





Internet Engineering Task Force (IETF)                         S. Bryant
Request for Comments: 7490                                   C. Filsfils
Category: Standards Track                                     S. Previdi
ISSN: 2070-1721                                            Cisco Systems
                                                                M. Shand
                                                 Independent Contributor
                                                                   N. So
                                                           Vinci Systems
                                                              April 2015


          Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)

Abstract

   This document describes an extension to the basic IP fast reroute
   mechanism, described in RFC 5286, that provides additional backup
   connectivity for point-to-point link failures when none can be
   provided by the basic mechanisms.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7490.

Copyright Notice

   Copyright (c) 2015 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
   (http://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.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   3.  Overview of Solution  . . . . . . . . . . . . . . . . . . . .   4
   4.  Repair Paths  . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Tunnels as Repair Paths . . . . . . . . . . . . . . . . .   6
     4.2.  Tunnel Requirements . . . . . . . . . . . . . . . . . . .   7
   5.  Construction of Repair Paths  . . . . . . . . . . . . . . . .   8
     5.1.  Identifying Required Tunneled Repair Paths  . . . . . . .   8
     5.2.  Determining Tunnel Endpoints  . . . . . . . . . . . . . .   8
       5.2.1.  Computing Repair Paths  . . . . . . . . . . . . . . .   9
       5.2.2.  Selecting Repair Paths  . . . . . . . . . . . . . . .  11
     5.3.  A Cost-Based RLFA Algorithm . . . . . . . . . . . . . . .  12
     5.4.  Interactions with IS-IS Overload, RFC 6987, and Costed
           Out Links . . . . . . . . . . . . . . . . . . . . . . . .  17
   6.  Example Application of Remote LFAs  . . . . . . . . . . . . .  17
   7.  Node Failures . . . . . . . . . . . . . . . . . . . . . . . .  18
   8.  Operation in an LDP Environment . . . . . . . . . . . . . . .  19
   9.  Analysis of Real World Topologies . . . . . . . . . . . . . .  21
     9.1.  Topology Details  . . . . . . . . . . . . . . . . . . . .  21
     9.2.  LFA Only  . . . . . . . . . . . . . . . . . . . . . . . .  22
     9.3.  RLFA  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     9.4.  Comparison of LFA and RLFA results  . . . . . . . . . . .  24
   10. Management and Operational Considerations . . . . . . . . . .  25
   11. Historical Note . . . . . . . . . . . . . . . . . . . . . . .  25
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  25
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     13.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29


















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1.  Introduction

   RFC 5714 [RFC5714] describes a framework for IP Fast Reroute (IPFRR)
   and provides a summary of various proposed IPFRR solutions.  A basic
   mechanism using Loop-Free Alternates (LFAs) is described in [RFC5286]
   that provides good repair coverage in many topologies [RFC6571],
   especially those that are highly meshed.  However, some topologies,
   notably ring-based topologies, are not well protected by LFAs alone.
   This is because there is no neighbor of the Point of Local Repair
   (PLR) that has a cost to the destination via a path that does not
   traverse the failure that is cheaper than the cost to the destination
   via the failure.

   The method described in this document extends the LFA approach
   described in [RFC5286] to cover many of these cases by tunneling the
   packets that require IPFRR to a node that is both reachable from the
   PLR and can reach the destination.

2.  Terminology

   This document uses the terms defined in [RFC5714].  This section
   defines additional terms that are used in this document.

   Repair tunnel:
      A tunnel established for the purpose of providing a virtual
      neighbor that is a Loop-Free Alternate.

   P-space:
      The P-space of a router with respect to a protected link is the
      set of routers reachable from that specific router using the pre-
      convergence shortest paths without any of those paths (including
      equal-cost path splits) transiting that protected link.

      For example, the P-space of S with respect to link S-E is the set
      of routers that S can reach without using the protected link S-E.

   Extended P-space:
      Consider the set of neighbors of a router protecting a link.
      Exclude from that set of routers the router reachable over the
      protected link.  The extended P-space of the protecting router
      with respect to the protected link is the union of the P-spaces of
      the neighbors in that set of neighbors with respect to the
      protected link (see Section 5.2.1.2).








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   Q-space:
      The Q-space of a router with respect to a protected link is the
      set of routers from which that specific router can be reached
      without any path (including equal-cost path splits) transiting
      that protected link.

   PQ node:
      A PQ node of a node S with respect to a protected link S-E is a
      node that is a member of both the P-space (or the extended
      P-space) of S with respect to that protected link S-E and the
      Q-space of E with respect to that protected link S-E.  A repair
      tunnel endpoint is chosen from the set of PQ-nodes.

   Remote LFA (RLFA):
      The use of a PQ node rather than a neighbor of the repairing node
      as the next hop in an LFA repair [RFC5286].

   In this document, the notation X-Y is used to mean the path from X to
   Y over the link directly connecting X and Y while the notation X->Y
   refers to the shortest path from X to Y via some set of unspecified
   nodes including the null set (i.e., including over a link directly
   connecting X and Y).

