Network Working Group                                    Pierre Francois
Internet-Draft                                            IMDEA Networks
Intended status: Standards Track                       Clarence Filsfils
Expires: August 4, 2014                              Cisco Systems, Inc.
                                                          Bruno Decraene
                                                              Rob Shakir
                                                        January 31, 2014

              Use-cases for Resiliency in Segment Routing


   This document describes the use cases for resiliency in SR networks.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Path protection . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Management-free local protection  . . . . . . . . . . . . . . . 4
   4.  Managed local protection  . . . . . . . . . . . . . . . . . . . 4
   5.  Co-existence  . . . . . . . . . . . . . . . . . . . . . . . . . 5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

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1.  Introduction

   Segment Routing (SR) aims at supporting services with tight SLA
   guarantees [1].  This document reviews alternative techniques to
   provide Fast Reroute (FRR) for SR traffic.  Note that these
   techniques can be applied to protect LSPs created with LDP as well as
   pure IP traffic.

   A FRR technique involves the pre-computation and dataplane pre-
   installation of backup path such as the repair traffic in 50msec upon
   failure detection.  The term "protection" is often used as a synonym
   for FRR.  Such technique supposes the existing of a sub-10msec
   failure detection technique.

   Three key alternatives are described: path protection, local
   protection without operator management and local protection with
   operator management.

   The purpose of this document is to illustrate the different
   techniques and explain how an operator could combine them in the same
   network.  Solutions are not defined in this document.

                             /  \
                            /    \
                          /|      | \  / | \  / |\
                         / |      |  \/  |  \/  | \
                        A  |      |  /\  |  /\  |  Z
                         \ |      | /  \ | /  \ | /
                          \|      |/    \|/    \|/

                       Figure 1: Reference topology

   We use Figure 1 as a reference topology throughout the document.  We
   describe the various use-cases in the next sections.  All link
   metrics are equal to 1, with the exception of the links of PE1 which
   are configured with a metric of 100.

2.  Path protection

   A first protection strategy consists in excluding any local repair
   but instead use end-to-end path protection.

   For example, a PW from A to Z can be "path protected" in the
   direction A to Z in the following manner: the operator configures two

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   SR tunnels T1 and T2 from A to Z. The two tunnels are installed in
   the forwarding plane of A and hence are ready to encapsulate and
   forward packets.  The two tunnels are installed over disjoint paths
   using adjacency segments (T1 over segment list {AB, BC, CD, DE, EZ}
   and T2 over segment list {AF, FG, GH, HI, IZ}).  When T1 is up, the
   packets of the PW are sent on T1.  When T1 fails, the packets of the
   PW are sent on T2.  When the tunnel T1 comes back up, the operator
   either allows for an automated reversion of the traffic onto T1 or
   selects an operator-driven reversion.  The end-to-end liveness of a
   tunnel is for example checked with BFD.

   From an SR viewpoint, we would like to highlight the following
   requirement: the adjacency segments used to support the tunnels T1
   and T2 MUST NOT benefit from local protection.  This is achieved by
   resetting the B-flag in the related AdjSID's as per the IGP
   extensions defined in [3].

3.  Management-free local protection

   An alternative protection strategy consists in management-free local

   For example, a PW from C to E transported by the single segment
   NodeSID(E) benefits from management-free local protection by having
   each node along the path (e.g.  C and D) to automatically pre-compute
   and pre-install backup path for the destination E. Upon local
   detection of the failure (e.g. link BFD), the traffic is repaired
   over the backup path in sub-50msec.

   The backup path computation should support the following

   o  100% link, node, and SRLG protection in any topology
   o  Automated computation by the IGP
   o  Selection of the backup path such as to minimize the chance for
      transient congestion and/or delay during the protection period, as
      reflected by the IGP metric configuration in the network.

   An SR solution aimed at supporting these requirements is defined in

4.  Managed local protection

   A final alternative protection strategy consists in managed local

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   For policy reasons, the operator may want very specific backup paths
   to be used.

   For example, the operator may want the backup path to end at the
   next-hop (or next-next-hop for node failure) hence excluding IPFRR/
   LFA types of backup path.  Also, the operator might want to tightly
   control the backup path to the next-hop: for the destination Z upon
   the failure of link CD, the backup path CGHD might be desired while
   the backup paths CGD and CHD are refused.

   The protection mechanism must support the explicit configuration of
   the backup path either under the form of high-level constraints (end
   at the next-hop, end at the next-next-hop, minimize this metric,
   avoid this SRLG...) or under the form of an explicit segment list.

5.  Co-existence

   The operator may want to support several very-different services on
   the same packet-switching infrastructure.

   The SR resiliency architecture allows the co-existence of different
   FRR techniques.

   Let us illustrate this for adjacency segments with the following

   o  Node C is configured with 3 adjacency segments for the connected
      interface to D: AdjSID(CD1), AdjSID(CD2) and AdjSID(CD3)
   o  SR Flow F1: from A to E over segment list
      {NodeSID(C), AdjSID(CD1), NodeSID(E)}
   o  SR Flow F2: from F to I over segment list
      {NodeSID(C), AdjSID(CD2), NodeSID(I)}
   o  SR Flow F3: from G to Z over segment list
      {NodeSID(C), AdjSID(CD3), NodeSID(Z)}
   o  Node C is configured with a distinct protection technique for each
      adjacency segment.  AdjSID(CD1) is configured without protection,
      AdjSID(CD2) is configured to benefit from management-free local
      protection and AdjSID(CD3) is configured for managed local
      protection over the path {AdjSID(CH), AdjSID(HD)}

   Such a co-existence is partially supported by the SR IGP extensions
   [ref tbd]
   o  Multiple adjacency segments can be advertised for the same
   o  The non-protected property of AdjSID(CD1) is signalled by a reset
      B flag.

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   o  The protected property of AdjSID(CD2) and AdjSID(CD3) are
      signalled by a set B flag.

   The SR IGP extension should be extended to discreminate between
   AdjSID(CD2) and AdjSID(CD3).  A single flag could be defined (managed
   path vs fully automated).

6.  References

   [1]  Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
        Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., Ytti,
        S., Henderickx, W., Tantsura, J., and E. Crabbe, "Segment
        Routing Architecture", draft-filsfils-rtgwg-segment-routing-00
        (work in progress), June 2013.

   [2]  Francois, P., Filsfils, C., Bashandy, A., Decraene, B., and S.
        Litkowski, "Topology Independent Fast Reroute using Segment
        Routing", November 2013.

   [3]  Previdi, S., Filsfils, C., and A. Bashandy, "IS-IS Segment
        Routing Extensions", October 2013.

Authors' Addresses

   Pierre Francois
   IMDEA Networks


   Clarence Filsfils
   Cisco Systems, Inc.


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   Bruno Decraene


   Rob Shakir


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