





Internet Engineering Task Force (IETF)                  C. Filsfils, Ed.
Request for Comments: 8355                               S. Previdi, Ed.
Category: Informational                              Cisco Systems, Inc.
ISSN: 2070-1721                                              B. Decraene
                                                                  Orange
                                                               R. Shakir
                                                                  Google
                                                              March 2018


                          Resiliency Use Cases
        in Source Packet Routing in Networking (SPRING) Networks

Abstract

   This document identifies and describes the requirements for a set of
   use cases related to Segment Routing network resiliency on Source
   Packet Routing in Networking (SPRING) networks.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8355.

















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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Path Protection . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Management-Free Local Protection  . . . . . . . . . . . . . .   6
     3.1.  Management-Free Bypass Protection . . . . . . . . . . . .   7
     3.2.  Management-Free Shortest-Path-Based Protection  . . . . .   8
   4.  Managed Local Protection  . . . . . . . . . . . . . . . . . .   8
     4.1.  Managed Bypass Protection . . . . . . . . . . . . . . . .   9
     4.2.  Managed Shortest Path Protection  . . . . . . . . . . . .   9
   5.  Loop Avoidance  . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Coexistence of Multiple Resilience Techniques in the Same
       Infrastructure  . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Manageability Considerations  . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13













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1.  Introduction

   This document reviews various use cases for the protection of
   services in a SPRING network.  The terminology used hereafter is in
   line with [RFC5286] and [RFC5714].

   The resiliency use cases described in this document can be applied
   not only to traffic that is forwarded according to the SPRING
   architecture, but also to traffic that originally is forwarded using
   other paradigms such as LDP signaling or pure IP traffic (IP-routed
   traffic).

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

   Path protection lets the ingress node be in charge of the failure
   recovery, as discussed in Section 2.

   The rest of the document focuses on approaches where protection is
   performed by the node adjacent to the failed component, commonly
   referred to as local protection techniques or fast-reroute techniques
   [RFC5286] [RFC5714].

   In Section 3, we discuss two different approaches providing unmanaged
   local protection, namely link/node bypass protection and shortest-
   path-based protection.

   Section 4 illustrates a case allowing the operator to manage the
   local protection behavior in order to accommodate specific policies.

   In Section 5, we discuss the opportunity for the SPRING architecture
   to provide loop-avoidance mechanisms such that transient forwarding
   state inconsistencies during routing convergence do not lead into
   traffic loss.

   The purpose of this document is to illustrate the different use cases
   and explain how an operator could combine them in the same network
   (see Section 6).  Solutions are not defined in this document.












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                          B------C------D------E
                         /|      | \  / | \  / |\
                        / |      |  \/  |  \/  | \
                       A  |      |  /\  |  /\  |  Z
                        \ |      | /  \ | /  \ | /
                         \|      |/    \|/    \|/
                          F------G------H------I

                       Figure 1: Reference Topology

   We use Figure 1 as a reference topology throughout the document.  The
   following link metrics are applied:

   o  Links from/to A and Z are configured with a metric of 100.

   o  CH, GD, DI, and HE links are configured with a metric of 6.

   o  All other links are configured with a metric of 5.

   Note: Link metrics are bidirectional; in other words, the same metric
   value is configured at both sides of each link.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Path Protection

   As a reminder, one of the major network operator requirements is path
   disjointness capability.  Network operators have deployed
   infrastructures with topologies that allow paths to be computed in a
   complete disjoint fashion where two paths wouldn't share any
   component (link or router), hence allowing an optimal protection
   strategy.

   A first protection strategy consists of excluding any local repair
   and instead uses end-to-end path protection where each SPRING path is
   protected by a second disjoint SPRING path.  In this case, local
   protection is not used along the path.

   For example, a pseudowire (PW) from A to Z can be "path protected" in
   the direction A to Z in the following manner: the operator configures
   two SPRING paths, T1 (primary) and T2 (backup), from A to Z.




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   The two paths may be used:

   o  concurrently, where the ingress router sends the same traffic over
      the primary and secondary path (this is usually known as 1+1
      protection);

   o  concurrently, where the ingress router splits the traffic over the
      primary and secondary path (this is usually known as Equal-Cost
      Multipath (ECMP) or Unequal-Cost Multipath (UCMP)); or

   o  as a primary and backup path, where the secondary path is used
      only when the primary failed (this is usually known as 1:1
      protection).

   T1 is established over path {AB, BC, CD, DE, EZ} as the primary path,
   and T2 is established over path {AF, FG, GH, HI, IZ} as the backup
   path.  The two paths MUST be disjoint in their links, nodes, and
   Shared Risk Link Groups (SRLGs) to satisfy the requirement of
   disjointness.

   In the case of primary/backup paths, when the primary path T1 is up,
   the packets of the PW are sent on T1.  When T1 fails, the packets of
   the PW are sent on the backup path T2.  When T1 comes back up, the
   operator either allows for an automated reversion of the traffic onto
   T1 or selects an operator-driven reversion.  Typically, the
   switchover from path T1 to path T2 is done in a fast-reroute fashion
   (e.g., sub-50 milliseconds) but, depending on the service that needs
   to be delivered, other restoration times may be used.