2.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Overview of Solution

   The problem of LFA IPFRR reachability in some networks is illustrated
   by the network fragment shown in Figure 1 below.

                                    S---E
                                   /     \
                                  A       D
                                   \     /
                                    B---C

                     Figure 1: A Simple Ring Topology

   If all link costs are equal, traffic that is transiting link S-E
   cannot be fully protected by LFAs.  The destination C is an Equal-
   Cost Multipath (ECMP) from S, and so traffic to C can be protected
   when S-E fails but traffic to D and E are not protectable using LFAs.





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   This document describes extensions to the basic repair mechanism in
   which tunnels are used to provide additional logical links that can
   then be used as loop-free alternates where none exist in the original
   topology.  In Figure 1, S can reach A, B, and C without going via
   S-E; these form S's extended P-space with respect to S-E.  The
   routers that can reach E without going through S-E will be in E's
   Q-space with respect to link S-E; these are D and C.  B has equal-
   cost paths to E via B-A-S-E and B-C-D-E, and so the forwarder at S
   might choose to send a packet to E via link S-E.  Hence, B is not in
   the Q-space of E with respect to link S-E.  The single node in both
   S's extended P-space and E's Q-space is C; thus, node C is selected
   as the repair tunnel's endpoint.  Thus, if a tunnel is provided
   between S and C as shown in Figure 2, then C, now being a direct
   neighbor of S, would become an LFA for D and E.  The definition of
   (extended) P-space and Q-space are provided in Section 2, and details
   of the calculation of the tunnel end points are provided in
   Section 5.2.

   The non-failure traffic distribution is not disrupted by the
   provision of such a tunnel since it is only used for repair traffic
   and MUST NOT be used for normal traffic.  Note that Operations,
   Administration, and Maintenance (OAM) traffic used specifically to
   verify the viability of the repair MAY traverse the tunnel prior to a
   failure.

                                    S---E
                                   / \   \
                                  A   \   D
                                   \   \ /
                                    B---C

                    Figure 2: The Addition of a Tunnel

   The use of this technique is not restricted to ring-based topologies
   but it is a general mechanism that can be used to enhance the
   protection provided by LFAs.  A study of the protection achieved
   using remote LFA in typical service provider core networks is
   provided in Section 9, and a side-by-side comparison between LFA and
   remote LFA is provided in Section 9.4.

   Remote LFA is suitable for incremental deployment within a network,
   including a network that is already deploying LFA.  Computation of
   the repair path requires acceptable CPU resources and takes place
   exclusively on the repairing node.  In MPLS networks, the targeted
   LDP protocol needed to learn the label binding at the repair tunnel
   endpoint (Section 8) is a well understood and widely deployed
   technology.




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   The technique described in this document is directed at providing
   repairs in the case of link failures.  Considerations regarding node
   failures are discussed in Section 7.  This memo describes a solution
   to the case where the failure occurs on a point-to-point link.  It
   covers the case where the repair first hop is reached via a broadcast
   or non-broadcast multi-access (NBMA) link such as a LAN and the case
   where the P or Q node is attached via such a link.  It does not,
   however, cover the more complicated case where the failed interface
   is a broadcast or NBMA link.

   This document considers the case when the repair path is confined to
   either a single area or to the level two routing domain.  In all
   other cases, the chosen PQ node should be regarded as a tunnel
   adjacency of the repairing node, and the considerations described in
   Section 6 of [RFC5286] should be taken into account.

4.  Repair Paths

   As with LFA FRR, when a router detects an adjacent link failure, it
   uses one or more repair paths in place of the failed link.  Repair
   paths are precomputed in anticipation of later failures so they can
   be promptly activated when a failure is detected.

   A tunneled repair path tunnels traffic to some staging point in the
   network from which it is known that, in the absence of a worse-than-
   anticipated failure, the traffic will travel to its destination using
   normal forwarding without looping back.  This is equivalent to
   providing a virtual loop-free alternate to supplement the physical
   loop-free alternates; hence the name "remote LFA FRR".  In its
   simplest form, when a link cannot be entirely protected with local
   LFA neighbors, the protecting router seeks the help of a remote LFA
   staging point.  Network manageability considerations may lead to a
   repair strategy that uses a remote LFA more frequently [LFA-MANAGE].

   Examples of worse failures are node failures (see Section 7), the
   failure of a Shared Risk Link Group (SRLG), the independent
   concurrent failures of multiple links, or broadcast or NBMA links
   (Section 3); protecting against such failures is out of scope for
   this specification.

