Subject: TI-LFA

From: Stewart Bryant <[email-masked]>
Date: Sat, 6 Nov 2021 17:34:19 +0000

As I noted earlier today, I took a second detailed look at this text.

I have concerns about the document and think that it could use a more considered, detailed and constructive dialogue between the authors and the WG or at least the members of the WG who understand IPFRR.

One particular concern that I have is that the document seems to start out as a document describing the repair of a regular IP network using SR then abruptly changes to a document on using SR to repair SR.

Also the document starts out setting out the general case for TI-LFA repairs, which includes the general case of SRLG, but does not describe SRLG repair and then talks about a special (undefined) type of SRLG repair in the results table.

I think that the community would be better served by two documents - a document describing the use of SR to repair a regular network and then a document describing the applicability of those techniques to the special case of an SR network.

My comments are below. (Marked SB>)

Best regards

- Stewart

On the 8th August Sasha Vainshtein raised the following point concerning this draft:

https://mailarchive.ietf.org/arch/msg/rtgwg/eHEvqzniwNpGFV7JNTQ1YCYy9jA/ <https://mailarchive.ietf.org/arch/msg/rtgwg/eHEvqzniwNpGFV7JNTQ1YCYy9jA/>

However I have not seen an answer. I think the point needs to be either shown to be invalid, or appropriate text needs to be added to the draft.

========
        Topology Independent Fast Reroute using Segment Routing
               draft-ietf-rtgwg-segment-routing-ti-lfa-07

Abstract

It extends these concepts to provide guaranteed coverage in
   any IGP network.  
SB> Strictly that should be 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, dramatically reducing the
   operational need to control the tie-breaks among various FRR options.
SB> If you are going to say "dramatically reducing” then I think there needs to be text in the body of the document justifying this and quantifying “dramatically”, although I am not a fan of the emotive term “dramatic” in a formal engineering text.
=========

1.  Acronyms

   o  DLFA: Remote LFA with Directed forwarding.

   o  FRR: Fast Re-route.

   o  IGP: Interior Gateway Protocol.

   o  LFA: Loop-Free Alternate.

   o  LSDB: Link State DataBase.

   o  PLR: Point of Local Repair.

   o  RL: Repair list.

   o  RLFA: Remote LFA.

   o  SID: Segment Identifier.

   o  SLA: Service Level Agreement.

   o  SPF: Shortest Path First.

   o  SPT: Shortest Path Tree.

   o  SR: Segment Routing.

   o  SRGB: Segment Routing Global Block.

   o  SRLG: Shared Risk Link Group.

   o  TI-LFA: Topology Independant LFA.

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 IGP shortest path capable of restoring end-to-
SB> I think that needs to be "link-state IGP". The method would not work in a RIP network.
   end connectivity in the case of a sudden directly connected failure
   of a network component.  
SB> In a two-connected network of course.

Non-SR mechanisms for local repair are

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   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].
SB> If you are going to evaluate against the other documents published by the IETF, you ought to also compare to RFC7812 and RFC6981 which could provide complete protection, and of course in MPLS the much deployed RSVP-TE tunnel protection method provided complete protection. Then of course there is draft-bryant-rtgwg-plfa-01 which can achieve the same result in non-SR networks including IPv4, and then there is the very first compete coverage IP protection scheme draft-bryant-ipfrr-tunnels-03 which was co-authored by one of the TI-LFA authors.

===========

   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.  
SB> I looked for the text and I cannot see where you deal with node failure. Since the TI-LFA path that is node avoiding is different from the TI-LFA path that is simply link avoiding. Same for SRLG.
SB> Also it is an advertised property of TI-LFA that the repair path is the same as the post convergence path and hence the result is loop-free. However if the failure is less severe than you are protecting against that property does not remain. This needs to be considered in the text.

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 routing domain.
SB> I think that should be a single link state routing domain. Maybe actually stricter, a single link state area?
   Protecting domain exit routers and/or links attached to another
   routing domains are beyond the scope of this document

   Thanks to 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.

