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Versions: (draft-francois-rtgwg-segment-routing-ti-lfa) 00 01 02 03 04 05

Network Working Group                                       A. Bashandy
Internet Draft                                                   Arrcus
Intended status: Standard Track                             C. Filsfils
Expires: April 2019                                       Cisco Systems
                                                         Bruno Decraene
                                                     Stephane Litkowski
                                                                  Orange
                                                         Pierre Francois
                                                               INSA Lyon
                                                                D. Voyer
                                                             Bell Canada
                                                           Francois Clad
                                                         Pablo Camarillo
                                                           Cisco Systems
                                                        October 4, 2018


          Topology Independent Fast Reroute using Segment Routing
              draft-bashandy-rtgwg-segment-routing-ti-lfa-05


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 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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   provisions of BCP 78 and BCP 79.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.



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Table of Contents

   1. Introduction...................................................3
      1.1. Conventions used in this document.........................5
   2. Terminology....................................................5
   3. Intersecting P-Space and Q-Space with post-convergence paths...6
      3.1. P-Space property computation for a resource X.............6
      3.2. Q-Space property computation for a link S-F, over post-
      convergence paths..............................................6
      3.3. Q-Space property computation for a set of links adjacent to
      S, over post-convergence paths.................................7
      3.4. Q-Space property computation for a node F, over post-
      convergence paths..............................................7
   4. TI-LFA Repair Tunnel...........................................7
      4.1. The repair node is a direct neighbor......................7

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      4.2. The repair node is a PQ node..............................8
      4.3. The repair is a Q node, neighbor of the last P node.......8
      4.4. Connecting distant P and Q nodes along post-convergence
      paths..........................................................8
   5. Protecting segments............................................8
      5.1. The active segment is a node segment......................8
      5.2. The active segment is an adjacency segment................9
         5.2.1. Protecting [Adjacency, Adjacency] segment lists......9
         5.2.2. Protecting [Adjacency, Node] segment lists...........9
      5.3. Protecting SR policy midpoints against node failure......10
         5.3.1. Protecting {F, T, D} or {S->F, T, D}................10
         5.3.2. Protecting {F, F->T, D} or {S->F, F->T, D}..........10
   6. Measurements on Real Networks.................................11
   7. Security Considerations.......................................17
   8. IANA Considerations...........................................17
   9. Conclusions...................................................17
   10. References...................................................17
      10.1. Normative References....................................17
      10.2. Informative References..................................17
   11. Acknowledgments..............................................18

1. Introduction

   Segment Routing aims at supporting services with tight SLA
   guarantees [1]. By relying on segment routing 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 [6].

   The term topology independent (Ti) refers to the ability to provide
   a loop free backup path irrespective of the topologies prior the
   failure and after the failure.

   For each destination in the network, TI-LFA prepares a data-plane
   switch-over 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 local 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 local 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).

   Protection techniques outlined in this document are limited to
   protecting links, nodes, and local SRLGs that are within a routing


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   domain. Protecting domain exit routers and/or links attached to
   another routing domains are beyond the scope of this document

   Using segment routing, there is no need to establish TLDP sessions
   with remote nodes in order to take advantage of the applicability of
   remote LFAs (RLFA) [4][5] or remote LFAs with directed forwarding
   (DLFA)[2]. As a result, preferring LFAs over RLFAs or DLFAs, as well
   as minimizing the number of RLFA or DLFA repair nodes is not
   required

   Using SR, there is no need to create state in the network in order
   to enforce an explicit FRR path thereby relieving the nodes from the
   extra state and the operator from having to deploy an extra protocol
   just to enhance FRR coverage.

   The FRR behavior suggested in this document tailors the repair paths
   over the post-convergence path from the PLR to the protected
   destination, given the enabled protection mode for the interface.
   Using the post-convergence path in TI-LFA resolves some of
   operational issues with LFA selection that are mentioned in Section
   3 of [5] (e.g. using PE routers to protect against core failures, or
   selecting links with low BW while links with high BW are available),
   because these issues presumably have been taken care of by the
   network operator as part of its original network engineering. Hence
   traffic that permanently uses the PLR after the failure achieves
   maximum benefits. Traffic that does not use the PLR prior to and
   after the failure remains unaffected. Traffic that temporarily
   continues to use the PLR after the failure benefits from the quick
   switching to the backup path by minimizing traffic loss until remote
   node(s) reacts.

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

                       Figure 1 TI-LFA Protection



   We use Figure 1 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 adjacent links including L (local 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.

