceding an
	adjacency SID with a Prefix-SID [RFC8402], and the resulting repair		Adjacency SID with a Prefix-SID [RFC8402], and the resulting RL is
	list is likely shorter.		likely shorter.
	If the traffic is protected at a Transit Node, then an SRv6 SID list		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		is added on the packet to apply the RL. The addition of the RL
	repair list follows the head-end behaviors as specified in Section 5		follows the head-end behaviors as specified in Section 5 of
	of [RFC8986].		[RFC8986].
	If the traffic is protected at an SR Segment Endpoint Node, first the		If the traffic is protected at an SR Segment Endpoint Node, first the
	Segment Endpoint packet processing is executed. Then, the packet is		Segment Endpoint packet processing is executed. Then, the packet is
	protected as if it were a transit packet.		protected as if it were a transit packet.
	8. TI-LFA and SR Algorithms		8. TI-LFA and SR Algorithms
	SR allows an operator to bind an algorithm to a Prefix-SID (as		SR allows an operator to bind an algorithm to a Prefix-SID (as
	defined in [RFC8402]). The algorithm value dictates how the path to		defined in [RFC8402]). The algorithm value dictates how the path to
	the prefix is computed. The SR default algorithm is known as the		the prefix is computed. The SR default algorithm is known as the
	skipping to change at line 665 ¶		skipping to change at line 656 ¶
	[RFC9350] defines a Flexible Algorithm framework to be associated		[RFC9350] defines a Flexible Algorithm framework to be associated
	with Prefix-SIDs. A Flexible Algorithm allows a user to associate a		with Prefix-SIDs. A Flexible Algorithm allows a user to associate a
	constrained path to a Prefix-SID rather than using the regular IGP		constrained path to a Prefix-SID rather than using the regular IGP
	shortest path. An implementation MAY support TI-LFA to protect Node-		shortest path. An implementation MAY support TI-LFA to protect Node-
	SIDs associated with a Flexible Algorithm. In such a case, rather		SIDs associated with a Flexible Algorithm. In such a case, rather
	than computing the expected post-convergence path based on the		than computing the expected post-convergence path based on the
	regular SPF, an implementation SHOULD use the constrained SPF		regular SPF, an implementation SHOULD use the constrained SPF
	algorithm bound to the Flexible Algorithm (using the Flexible		algorithm bound to the Flexible Algorithm (using the Flexible
	Algorithm Definition) instead of the regular Dijkstra in all the SPF/		Algorithm Definition) instead of the regular Dijkstra in all the SPF/
	rSPF computations that are occurring during the TI-LFA computation.		reverse SPF computations that are occurring during the TI-LFA
	This includes the computation of the P-Space and Q-Space as well as		computation. This includes the computation of the P-space and
	the post-convergence path. Furthermore, the implementation SHOULD		Q-space as well as the post-convergence path. Furthermore, the
	only use Node-SIDs/Adj-SIDs bound to the Flexible Algorithm and/or		implementation SHOULD only use Node-SIDs/Adj-SIDs bound to the
	unprotected Adj-SIDs of the regular SPF to build the repair list.		Flexible Algorithm and/or unprotected Adj-SIDs of the regular SPF to
	The use of regular Dijkstra for the TI-LFA computation or for		build the RL. The use of regular Dijkstra for the TI-LFA computation
	building the repair path using SIDs other than those recommended does		or for building the repair path using SIDs other than those
	not ensure that the traffic going over the TI-LFA repair path during		recommended does not ensure that the traffic going over the TI-LFA
	the FRR period is honoring the Flexible Algorithm constraints.		repair path during the FRR period is honoring the Flexible Algorithm
			constraints.
	9. Usage of Adjacency Segments in the Repair List		9. Usage of Adjacency Segments in the Repair List
	The repair list of segments computed by TI-LFA may contain one or		The RL of segments computed by TI-LFA may contain one or more
	more adjacency segments. An adjacency segment may be protected or		Adjacency segments. An Adjacency segment may be protected or not
	not protected.		protected.