   Thanks to 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

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   tuning manually the best alternate election based on its own
   requirements.
SB> There needs to be some follow on text saying how TI-LFA knows not to do this.

   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). 
SB> An important point which is skated over in TI-LFA is that this assumption is not always valid
   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 only 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. 
SB> I am concerned about the previous text. This is an IPFRR text and so X WILL re-route i.e. repair its traffic before PE1 even learns of the X-B failure.
   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 1

   Based on this assumption, in order to facilitate the operation of
   FRR, and limit the implementation of local FRR policies, it looks
   interesting to steer the traffic onto the post-convergence path from
   the PLR point of view during the FRR phase.  In our example, when

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   link L fails, X switches the traffic destined to PE3 and PE2 on the
   post-convergence paths.  
SB> There is an important point here that we need to check. The implication in the above text that you ECMP into the repair paths. We need to check that this is considered later.
   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.
SB> I am not sure why we consider convergence for capacity planning here. In any IPFRR design we have to consider that X will redistribute the traffic onto the repair path before any another node acts.

   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
   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.
SB> It is surely more than a matter of optimisation. The loop-free strategy is also compromised.

   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
   ([I-D.ietf-spring-segment-routing-policy]) 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 ([I-D.ietf-lsr-flex-algo]) to

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   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 7.

========
   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, in an MPLS world, this may create a
   long stack of labels to be pushed that some hardware may not be able
   to push.  
SB> Surely not just in the MPLS world?

   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 path
   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 4 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.

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   Using the properties defined in Section 4, Section 5 describes how to
   compute protection lists that encode a loop-free post-convergence
   path towards the destination.

   Section 6 defines the segment operations to be applied by the PLR to
   ensure consistency with the forwarding state of the repair node.

   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 9
   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 node R w.r.t. a resource X (e.g. a link S-F,
   a node F, or a SRLG) is the set of nodes that are reachable from R
   without passing through X.  It is the set of nodes that are not
   downstream of X in SPT_old(R).
SB> This does not look right
SB> Here is the RFC7490 definition of P-space
   P-space:
      The P-space of a router with respect to a protected link is the
      set of routers reachable from that specific router using the pre-
      convergence shortest paths without any of those paths (including
      equal-cost path splits) transiting that protected link.

SB> I think you need the whole definition but with s/link/resource/ because ECMP is something that has tripped up many FRR schemes.
SB> However if you have an ECMP to the destination from R then that node is in P-space so the second part of the definition seems wrong.
SB> Unless you have a good reason to change the definition beyond the generalisation above I think you should use the original without what looks like an incorrect clarification.

   The Extended P-Space P'(R,X) of a node R w.r.t. a resource X is the
   set of nodes that are reachable from R or a neighbor of R, without
   passing through X.

SB> Again I think you need to use the RFC7490 definition
     The extended P-space of the protecting router
      with respect to the protected link is the union of the P-spaces of
      the neighbors in that set of neighbors with respect to the
      protected link (see Section 5.2.1.2 <https://datatracker.ietf.org/doc/html/rfc7490#section-5.2.1.2>).

SB> With the substitution of protected resource for protected link. The important difference is that extended P space includes nodes that are not ordinarily reachable from R and need to be forced over the first hop in contravention of the prefailure SPT.

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   The Q-Space Q(D,X) of a destination node D w.r.t. a resource X is the
   set of nodes which do not use X to reach D in the initial state of
   the network.  
SB> Again I would be happier with a generalisation of the much thought about RFC7490 definition
   Q-space:
      The Q-space of a router with respect to a protected link is the
      set of routers from which that specific router can be reached
      without any path (including equal-cost path splits) transiting
      that protected link.

SB> Again with the simple substitution of protected resource for protected link. Again it is important to worry about ECMP.