   It is essential that any path, primary or backup, benefit from an
   end-to-end liveness monitoring/verification.  The method and
   mechanisms that provide such a liveness check are outside the scope
   of this document.  An example is given by [RFC5880].

   There are multiple options for a liveness check, e.g., path liveness,
   where the path is monitored at the network level (either by the head-
   end node or by a network controller/monitoring system).  Another
   possible approach consists of a service-based path monitored by the
   service instance (verifying reachability of the endpoint).  All these
   options are given here as examples.  While this document does express
   the requirement for a liveness mechanism, it does not mandate, nor
   define, any specific one.









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   From a SPRING viewpoint, we would like to highlight the following
   requirements:

   o  SPRING architecture MUST provide a way to compute paths that are
      not protected by local repair techniques (as illustrated in the
      example of paths T1 and T2).

   o  SPRING architecture MUST provide a way to instantiate pairs of
      disjoint paths on a topology based on a protection strategy (link,
      node, or SRLG protection) and allow the validation or
      recomputation of these paths upon network events.

   o  The SPRING architecture MUST provide an end-to-end liveness check
      of SPRING-based paths.

3.  Management-Free Local Protection

   This section describes two alternatives that provide local protection
   without requiring operator management, namely bypass protection and
   shortest-path-based protection.

   For example, traffic from A to Z, transported over the shortest paths
   provided by the SPRING architecture, benefits from management-free
   local protection by having each node along the path automatically
   precompute and preinstall a backup path for the destination Z.  Upon
   local detection of the failure, the traffic is repaired over the
   backup path in sub-50 milliseconds.  When the primary path comes back
   up, the operator either allows for an automated reversion of the
   traffic onto it or selects an operator-driven reversion.

   The backup path computation SHOULD support the following
   requirements:

   o  100% link, node, and SRLG protection in any topology;

   o  automated computation by the IGP; and

   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.











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3.1.  Management-Free Bypass Protection

   One way to provide local repair is to enforce a failover along the
   shortest path around the failed component.

   In case of link protection, the point of local repair will create a
   repair path avoiding the protected link and merging back to the
   primary path at the next hop.

   In case of node protection, the repair path will avoid the protected
   node and merge back to the primary path at the next-next hop.

   In case of SRLG protection, the repair path will avoid members of the
   same group and merge back to the primary path just after.

   In our example, C protects destination Z against a failure of the CD
   link by enforcing the traffic over the bypass {CH, HD}.  The
   resulting end-to-end path between A and Z, upon recovery from the
   failure of CD, is depicted in Figure 2.

                          B * * *C------D * * *E
                         *|      | *  / * \  / |*
                        * |      |  */  *  \/  | *
                       A  |      |  /*  *  /\  |  Z
                        \ |      | /  * * /  \ | /
                         \|      |/    **/    \|/
                          F------G------H------I

                Figure 2: Bypass Protection around Link CD

   When the primary path comes back up, the operator either allows for
   an automated reversion of the traffic onto the primary path or
   selects an operator-driven reversion.


















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3.2.  Management-Free Shortest-Path-Based Protection

   An alternative protection strategy consists in management-free local
   protection that is aimed at providing a repair for the destination
   based on the shortest path to the destination.

   In our example, C protects Z (which the traffic initially reaches via
   CD) by enforcing the traffic over its shortest path to Z and
   considering the failure of the protected component.  The resulting
   end-to-end path between A and Z, upon recovery from the failure of
   CD, is depicted in Figure 3.

                          B * * *C------D------E
                         *|      | *  / | \  / |\
                        * |      |  */  |  \/  | \
                       A  |      |  /*  |  /\  |  Z
                        \ |      | /  * | /  \ | *
                         \|      |/    *|/    \|*
                          F------G------H * * *I

             Figure 3: Shortest Path Protection around Link CD

   When the primary path comes back up, the operator either allows for
   an automated reversion of the traffic onto the primary path or
   selects an operator-driven reversion.

4.  Managed Local Protection

   There may be cases where a management-free repair does not fit the
   policy of the operator.  For example, in our illustration, the
   operator may not want to have CD and CH used to protect each other
   due to the bandwidth (BW) availability in each link that could not
   suffice to absorb the other link traffic.

   In this context, 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, etc.) or under the form of an explicit
   path.  Upon local detection of the failure, the traffic is repaired
   over the backup path in sub-50 milliseconds.  When the primary path
   comes back up, the operator either allows for an automated reversion
   of the traffic onto it or selects an operator-driven reversion.

   We discuss such aspects for both bypass and shortest-path-based
   protection schemes.






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4.1.  Managed Bypass Protection

   Let us illustrate the case using our reference example.  For the
   demand from A to Z, the operator does not want to use the shortest
   failover path to the next hop, {CH, HD}, but rather the path {CG, GH,
   HD}, as illustrated in Figure 4.

                          B * * *C------D * * *E
                         *|      * \  / * \  / |*
                        * |      *  \/  *  \/  | *
                       A  |      *  /\  *  /\  |  Z
                        \ |      * /  \ * /  \ | /
                         \|      */    \*/    \|/
                          F------G * * *H------I

                    Figure 4: Managed Bypass Protection

   The computation of the repair path SHOULD be possible in an automated
   fashion as well as statically expressed in the point of local repair.

