Topology Independent Fast Reroute using Segment Routing
draft-ietf-rtgwg-segment-routing-ti-lfa-10

Network Working Group                                       S. Litkowski
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                             A. Bashandy
Expires: 29 October 2023                                      Individual
                                                             C. Filsfils
                                                           Cisco Systems
                                                             P. Francois
                                                               INSA Lyon
                                                             B. Decraene
                                                                  Orange
                                                                D. Voyer
                                                             Bell Canada
                                                           27 April 2023

        Topology Independent Fast Reroute using Segment Routing
               draft-ietf-rtgwg-segment-routing-ti-lfa-10

Abstract

   This document presents Topology Independent Loop-free Alternate Fast
   Re-route (TI-LFA), aimed at providing protection of node and
   adjacency segments within the Segment Routing (SR) framework.  This
   Fast Re-route (FRR) behavior builds on proven IP-FRR concepts being
   LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding
   (DLFA).  It extends these concepts to provide guaranteed coverage in
   any two connected network using a link-state IGP.  A key aspect of
   TI-LFA is the FRR path selection approach establishing protection
   over the expected post-convergence paths from the point of local
   repair, reducing the operational need to control the tie-breaks among
   various FRR options.

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   This Internet-Draft will expire on 29 October 2023.

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Copyright Notice

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Table of Contents

   1.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Conventions used in this document . . . . . . . . . . . .   9
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Base principle  . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Intersecting P-Space and Q-Space with post-convergence
           paths . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Extended P-Space property computation for a resource X,
           over post-convergence paths . . . . . . . . . . . . . . .  10
     5.2.  Q-Space property computation for a resource X, over
           post-convergence paths  . . . . . . . . . . . . . . . . .  11
     5.3.  Scaling considerations when computing Q-Space . . . . . .  11
   6.  TI-LFA Repair path  . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  FRR path using a direct neighbor  . . . . . . . . . . . .  11
     6.2.  FRR path using a PQ node  . . . . . . . . . . . . . . . .  12
     6.3.  FRR path using a P node and Q node that are adjacent  . .  12
     6.4.  Connecting distant P and Q nodes along post-convergence
           paths . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Building TI-LFA repair lists  . . . . . . . . . . . . . . . .  12
     7.1.  The active segment is a node segment  . . . . . . . . . .  13
     7.2.  The active segment is an adjacency segment  . . . . . . .  13
       7.2.1.  Protecting [Adjacency, Adjacency] segment lists . . .  13
       7.2.2.  Protecting [Adjacency, Node] segment lists  . . . . .  14
   8.  Dataplane specific considerations . . . . . . . . . . . . . .  14
     8.1.  MPLS dataplane considerations . . . . . . . . . . . . . .  14
     8.2.  SRv6 dataplane considerations . . . . . . . . . . . . . .  15
   9.  TI-LFA and SR algorithms  . . . . . . . . . . . . . . . . . .  15
   10. Usage of Adjacency segments in the repair list  . . . . . . .  16
   11. Analysis based on real network topologies . . . . . . . . . .  16
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   14. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  22

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   15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  22
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     16.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

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.

   *  RSPT: Reverse Shortest Path Tree.

   *  SID: Segment Identifier.

   *  SLA: Service Level Agreement.

   *  SPF: Shortest Path First.

   *  SPT: Shortest Path Tree.

   *  SR: Segment Routing.

   *  SRGB: Segment Routing Global Block.

   *  SRLG: Shared Risk Link Group.

   *  TI-LFA: Topology Independant LFA.

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2.  Introduction

   Segment Routing aims at supporting services with tight SLA guarantees
   [RFC8402].  By relying on SR this document provides a local repair
   mechanism for standard link-state IGP shortest path capable of
   restoring end-to-end connectivity in the case of a sudden directly
   connected failure of a network component.  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].

   The term topology independent (TI) refers to the ability to provide a
   loop free backup path irrespective of the topologies used in the
   network.  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].

   For each destination 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 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 using SR, TI-LFA does not require the establishment of TLDP
   sessions with remote nodes in order to take advantage of the
   applicability of remote LFAs (RLFA) [RFC7490][RFC7916] or remote LFAs
   with directed forwarding (DLFA)[RFC5714].  All the Segment
   Identifiers (SIDs) are available in the link state database (LSDB) of
   the IGP.  As a result, preferring LFAs over RLFAs or DLFAs, as well
   as minimizing the number of RLFA or DLFA repair nodes is not required
   anymore.

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   By using SR, there is no need to create state in the network in order
   to enforce an explicit FRR path.  This relieves the nodes themselves
   from having to maintain extra state, and it relieves the operator
   from having to deploy an extra protocol or extra protocol sessions
   just to enhance the protection coverage.

   [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
   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 1, 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 1, considering that the source of traffic is only from PE1
   and PE4, when the link L fails, depending on the convergence speed of
   the nodes, X may reroute its forwarding entries to the remote PEs
   onto X-H or X-D; 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).

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

                                  Figure 1

   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.

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   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
   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
   the 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.

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   Thanks to SR and the combination of Adjacency segments and Node
   segments, the expression of the expected post-convergence path rooted
   at the PLR is facilitated and does not create any additional state on
   intermediate nodes.  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 SID list that
   some hardware may not be able to push.  One of the challenges of TI-
   LFA is to encode the expected post-convergence path by combining
   adjacency segments and node segments.  Each implementation will be
   free to have its own SID list compression optimization algorithm.
   This document details the basic concepts that could be used to build
   the SR backup path as well as the associated dataplane procedures.

