Topology Independent Fast Reroute using Segment Routing
draft-ietf-rtgwg-segment-routing-ti-lfa-17

Network Working Group                                        A. Bashandy
Internet-Draft                                              S. Litkowski
Intended status: Standards Track                             C. Filsfils
Expires: 6 January 2025                                    Cisco Systems
                                                             P. Francois
                                                               INSA Lyon
                                                             B. Decraene
                                                                  Orange
                                                                D. Voyer
                                                             Bell Canada
                                                             5 July 2024

        Topology Independent Fast Reroute using Segment Routing
               draft-ietf-rtgwg-segment-routing-ti-lfa-17

Abstract

   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).  It extends these concepts to provide guaranteed
   coverage in any two-connected networks using a link-state IGP.  A key
   aspect of TI-LFA is the FRR path selection approach establishing
   protection over the expected post-convergence paths from the point of
   local repair, reducing the operational need to control the tie-breaks
   among various FRR options.

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   This Internet-Draft will expire on 6 January 2025.

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Table of Contents

   1.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Conventions used in this document . . . . . . . . . . . .   8
   4.  Base principle  . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Intersecting P-Space and Q-Space with post-convergence
           paths . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Extended P-Space property computation for a resource X,
           over post-convergence paths . . . . . . . . . . . . . . .   8
     5.2.  Q-Space property computation for a resource X, over
           post-convergence paths  . . . . . . . . . . . . . . . . .   9
     5.3.  Scaling considerations when computing Q-Space . . . . . .   9
   6.  TI-LFA Repair path  . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  FRR path using a direct neighbor  . . . . . . . . . . . .  11
     6.2.  FRR path using a PQ node  . . . . . . . . . . . . . . . .  11
     6.3.  FRR path using a P node and Q node that are adjacent  . .  11
     6.4.  Connecting distant P and Q nodes along post-convergence
           paths . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Building TI-LFA repair lists  . . . . . . . . . . . . . . . .  12
     7.1.  The active segment is a node segment  . . . . . . . . . .  12
     7.2.  The active segment is an adjacency segment  . . . . . . .  12
       7.2.1.  Protecting [Adjacency, Adjacency] segment lists . . .  13
       7.2.2.  Protecting [Adjacency, Node] segment lists  . . . . .  13
   8.  Dataplane specific considerations . . . . . . . . . . . . . .  13
     8.1.  MPLS dataplane considerations . . . . . . . . . . . . . .  13
     8.2.  SRv6 dataplane considerations . . . . . . . . . . . . . .  14
   9.  TI-LFA and SR algorithms  . . . . . . . . . . . . . . . . . .  14
   10. Usage of Adjacency segments in the repair list  . . . . . . .  15
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  16
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16

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   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     15.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Advantages of using the expected post-convergence path
           during FRR  . . . . . . . . . . . . . . . . . . . . . . .  19
   Appendix B.  Analysis based on real network topologies  . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Acronyms

   *  DLFA: Remote LFA with Directed forwarding.

   *  FRR: Fast Re-route.

   *  IGP: Interior Gateway Protocol.

   *  LFA: Loop-Free Alternate.

   *  LSDB: Link State DataBase.

   *  PLR: Point of Local Repair.

   *  RL: Repair list.

   *  RLFA: Remote LFA.

   *  SID: Segment Identifier.

   *  SPF: Shortest Path First.

   *  SR: Segment Routing.

   *  SRLG: Shared Risk Link Group.

   *  TI-LFA: Topology Independent LFA.

2.  Introduction

   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
   [I-D.bashandy-rtgwg-segment-routing-uloop].

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   The term topology independent (TI) describes the capability providing
   a loop free backup path that is effective accross all network
   topologies.  This provides a major improvement compared to LFA
   [RFC5286] and remote LFA [RFC7490] which cannot provide a complete
   protection coverage in some topologies as described in [RFC6571].

   When the network reconverges after failure, micro-loops [RFC5715] 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
   convergence method, such as one of those described in [RFC5715],
   [RFC6976], [RFC8333], or [I-D.bashandy-rtgwg-segment-routing-uloop]
   should be implemented.