4.1.  Tunnels as Repair Paths

   Consider an arbitrary protected link S-E.  In LFA FRR, if a path to
   the destination from a neighbor N of S does not cause a packet to
   loop back over the link S-E (i.e., N is a loop-free alternate), then
   S can send the packet to N and the packet will be delivered to the
   destination using the pre-failure forwarding information.  If there
   is no such LFA neighbor, then S may be able to create a virtual LFA



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   by using a tunnel to carry the packet to a point in the network that
   is not a direct neighbor of S from which the packet will be delivered
   to the destination without looping back to S.  In this document, such
   a tunnel is termed a repair tunnel.  The tail end of this tunnel (the
   repair tunnel endpoint) is a "PQ node", and the repair mechanism is a
   "remote LFA".  This tunnel MUST NOT traverse the link S-E.

   Note that the repair tunnel terminates at some intermediate router
   between S and E, and not E itself.  This is clearly the case, since
   if it were possible to construct a tunnel from S to E, then a
   conventional LFA would have been sufficient to effect the repair.

4.2.  Tunnel Requirements

   There are a number of IP-in-IP tunnel mechanisms that may be used to
   fulfill the requirements of this design, such as IP-in-IP [RFC1853]
   and Generic Routing Encapsulation (GRE) [RFC1701].

   In an MPLS-enabled network using LDP [RFC5036], a simple label stack
   [RFC3032] may be used to provide the required repair tunnel.  In this
   case, the outer label is S's neighbor's label for the repair tunnel
   endpoint, and the inner label is the repair tunnel endpoint's label
   for the packet destination.  In order for S to obtain the correct
   inner label, it is necessary to establish a targeted LDP session
   [RFC5036] to the tunnel endpoint.

   The selection of the specific tunneling mechanism (and any necessary
   enhancements) used to provide a repair path is outside the scope of
   this document.  The deployment in an MPLS/LDP environment is
   relatively simple in the data plane, as an LDP Label Switched Path
   (LSP) from S to the repair tunnel endpoint (the selected PQ node) is
   readily available and hence does not require any new protocol
   extension or design change.  This LSP is automatically established as
   a basic property of LDP behavior.  The performance of the
   encapsulation and decapsulation is efficient, as encapsulation is
   just a push of one label (like conventional MPLS-TE FRR) and the
   decapsulation is normally configured to occur at the penultimate hop
   before the repair tunnel endpoint.  In the control plane, a Targeted
   LDP (TLDP) session is needed between the repairing node and the
   repair tunnel endpoint, which will need to be established and the
   labels processed before the tunnel can be used.  The time to
   establish the TLDP session and acquire labels will limit the speed at
   which a new tunnel can be put into service.  This is not anticipated
   to be a problem in normal operation since the managed introduction
   and removal of links is relatively rare, as is the incidence of
   failure in a well-managed network.





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   When a failure is detected, it is necessary to immediately redirect
   traffic to the repair path.  Consequently, the repair tunnel used
   MUST be provisioned beforehand in anticipation of the failure.  Since
   the location of the repair tunnels is dynamically determined, it is
   necessary to automatically establish the repair tunnels.  Multiple
   repair tunnels may share a tunnel endpoint.

5.  Construction of Repair Paths

5.1.  Identifying Required Tunneled Repair Paths

   Not all links will require protection using a tunneled repair path.
   Referring to Figure 1, if E can already be protected via an LFA, S-E
   does not need to be protected using a repair tunnel since all
   destinations normally reachable through E must therefore also be
   protectable by an LFA; such an LFA is frequently termed a "link LFA".
   Tunneled repair paths (which may be calculated per prefix) are only
   required for links that do not have a link or per-prefix LFA.

   It should be noted that using the Q-space of E as a proxy for the
   Q-space of each destination can result in failing to identify valid
   remote LFAs.  The extent to which this reduces the effective
   protection coverage is topology dependent.

5.2.  Determining Tunnel Endpoints

   The repair tunnel endpoint needs to be a node in the network
   reachable from S without traversing S-E.  In addition, the repair
   tunnel endpoint needs to be a node from which packets will normally
   flow towards their destination without being attracted back to the
   failed link S-E.

   Note that once released from the tunnel, the packet will be
   forwarded, as normal, on the shortest path from the release point to
   its destination.  This may result in the packet traversing the router
   E at the far end of the protected link S-E, but this is obviously not
   required.

   The properties that are required of repair tunnel endpoints are as
   follows:

   o  The repair tunneled point MUST be reachable from the tunnel source
      without traversing the failed link; and

   o  when released from the tunnel, packets MUST proceed towards their
      destination without being attracted back over the failed link.





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   Provided both these requirements are met, packets forwarded over the
   repair tunnel will reach their destination and will not loop after a
   single link failure.

   In some topologies it will not be possible to find a repair tunnel
   endpoint that exhibits both the required properties.  For example, if
   the ring topology illustrated in Figure 1 had a cost of four for the
   link B-C while the remaining links were the cost of one, then it
   would not be possible to establish a tunnel from S to C (without
   resorting to some form of source routing).

5.2.1.  Computing Repair Paths

   To compute the repair path for link S-E, it is necessary to determine
   the set of routers that can be reached from S without traversing S-E
   and match this with the set of routers from which the node E can be
   reached by normal forwarding without traversing the link S-E.