	S --- R2 --- R3 ---- R4 --- R5 --- D		S --- R2 --- R3 ---- R4 --- R5 --- D
	* | \ *		* | \ *
	* | \ *		* | \ *
	R7 ** R8		R7 ** R8
	* |		* |
	* |		* |
	R9 -- R10		R9 -- R10
	Figure 2		Figure 2
	In Figure 2, all the metrics are equal to 1 except		In Figure 2, all the metrics are equal to 1 except
	R2-R7,R7-R8,R8-R4,R7-R9, which have a metric of 1000. Considering R2		R2-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		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-		S->D, the RL computed by R2 will be [adj-sid(R7-R8), adj-sid(R8-R4)],
	sid(R8-R4)], and the outgoing interface will be to R7. If R3 fails,		and the outgoing interface will be to R7. If R3 fails, R2 pushes the
	R2 pushes the repair list onto the incoming packet to D. During the		RL onto the incoming packet to D. During the FRR, if R7-R8 fails and
	FRR, if R7-R8 fails and if TI-LFA has picked a protected adjacency		if TI-LFA has picked a protected Adjacency segment for Adj-
	segment for adj-sid(R7-R8), R7 will push an additional repair list		SID(R7-R8), R7 will push an additional RL onto the packet following
	onto the packet following the procedures defined in Section 6.		the procedures defined in Section 6.
	To avoid the possibility of this double FRR activation, an		To avoid the possibility of this double FRR activation, an
	implementation of TI-LFA MAY pick only non-protected adjacency		implementation of TI-LFA MAY pick only unprotected Adjacency segments
	segments when building the repair list. However, it is important to		when building the RL. However, it is important to note that FRR in
	note that FRR in general is intended to protect for a single pre-		general is intended to protect for a single pre-planned failure. If
	planned failure. If the failure that happens is worse than expected		the failure that happens is worse than expected or multiple failures
	or multiple failures happen, FRR is not guaranteed to work. In such		happen, FRR is not guaranteed to work. In such a case, fast IGP
	a case, fast IGP convergence remains important to restore traffic as		convergence remains important to restore traffic as quickly as
	quickly as possible.		possible.
	10. Security Considerations		10. Security Considerations
	The techniques described in this document are internal		The techniques described in this document are internal
	functionalities to a router that can guarantee an upper bound on the		functionalities to a router that can guarantee an upper bound on the
	time taken to restore traffic flow upon the failure of a directly		time taken to restore traffic flow upon the failure of a directly
	connected link or node. As these techniques steer traffic to the		connected link or node. As these techniques steer traffic to the
	post-convergence path as quickly as possible, this serves to minimize		post-convergence path as quickly as possible, this serves to minimize
	the disruption associated with a local failure, which can be seen as		the disruption associated with a local failure, which can be seen as
	a modest security enhancement. The protection mechanism does not		a modest security enhancement. The protection mechanism does not
	protect external destinations, but rather provides quick restoration		protect external destinations, but rather provides quick restoration
	for destinations that are internal to a routing domain.		for destinations that are internal to a routing domain.
	The security considerations described in [RFC5286] and [RFC7490]		The security considerations described in [RFC5286] and [RFC7490]
	apply to this document. Similarly, as the solution described in this		apply to this document. Similarly, as the solution described in this
	document is based on SR technology, the reader should be aware of the		document is based on SR technology, the reader should be aware of the
	security considerations related to this technology (see [RFC8402])		security considerations related to this technology (see [RFC8402])
	and its dataplane instantiations (see [RFC8660], [RFC8754], and		and its data plane instantiations (see [RFC8660], [RFC8754], and
	[RFC8986]). However, this document does not introduce additional		[RFC8986]). However, this document does not introduce additional
	security concerns.		security concerns.
	11. IANA Considerations		11. IANA Considerations
	This document has no IANA actions.		This document has no IANA actions.
	12. References		12. References
	12.1. Normative References		12.1. Normative References
	skipping to change at line 776 ¶		skipping to change at line 768 ¶
	<https://www.rfc-editor.org/info/rfc8754>.		<https://www.rfc-editor.org/info/rfc8754>.
	[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,		[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
	D., Matsushima, S., and Z. Li, "Segment Routing over IPv6		D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
	(SRv6) Network Programming", RFC 8986,		(SRv6) Network Programming", RFC 8986,
	DOI 10.17487/RFC8986, February 2021,		DOI 10.17487/RFC8986, February 2021,
	<https://www.rfc-editor.org/info/rfc8986>.		<https://www.rfc-editor.org/info/rfc8986>.