4.2.  Managed Shortest Path Protection

   In the case of shortest path protection, the operator does not want
   to use the shortest failover via link CH, but rather the traffic
   should reach H via {CG, GH} due to constraints such as delay, BW, or
   SRLG.

   The resulting end-to-end path upon activation of the protection is
   illustrated in Figure 5.

                          B * * *C------D------E
                         *|      * \  / | \  / |\
                        * |      *  \/  |  \/  | \
                       A  |      *  /\  |  /\  |  Z
                        \ |      * /  \ | /  \ | *
                         \|      */    \|/    \|*
                          F------G * * *H * * *I

                Figure 5: Managed Shortest Path Protection

   The computation of the repair path SHOULD be possible in an automated
   fashion as well as statically expressed in the point of local repair.

   The computation of the repair path based on a specific constraint
   SHOULD be possible on a per-destination prefix base.






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5.  Loop Avoidance

   It is part of routing protocols' behavior to have what are called
   "transient routing inconsistencies".  This is due to the routing
   convergence that happens in each node at different times and during a
   different lapse of time.

   These inconsistencies may cause routing loops that last the time that
   it takes for the node impacted by a network event to converge.  These
   loops are called "micro-loops".

   Usually, in normal routing protocol operations, micro-loops do not
   last long and are only noticed during the time it takes the network
   to converge.  However, with the emergence of fast-convergence and
   fast-reroute technologies, micro-loops can be an issue in networks
   where sub-50 millisecond convergence/reroute is required.  Therefore,
   the micro-loop problem needs to be addressed.

   Networks may be affected by micro-loops during convergence depending
   of their topologies.  Detecting micro-loops can be done during
   topology computation (e.g., Shortest Path First (SPF) computation),
   and therefore techniques to avoid micro-loops may be applied.  An
   example of such technique is to compute a path free of micro-loops
   that would be used during network convergence.

   The SPRING architecture SHOULD provide solutions to prevent the
   occurrence of micro-loops during convergence following a change in
   the network state.  Traditionally, the lack of packet steering
   capability made it difficult to apply efficient solutions to micro-
   loops.  A SPRING-enabled router could take advantage of the increased
   packet steering capabilities offered by SPRING in order to steer
   packets in a way that packets do not enter such loops.

6.  Coexistence of Multiple Resilience Techniques in the Same
    Infrastructure

   The operator may want to support several very different services on
   the same packet-switching infrastructure.  As a result, the SPRING
   architecture SHOULD allow for the coexistence of the different use
   cases listed in this document, in the same network.











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   Let us illustrate this with the following example:

   o  Flow F1 is supported over path {C, CD, E}

   o  Flow F2 is supported over path {C, CD, I}

   o  Flow F3 is supported over path {C, CD, Z}

   o  Flow F4 is supported over path {C, CD, Z}

   It should be possible for the operator to configure the network to
   achieve path protection for F1, management-free shortest path local
   protection for F2, managed protection over path {CG, GH, Z} for F3,
   and management-free bypass protection for F4.

7.  Security Considerations

   This document describes requirements for the SPRING architecture to
   provide resiliency in SPRING networks.  As such, it does not
   introduce any new security considerations beyond those discussed in
   [RFC7855].

8.  IANA Considerations

   This document has no IANA actions.

9.  Manageability Considerations

   This document provides use cases.  Solutions aimed at supporting
   these use cases should provide the necessary mechanisms in order to
   allow for manageability as described in [RFC7855].

   Manageability concerns the computation, installation, and
   troubleshooting of the repair path.  Also, necessary mechanisms
   SHOULD be provided in order for the operator to control when a repair
   path is computed, how it has been computed, and if it's installed and
   used.














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10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7855]  Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
              Litkowski, S., Horneffer, M., and R. Shakir, "Source
              Packet Routing in Networking (SPRING) Problem Statement
              and Requirements", RFC 7855, DOI 10.17487/RFC7855,
              May 2016, <https://www.rfc-editor.org/info/rfc7855>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2.  Informative References

   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <https://www.rfc-editor.org/info/rfc5286>.

   [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework",
              RFC 5714, DOI 10.17487/RFC5714, January 2010,
              <https://www.rfc-editor.org/info/rfc5714>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

Acknowledgements

   The authors would like to thank Stephane Litkowski and Alexander
   Vainshtein for the comments and review of this document.

Contributors

   Pierre Francois contributed to the writing of the first draft version
   of this document.








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Authors' Addresses

   Clarence Filsfils (editor)
   Cisco Systems, Inc.
   Brussels
   Belgium

   Email: cfilsfil@cisco.com


   Stefano Previdi (editor)
   Cisco Systems, Inc.
   Via Del Serafico, 200
   Rome  00142
   Italy

   Email: stefano@previdi.net


   Bruno Decraene
   Orange
   France

   Email: bruno.decraene@orange.com


   Rob Shakir
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   United States of America

   Email: robjs@google.com


















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