   The approach used in this memo is as follows:

   o  The method of computing the set of routers that can be reached
      from S on the shortest path tree without traversing S-E is
      described.  This is called the S's P-space with respect to the
      failure of link S-E.

   o  The distance of the tunnel endpoint from the PLR is increased by
      noting that S is able to use the P-space of its neighbors with
      respect to the failure of link S-E since S can determine which
      neighbor it will use as the next hop for the repair.  This is
      called the S's extended P-space with respect to the failure of
      link S-E.  The use of extended P-space allows greater repair
      coverage and is the preferred approach.

   o  Finally, two methods of computing the set of routers from which
      the node E can be reached by normal forwarding without traversing
      the link S-E.  This is called the Q-space of E with respect to the
      link S-E.

   The selection of the preferred node from the set of nodes that are in
   both extended P-space and Q-space with respect to the S-E is
   described in Section 5.2.2.

   A suitable cost-based algorithm to compute the set of nodes common to
   both extended P-space and Q-space with respect to the S-E is provided
   in Section 5.3.






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5.2.1.1.  P-space

   The set of routers that can be reached from S on the shortest path
   tree without traversing S-E is termed the P-space of S with respect
   to the link S-E.  This P-space can be obtained by computing a
   Shortest Path Tree (SPT) rooted at S and excising the subtree reached
   via the link S-E (including those routers that are members of an ECMP
   that includes link S-E).  The exclusion of routers reachable via an
   ECMP that includes S-E prevents the forwarding subsystem from
   attempting to execute a repair via the failed link S-E.  Thus, for
   example, if the Shortest Path First (SPF) computation stores at each
   node the next hops to be used to reach that node from S, then the
   node can be added to P-space if none of its next hops are link S-E.
   In the case of Figure 1, this P-space comprises nodes A and B only.
   Expressed in cost terms, the set of routers {P} are those for which
   the shortest path cost S->P is strictly less than the shortest path
   cost S->E->P.

5.2.1.2.  Extended P-space

   The description in Section 5.2.1.1 calculated router S's P-space
   rooted at S itself.  However, since router S will only use a repair
   path when it has detected the failure of the link S-E, the initial
   hop of the repair path need not be subject to S's normal forwarding
   decision process.  Thus, the concept of extended P-space is
   introduced.  Router S's extended P-space is the union of the P-spaces
   of each of S's neighbors (N).  This may be calculated by computing an
   SPT at each of S's neighbors (excluding E) and excising the subtree
   reached via the path N->S->E.  Note this will excise those routers
   that are reachable through all ECMPs that include link S-E.  The use
   of extended P-space may allow router S to reach potential repair
   tunnel endpoints that were otherwise unreachable.  In cost terms, a
   router (P) is in extended P-space if the shortest path cost N->P is
   strictly less than the shortest path cost N->S->E->P.  In other
   words, once the packet is forced to N by S, it is a lower cost for it
   to continue on to P by any path except one that takes it back to S
   and then across the S->E link.

   Since in the case of Figure 1 node A is a per-prefix LFA for the
   destination node C, the set of extended P-space nodes with respect to
   link S-E comprises nodes A, B, and C.  Since node C is also in E's
   Q-space with respect to link S-E, there is now a node common to both
   extended P-space and Q-space that can be used as a repair tunnel
   endpoint to protect the link S-E.







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5.2.1.3.  Q-space

   The set of routers from which the node E can be reached, by normal
   forwarding without traversing the link S-E, is termed the Q-space of
   E with respect to the link S-E.  The Q-space can be obtained by
   computing a reverse Shortest Path Tree (rSPT) rooted at E, with the
   subtree that might traverse the protected link S-E excised (i.e.,
   those nodes that would send the packet via S-E plus those nodes that
   have an ECMP set to E with one or more members of that ECMP set
   traversing the protected link S-E).  The rSPT uses the cost towards
   the root rather than from it and yields the best paths towards the
   root from other nodes in the network.  In the case of Figure 1, the
   Q-space of E with respect to S-E comprises nodes C and D only.
   Expressed in cost terms, the set of routers {Q} are those for which
   the shortest path cost Q<-E is strictly less than the shortest path
   cost Q<-S<-E.  In Figure 1, the intersection of the E's Q-space with
   respect to S-E with S's P-space with respect to S-E defines the set
   of viable repair tunnel endpoints, known as "PQ nodes".  As can be
   seen in the case of Figure 1, there is no common node and hence no
   viable repair tunnel endpoint.  However, when the extended P-space
   (Section 5.2.1.2) at S with respect to S-E is considered, a suitable
   intersection is found at C.

