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Versions: 00 01 02 03 04 draft-bashandy-rtgwg-segment-routing-ti-lfa

Network Working Group                                    Pierre Francois
Internet-Draft                                         Clarence Filsfils
Intended status: Standards Track                          Ahmed Bashandy
Expires: February 18, 2016                           Cisco Systems, Inc.
                                                          Bruno Decraene
                                                      Stephane Litkowski
                                                                  Orange
                                                            Aug 17, 2015


        Topology Independent Fast Reroute using Segment Routing
             draft-francois-rtgwg-segment-routing-ti-lfa-00

Abstract

   This document presents Topology Independent Loop-free Alternate Fast
   Re-route (TI-LFA), aimed at providing link and node 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.  We accommodate the FRR
   discovery and selection approaches in order to establish 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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on February 18, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.


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  . . . . . . . . . . . . . . . . . . 5
     3.3.  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  . . . . . . . . . . . 6
     4.2.  The repair node is a PQ node  . . . . . . . . . . . . . . . 6
     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  . . . . . . . . 7
       5.2.1.  Protecting [Adjacency, Adjacency] segment lists . . . . 8
       5.2.2.  Protecting [Adjacency, Node] segment lists  . . . . . . 8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9
















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1.  Introduction

   Segment Routing aims at supporting services with tight SLA guarantees
   [1].  This document provides local repair mechanisms using SR,
   capable of restoring end-to-end connectivity in the case of a sudden
   failure of a link or a node, with guaranteed coverage properties.

   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.
   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 possible
   failure in the network without creating additional forwarding state.

   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.

   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).

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





                                 L         ____
                            S----------F--{____}--D
                           _|_       ___________ /
                          {___}--Q--{___________}



                        Figure 1: TI-LFA Protection




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   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, or
   node F. 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.
   The failure of a link S-F as well as the failure of a neighbor F is
   discussed.

   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.

   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.




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   The P-Space P(R,X) of a node R w.r.t. a resource X (e.g. a link S-F,
   or a node F) 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 (or F).

   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 on 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).

   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 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).

   The post-convergence path to D requires to compute SPT_new(S, S-F).

   A node N is in Q(D,S-F) if it is not downstream of S-F in
   rSPT_old(D).




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3.3.  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).

   The post-convergence path to D requires to compute SPT_new(S, F).

   A node N is in Q(D,F) if it is not downstream of F in rSPT_old(D).


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
   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 an 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 an RLFA repair tunnel.







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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 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.

   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.

   A future version of the document will describe the FRR behavior when
   the active segment is a node segment destined to F, and F has failed.

   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.

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.







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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.

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, similarly to the previous case.  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.

   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), ...].


6.  References

   [1]  Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and R.
        Shakir, "Segment Routing Architecture",
        draft-ietf-spring-segment-routing-04 (work in progress),
        July 2015.




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   [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


   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





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   Stephane Litkowski
   Orange
   FR

   Email: bruno.decraene@orange.com














































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