Topology Independent Fast Reroute using Segment Routing
draft-ietf-rtgwg-segment-routing-ti-lfa-02

Network Working Group                                      S. Litkowski
Internet-Draft                                                    Cisco
Intended status: Standards Track                            A. Bashandy
Expires: July 2020                                           Individual
                                                            C. Filsfils
                                                          Cisco Systems
                                                            B. Decraene
                                                                 Orange
                                                             P. Francois
                                                               INSA Lyon
                                                                D. Voyer
                                                             Bell Canada
                                                                 F. Clad
                                                            P. Camarillo
                                                           Cisco Systems
                                                       January 18, 2020

          Topology Independent Fast Reroute using Segment Routing
                draft-ietf-rtgwg-segment-routing-ti-lfa-02

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 IGP network.  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.

Status of This Memo

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   This Internet-Draft will expire on July 18, 2020.

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Table of Contents

   1. Introduction...................................................3
      1.1. Conventions used in this document.........................7
   2. Terminology....................................................7
   3. Intersecting P-Space and Q-Space with post-convergence paths...8
      3.1. P-Space property computation for a resource X.............8
      3.2. Q-Space property computation for a link S-F, over post-
      convergence paths..............................................8
      3.3. Q-Space property computation for a set of links adjacent to
      S, over post-convergence paths.................................9
      3.4. Q-Space property computation for a node F, over post-
      convergence paths..............................................9
      3.5. Scaling considerations when computing Q-Space.............9
   4. TI-LFA Repair Tunnel...........................................9
      4.1. FRR path using a direct neighbor.........................10
      4.2. FRR path using a PQ node.................................10
      4.3. FRR path using a P node and Q node that are adjacent.....10
      4.4. Connecting distant P and Q nodes along post-convergence paths
      ..............................................................10
   5. Protecting segments...........................................10
      5.1. The active segment is a node segment.....................11
      5.2. The active segment is an adjacency segment...............11
         5.2.1. Protecting [Adjacency, Adjacency] segment lists.....11
         5.2.2. Protecting [Adjacency, Node] segment lists..........12
      5.3. Protecting SR policy midpoints against node failure......13
         5.3.1. Protecting {F, T, D} or {S->F, T, D}................13
         5.3.2. Protecting {F, F->T, D} or {S->F, F->T, D}..........14
   6. TI-LFA and SR Algorithms......................................15
   7. Usage of Adjacency segments in the repair list................15
   8. Measurements on Real Networks.................................16

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   9. Security Considerations.......................................21
   10. IANA Considerations..........................................21
   11. Conclusions..................................................21
   12. Acknowledgments..............................................22
   13. References...................................................22
      13.1. Normative References....................................22
      13.2. Informative References..................................22

1. 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-
   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 be applicable in
   some topologies ([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 routing domain.
   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.

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   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
   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 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.  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, 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
   link L fails, X switches the traffic destined to PE3 and PE2 on the
   post-convergence paths.  This is perfectly inline with the capacity
   planning rule that was presented before and also inline with the fact
   X may converge before PE1 (or any other upstream router) and may
   spread the X-B traffic onto the post-convergence paths rooted at X.

   It should be noted, that some networks may have a different capacity
   planning rule, leading to an allocation of less bandwidth on X-H and
   X-D links.  In such a case, using the post-convergence paths rooted
   at X during FRR may introduce some congestion on X-H and X-D links.
   However it is important to note, that a transient congestion may
   possibly happen, even without FRR activated, for instance when X
   converges before the upstream routers.  Operators are still free to
   use the policy framework defined in [RFC7916] if the usage of the
   post-convergence paths rooted at the PLR is not suitable.

   Readers should be aware that FRR protection is pre-computing a backup
   path to protect against a particular type of failure (link, node,
   SRLG). When using the post-convergence path as FRR backup path, the
   computed post-convergence path is the one considering the failure we
   are protecting against.  This means that FRR is using an expected
   post-convergence path, and this expected post-convergence path may be
   actually different from the post-convergence path used if the failure
   that happened is different from the failure FRR was protecting
   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.

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   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
   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 6.

   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.  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.