   In other words, it is the set of nodes which have D in
   their P-Space w.r.t.  S-F, F, or a set of links adjacent to S).
SB> That does not look quite right. I think that it is “the set of links”, but I am worried that the example is not as precise as the base definition.
Perhaps:
   In the example shown in Figure 1 it is the set of nodes which have D in
   their P-Space w.r.t. any of S’s links, F in the case of node protection, and the SRLG that includes S-F in the case of SRLG protectio

   A symmetric network is a network such that the IGP metric of each
   link is the same in both directions of the link.
SB> TI-LFA could work with asymmetric metrics. So we really need to think about how asymmetry is discussed in the text.

4.  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 5 to compute the TI-LFA repair list.

4.1.  P-Space property computation for a resource X

   A node N is in P(R, X) if it is not downstream of X in SPT_old(R).  
SB> I think that needs to be “a downstream neighbour of X” a node that is further away can surely be downstream in the SPT_old(R).
SB> I looked for a definition of downstream in the previous work that would have avoided that ambiguity but did not see it,
   X
   can be a link, a node, or a set of links adjacent to the PLR.  A node
   N is in P'(R,X) if it is not downstream of X in SPT_old(N), for at
   least one neighbor N of R.
SB> I find the above hard to parse do you mean:
SB> if it is not downstream of X in SPT_old(N), for any neighbor N of R.
SB> However I think we would be better pointing the reader to RFC7490 and saying that they should calculate the P space as per RFC7490 * by additionally excluding nodes reachable by the protected resource X where X is other than the next hop link from R to D.

* thus dar we got link failure, so the rest of the sentence deals with the other types of resource.

SB> However I am wondering why we need to go though this step in the explanation.

SB> What you want to end up with is the Ti-LFA P space, call it the TP space. Which is the set of nodes N that are in the P-space of R wrt X where the path to N, including ECMP, is congruent with the path to N in the post failure SPT.

SB> Is it necessary to say any more at this stage of the text?

4.2.  Q-Space property computation for a link S-F, over post-convergence
      paths

   We want to determine which nodes on the post-convergence path from
   the PLR to the destination D are in the Q-Space of destination D
   w.r.t. link S-F.

   This can be found by intersecting the post-convergence path to D,
   assuming the failure of S-F, with Q(D, S-F).
SB> I am not sure what you mean by “intersecting”
SB> However as this is just link failure why do we not just say do what  RFC7490 does?
SB> Again as this is Ti-LFA we can usefully note that these paths will be congruent with the paths computed in the new SPT.

4.3.  Q-Space property computation for a set of links adjacent to S,
      over post-convergence paths

   We want to determine which nodes on the post-convergence path from
   the PLR to the destination D are in the Q-Space of destination D
   w.r.t. a set of links adjacent to S (S being the PLR).  That is, we
   aim to find the set of nodes on the post-convergence path that use
   none of the members of the protected set of links, to reach D.
SB> Isn’t that by definition true of any node that is in the Q space of R wrt X?

   This can be found by intersecting the post-convergence path to D,
   assuming the failure of the set of links, with the intersection among
   Q(D, S->X) for all S->X belonging to the set of links.
SB> Again I (nor I suspect will many readers) do not understand the term intersecting.

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4.4.  Q-Space property computation for a node F, over post-convergence
      paths

   We want to determine which nodes on the post-convergence from the PLR
   to the destination D are in the Q-Space of destination D w.r.t. node
   F.

   This can be found by intersecting the post-convergence path to D,
   assuming the failure of F, with Q(D, F).
SB> Isn’t the notation the failure of X in the earlier text. However for the moment I am at a lost to understand why the RFC7490 Q space is not always congruent post convergence paths.
SB> Is there something subtle to do with the fragmentation of the paths that discontiguous SRLG paths may produce in the general case?
SB> In any case I think there needs to be text in the discontiguous SRLG case because it could produce some very interesting corner cases unless dealt with correctly. Maybe you should actually exclude that case from Ti-LFA?

4.5.  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 SPT 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.
SB> We have not introduced the reader to the term “reverse SPT” yet.