   Note that the Q-space calculation could be conducted for each
   individual destination and a per-destination repair tunnel end point
   determined.  However, this would, in the worst case, require an SPF
   computation per destination that is not currently considered to be
   scalable.  Therefore, the Q-space of E with respect to link S-E is
   used as a proxy for the Q-space of each destination.  This
   approximation is obviously correct since the repair is only used for
   the set of destinations which were, prior to the failure, routed
   through node E.  This is analogous to the use of link LFAs rather
   than per-prefix LFAs.

5.2.2.  Selecting Repair Paths

   The mechanisms described above will identify all the possible repair
   tunnel endpoints that can be used to protect a particular link.  In a
   well-connected network, there are likely to be multiple possible
   release points for each protected link.  All will deliver the packets
   correctly, so arguably, it does not matter which is chosen.  However,
   one repair tunnel endpoint may be preferred over the others on the
   basis of path cost or some other selection criteria.

   There is no technical requirement for the selection criteria to be
   consistent across all routers, but such consistency may be desirable
   from an operational point of view.  In general, there are advantages
   in choosing the repair tunnel endpoint closest (shortest metric) to



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   S.  Choosing the closest maximizes the opportunity for the traffic to
   be load balanced once it has been released from the tunnel.  For
   consistency in behavior, it is RECOMMENDED that the member of the set
   of routers {PQ} with the lowest cost S->P be the default choice for
   P.  In the event of a tie, the router with the lowest node identifier
   SHOULD be selected.

   It is a local matter whether the repair path selection policy used by
   the router favors LFA repairs over RLFA repairs.  An LFA repair has
   the advantage of not requiring the use of a tunnel; however, network
   manageability considerations may lead to a repair strategy that uses
   a remote LFA more frequently [LFA-MANAGE].

   As described in [RFC5286], always selecting a PQ node that is
   downstream to the destination with respect to the repairing node
   prevents the formation of loops when the failure is worse than
   expected.  The use of downstream nodes reduces the repair coverage,
   and operators are advised to determine whether adequate coverage is
   achieved before enabling this selection feature.

5.3.  A Cost-Based RLFA Algorithm

   The preceding text has described the computation of the remote LFA
   repair target (PQ) in terms of the intersection of two reachability
   graphs computed using an SPF algorithm.  This section describes a
   method of computing the remote LFA repair target for a specific
   failed link using a cost-based algorithm.  The pseudocode provided in
   this section avoids unnecessary SPF computations; for the sake of
   readability, it does not otherwise try to optimize the code.  The
   algorithm covers the case where the repair first hop is reached via a
   broadcast or NBMA link such as a LAN.  It also covers the case where
   the P or Q node is attached via such a link.  It does not cover the
   case where the failed interface is a broadcast or NBMA link.  To
   address that case it is necessary to compute the Q-space of each
   neighbor of the repairing router reachable through the LAN, i.e., to
   treat the pseudonode [RFC1195] as a node failure; this is because the
   Q-spaces of the neighbors of the pseudonode may be disjoint and
   require use of a neighbor-specific PQ node.  The reader is referred
   to [NODE-PROTECTION] for further information on the use of RLFA for
   node repairs.

   The following notation is used:

   o  D_opt(a,b) is the shortest distance from node a to node b as
      computed by the SPF.

   o  dest is the packet destination.




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   o  fail_intf is the failed interface (S-E in the example).

   o  fail_intf.remote_node is the node reachable over interface
      fail_intf (node E in the example).

   o  intf.remote_node is the set of nodes reachable over interface
      intf.

   o  root is the root of the SPF calculation.

   o  self is the node carrying out the computation.

   o  y is the node in the network under consideration.

   o  y.pseudonode is true if y is a pseudonode.

      //////////////////////////////////////////////////////////////////
      //
      //   Main Function



      //////////////////////////////////////////////////////////////////
      //
      // We have already computed the forward SPF from self to all nodes
      // y in network and thus we know D_opt (self, y).  This is needed
      // for normal forwarding.
      // However, for completeness:

      Compute_and_Store_Forward_SPF(self)

      // To extend P-space, we compute the SPF at each neighbor except
      // the neighbor that is reached via the link being protected.
      // We will also need D_opt(fail_intf.remote_node,y), so we
      // compute that at the same time.

      Compute_Neighbor_SPFs()

      // Compute the set of nodes {P} reachable other than via the
      // failed link.

      Compute_Extended_P_Space(fail_intf)

      // Compute the set of nodes that can reach the node on the far
      // side of the failed link without traversing the failed link.

      Compute_Q_Space(fail_intf)




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      // Compute the set of candidate RLFA tunnel endpoints.

      Intersect_Extended_P_and_Q_Space()

      // Make sure that we cannot get looping repairs when the
      // failure is worse than expected.

      if (guarantee_no_looping_on_worse_than_protected_failure)
          Apply_Downstream_Constraint()

      //
      //  End of Main Function
      //
      //////////////////////////////////////////////////////////////////

      //////////////////////////////////////////////////////////////////
      //
      //  Procedures
      //


      /////////////////////////////////////////////////////////////////
      //
      // This computes the SPF from root and stores the optimum
      // distance from root to each node y.