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

   Section 3 suggests to compute the P-Space and Q-Space properties
   defined in Section 2, for the specific case of nodes lying over the
   post-convergence paths towards the protected destinations.

   Using the properties defined in Section 3, Section 4 describes how
   to compute protection lists that encode a loop-free post-convergence
   path towards the destination.

   Section 5 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 8
   summarizes these measurements.

1.1. 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.

2. 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.

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   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).

   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.

   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.  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).

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

3. 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 4 to compute the TI-LFA repair list.

3.1. 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).  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.

3.2. 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.

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   This can be found by intersecting the post-convergence path to D,
   assuming the failure of S-F, with Q(D, S-F).

3.3. 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.

   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.

3.4. 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).

3.5. 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.

4. TI-LFA Repair Tunnel

   The TI-LFA repair tunnel consists of an outgoing interface and a list
   of segments (repair list) 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. 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 tunnel 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

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   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 8 provides statistics for the
   number of SIDs in the explicit path to protect against various
   failures.

4.1. 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 MUST be empty.

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

4.2. 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 MUST be set to the outgoing
   interface used to reach R.

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

4.3. 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 MUST 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.

4.4. 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.

5. Protecting segments

   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 behavior depends on the type of active segment to be protected.

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5.1. The active segment is a node segment

   The active segment MUST be kept on the SR header unchanged and the
   repair list MUST be inserted at the head of the list.  The active
   segment becomes the first segment of the inserted repair list.

   This behavior is slightly modified when SR-MPLS is used:

   o  If the repair list ends with an adjacency segment terminating on
      the tail-end of the active segment, and if the active segment has
      been signalled with penultimate hop popping, the active segment
      MUST be popped before pushing the repair list.

   o  If the SRGB at the Q node is different from the SRGB at the PLR,
      then the active segment (before the insertion of the repair list)
      MUST be updated to fit the SRGB of the Q node.

   In Section 5.3, we describe the node protection behavior of PLR S,
   for the specific case where the active segment is a prefix segment
   for the neighbor F itself.

5.2. 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.

   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.

5.2.1. 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 two
   consecutive "PUSH" operations: first it pushes a node segment for F,
   and then it pushes a repair list allowing to reach F while bypassing
   S-F. 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 [RT(F),node(F),adj(F-M)], where RT(F) is the repair tunnel
   for destination F.

   This behavior is slightly modified when SR-MPLS is used:

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   o  If the repair list ends with an adjacency segment terminating on
      F, and if the node segment of F has been signalled with
      penultimate hop popping, the implementation MUST pop Adj(S-F) and
      then push the repair list (the node segment of F is not pushed).
      The packet will leave S with a segment list matching
      [RT(F),adj(F-M)].

   o  If the SRGB at the Q node is different from the SRGB at the PLR,
      then MPLS label representing node(F) MUST be calculated as per the
      SRGB of the Q node.

   In Section 5.3.2, we describe the TI-LFA behavior of PLR S when
   node protection is applied and the two first segments are Adjacency
   Segments.

5.2.2. 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),...].

   A first solution would consist in steering the packet back to F while
   avoiding S-F.  To do so, S MUST apply a "NEXT" operation on Adj(S-F)
   and then two consecutive "PUSH" operations: first it pushes a node
   segment for F, and then it pushes a repair list allowing to reach F
   while bypassing S-F.

   Upon failure of S-F, a packet reaching S with a segment list matching
   [adj(S-F),node(T),...] will thus leave S with a segment list matching
   [RT(F),node(F),node(T)].

   This behavior is slightly modified when SR-MPLS is used:

   o  If the repair list ends with an adjacency segment terminating on
      F, and if the node segment of F has been signalled with
      penultimate hop popping, the implementation MUST pop Adj(S-F) and
      then push the repair list (the node segment of F is not pushed).
      The packet will leave S with a segment list matching
      [RT(F),node(T)].

   o  If the SRGB at the Q node is different from the SRGB at the PLR,
      then MPLS label representing node(F) MUST be calculated as per the
      SRGB of the Q node.

   Another solution is to not steer the packet back via F but rather
   follow the new shortest path to 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.