5.  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.  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. 
SB> Guarantee is absolute, and I don’t think you can make that assumption other than for the failure of a single protected link.
SB> If you are node of SRLG protecting and the failure is a simple link failure repair path and the post failure path may not be congruent. Now you would be OK if you only used adjacency segments, but as soon as you use loose SR to reduce the the size of the segment list I think you risk a micro-loop problem. Is there a mathematical proof that this would not be the case?
   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)).
SB> OK so now I see what you mean by intersecting, but I still think you should explain this to the general reader.

   Most often, the TI-LFA repair list has a simpler form, as described
   in the following sections.  Section 9 provides statistics for the
   number of SIDs in the explicit path to protect against various
   failures.

5.1.  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.
SB> Why do we need to talk about the segment list at all, by not say use an LFA repair?

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

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5.2.  FRR path using a PQ node

   When a remote node R is in P(S,X) and Q(D,x) and on the post-
   convergence path, the repair list MUST 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.
SB> Given your earlier discussion about congestion management should you not talk about ECMP here rather than a single path?

5.3.  FRR path using a P node and Q node that are adjacent

   When a node P is in P(S,X) and a node Q is in Q(D,x) and both are on
   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 repair tunnel.
SB> I do not think we have yet pointed the reader to a definition of a DLFA repair tunnel have we?
SB> The closest I can find with google is draft-bryant-ipfrr-tunnels-00.txt
 <https://datatracker.ietf.org/doc/html/draft-bryant-ipfrr-tunnels-00.txt>

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

   In some cases, there is no adjacent P and Q node along the post-
   convergence path.  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.
SB> That seems to me to be a big omission in one of the major selling points of TI-LFA that it can repair any failure.
SB> You should probably say that you do this with a full set of adjacency SIDs, but optimisations may be possible. Such optimisations are outside the scope of this text. However I think that leaves the reader short of the headline claims for this approach.

6.  Building TI-LFA repair lists
SB> Until this point in the draft everything is applicable to the use of TI-LFA to repair regular IP/MPLS as well as SR.
SB> I think that this should be called out in the draft.
SB> Then I think there needs to be a discussion of what it means to do SR FRR. For example whether the goal is to simply get the packet to its destination or to get the packet to the next segment endpoint. There are both design and operational implications with the decision and these need to be called out so that the reader fully understand the issues.
SB> Arguably repairing to the segment endpoint is what should be done, on the basis that the segments are created for a reason that the FRR engine is not privy to. In this case the solution for the use of TI-LFA to repair non SR and SR is identical.
SB> There is another point which is moot but perhaps ought to be called out to reassure the reader and that is that the post convergence SR path may not be the same as the pre-convergence SR path. I do not think that this will cause micro loops because the packets travel in the underlay rather than the SR overlay, but it has implications for traffic engineering and order of packet arrival which may be important in some service overlays such as DetNet.

   The following sections describe how to build the repair lists using
   the terminology defined in [RFC8402].  The procedures described in
   Section 6.1 are equally applicable to both SR-MPLS and SRv6
   dataplane, while the dataplane-specific considerations are described
   in Section 6.2.

6.1.  Link protection

   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.

6.1.1.  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 6.2).
SB> The active segment becomes the first segment of the repair list. - I do not understand this. Surely it is the last segment on the repair list in order to eventually deliver the packet back to the SR path?

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6.1.2.  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
   signalled in the IGP, any SR policy using this segment knows that it
   may be transiently rerouted out of S-F in case of S-F failure.
SB> You have introduced this control plane concept without reference to the control plane. I think this needs a reference.

   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 two "PUSH" operations are performed.  
SB> I assume that this limit is because of the symmetric cost constraint. It would be useful to advise the reader.
   If the repair list,
   while avoiding S-F, terminates on F, S only pushes the repair list.
   Otherwise, S pushes a node segment of F, followed by by push 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. 
SB> It also constrains the packet to the SR segment set which may be important for other reasons.
    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.