   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:

   *  Micro-loops that may or may not form during the distributed
      Interior Gateway Protocol (IGP) convergence as delineated in
      [RFC5715]:

      -  These micro-loops occur on routes directred towards the
         destination that do not traverse TI-LFA-configured paths.
         According to [RFC5714], the formation of such micro-loops can
         prevent traffic from reaching the PLR, thereby bypassing the
         TI-LFA paths establised for rerouting.

   *  Micro-loops that may or may not develop when the previously failed
      link is restored to functionality.

   TI-LFA paths are activated from the instant the PLR detects a failure
   in a local link and remain in effect until the Interior Gateway
   Protocol (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.

   TI-LFA paths are applied from the moment the PLR detects failure of a
   local link and until IGP convergence at the PLR is completed.
   Therefore, early (relative to the other nodes) IGP convergence at the
   PLR and the consecutive ”early” release of TI-LFA paths may cause
   micro-loops, especially if these paths have been computed using the
   methods described in Section Section 6.2, Section 6.3, or Section 6.4
   of the document.  One of the possible ways to prevent such micro-
   loops is local convergence delay ([RFC8333]).

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   TI-LFA procedures are complementary to application of any micro-loop
   avoidance procedures in the case of link or node failure:

   *  Link or node failure requires some urgent action to restore the
      traffic that passed thru the failed resource.  TI-LFA paths are
      pre-computed and pre-installed and therefore suitable for urgent
      recovery

   *  The paths used in the micro-loop avoidance procedures typically
      cannot be pre-computed.

   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 single 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 Section 3 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. a linecard or a
   set of links sharing a common transmission pipe).

   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 routing domains are beyond the scope of this document

   By utilizing Segment Routing (SR), T-LFA eliminates the need to
   establish Targeted Label Distribution Protocol sessions with remote
   nodes for leveraging the benefits of Remote Loop-Free Alternates
   (RLFA) [RFC7490][RFC7916] or Directed Loop-Free Alternates (DLFA)
   [RFC5714].  All the Segment Identifiers (SIDs) required are present
   within the Link State Database (LSDB) of the Interior Gateway
   Protocol (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.

   Utilizing SR makes the requirement unnecessary to establish
   additional state within the network for enforcing explicit Fast
   Reroute (FRR) paths.  This spares the nodes from maintaining
   supplementary state and frees the operator from the necessity to
   implement additional protocols or protocol sessions solely to augment
   protection coverage.

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   Although not a TI-LFA requirement or constraint, TI-LFA also brings
   the benefit of the ability to provide a backup path that follows the
   expected post-convergence path considering a particular failure which
   reduces the need of locally configured policies that influence the
   backup path selection ([RFC7916]).  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.

   Section 3 defines the main notations used in the document.  They are
   in line with [RFC5714].

   Section 4 defines the main principles of TI-LFA backup path
   computation.

   Section 5 suggests to compute the P-Space and Q-Space properties
   defined in Section 3, for the specific case of nodes lying over the
   post-convergence paths towards the protected destinations.

   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.

   Section 7 defines the segment operations to be applied by the PLR to
   ensure consistency with the forwarding state of the repair node.

   Section 8 discusses aspects that are specific to the dataplane.

   Section 9 discusses relationship between TI-LFA and the SR-algorithm.

   Certain considerations are needed when adjacency segments are used in
   a repare list.  Section 10 provides an overview of these
   considerations.

   Section 11 discusses security considerations.

   Appendix A highlights advantages of using the expected post-
   convergence path during FRR.

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   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.

3.  Terminology

   The main notations used in this document are defined as follows.

   The terms "old" and "new" topologies refer to the Link State Database
   (LSDB) state before and after the considered failure, respectively.

   SPT_old(R) is the Shortest Path Tree rooted at node R in the initial
   state of the network.

   SPT_new(R, X) is the Shortest Path Tree rooted at node R in the state
   of the network after the resource X has failed.