      Compute_and_Store_Forward_SPF(root)
          Compute_Forward_SPF(root)
          foreach node y in network
              store D_opt(root,y)



      /////////////////////////////////////////////////////////////////
      //
      // This computes the optimum distance from each neighbor (other
      // than the neighbor reachable through the failed link) and
      // every other node in the network.
      //
      // Note that we compute this for all neighbors, including the
      // neighbor on the far side the failure.  This is done on the
      // expectation that more than one link will be protected and
      // that the results are stored for later use.
      //

      Compute_Neighbor_SPFs()
          foreach interface intf in self
              Compute_and_Store_Forward_SPF(intf.remote_node)



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      /////////////////////////////////////////////////////////////////
      //
      // The reverse SPF computes the cost from each remote node to
      // root. This is achieved by running the normal SPF algorithm
      // but using the link cost in the direction from the next hop
      // back towards root in place of the link cost in the direction
      // away from root towards the next hop.

      Compute_and_Store_Reverse_SPF(root)
          Compute_Reverse_SPF(root)
          foreach node y in network
              store D_opt(y,root)



      /////////////////////////////////////////////////////////////////
      //
      // Calculate Extended P-space
      //
      // Note that the "strictly less than" operator is needed to
      // avoid ECMP issues.

      Compute_Extended_P_Space(fail_intf)
          foreach node y in network
              y.in_extended_P_space = false
              // Extend P-space to the P-spaces of all reachable
              // neighbors
              foreach interface intf in self
                  // Exclude failed interface, noting that
                  // the node reachable via that interface may be
                  // reachable via another interface (parallel path)
                  if (intf != fail_intf)
                      foreach neighbor n in intf.remote_node
                          // Apply RFC 5286 Inequality 1
                          if ( D_opt(n, y) <
                                  D_opt(n,self) + D_opt(self, y))
                              y.in_extended_P_space = true

      /////////////////////////////////////////////////////////////////
      //
      // Compute the Nodes in Q-space
      //

      Compute_Q_Space(fail_intf)
          // Compute the cost from every node in the network to the
          // node normally reachable across the failed link
          Compute_and_Store_Reverse_SPF(fail_intf.remote_node)




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          // Compute the cost from every node in the network to self
          Compute_and_Store_Reverse_SPF(self)

          foreach node y in network
              if ( D_opt(y,fail_intf.remote_node) < D_opt(y,self) +
                      D_opt(self,fail_intf.remote_node) )
                  y.in_Q_space = true
              else
                  y.in_Q_space = false



      /////////////////////////////////////////////////////////////////
      //
      // Compute Set of Nodes in Both Extended P-space and in Q-space

      Intersect_Extended_P_and_Q_Space()
          foreach node y in network
              if ( y.in_extended_P_space && y.in_Q_space &&
                      y.pseudonode == False)
                  y.valid_tunnel_endpoint = true
              else
                  y.valid_tunnel_endpoint = false


      /////////////////////////////////////////////////////////////////
      //
      // A downstream route is one where the next hop is strictly
      // closer to the destination.  By sending the packet to a
      // PQ node that is downstream, we know that if the PQ node
      // detects a failure it will not loop the packet back to self.
      // This is useful when there are two failures or when a node has
      // failed rather than a link.

      Apply_Downstream_Constraint()
          foreach node y in network
              if (y.valid_tunnel_endpoint)
                  Compute_and_Store_Forward_SPF(y)
                  if ((D_opt(y,dest) < D_opt(self,dest))
                      y.valid_tunnel_endpoint = true
                  else
                      y.valid_tunnel_endpoint = false


   //
   /////////////////////////////////////////////////////////////////





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5.4.  Interactions with IS-IS Overload, RFC 6987, and Costed Out Links

   Since normal link state routing takes into account the IS-IS overload
   bit, OSPF stub router advertisement [RFC6987], and costed out links
   (as described in Section 3.5 of [RFC5286]), the forward SPFs
   performed by the PLR rooted at the neighbors of the PLR also need to
   take this into account.  A repair tunnel path from a neighbor of the
   PLR to a repair tunnel endpoint will generally avoid the nodes and
   links excluded by the IGP overload/costing-out rules.  However, there
   are two situations where this behavior may result in a repair path
   traversing a link or router that should be excluded:

   1.  One situation is when the first hop on the repair tunnel path
       (from the PLR to a direct neighbor) does not follow the IGP
       shortest path.  In this case, the PLR MUST NOT use a repair
       tunnel path whose first hop is along a link that has a cost or
       reverse cost equal to MaxLinkMetric (for OSPF) or the maximum
       cost (for IS-IS) or whose first hop has the overload bit set (for
       IS-IS).