	12.2. Informative References		12.2. Informative References
			[IPFRR-TUNNELS]
			Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
			Fast Reroute using tunnels", Work in Progress, Internet-
			Draft, draft-bryant-ipfrr-tunnels-03, 16 November 2007,
			<https://datatracker.ietf.org/doc/html/draft-bryant-ipfrr-
			tunnels-03>.
	[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for		[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
	IP Fast Reroute: Loop-Free Alternates", RFC 5286,		IP Fast Reroute: Loop-Free Alternates", RFC 5286,
	DOI 10.17487/RFC5286, September 2008,		DOI 10.17487/RFC5286, September 2008,
	<https://www.rfc-editor.org/info/rfc5286>.		<https://www.rfc-editor.org/info/rfc5286>.
	[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",		[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
	RFC 5714, DOI 10.17487/RFC5714, January 2010,		RFC 5714, DOI 10.17487/RFC5714, January 2010,
	<https://www.rfc-editor.org/info/rfc5714>.		<https://www.rfc-editor.org/info/rfc5714>.
	[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free		[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
	skipping to change at line 880 ¶		skipping to change at line 879 ¶
	and PE4, when the link L fails, depending on the convergence speed of		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		the nodes, X may reroute its forwarding entries to the remote PEs
	onto X-H or X-D; however, in a similar timeframe, PE1 will also		onto X-H or X-D; however, in a similar timeframe, PE1 will also
	reroute a subset of its traffic (the subset destined to PE2) out of		reroute a subset of its traffic (the subset destined to PE2) out of
	its nominal path, reducing the quantity of traffic received by X.		its nominal path, reducing the quantity of traffic received by X.
	The capacity planning rule presented previously has the drawback of		The capacity planning rule presented previously has the drawback of
	oversizing the network; however, it allows for preventing any		oversizing the network; however, it allows for preventing any
	transient congestion (for example, when X reroutes traffic before PE1		transient congestion (for example, when X reroutes traffic before PE1
	does).		does).
	H --- I --- J		H --- I --- J *
	| | \		| | *
	PE4 | | PE3		PE4 | | PE3
	\ | (L) | /		\ | (L) | *
	A --- X --- B --- G		* A --- X --- B --- G *
	/ | | \		* | | *
	PE1 | | PE2		PE1 | | PE2
	\ | | /		* | | *
	C --- D --- E --- F		* C --- D --- E --- F *
	Figure 3		Figure 3
	Based on this assumption, in order to facilitate the operation of FRR		Based on this assumption, in order to facilitate the operation of FRR
	and limit the implementation of local FRR policies, traffic can be		and limit the implementation of local FRR policies, traffic can be
	steered by the PLR onto its expected post-convergence path during the		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		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		destined to PE3 and PE2 on the post-convergence paths. This is
	perfectly in line with the capacity planning rule that was presented		perfectly in line with the capacity planning rule that was presented
	before and also in line with the fact that X may converge before PE1		before and also in line with the fact that X may converge before PE1
	skipping to change at line 929 ¶		skipping to change at line 928 ¶
	that happened is different from the failure FRR was protecting		that happened is different from the failure FRR was protecting
	against. As an example, if the operator has implemented a protection		against. As an example, if the operator has implemented a protection
	against a node failure, the expected post-convergence path used		against a node failure, the expected post-convergence path used
	during FRR will be the one considering that the node has failed.		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		However, even if a single link is failing or a set of links is
	failing (instead of the full node), the node-protecting post-		failing (instead of the full node), the node-protecting post-
	convergence path will be used. The consequence is that the path used		convergence path will be used. The consequence is that the path used
	during FRR is not optimal with respect to the failure that has		during FRR is not optimal with respect to the failure that has
	actually occurred.		actually occurred.