6.1.2.1.  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(S-F) and then one or
   two "PUSH" operations.  If the repair list, while avoiding S-F,
   terminates on F, S only pushes the repair list.  Otherwise, S pushes
   a node segment of F, followed by push of the repair list..  For
   details on the "NEXT" and "PUSH" operations, refer to [RFC8402].
SB> Please can you explain why you need the two approaches. An adj to F was ok in the original packet why do we need the node?

   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.

SB>[RL(F),node(F),adj(F-M),. . .] to match above.

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6.1.2.2.  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), …].
SB> So what about SRLG that was discussed earlier in the draft?
SB> Is this the entirety of the SR cases?

6.2.  Dataplane specific considerations

6.2.1.  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.
SB> Do you have to PHP the repair list? Do you always PHP the repair list? I do not see this documented.

   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.
SB> A couple of figures would be useful

6.2.2.  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

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

SB> This could usefully include some example using the RFC8986 notation.

   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.

7.  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.

   [I-D.ietf-lsr-flex-algo] 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.
================
8.  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.

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           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 6.

   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.

SB> I am worried about this text because it talks about one specific case and it is not clear how this works. I assume that there is some unreferenced IPG extension, but I am worried how well this scales in practise. In the preceding works on IPFRR there has been an assumption that protecting against a single failure is OK, but protecting against a multiple failure simply does not scale. The normal practise is to abandon all hope (AAH) i.e. abort the repair if there are two or more failures.

SB> I think that there needs to be a much more comprehensive discussion of multiple failures if an approach beyond AAH is to be described.

9.  Measurements on Real Networks

   This section presents measurements performed on real service provider
   and large enterprise networks.  
SB> Were these measurements? I thought this was a simulation of the algorithms on the LSP DB from these networks.

The objective of the measurements 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.

SB> Now there an important note to make here. These tables were in the document back in the days when it was describing the use of SR to protect an IP network. The document has been changed to be a description of how to use SR to protect an SR network. I think that the networks are regular IP networks but if not that should be clarified. As a method of protecting a segment I *think* that this is likely to be pessimistic, but without running the tests on some SR networks that is an assumption.
   The measurements below indicate that for link and local SRLG
   protection, 
SB> What is a local SRLG?
   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:

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

   o  For each prefix, the number of SIDs used by the repair path is
      recored

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   o  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 paths are 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

   o  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 5.1.

   o  1 SIDs: the repair node is a PQ node, in which case only 1 SID is
      needed to guarantee loop-freeness.  This scenario is covered in
      Section 5.2.

   o  2 or more SIDs: The repair path consists of 2 or more SIDs as
      described in Sections 4.3 and 4.4.  We do not cover the case for 2
      SIDs (Section 5.3) separately because there was no granularity in
      the result.  Also we treat the node-SID+adj-SID and node-SID +

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

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

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

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   |    T9   |  98.9%   | 100.0%   | 100.0%   | 100.0%   |  100%    |
   +---------+----------+----------+----------+----------+----------+
      Table 4B: Node protection (repair size cumulative distribution)
SB> What are the stats for node protection in the presence of SRLGs?

10.  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.

11.  IANA Considerations

   No requirements for IANA

12.  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

13.  Acknowledgments

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

14.  References

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14.1.  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>.

14.2.  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", draft-bashandy-rtgwg-segment-routing-uloop-10
              (work in progress), December 2020.

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   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
              algo-15 (work in progress), April 2021.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-11 (work in progress),
              April 2021.

   [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>.

Authors' Addresses

   Stephane Litkowski
   Cisco Systems
   France

   Email: [email-masked]

   Ahmed Bashandy
   Individual

   Email: [email-masked]

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   Clarence Filsfils
   Cisco Systems
   Brussels
   Belgium

   Email: [email-masked]

   Pierre Francois
   INSA Lyon

   Email: [email-masked]

   Bruno Decraene
   Orange
   Issy-les-Moulineaux
   France

   Email: [email-masked]

   Daniel Voyer
   Bell Canada
   Canada

   Email: [email-masked]

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