   2.  The other situation is when the IS-IS overload bit and the
       mechanism of [RFC6987] only prevent transit traffic from
       traversing a node; they do not prevent traffic destined to a
       node.  The per-neighbor forward SPFs using the standard IGP
       overload rules will not prevent a PLR from choosing a repair
       tunnel endpoint that is advertising a desire to not carry transit
       traffic.  Therefore, the PLR MUST NOT use a repair tunnel
       endpoint with the IS-IS overload bit set or where all outgoing
       interfaces have the cost set to MaxLinkMetric for OSPF.

6.  Example Application of Remote LFAs

   An example of a commonly deployed topology that is not fully
   protected by LFAs alone is shown in Figure 3.  Provider Edge (PE)1
   and PE2 are connected in the same site.  P1 and P2 may be
   geographically separated (intersite).  In order to guarantee the
   lowest latency path from/to all other remote PEs, normally the
   shortest path follows the geographical distance of the site
   locations.  Therefore, to ensure this, a lower IGP metric (5) is
   assigned between PE1 and PE2.  A high metric (1000) is set on the
   P-PE links to prevent the PEs being used for transit traffic.  The
   PEs are not individually dual-homed in order to reduce costs.

   This is a common topology in Service Provider (SP) networks.







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   When a failure occurs on the link between PE1 and P1, PE1 does not
   have an LFA for traffic reachable via P1.  Similarly, by symmetry, if
   the link between PE2 and P2 fails, PE2 does not have an LFA for
   traffic reachable via P2.

   Increasing the metric between PE1 and PE2 to allow the LFA would
   impact the normal traffic performance by potentially increasing the
   latency.

                               |    100    |
                              -P1---------P2-
                                \         /
                            1000 \       / 1000
                                 PE1---PE2
                                     5

                       Figure 3: Example SP Topology

   Clearly, full protection can be provided using the techniques
   described in this document by PE1 choosing P2 as the remote LFA
   repair target node and PE2 choosing P1 as the remote LFA repair
   target.

7.  Node Failures

   When the failure is a node failure rather than a point-to-point link
   failure, there is a danger that the RLFA repair will loop.  This is
   discussed in detail in [IP-FRR].  In summary, the problem is that two
   or more of E's neighbors, each with E as the next hop to some
   destination D, may attempt to repair a packet addressed to
   destination D via the other neighbor and then E, thus causing a loop
   to form.  A similar problem exists in the case of a shared risk link
   group failure where the PLR for each failure attempts to repair via
   the other failure.  As will be noted from [IP-FRR], this can rapidly
   become a complex problem to address.

   There are a number of ways to minimize the probability of a loop
   forming when a node failure occurs, and there exists the possibility
   that two of E's neighbors may form a mutual repair.

   1.  Detect when a packet has arrived on some interface I that is also
       the interface used to reach the first hop on the RLFA path to the
       remote LFA repair target, and drop the packet.  This is useful in
       the case of a ring topology.







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   2.  Require that the path from the remote LFA repair target to
       destination D never passes through E (including in the ECMP
       case), i.e., only use node protecting paths in which the cost
       from the remote LFA repair target to D is strictly less than the
       cost from the remote LFA repair target to E plus the cost E to D.

   3.  Require that where the packet may pass through another neighbor
       of E, that node is down stream (i.e., strictly closer to D than
       the repairing node).  This means that some neighbor of E (X) can
       repair via some other neighbor of E (Y), but Y cannot repair via
       X.

   Case 1 accepts that loops may form and suppresses them by dropping
   packets.  Dropping packets may be considered less detrimental than
   looping packets.  This approach may also lead to dropping some
   legitimate packets.  Cases 2 and 3 above prevent the formation of a
   loop but at the expense of a reduced repair coverage and at the cost
   of additional complexity in the algorithm to compute the repair path.
   Alternatively, one might choose to assume that the probability of a
   node failure is sufficiently rare that the issue of looping RLFA
   repairs can be ignored.

   The probability of a node failure and the consequences of node
   failure in any particular topology will depend on the node design,
   the particular topology in use, and the strategy adopted under node
   failure.  It is recommended that a network operator perform an
   analysis of the consequences and probability of node failure in their
   network and determine whether the incidence and consequence of
   occurrence are acceptable.

   This topic is further discussed in [NODE-PROTECTION].

8.  Operation in an LDP Environment

   Where this technique is used in an MPLS network using LDP [RFC5036],
   and S is a transit node, S will need to swap the top label in the
   stack for the remote LFA repair target's (PQ's) label to the
   destination and to then push its own label for the remote LFA repair
   target.

   In the example, S in Figure 2 already has the first hop (A) label for
   the remote LFA repair target (C) as a result of the ordinary
   operation of LDP.  To get the remote LFA repair target's label (C's
   label) for the destination (D), S needs to establish a targeted LDP
   session with C.  The label stack for normal operation and RLFA
   operation is shown below in Figure 4.