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   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
   [RT(Q),node(T), ...].  Note that this second behavior is the one
   followed for node protection, as described in Section 5.3.1.

   This behavior is slightly modified when SR-MPLS is used:

   o  If the repair list ends with an adjacency segment terminating on T
      (T being the Q node), and if the node segment of T has been
      signalled with penultimate hop popping, the implementation MUST
      pop Adj(S-F) and then push the repair list (the node segment of T
      is not pushed).  The packet will leave S with a segment list
      matching [RT(Q=T), ...].

   o  If the SRGB at the Q node is different from the SRGB at the PLR,
      then the MPLS label representing node(T) MUST be calculated as per
      the SRGB of the Q node.

   The first proposal 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 existing path.  As stated in Section 1,
   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.

5.3. Protecting SR policy midpoints against node failure

   In this section, we describe the behavior of a node S configured to
   interpret the failure of link S->F as the node failure of F, in the
   specific case where the active segment of the packet received by S is
   a Prefix SID of F represented as "F"), or an Adjacency SID for the
   link S-F (represented as "S->F").

5.3.1. Protecting {F, T, D} or {S->F, T, D}

   This section describes the protection behavior of S when all of the
   following conditions are true:

   1. the active segment is a prefix SID for a neighbor F, or an
      adjacency segment S->F

   2. the primary interface used to forward the packet failed

   3. the segment following the active segment is a prefix SID (for
      node T)

   4. node protection is active for that interface.

   In such a case, the PLR MUST:

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   1. apply a NEXT operation; the segment F or S->F is removed

   2. Confirm that the next segment is in the SRGB of F, meaning that
      the next segment is a prefix segment, e.g. for node T

   3. Retrieve the segment ID of T (as per the SRGB of F)

   4. Apply a NEXT operation followed by a PUSH operation of T's segment
      based on the SRGB of node S.

   5. Look up T's segment (based on the updated label value) and
      forward accordingly.

5.3.2. Protecting {F, F->T, D} or {S->F, F->T, D}

   This section describes the protection behavior of S when all of the
   following conditions are true:

   1. the active segment is a prefix SID for a neighbor F, or an
      adjacency segment S->F

   2. the primary interface used to forward the packet failed

   3. the segment following the active segment is an adjacency SID (F-
      >T)

   4. node protection is active for that interface.

   In such a case, the PLR MUST:

   1. Apply a NEXT operation; the segment F or S->F is removed

   2. Confirm that the next segment is an adjacency SID of F, say F->T

   3. Retrieve the node segment ID associated to T (as per the set of
      Adjacency Segments of F)

   4. Apply a NEXT operation on the next segment followed by a PUSH of
      T's segment based on the SRGB of the node S.

   5. Look up T's segment (based on the updated label value) and forward
      accordingly.

   It is noteworthy to mention that node "S" in the procedures described
   in Sections 5.3.1 and 5.3.2 can always determine whether the
   segment after popping the top segment is an adjacency SID or a
   prefix-SID of the next-hop "F" as follows:

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   1. In a link state environment, the node "S" knows the SRGB and the
      adj-SIDs of the neighboring node "F"

   2. If the new segment after popping the top segment is within the
      SRGB or the adj-SIDs of "F", then node "S" is certain that the
      failure of node "F" is a midpoint failure and hence node "S"
      applies the procedures specified in Sections 5.3.1 or 5.3.2,
      respectively.

   3. Otherwise the failure is not a midpoint failure and hence the node
      "S" may apply other protection techniques that are beyond the
      scope of this document or simply drop the packet and wait for
      normal protocol convergence.

6. 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.

   When a Node-SID is associated with the SR default algorithm,
   enforcing TI-LFA to use Node-SIDs associated with a strict SPF
   algorithm is a definitive solution to this problem.

   [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 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.

7. 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 5.

   To avoid the possibility of this double FRR, an implementation of TI-
   LFA MAY pick only non protected adjacency segments when building the
   repair list.

8. Measurements on Real Networks

   This section presents measurements performed on real service provider
   and large enterprise 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.

   The measurements 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 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

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

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   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   |  Circuits  |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 4.1.

   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 4.2.