	Another consideration to take into account is as follows: While using		Another consideration to take into account is as follows: while using
	the expected post-convergence path for SR traffic using node segments		the expected post-convergence path for SR traffic using node segments
	only (for instance, PE to PE traffic using the shortest path) has		only (for instance, PE to PE traffic using the shortest path) has
	some advantages, these advantages reduce when SR policies [RFC9256]		some advantages, these advantages reduce when SR policies [RFC9256]
	are involved. A segment list used in an SR policy is computed to		are involved. A segment list used in an SR policy is computed to
	obey a set of path constraints defined locally at the head-end or		obey a set of path constraints defined locally at the head-end or
	centrally in a controller. TI-LFA cannot be aware of such path		centrally in a controller. TI-LFA cannot be aware of such path
	constraints, and there is no reason to expect the TI-LFA backup path		constraints, and there is no reason to expect the TI-LFA backup path
	protecting one segment in that segment list to obey those		protecting one segment in that segment list to obey those
	constraints. When SR policies are used and the operator wants to		constraints. When SR policies are used and the operator wants to
	have a backup path that still follows the policy requirements, this		have a backup path that still follows the policy requirements, this
	skipping to change at line 961 ¶		skipping to change at line 960 ¶
	This section presents an analysis performed on real service provider		This section presents an analysis performed on real service provider
	and large enterprise network topologies. The objective of the		and large enterprise network topologies. The objective of the
	analysis is to assess the number of SIDs required in an explicit path		analysis is to assess the number of SIDs required in an explicit path
	when the mechanisms described in this document are used to protect		when the mechanisms described in this document are used to protect
	against the failure scenarios within the scope of this document. The		against the failure scenarios within the scope of this document. The
	number of segments described in this section are applicable to		number of segments described in this section are applicable to
	instantiating SR over the MPLS forwarding plane.		instantiating SR over the MPLS forwarding plane.
	The measurement below indicates that, for link and local SRLG		The measurement below indicates that, for link and local SRLG
	protection, a 1-SID repair path delivers more than 99% coverage. For		protection, a repair path of 1 SID or less delivers more than 99%
	node protection, a 2-SID repair path yields 99% coverage.		coverage. For node protection, a repair path of 2 SIDs or less
			yields 99% coverage.
	Table 1 below lists the characteristics of the networks used in our		Table 1 below lists the characteristics of the networks used in our
	measurements. The number of links refers to the number of		measurements. The number of links refers to the number of
	"bidirectional" links (not directed edges of the graph). The		"bidirectional" links (not directed edges of the graph). The
	measurements are carried out as follows:		measurements are carried out as follows:
	* For each network, the algorithms described in this document are		* For each network, the algorithms described in this document are
	applied to protect all prefixes against link, node, and local SRLG		applied to protect all prefixes against link, node, and local SRLG
	failure.		failure.
	* For each prefix, the number of SIDs used by the repair path is		* For each prefix, the number of SIDs used by the repair path is
	recorded.		recorded.
	* The percentage of number of SIDs are listed in Tables 2, 3, 4, 5,		* The percentage of number of SIDs are listed in Tables 2, 3, 4, 5,
	6, and 7.		6, and 7.
	The measurements listed in the tables indicate that for link and
	local SRLG protection, a 1-SID repair path is sufficient to protect
	more than 99% of the prefix in almost all cases. For node
	protection, 2-SID repair paths yield 99% coverage.
	+=========+=======+=======+====================+============+		+=========+=======+=======+====================+============+
	| Network | Nodes | Links | Node-to-Link Ratio | SRLG Info? |		| Network | Nodes | Links | Node-to-Link Ratio | SRLG Info? |
	+=========+=======+=======+====================+============+		+=========+=======+=======+====================+============+
	| T1 | 408 | 665 | 1.63 | Yes |		| T1 | 408 | 665 | 1.63 | Yes |
	+---------+-------+-------+--------------------+------------+		+---------+-------+-------+--------------------+------------+
	| T2 | 587 | 1083 | 1.84 | No |		| T2 | 587 | 1083 | 1.84 | No |
	+---------+-------+-------+--------------------+------------+		+---------+-------+-------+--------------------+------------+
	| T3 | 93 | 401 | 4.31 | Yes |		| T3 | 93 | 401 | 4.31 | Yes |
	+---------+-------+-------+--------------------+------------+		+---------+-------+-------+--------------------+------------+
	| T4 | 247 | 393 | 1.59 | Yes |		| T4 | 247 | 393 | 1.59 | Yes |
	skipping to change at line 1023 ¶		skipping to change at line 1018 ¶
	case, there is no need to explicitly route the traffic using		case, there is no need to explicitly route the traffic using
	additional SIDs. This scenario is described in Section 5.1.		additional SIDs. This scenario is described in Section 5.1.
