Network Working Group                                       S. Litkowski
Internet-Draft                                                    Orange
Intended status: Standards Track                             A. Bashandy
Expires: September 6, 2019                                    Individual
                                                             C. Filsfils
                                                           Cisco Systems
                                                             B. Decraene
                                                                  Orange
                                                             P. Francois
                                                               INSA Lyon
                                                                D. Voyer
                                                             Bell Canada
                                                                 F. Clad
                                                            P. Camarillo
                                                           Cisco Systems
                                                           March 5, 2019


        Topology Independent Fast Reroute using Segment Routing
               draft-ietf-rtgwg-segment-routing-ti-lfa-01

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 September 6, 2019.

Copyright Notice

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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  . . . . . . .  16
   8.  Measurements on Real Networks . . . . . . . . . . . . . . . .  16



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   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21