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   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 4.3) 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       |  74.227%   |   25.256%  |   0.517%   |   0.001%   |
   +-------------+------------+------------+------------+------------+
   |    T2       |  81.097%   |   18.738%  |   0.165%   |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T3       |  95.878%   |    4.067%  |   0.056%   |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T4       |  62.547%   |   35.666%  |   1.788%   |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T5       |  85.733%   |   14.267%  |   0.0%     |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T6       |  81.252%   |   18.714%  |   0.033%   |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T7       |  98,857%   |   1.143%   |   0.0%     |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T8       |  94,118%   |   5.882%   |   0.0%     |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T9       |  98.950%   |   1.050%   |   0.0%     |   0.0%     |
   +-------------+------------+------------+------------+------------+
           Table 2A: Link protection (repair size distribution)

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   +-------------+------------+------------+------------+------------+
   |   Network   |    0 SIDs  |    1 SID   |   2 SIDs   |   3 SIDs   |
   +-------------+------------+------------+------------+------------+
   |    T1       |  74.227%   |   99.482%  |    99.999% |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T2       |  81.097%   |   99.835%  |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T3       |  95.878%   |   99.944%  |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T4       |  62.547%   |   98.212%  |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T5       |  85.733%   |  100.000%  |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T6       |  81.252%   |   99.967%  |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T7       |  98,857%   |  100.000%  |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T8       |  94,118%   |  100.000%  |   100.0%   |   100.0%   |
   +-------------+------------+------------+------------+------------+
   |    T9       |  98,950%   |  100.000%  |   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       |  74.177%   |   25.306%  |   0.517%   |   0.001%   |
   +-------------+------------+------------+------------+------------+
   |    T2       |                No SRLG Information                |
   +-------------+------------+------------+------------+------------+
   |    T3       |  93.650%   |    6.301%  |   0.049%   |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T4       |  62,547%   |   35.666%  |   1.788%   |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T5       |  83.139%   |   16.861%  |   0.0%     |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T6       |                No SRLG Information                |
   +-------------+---------------------------------------------------+
   |    T7       |                No SRLG Information                |
   +-------------+------------+------------+------------+------------+
   |    T8       |  85.185%   |   14.815%  |   0.0%     |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T9       |  98,940%   |    1.060%  |   0.0%     |   0.0%     |
   +-------------+------------+------------+------------+------------+
         Table 3A: Local SRLG protection repair size distribution

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   +-------------+------------+------------+------------+------------+
   |   Network   |    0 SIDs  |    1 SID   |   2 SIDs   |   3 SIDs   |
   +-------------+------------+------------+------------+------------+
   |    T1       |  74.177%   |   99.482%  |  99.999%   | 100.001%   |
   +-------------+------------+------------+------------+------------+
   |    T2       |                No SRLG Information                |
   +-------------+------------+------------+------------+------------+
   |    T3       |  93.650%   |    99.951% | 100.000%   |   0.0%     |
   +-------------+------------+------------+------------+------------+
   |    T4       |  62,547%   |   98.212%  | 100.000%   | 100.0%     |
   +-------------+------------+------------+------------+------------+
   |    T5       |  83.139%   |  100.000%  | 100.0%     | 100.0%     |
   +-------------+------------+------------+------------+------------+
   |    T6       |                No SRLG Information                |
   +-------------+---------------------------------------------------+
   |    T7       |                No SRLG Information                |
   +-------------+------------+------------+------------+------------+
   |    T8       |  85.185%   |   100,000% | 100.000%   | 100.0%     |
   +-------------+------------+------------+------------+------------+
   |    T9       |  98,940%   |   100,000% | 100.000%   | 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   |  49.771% | 47.902%  | 2.156%   |  0.148%  |  0.023%  |
   +---------+----------+----------+----------+----------+----------+
   |    T2   |  36,528% | 59.625%  | 3.628%   |  0.194%  |  0.025%  |
   +---------+----------+----------+----------+----------+----------+
   |    T3   |  73,287% | 25,574%  | 1,128%   |  0.010%  |  0%      |
   +---------+----------+----------+----------+----------+----------+
   |    T4   |  36.112% | 57.350%  | 6.329%   |  0.199%  |  0.010%  |
   +---------+----------+----------+----------+----------+----------+
   |    T5   |  73.185% | 26.815%  | 0%       |  0%      |  0%      |
   +---------+----------+----------+----------+----------+----------+
   |    T6   |  78.362% | 21.320%  | 0.318%   |  0%      |  0%      |
   +---------+----------+----------+----------+----------+----------+
   |    T7   |  66.106% | 32.813%  | 1.082%   |  0%      |  0%      |
   +---------+----------+----------+----------+----------+----------+
   |    T8   |  59.712% | 40.288%  | 0%       |  0%      |  0%      |
   +---------+----------+----------+----------+----------+----------+
   |    T9   |  98.950% | 1.050%   | 0%       |  0%      |  0%      |
   +---------+----------+----------+----------+----------+----------+
           Table 4A: Node protection (repair size distribution)