	* 1 SID: The repair node is a PQ node; in which case, only 1 SID is		* 1 SID: The repair node is a PQ node; in which case, only 1 SID is
	needed to guarantee a loop-free path. This scenario is covered in		needed to guarantee a loop-free path. This scenario is covered in
	Section 5.2.		Section 5.2.
	* 2 or more SIDs: The repair path consists of 2 or more SIDs as		* 2 or more SIDs: The repair path consists of 2 or more SIDs as
	described in Sections 5.3 and 5.4. We do not cover the case for 2		described in Sections 5.3 and 5.4. We do not cover the case for 2
	SIDs (Section 5.3) separately because there was no granularity in		SIDs (Section 5.3) separately because there was no granularity in
	the result. Also, we treat the node-SID + adj-SID and node-SID +		the result. Also, we treat the Node-SID + Adj-SID and Node-SID +
	node-SID the same because they do not differ from the data plane		Node-SID the same because they do not differ from the data plane
	point of view.		point of view.
	Tables 2 and 3 below summarize the measurements on the number of SIDs		Tables 2 and 3 below summarize the measurements on the number of SIDs
	needed for link protection.		needed for link protection.
	+=========+========+=======+========+========+		+=========+========+=======+========+========+
	| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |		| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
	+=========+========+=======+========+========+		+=========+========+=======+========+========+
	| T1 | 74.3% | 25.3% | 0.5% | 0.0% |		| T1 | 74.3% | 25.3% | 0.5% | 0.0% |
	+---------+--------+-------+--------+--------+		+---------+--------+-------+--------+--------+
	skipping to change at line 1055 ¶		skipping to change at line 1050 ¶
	| T7 | 98.9% | 1.1% | 0.0% | 0.0% |		| T7 | 98.9% | 1.1% | 0.0% | 0.0% |
	+---------+--------+-------+--------+--------+		+---------+--------+-------+--------+--------+
	| T8 | 94.1% | 5.9% | 0.0% | 0.0% |		| T8 | 94.1% | 5.9% | 0.0% | 0.0% |
	+---------+--------+-------+--------+--------+		+---------+--------+-------+--------+--------+
	| T9 | 98.9% | 1.0% | 0.0% | 0.0% |		| T9 | 98.9% | 1.0% | 0.0% | 0.0% |
	+---------+--------+-------+--------+--------+		+---------+--------+-------+--------+--------+
	Table 2: Link Protection (Repair Size		Table 2: Link Protection (Repair Size
	Distribution)		Distribution)
	+=========+========+========+========+========+		+=========+========+=======+========+========+
	| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |		| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
	+=========+========+========+========+========+		+=========+========+=======+========+========+
	| T1 | 74.2% | 99.5% | 99.9% | 100.0% |		| T1 | 74.2% | 99.5% | 99.9% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T2 | 81.1% | 99.8% | 100.0% | 100.0% |		| T2 | 81.1% | 99.8% | 100% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T3 | 95.9% | 99.9% | 100.0% | 100.0% |		| T3 | 95.9% | 99.9% | 100% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T4 | 62.5% | 98.2% | 100.0% | 100.0% |		| T4 | 62.5% | 98.2% | 100% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T5 | 85.7% | 100.0% | 100.0% | 100.0% |		| T5 | 85.7% | 100% | 100% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T6 | 81.2% | 99.9% | 100.0% | 100.0% |		| T6 | 81.2% | 99.9% | 100% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T7 | 98.8% | 100.0% | 100.0% | 100.0% |		| T7 | 98.8% | 100% | 100% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T8 | 94.1% | 100.0% | 100.0% | 100.0% |		| T8 | 94.1% | 100% | 100% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T9 | 98.9% | 100.0% | 100.0% | 100.0% |		| T9 | 98.9% | 100% | 100% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	Table 3: Link Protection (Repair Size		Table 3: Link Protection (Repair Size
	Cumulative Distribution)		Cumulative Distribution)
	Tables 4 and 5 summarize the measurements on the number of SIDs		Tables 4 and 5 summarize the measurements on the number of SIDs
	needed for local SRLG protection.		needed for local SRLG protection.