   PLR stands for "Point of Local Repair".  It is the router that
   applies fast traffic restoration after detecting failure in a
   directly attached link, set of links, and/or node.

   Similar to [RFC7490], the concept of P-Space and Q-Space is used for
   TI-LFA.

   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 a 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 the P-space.

   Consider the set of neighbors of a router R and a resource X.
   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 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.

   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 the Q-space

   EP(P, Q) is an explicit SR path from a node P to a node Q.

   Primary Interface: Primary Outgoing Interface: One of the outgoing
   interfaces towards a destination according to the IGP link-state
   protocol

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   Primary Link: A link connected to the primary interface

   adj-sid(S-F): Adjacency Segment from node S to node F

3.1.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

4.  Base principle

   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
   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 to
   MSD (Maximum SID Depth) limitations.

   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 allow to maximize ECMPs over the
   backup path.  These optimizations are out of scope of this document,
   however the subsequent sections provide some guidance on how to
   leverage P-Spaces and Q-Spaces to optimize the size of the segment
   list.

5.  Intersecting P-Space and Q-Space with post-convergence paths

   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 implementation MAY determine
   the P-Space/Extended P-Space and Q-Space properties (defined in
   [RFC7490]) 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 Section 6 to compute the TI-LFA repair list.

5.1.  Extended P-Space property computation for a resource X, over post-
      convergence paths

   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 the PLR, or a neighbor node of the PLR).

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   This can be found by:

   *  Excluding neighbors which are not on the post-convergence path
      when computing P'(R,X)

   *  Then, intersecting the set of nodes belonging to the post-
      convergence path from R to D, assuming the failure of X, with
      P'(R, X).

5.2.  Q-Space property computation for a resource X, over post-
      convergence paths

   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).

   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).

5.3.  Scaling considerations when computing Q-Space

   [RFC7490] 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
   ([RFC7490]) 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.

6.  TI-LFA Repair path

   The TI-LFA repair path (RP) consists of an outgoing interface and a
   list of segments (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 any co-ordination or message exchange between the PLR and
   any other router in the network.

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   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-
   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.
   The TI-LFA repair list is expressed generally as (Node-SID(P), EP(P,
   Q)).

     S ------- N1 ----------- D
     *\         |  \          |
     * \        |   \         |
     *  \       |    \        |
     *   N2-----R1****R2 *** R3
     *          *
     N3 *********

       ***** : link with high metric (1k)
       ----- : link with metric 1

                   Figure 1: Sample topology with TI-LFA

   As an example, in Figure 1, the focus is on the TI-LFA backup from S
   to D, considering the failure of node N1.

   *  First, P(S, N1) is computed and results in [N3, N2, R1].

   *  Then, Q(D, N1) is computed and results in [R3].

   *  The expected post-convergence path from S to D considering the
      failure of N1 is <N2 -> R1 -> R2 -> R3 -> D> (we are naming it
      PCPath in this example).

   *  P(S, N1) intersection with PCPath is [N2, R1], R1 being the deeper
      downstream node in PCP, it can be assumed to be used as P node
      (this is an example and an implementation could use a different
      strategy to choose the P node).

   *  Q(D, N1) intersection with PCPath is [R3], so R3 is picked as Q
      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-
      Sid(R2-R3)>.

   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)>.

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   Most often, the TI-LFA repair list has a simpler form, as described
   in the following sections.  Appendix B provides statistics for the
   number of SIDs in the explicit path to protect against various
   failures.

6.1.  FRR path using a direct neighbor

   When a direct neighbor is in P(S,X) and Q(D,x) and the link to that
   direct neighbor is on the post-convergence path, the outgoing
   interface is set to that neighbor and the repair segment list SHOULD
   be empty.

   This is comparable to a post-convergence LFA FRR repair.

6.2.  FRR path using a PQ node

   When a remote node R is in P(S,X) and Q(D,x) and on the post-
   convergence path, the repair list SHOULD be made of a single node
   segment to R and the outgoing interface SHOULD be set to the outgoing
   interface used to reach R.

   This is comparable to a post-convergence RLFA repair tunnel.

