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

Network Working Group                                    Pierre Francois
Internet-Draft                                         Clarence Filsfils
Intended status: Standards Track                          Ahmed Bashandy
Expires: November 18, 2016                           Cisco Systems, Inc.
                                                          Bruno Decraene
                                                      Stephane Litkowski
                                                                  Orange
                                                            May 17, 2016

        Topology Independent Fast Reroute using Segment Routing
             draft-francois-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 establising 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.

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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Intersecting P-Space and Q-Space with post-convergence
       paths  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  P-Space property computation for a resource X  . . . . . .  5
     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 . . . . . . . .  6
     3.4.  Q-Space property computation for a node F, over
           post-convergence paths . . . . . . . . . . . . . . . . . .  6
   4.  TI-LFA Repair Tunnel . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  The repair node is a direct neighbor . . . . . . . . . . .  7
     4.2.  The repair node is a PQ node . . . . . . . . . . . . . . .  7
     4.3.  The repair is a Q node, neighbor of the last P node  . . .  7
     4.4.  Connecting distant P and Q nodes along
           post-convergence paths . . . . . . . . . . . . . . . . . .  7
   5.  Protecting segments  . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  The active segment is a node segment . . . . . . . . . . .  7
     5.2.  The active segment is an adjacency segment . . . . . . . .  8
       5.2.1.  Protecting [Adjacency, Adjacency] segment lists  . . .  8
       5.2.2.  Protecting [Adjacency, Node] segment lists . . . . . .  8
     5.3.  Protecting SR policy midpoints against node failure  . . .  9
       5.3.1.  Protecting {F, T, D} or {S->F, T, D} . . . . . . . . .  9
       5.3.2.  Protecting {F, F->T, D} or {S->F, F->T, D} . . . . . .  9
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   Segment Routing aims at supporting services with tight SLA guarantees
   [1].  This document provides a local repair mechanism relying on SR,
   capable of restoring end-to-end connectivity in the case of a sudden
   failure of a network component.

   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 against
   link failure, node failure, and local SRLG failures.  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 neighbour 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).

   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) 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.
   This allows for a protection path selection approach meeting
   operational needs rather than a topologically constrained one.

   Using SR, there is no need to create state in the network in order to
   enforce an explicit FRR path.  As a result, we can use optimized
   detour paths for each specific destination and for each type of
   failure without creating additional forwarding state.  Also, the mode
   of protection (link, node, SRLG) is not constrained to be network
   wide or node wide, but can be managed on a per interface basis.

   Building on such an easier forwarding environment, 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.

   As the capacity of the post-convergence path is typically planned by
   the operator to support the post-convergence routing of the traffic
   for any expected failure, there is much less need for the operator to
   tune the decision among which protection path to choose.  The
   protection path will automatically follow the natural backup path
   that would be used after local convergence.  This also helps to
   reduce the amount of path changes and hence service transients: one
   transition (pre-convergence to post-convergence) instead of two (pre-
   convergence to FRR and then post-convergence).

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   We provide the TI-LFA approach that achieves guaranteed coverage
   against link, node, and local SRLG failure, in any IGP network,
   relying on the flexibility of SR.

                                 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.

   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, we describe how to compute
   protection lists that encode a loopfree post-convergence towards the
   destination, in Section 4.

   Finally, we define the segment operations to be applied by the PLR to
   ensure consistency with the forwarding state of the repair node, in
   Section 5.

2.  Terminology

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

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   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 distance from node A to node B in SPT_old(A).

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

   Similarly to [4], we rely on 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

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   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).

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.

   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

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   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.

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.

   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.

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   Note (1): If 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 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 neighbour 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.  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.

   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 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

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   [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.

   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 behaviour is the one
   followed for node protection, as described in section Section 5.3.1.

5.3.  Protecting SR policy midpoints against node failure

   As planned in the previous version of this document, we describe the
   behaviour 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}

   We describe the protection behaviour of S when

   1.  the active segment is a prefix SID for a neighbour 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 protetion 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}

   We describe the protection behaviour of S when

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   1.  the active segment is a prefix SID for a neighbour 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 protetion 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 Adjancey 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.

6.  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.

   [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>.

Authors' Addresses

   Pierre Francois
   Cisco Systems, Inc.
   Vimercate
   IT

   Email: pifranco@cisco.com

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   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   BE

   Email: cfilsfil@cisco.com

   Ahmed Bashandy
   Cisco Systems, Inc.
   San Jose
   US

   Email: bashandy@cisco.com

   Bruno Decraene
   Orange
   Issy-les-Moulineaux
   FR

   Email: bruno.decraene@orange.com

   Stephane Litkowski
   Orange
   FR

   Email: bruno.decraene@orange.com

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