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   In Section 2 we define the main notations used in the document.
   They are in line with [2].

   In Section 3, we suggest 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 loopfree 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 6 summarizes these measurements.

1.1. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119

   In this document, these words will appear with that interpretation
   only when in ALL CAPS. Lower case uses of these words are not to be
   interpreted as carrying RFC-2119 significance.

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.

   Dist_old(A,B) is the shortest distance from node A to node B in
   SPT_old(A).

   Dist_new(A,B, X) is the shortest distance from node A to node B in
   SPT_new(A,X).



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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 [4], 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 local 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

   In this section, we suggest to determine the P-Space and Q-Space
   properties of the nodes along the post-convergence paths 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.

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



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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).

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

4.1. The repair node is a direct neighbor

   When the repair node is a direct neighbor, the outgoing interface is
   set to that neighbor and the repair segment list is empty.


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

4.2. The repair node is a PQ node

   When the repair node is in P(S,X), the repair list is made of a
   single node segment to the repair node.

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

4.3. The repair is a Q node, neighbor of the last P node

   When the repair node is adjacent to P(S,X), the repair list is made
   of two segments: A node segment to the adjacent P node, and an
   adjacency segment from that node to the repair node.

   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 loopfree
   path from P to Q.


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.

5.1. The active segment is a node segment

   The active segment is kept on the SR header, unchanged (1).  The
   repair list is inserted at the head of the list.  The active segment
   becomes the first segment of the inserted repair list.

   Note (1): If SR-MPLS is being used and the SRGB at the repair node
   is different from the SRGB at the PLR, then the active segment MUST
   be updated to fit the SRGB of the repair 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.



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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.  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 applies 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 protection list allowing to reach F while
   bypassing S-F. For details on the "NEXT" and "PUSH" operations,
   refer to [7].

   Upon failure of S-F, a packet reaching S with a segment list
   matching [adj(S-F),adj(M),...] will thus leave S with a segment list
   matching [RT(F),node(F),adj(M)], where RT(F) is the repair tunnel
   for destination F.

   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 packet segment list 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 applies 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)].

   Another solution is to not steer the packet back via F but rather
   follow the new shortest path to T. In this case, S just needs to
   apply a "NEXT" operation on the Adjacency segment related to S-F,
   and push 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(L), 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.



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.

   The TILFA Node FRR behavior becomes equivalent to:

   1. Pop; 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. Identify T (as per the SRGB of F)

   4. Pop the next segment and push T's segment based on the local SRGB

   5. forward the packet according to T.

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

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   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.

   The TILFA Node FRR behavior becomes equivalent to:

   1. Pop; the segment F or S->F is removed

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

   3. Identify T (as per the set of Adjacency Segments of F)

   4. Pop the next segment and push T's segment based on the local SRGB

   5. forward the packet according to T.



   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:

   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 conversion.





6. 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 mechanism described in this document are used to

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   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



   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.























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   +-------------+------------+------------+------------+------------+
   |   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.

   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






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   +-------------+------------+------------+------------+------------+
   |   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)



   +-------------+------------+------------+------------+------------+
   |   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




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   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

   +-------------+------------+------------+------------+------------+
   |   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



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   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)



   +---------+----------+----------+----------+----------+----------+
   | 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)

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7. Security Considerations

   The techniques described in this document is internal
   functionality 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.

8. IANA Considerations

   No requirements for IANA

9. 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
   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.

10. References

10.1. Normative References

10.2. Informative References

   [1]   Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and R.
         Shakir, "Segment Routing Architecture", draft-ietf-spring-
         segment-routing-08 (work in progress), May 2016.

   [2]   Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
         5714, January 2010.

   [3]   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,
         June 2012.





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   [4]   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>.

   [5]   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>.

   [6]   Bashandy, A., Filsfils, C., and Litkowski, S., " Loop
         avoidance using Segment Routing", draft-bashandy-rtgwg-
         segment-routing-uloop-00, (work in progress), May 2017

   [7]   Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and
         Shakir, R, "Segment Routing Architecture", draft-ietf-spring-
         segment-routing-11 (work in progress), February 2017



11. Acknowledgments

   We would like to give Les Ginsberg special thanks for the valuable
   comments and contribution

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

























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Authors' Addresses

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

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

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

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

   Stephane Litkowski
   Orange
   FR
   Email: stephane.litkowski@orange.com

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

   Pablo Camarillo
   Cisco Systems
   Email: pcamaril@cisco.com

   Francois Clad
   Cisco Systems
   Email: fclad@cisco.com













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