6.3.  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
   the post-convergence path and both are adjacent to each other, the
   repair list SHOULD be made of two segments: A node segment to P (to
   be processed first), followed by an adjacency segment from P to Q.

   This is comparable to a post-convergence DLFA (LFA with directed
   forwarding) repair tunnel.

6.4.  Connecting distant P and Q nodes along post-convergence paths

   In some cases, there is no adjacent P and Q node along the post-
   convergence path.  As mentioned in Section 4, 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.

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7.  Building TI-LFA repair lists

   The following sections describe how to build the repair lists using
   the terminology defined in [RFC8402].  The procedures described in
   this section are equally applicable to both SR-MPLS and SRv6
   dataplane, while the dataplane-specific considerations are described
   in Section 8.

   In this section, the process by which a protecting router S handles
   the active segment of a packet upon the failure of its primary
   outgoing interface for the packet, S-F, is explained.  The failure of
   the primary outgoing interface may occur due to various triggers,
   such as link failure, neighbor node failure, and others.

7.1.  The active segment is a node segment

   The active segment MUST be kept on the SR header unchanged and the
   repair list MUST 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 Section 8).

7.2.  The active segment is an adjacency segment

   The FRR behavior applied by S for any packet received with an active
   adjacency segment S-F, for which protection was enabled, is defined
   here.  Since protection has been enabled for the segment S-F and
   signaled in the IGP (for instance, using protocol extensions from
   [RFC8667] and [RFC8665]), 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.

   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” 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 [RFC8402].

   This method which merges back the traffic at the remote end of the
   adjacency segment has the advantage of keeping as much as possible
   the traffic on the pre-failure path.  When SR policies are involved
   and a strict compliance of the policy is required, an end-to-end
   protection should be preferred over a local repair mechanism.
   However, this method may not provide the expected post-convergence
   path to the final destination as the expected post-convergence path
   may not go through F.  Another method requires to look to the next
   segment in the segment list.

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   The case where this active segment is followed by another adjacency
   segment is distinguished from the case where it is followed by a node
   segment.

7.2.1.  Protecting [Adjacency, Adjacency] segment lists

   If the next segment in the list is an Adjacency segment, then the
   packet has to be conveyed to F.

   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
   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 [RFC8402].

   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
   path for destination F.

7.2.2.  Protecting [Adjacency, Node] segment lists

   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),...].

   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
   redirecting the traffic to a node Q, whose path to node segment T is
   not affected by the failure.

   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), ...].

8.  Dataplane specific considerations

8.1.  MPLS dataplane considerations

   MPLS dataplane for Segment Routing is described in [RFC8660].

   The following dataplane behaviors apply when creating a repair list
   using an MPLS dataplane:

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   1.  If the active segment is a node segment that has been signaled
       with penultimate hop popping and the repair list ends with an
       adjacency segment terminating on a node that advertised NEXT
       operation [RFC8402] of the active segment, then the active
       segment MUST be popped before pushing the repair list.

   2.  If the active segment is a node segment but the other conditions
       in 1. are not met, the active segment MUST be popped then pushed
       again with a label value computed according to the Segment
       Routing Global Block of Q, where Q is the endpoint of the repair
       list.  Finally, the repair list MUST be pushed.

8.2.  SRv6 dataplane considerations

   SRv6 dataplane and programming instructions are described
   respectively in [RFC8754] and [RFC8986].

   The TI-LFA path computation algorithm is the same as in the SR-MPLS
   dataplane.  Note however that the Adjacency SIDs are typically
   globally routed.  In such case, there is no need for preceding an
   adjacency SID with a Prefix-SID [RFC8402] and the resulting repair
   list is likely shorter.

   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 the headend behaviors as specified in section 5
   of [RFC8986].

   If the traffic is protected at an SR Segment Endpoint Node, first the
   Segment Endpoint packet processing is executed.  Then the packet is
   protected as if its were a transit packet.