                                    L     ____
                                 S----F--{____}----D
                                /\    |          /
                               |  |   | _______ /
                               |__}---Q{_______}

                        Figure 2: TI-LFA Protection

   We use Figure 2 to illustrate the TI-LFA approach.

   The Point of Local Repair (PLR), S, needs to find a node Q (a repair
   node) that is capable of safely forwarding the traffic to a
   destination D affected by the failure of the protected link L, a set
   of links including L (SRLG), or the node F itself.  The PLR also
   needs to find a way to reach Q without being affected by the
   convergence state of the nodes over the paths it wants to use to
   reach Q: the PLR needs a loop-free path to reach Q.

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

   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.

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   By applying the algorithms specified in this document to actual
   service providers and large enterprise networks, we provide real life
   measurements for the number of SIDs used by repair paths.  Section 11
   summarizes these measurements.

2.1.  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.

3.  Terminology

   We define the main notations used in this document as the following.

   We refer to "old" and "new" topologies as the LSDB state before and
   after the considered failure.

   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], we use the concept of P-Space and Q-Space 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.

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

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   A symmetric network is a network such that the IGP metric of each
   link is the same in both directions of the link.

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 SID list.  One way to provide a loop-free SID 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 SID 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 SID 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-based 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

   We want 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 (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:

   *  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).

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

   We want to determine which nodes on the post-convergence path from
   the PLR R to the destination D are in the Q-Space of destination D
   with regard to resource X (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 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.  Thus, there is no need for any co-ordination or
   message exchange between the PLR and any other router in the network.

   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 P to Q 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)).

   Most often, the TI-LFA repair list has a simpler form, as described
   in the following sections.  Section 11 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 on the post-
   convergence path, the outgoing interface is set to that neighbor and
   the repair segment list SHOULD be empty.

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   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.

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, we explain how a protecting router S processes 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 may happen due to different triggers (e.g.: link failure,
   neighbor node failure...)

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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 of 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

   We define hereafter the FRR behavior applied by S for any packet
   received with an active adjacency segment S-F for which protection
   was enabled.  As protection has been enabled for the segment S-F and
   signaled in the IGP (for instance using protocol extensions from
   [RFC8667] and [RFC8665]), any SR policy using this segment knows that
   it may be transiently rerouted out of S-F in case of 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.  As stated in Section 2, 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.

   We distinguish the case where this active segment is followed by
   another adjacency segment 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.

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   To do so, S MUST apply a "NEXT" operation on Adj(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(S-F),adj(F-M),...] will thus leave S with a segment list
   matching [RL(F),node(F),adj(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(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(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:

   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 the tail-end 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 SRGB of Q,
       where Q is the endpoint of the repair list.  Finally, the repair
       list MUST be pushed.

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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 a preceding
   Prefix SID 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.

   [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 to build the repair list.

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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 3

   In Figure 3, 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(R7-R8),adj(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(R7-R8), R7 will push an additional repair list onto the packet
   following the procedures defined in Section 7.

   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.  Implementations of TI-LFA should deal with the
   occurence of multiple unrelated failures in accordance to the IP Fast
   Reroute Framework [RFC5714].

11.  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.

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   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:

   *  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

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   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.

   *  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   |
   +-------------+------------+------------+------------+------------+

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   |    T1       |  74.2%     |   99.5%    |    99.9%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T2       |  81.1%     |   99.8%    |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T3       |  95.9%     |   99.9%    |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T4       |  62.5%     |   98.2%    |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T5       |  85.7%     |  100.0%    |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T6       |  81.2%     |   99.9%    |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T7       |  98,8%     |  100.0%    |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T8       |  94,1%     |  100.0%    |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T9       |  98,9%     |  100.0%    |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
       Table 2B: Link protection repair size cumulative distribution
   Table 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   |
   +-------------+------------+------------+------------+------------+

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

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   |    T1   |  49.7%   |  97.6%   |  99.8%   | 99.9%    |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T2   |  36.5%   |  96.1%   |  99.7%   | 99.9%    |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T3   |  73.3%   |  98.9%   |  99.9%   | 100.0%   |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T4   |  36.1%   |  93.4%   |  99.8%   | 99.9%    |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T5   |  73.2%   | 100.0%   | 100.0%   | 100.0%   |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T6   |  78.4%   | 99.7%    | 100.0%   | 100.0%   |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T7   |  66.1%   | 98.9%    | 100.0%   | 100.0%   |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T8   |  59.7%   | 100.0%   | 100.0%   | 100.0%   |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T9   |  98.9%   | 100.0%   | 100.0%   | 100.0%   |  100%    |
   +---------+----------+----------+----------+----------+----------+
      Table 4B: Node protection (repair size cumulative distribution)

12.  Security Considerations

   The techniques described in this document are internal
   functionalities to a router that result in the ability to 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.

13.  IANA Considerations

   No requirements for IANA

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14.  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

15.  Acknowledgments

   We would like to thank Les Ginsberg, Stewart Bryant, Alexander
   Vainsthein, Chris Bowers, Shraddha Hedge for their valuable comments.

16.  References

16.1.  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>.

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   [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>.

16.2.  Informative References

   [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-14, 18 December 2022,
              <https://datatracker.ietf.org/doc/html/draft-bashandy-
              rtgwg-segment-routing-uloop-14>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

Litkowski, et al.        Expires 29 October 2023               [Page 23]
Internet-Draft                  SR TI-LFA                     April 2023

   [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>.

Authors' Addresses

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

   Ahmed Bashandy
   Individual
   Email: abashandy.ietf@gmail.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

Litkowski, et al.        Expires 29 October 2023               [Page 24]