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   +---------+----------+----------+----------+----------+----------+
   | Network |  0 SIDs  |   1 SID  | 2 SIDs   |  3 SIDs  |  4 SIDs  |
   +---------+----------+----------+----------+----------+----------+
   |    T1   |  49.771% |  97.673% |  99.829% | 99.977%  |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T2   |  36,528% |  96.153% |  99.781% | 99.975%  |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T3   |  73,287% |  98.862% |  99.990% |100.0%    |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T4   |  36.112% |  93.461% |  99.791% | 99.990%  |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T5   |  73.185% | 100.0%   | 100.0%   |100.0%    |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T6   |  78.362% | 99.682%  | 100.0%   |100.0%    |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T7   |  66.106% | 98,918%  | 100.0%   |100.0%    |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T8   |  59.712% | 100.0%   | 100.0%   |100.0%    |  100%    |
   +---------+----------+----------+----------+----------+----------+
   |    T9   |  98.950% | 100.0%   | 100.0%   |100.0%    |  100%    |
   +---------+----------+----------+----------+----------+----------+
      Table 4B: Node protection (repair size cumulative distribution)

9. 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.

10. IANA Considerations

   No requirements for IANA

11. Conclusions

   This document proposes a mechanism that is able to pre-calculate a
   backup path for every primary path so as to be able to protect

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   against the failure of a directly connected link, node, or SRLG.
   The mechanism is able to calculate the backup path irrespective of
   the topology as long as the topology is sufficiently redundant.

12. Acknowledgments

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

   This document was prepared using 2-Word-v2.0.template.dot.

13. References

13.1. 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., Previdi, S., Decraene, B., Litkowski, S., and
             R. Shakir, "Segment Routing Architecture", RFC 8402, DOI
             10.17487/RFC8402 July 2018, <http://www.rfc-
             editor.org/info/rfc8402>.

13.2. Informative References

   [I-D.bashandy-rtgwg-segment-routing-uloop] Bashandy, A., Filsfils,
             C., Litkowski, S., Decraene, B., Francois, P., and Psenak,
             P. " Loop avoidance using Segment Routing", draft-bashandy-
             rtgwg-segment-routing-uloop-07, (work in progress), January
             2020

   [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-05 (work in progress), November
             2019.

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   [I-D.ietf-spring-segment-routing-policy] Filsfils, C., Sivabalan, S.,
             daniel.voyer@bell.ca, d., bogdanov@google.com, b., and P.
             Mattes, "Segment Routing Policy Architecture", draft-ietf-
             spring-segment-routing-policy-06 (work in progress),
             December 2019.

   [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, <http://www.rfc-
             editor.org/info/rfc5714>.

   [RFC6571] Filsfils, C., Francois, P., 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,
             <http://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,
             <http://www.rfc-editor.org/info/rfc7490>.

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Authors' Addresses

   Stephane Litkowski
   Cisco
   Email: slitkows.ietf@gmail.com

   Ahmed Bashandy
   Individual
   Email: abashandy.ietf@gmail.com

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

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

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

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

   Francois Clad
   Cisco Systems
   Email: fclad@cisco.com

   Pablo Camarillo
   Cisco Systems
   Email: pcamaril@cisco.com

Bashandy                Expires July 13, 2020                 [Page 24]