9.  TI-LFA and SR algorithms

   SR allows an operator to bind an algorithm to a prefix-SID (as
   defined in [RFC8402].  The algorithm value dictates how the path to
   the prefix is computed.  The SR default algorithm is known has 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 be loop-free as a local
   policy may have overriden 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 situation can
   already happen today with existing mechanisms as remote LFA.

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   [RFC9350] defines a flexible algorithm (FlexAlgo) framework to be
   associated with Prefix-SIDs.  FlexAlgo allows a user to associate a
   constrained path to a Prefix-SID rather than using the regular IGP
   shortest path.  An implementation MAY support TI-LFA to protect Node-
   SIDs associated to a FlexAlgo.  In such a case, rather than computing
   the expected post-convergence path based on the regular SPF, an
   implementation SHOULD use the constrained SPF algorithm bound to the
   FlexAlgo (using the Flex Algo 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.  An implementation
   MUST only use Node-SIDs bound to the FlexAlgo and/or Adj-SIDs that
   are unprotected or, in case of SRv6, adj-SIDs that are bound to the
   FlexAlgo to build the repair list.

10.  Usage of Adjacency segments in the repair list

   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.

           S --- R2 --- R3 ---- R4 --- R5 --- D
                    *   |  \   *
                      * |   \ *
                       R7 ** R8
                        *    |
                        *    |
                       R9 -- R10

                                  Figure 2

   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
   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)] 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 Section 7.

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   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, this 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.

11.  Security Considerations

   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 local failure which can be seen as a
   modest security enhancement.  The protection mechanisms does not
   protect external destinations, but rather provides quick restoration
   for destination that are internal to a routing domain.

   Security considerations described in [RFC5286] and [RFC7490] apply to
   this document.  Similarly, as the solution described in the document
   is based on Segment Routing technology, reader should be aware of the
   security considerations related to this technology ([RFC8402]) and
   its dataplane instantiations ([RFC8660], [RFC8754] and [RFC8986]).
   However, this document does not introduce additional security
   concern.

12.  IANA Considerations

   No requirements for IANA

13.  Contributors

   In addition to the authors listed on the front page, the following
   co-authors have also contributed to this document:

   *  Francois Clad, Cisco Systems

   *  Pablo Camarillo, Cisco Systems

14.  Acknowledgments

   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.

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15.  References

15.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7916]  Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K.,
              Horneffer, M., and P. Sarkar, "Operational Management of
              Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916,
              July 2016, <https://www.rfc-editor.org/info/rfc7916>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

15.2.  Informative References

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   [I-D.bashandy-rtgwg-segment-routing-uloop]
              Bashandy, A., Filsfils, C., Litkowski, S., Decraene, B.,
              Francois, P., and P. Psenak, "Loop avoidance using Segment
              Routing", Work in Progress, Internet-Draft, draft-
              bashandy-rtgwg-segment-routing-uloop-16, 17 December 2023,
              <https://datatracker.ietf.org/doc/html/draft-bashandy-
              rtgwg-segment-routing-uloop-16>.

   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <https://www.rfc-editor.org/info/rfc5286>.

   [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework",
              RFC 5714, DOI 10.17487/RFC5714, January 2010,
              <https://www.rfc-editor.org/info/rfc5714>.

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, DOI 10.17487/RFC5715, January
              2010, <https://www.rfc-editor.org/info/rfc5715>.

   [RFC6571]  Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
              B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
              Alternate (LFA) Applicability in Service Provider (SP)
              Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
              <https://www.rfc-editor.org/info/rfc6571>.

   [RFC6976]  Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
              Francois, P., and O. Bonaventure, "Framework for Loop-Free
              Convergence Using the Ordered Forwarding Information Base
              (oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
              2013, <https://www.rfc-editor.org/info/rfc6976>.

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,
              <https://www.rfc-editor.org/info/rfc7490>.

   [RFC8333]  Litkowski, S., Decraene, B., Filsfils, C., and P.
              Francois, "Micro-loop Prevention by Introducing a Local
              Convergence Delay", RFC 8333, DOI 10.17487/RFC8333, March
              2018, <https://www.rfc-editor.org/info/rfc8333>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

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   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
              A., and P. Mattes, "Segment Routing Policy Architecture",
              RFC 9256, DOI 10.17487/RFC9256, July 2022,
              <https://www.rfc-editor.org/info/rfc9256>.