	+=========+========+=======+========+========+		+=========+========+=======+========+========+
	| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |		| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
	+=========+========+=======+========+========+		+=========+========+=======+========+========+
	| T1 | 74.2% | 25.3% | 0.5% | 0.0% |		| T1 | 74.2% | 25.3% | 0.5% | 0.0% |
	+---------+--------+-------+--------+--------+		+---------+--------+-------+--------+--------+
	| T2 | No SRLG information |		| T2 | No SRLG information |
	skipping to change at line 1108 ¶		skipping to change at line 1103 ¶
	| T7 | No SRLG information |		| T7 | No SRLG information |
	+---------+--------+-------+--------+--------+		+---------+--------+-------+--------+--------+
	| T8 | 85.2% | 14.8% | 0.0% | 0.0% |		| T8 | 85.2% | 14.8% | 0.0% | 0.0% |
	+---------+--------+-------+--------+--------+		+---------+--------+-------+--------+--------+
	| T9 | 98.9% | 1.1% | 0.0% | 0.0% |		| T9 | 98.9% | 1.1% | 0.0% | 0.0% |
	+---------+--------+-------+--------+--------+		+---------+--------+-------+--------+--------+
	Table 4: Local SRLG Protection (Repair		Table 4: Local SRLG Protection (Repair
	Size Distribution)		Size Distribution)
	+=========+========+========+========+========+		+=========+========+=======+========+========+
	| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |		| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
	+=========+========+========+========+========+		+=========+========+=======+========+========+
	| T1 | 74.2% | 99.5% | 99.9% | 100.0% |		| T1 | 74.2% | 99.5% | 99.9% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T2 | No SRLG information |		| T2 | No SRLG information |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T3 | 93.6% | 99.9% | 100.0% | 0.0% |		| T3 | 93.6% | 99.9% | 100% | 0.0% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T4 | 62.5% | 98.2% | 100.0% | 100.0% |		| T4 | 62.5% | 98.2% | 100% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T5 | 83.1% | 100.0% | 100.0% | 100.0% |		| T5 | 83.1% | 100% | 100% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T6 | No SRLG information |		| T6 | No SRLG information |
	+---------+-----------------------------------+		+---------+----------------------------------+
	| T7 | No SRLG information |		| T7 | No SRLG information |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T8 | 85.2% | 100.0% | 100.0% | 100.0% |		| T8 | 85.2% | 100% | 100% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	| T9 | 98.9% | 100.0% | 100.0% | 100.0% |		| T9 | 98.9% | 100% | 100% | 100% |
	+---------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+
	Table 5: Local SRLG Protection (Repair Size		Table 5: Local SRLG Protection (Repair
	Cumulative Distribution)		Size Cumulative Distribution)
	The remaining two tables summarize the measurements on the number of		The remaining two tables summarize the measurements on the number of
	SIDs needed for node protection.		SIDs needed for node protection.
	+=========+========+=======+========+========+========+		+=========+========+=======+========+========+========+
	| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | 4 SIDs |		| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | 4 SIDs |
	+=========+========+=======+========+========+========+		+=========+========+=======+========+========+========+
	| T1 | 49.8% | 47.9% | 2.1% | 0.1% | 0.0% |		| T1 | 49.8% | 47.9% | 2.1% | 0.1% | 0.0% |
	+---------+--------+-------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	| T2 | 36.5% | 59.6% | 3.6% | 0.2% | 0.0% |		| T2 | 36.5% | 59.6% | 3.6% | 0.2% | 0.0% |
	skipping to change at line 1160 ¶		skipping to change at line 1155 ¶
	+---------+--------+-------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	| T7 | 66.1% | 32.8% | 1.1% | 0.0% | 0.0% |		| T7 | 66.1% | 32.8% | 1.1% | 0.0% | 0.0% |
	+---------+--------+-------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	| T8 | 59.7% | 40.2% | 0.0% | 0.0% | 0.0% |		| T8 | 59.7% | 40.2% | 0.0% | 0.0% | 0.0% |
	+---------+--------+-------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	| T9 | 98.9% | 1.0% | 0.0% | 0.0% | 0.0% |		| T9 | 98.9% | 1.0% | 0.0% | 0.0% | 0.0% |
	+---------+--------+-------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	Table 6: Node Protection (Repair Size Distribution)		Table 6: Node Protection (Repair Size Distribution)
	+=========+========+========+========+========+========+		+=========+========+=======+========+========+========+
	| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | 4 SIDs |		| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | 4 SIDs |
	+=========+========+========+========+========+========+		+=========+========+=======+========+========+========+
	| T1 | 49.7% | 97.6% | 99.8% | 99.9% | 100.0% |		| T1 | 49.7% | 97.6% | 99.8% | 99.9% | 100% |
	+---------+--------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	| T2 | 36.5% | 96.1% | 99.7% | 99.9% | 100.0% |		| T2 | 36.5% | 96.1% | 99.7% | 99.9% | 100% |
	+---------+--------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	| T3 | 73.3% | 98.9% | 99.9% | 100.0% | 100.0% |		| T3 | 73.3% | 98.9% | 99.9% | 100% | 100% |
	+---------+--------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	| T4 | 36.1% | 93.4% | 99.8% | 99.9% | 100.0% |		| T4 | 36.1% | 93.4% | 99.8% | 99.9% | 100% |
	+---------+--------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	| T5 | 73.2% | 100.0% | 100.0% | 100.0% | 100.0% |		| T5 | 73.2% | 100% | 100% | 100% | 100% |
	+---------+--------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	| T6 | 78.4% | 99.7% | 100.0% | 100.0% | 100.0% |		| T6 | 78.4% | 99.7% | 100% | 100% | 100% |
	+---------+--------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	| T7 | 66.1% | 98.9% | 100.0% | 100.0% | 100.0% |		| T7 | 66.1% | 98.9% | 100% | 100% | 100% |
	+---------+--------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	| T8 | 59.7% | 100.0% | 100.0% | 100.0% | 100.0% |		| T8 | 59.7% | 100% | 100% | 100% | 100% |
	+---------+--------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	| T9 | 98.9% | 100.0% | 100.0% | 100.0% | 100.0% |		| T9 | 98.9% | 100% | 100% | 100% | 100% |
	+---------+--------+--------+--------+--------+--------+		+---------+--------+-------+--------+--------+--------+
	Table 7: Node Protection (Repair Size Cumulative		Table 7: Node Protection (Repair Size Cumulative
	Distribution)		Distribution)
	Acknowledgments		Acknowledgments
	The authors would like to thank Les Ginsberg, Stewart Bryant,		The authors would like to thank Les Ginsberg, Stewart Bryant,
	Alexander Vainsthein, Chris Bowers, Shraddha Hedge, Wes Hardaker,		Alexander Vainsthein, Chris Bowers, Shraddha Hedge, Wes Hardaker,
	Gunter Van de Velde, and John Scudder for their valuable comments.		Gunter Van de Velde, and John Scudder for their valuable comments.
	Contributors		Contributors
	In addition to the authors listed on the front page, the following		In addition to the authors listed on the front page, the following
	skipping to change at line 1205 ¶		skipping to change at line 1200 ¶
	Francois Clad		Francois Clad
	Cisco Systems		Cisco Systems
	Pablo Camarillo		Pablo Camarillo
	Cisco Systems		Cisco Systems
	Authors' Addresses		Authors' Addresses
	Ahmed Bashandy		Ahmed Bashandy
	Individual		HPE
			United States of America
	Email: abashandy.ietf@gmail.com		Email: abashandy.ietf@gmail.com
	Stephane Litkowski		Stephane Litkowski
	Cisco Systems		Cisco Systems
	France		France
	Email: slitkows@cisco.com		Email: slitkows@cisco.com
	Clarence Filsfils		Clarence Filsfils
	Cisco Systems		Cisco Systems
	Brussels		Brussels
	Belgium		Belgium
	Email: cfilsfil@cisco.com		Email: cfilsfil@cisco.com
	Pierre Francois		Pierre Francois
	INSA Lyon		INSA Lyon
	Email: pierre.francois@insa-lyon.fr		Email: pierre.francois@insa-lyon.fr
	Bruno Decraene		Bruno Decraene
	Orange		Orange
	Issy-les-Moulineaux
	France		France
	Email: bruno.decraene@orange.com		Email: bruno.decraene@orange.com
	Daniel Voyer		Daniel Voyer
	Bell Canada		Bell Canada
	Canada		Canada
	Email: daniel.voyer@bell.ca		Email: daniel.voyer@bell.ca
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