   [RFC9350]  Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
              and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
              DOI 10.17487/RFC9350, February 2023,
              <https://www.rfc-editor.org/info/rfc9350>.

Appendix A.  Advantages of using the expected post-convergence path
             during FRR

   [RFC7916] raised several operational considerations when using LFA or
   remote LFA.  [RFC7916] Section 3 presents a case where a high
   bandwidth link between two core routers is protected through a PE
   router connected with low bandwidth links.  In such a case,
   congestion may happen when the FRR backup path is activated.
   [RFC7916] introduces a local policy framework to let the operator
   tuning manually the best alternate election based on its own
   requirements.

   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
   Figure 3, we consider a network with all metrics equal to 1 except
   the metrics on links used by PE1, PE2 and PE3 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 policy works, however it is not fully
   accurate.

   In Figure 3, 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

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   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
   its nominal path reducing the quantity of traffic received by X.  The
   capacity planning rule presented previously has the drawback of
   oversizing the network, however it allows to prevent any transient
   congestion (when for example X reroutes traffic before PE1 does).

              H --- I --- J
              |           | \
   PE4        |           |  PE3
      \       | (L)       | /
        A --- X --- B --- G
       /      |           | \
    PE1       |           |  PE2
       \      |           | /
        C --- D --- E --- F

                                  Figure 3

   Based on this assumption, in order to facilitate the operation of
   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 perfectly inline with the capacity planning rule that was
   presented before and also inline with the fact 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.

   It should be 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.
   However it is important to note, that a transient congestion may
   possibly happen, even without FRR activated, for instance when X
   converges before the upstream routers.  Operators are still free to
   use the policy framework defined in [RFC7916] if the usage of the
   post-convergence paths rooted at the PLR is not suitable.

   Readers should be aware that FRR protection is pre-computing a backup
   path to protect against a particular type of failure (link, node,
   SRLG).  When using the post-convergence path as 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

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   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.

   Another consideration to take into account is: while using the
   expected post-convergence path for SR traffic using node segments
   only (for instance, PE to PE traffic using shortest path) has some
   advantages, these advantages reduce when SR policies ([RFC9256]) 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 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 protecting one
   segments in that segment list to obey those constraints.  When SR
   policies are used and the operator wants to have a backup path which
   still follows the policy requirements, this backup path should be
   computed as part of the SR policy in the ingress node (or central
   controller) and the SR policy should not rely on local protection.
   Another option could be to use FlexAlgo ([RFC9350]) to express the
   set of constraints and use a single node segment associated with a
   FlexAlgo to reach the destination.  When using a node segment
   associated with a FlexAlgo, TI-LFA keeps providing an optimal backup
   by applying the appropriate set of constraints.  The relationship
   between TI-LFA and the SR-algorithm is detailed in Section 9.

Appendix B.  Analysis based on real network topologies

   This section presents 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 instantiating
   segment routing over the MPLS forwarding plane.

   The measurement below indicate that for link and local SRLG
   protection, a 1 SID repair path delivers more than 99% coverage.  For
   node protection a 2 SIDs repair path yields 99% coverage.

   Table 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:

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   *  For each network, the algorithms described in this document are
      applied to protect all prefixes against link, node, and local SRLG
      failure

   *  For each prefix, the number of SIDs used by the repair path is
      recorded

   *  The percentage of number of SIDs are listed in Tables 2A/B, 3A/B,
      and 4A/B

   The measurements listed in the tables indicate that for link and
   local SRLG protection, 1 SID repair path is sufficient to protect
   more than 99% of the prefix in almost all cases.  For node protection
   2 SIDs repair paths yield 99% coverage.

   +-------------+------------+------------+------------+------------+
   |   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 Set Definition

   The rest of this section presents the measurements done on the actual
   topologies.  The convention that we use is as follows

   *  0 SIDs: the calculated repair path starts with a directly
      connected neighbor that is also a loop free alternate, in which
      case there is no need to explicitly route the traffic using
      additional SIDs.  This scenario is described in Section 6.1.

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   *  1 SIDs: 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
      Section 6.2.

   *  2 or more SIDs: The repair path consists of 2 or more SIDs as
      described in Section 6.3 and Section 6.4.  We do not cover the
      case for 2 SIDs (Section 6.3) separately because there was no
      granularity in 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 point of view.

   Table 2A and 2B below summarize the measurements on the number of
   SIDs needed for link protection

   +-------------+------------+------------+------------+------------+
   |   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%   |
   +-------------+------------+------------+------------+------------+

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   |    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 3A and 3B summarize the measurements on the number of SIDs
   needed for local SRLG protection.

   +-------------+------------+------------+------------+------------+
   |   Network   |    0 SIDs  |    1 SID   |   2 SIDs   |   3 SIDs   |
   +-------------+------------+------------+------------+------------+
   |    T1       |  74.2%     |   25.3%    |   0.5%     |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T2       |                No SRLG Information                |
   +-------------+------------+------------+------------+------------+
   |    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       |                No SRLG Information                |
   +-------------+---------------------------------------------------+
   |    T7       |                No SRLG Information                |
   +-------------+------------+------------+------------+------------+
   |    T8       |  85.2%     |   14.8%    |   0.0%     |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T9       |  98,9%     |    1.1%    |   0.0%     |   0.0%     |
   +-------------+------------+------------+------------+------------+
         Table 3A: Local SRLG protection repair size distribution

   +-------------+------------+------------+------------+------------+
   |   Network   |    0 SIDs  |    1 SID   |   2 SIDs   |   3 SIDs   |
   +-------------+------------+------------+------------+------------+
   |    T1       |  74.2%     |   99.5%    |  99.9%     | 100.0%     |
   +-------------+------------+------------+------------+------------+
   |    T2       |                No SRLG Information                |
   +-------------+------------+------------+------------+------------+
   |    T3       |  93.6%     |   99.9%    | 100.0%     |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T4       |  62.5%     |   98.2%    | 100.0%     | 100.0%     |
   +-------------+------------+------------+------------+------------+

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   |    T5       |  83.1%     |  100.0%    | 100.0%     | 100.0%     |
   +-------------+------------+------------+------------+------------+
   |    T6       |                No SRLG Information                |
   +-------------+---------------------------------------------------+
   |    T7       |                No SRLG Information                |
   +-------------+------------+------------+------------+------------+
   |    T8       |  85.2%     |   100.0%   | 100.0%     | 100.0%     |
   +-------------+------------+------------+------------+------------+
   |    T9       |  98.9%     |   100.0%   | 100.0%     | 100.0%     |
   +-------------+------------+------------+------------+------------+
    Table 3B: Local SRLG protection repair size Cumulative distribution
   The remaining two tables summarize the measurements on the number of
   SIDs needed for node protection.

   +---------+----------+----------+----------+----------+----------+
   | Network |  0 SIDs  |   1 SID  | 2 SIDs   |  3 SIDs  |  4 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: Node protection (repair size distribution)

   +---------+----------+----------+----------+----------+----------+
   | Network |  0 SIDs  |   1 SID  | 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%    |
   +---------+----------+----------+----------+----------+----------+

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   |    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)

Authors' Addresses

   Ahmed Bashandy
   Cisco Systems
   Email: abashandy.ietf@gmail.com

   Stephane Litkowski
   Cisco Systems
   France
   Email: slitkows@cisco.com

   Clarence Filsfils
   Cisco Systems
   Brussels
   Belgium
   Email: cfilsfil@cisco.com

   Pierre Francois
   INSA Lyon
   Email: pierre.francois@insa-lyon.fr

   Bruno Decraene
   Orange
   Issy-les-Moulineaux
   France
   Email: bruno.decraene@orange.com

   Daniel Voyer
   Bell Canada
   Canada
   Email: daniel.voyer@bell.ca

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