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   +-----------------+     +-----------------+     +-----------------+
   |    datalink     |     |    datalink     |     |    datalink     |
   +-----------------+     +-----------------+     +-----------------+
   | S's label for D |     | E's label for D |     | A's label for C |
   +-----------------+     +-----------------+     +-----------------+
   |    Payload      |     |    Payload      |     | C's label for D |
   +-----------------+     +-----------------+     +-----------------+
           X                       Y               |    Payload      |
                                                   +-----------------+
                                                            Z

   X = Normal label stack packet arriving at S
   Y = Normal label stack packet leaving S
   Z = RLFA label stack to D via C as the remote LFA repair target

                                 Figure 4

   To establish a targeted LDP session with a candidate remote LFA
   repair target node, the repairing node (S) needs to know what IP
   address the remote LFA repair target is willing to use for targeted
   LDP sessions.  Ideally, this is provided by the remote LFA repair
   target advertising this address in the IGP in use.  Which address is
   used, how this is advertised in the IGP, and whether this is a
   special IP address or an IP address also used for some other purpose
   is out of scope for this document and must be specified in an
   IGP-specific RFC.

   In the absence of a protocol to learn the preferred IP address for
   targeted LDP, an LSR should attempt a targeted LDP session with the
   Router ID [RFC2328] [RFC5305] [RFC5340] [RFC6119] [OSPF-RI] unless it
   is configured otherwise.

   No protection is available until the TLDP session has been
   established and a label for the destination has been learned from the
   remote LFA repair target.  If for any reason the TLDP session cannot
   be established, an implementation SHOULD advise the operator about
   the protection setup issue through the network management system.














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9.  Analysis of Real World Topologies

   This section gives the results of analyzing a number of real world
   service provider topologies collected between the end of 2012 and
   early 2013.

9.1.  Topology Details

   The figure below characterizes each topology (topo) studied in terms
   of:

   o  the number of nodes (# nodes) excluding pseudonodes;

   o  the number of bidirectional links (# links) including parallel
      links and links to and from pseudonodes;

   o  the number of node pairs that are connected by one or more links
      (# pairs);

   o  the number of node pairs that are connected by more than one
      (i.e., parallel) link (# para); and

   o  the number of links (excluding pseudonode links, which are by
      definition asymmetric) that have asymmetric metrics (# asym).

      +------+---------+---------+---------+--------+--------+
      | topo | # nodes | # links | # pairs | # para | # asym |
      +------+---------+---------+---------+--------+--------+
      |    1 |     315 |     570 |     560 |     10 |      3 |
      |    2 |     158 |     373 |     312 |     33 |      0 |
      |    3 |     655 |    1768 |    1314 |    275 |   1195 |
      |    4 |    1281 |    2326 |    2248 |     70 |     10 |
      |    5 |     364 |     811 |     659 |     80 |     86 |
      |    6 |     114 |     318 |     197 |    101 |      4 |
      |    7 |      55 |     237 |     159 |     67 |      2 |
      |    8 |     779 |    1848 |    1441 |    199 |    437 |
      |    9 |     263 |     482 |     413 |     41 |     12 |
      |   10 |      86 |     375 |     145 |     64 |     22 |
      |   11 |     162 |    1083 |     351 |    201 |     49 |
      |   12 |     380 |    1174 |     763 |    231 |      0 |
      |   13 |    1051 |    2087 |    2037 |     48 |     64 |
      |   14 |      92 |     291 |     204 |     64 |      2 |
      +------+---------+---------+---------+--------+--------+








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9.2.  LFA Only

   The figure below shows the percentage of protected destinations (%
   prot) and the percentage of guaranteed node protected destinations (%
   gtd N) for the set of topologies characterized in Section 9.1
   achieved using only LFA repairs.

   These statistics were generated by considering each node and then
   considering each link to each next hop to each destination.  The
   percentage of such links across the entire network that are protected
   against link failure was determined.  This is the percentage of
   protected destinations.  If a link is protected against the failure
   of the next hop node, this is considered Guaranteed Node Protecting
   (GNP) and the percentage of guaranteed node protected destinations is
   calculated using the same method used for calculating the link
   protection coverage.

   GNP is identical to node-protecting as defined in [RFC6571] and does
   not include the additional node protection coverage obtained by the
   de facto node-protecting condition described in [RFC6571].

      +------+--------+---------+
      | topo | % prot | % gtd N |
      +------+--------+---------+
      |    1 | 78.5   | 36.9    |
      |    2 | 97.3   | 52.4    |
      |    3 | 99.3   | 58      |
      |    4 | 83.1   | 63.1    |
      |    5 | 99     | 59.1    |
      |    6 | 86.4   | 21.4    |
      |    7 | 93.9   | 35.4    |
      |    8 | 95.3   | 48.1    |
      |    9 | 82.2   | 49.5    |
      |   10 | 98.5   | 14.9    |
      |   11 | 99.6   | 24.8    |
      |   12 | 99.5   | 62.4    |
      |   13 | 92.4   | 51.6    |
      |   14 | 99.3   | 48.6    |
      +------+--------+---------+

9.3.  RLFA

   The figure below shows the percentage of protected destinations (%
   prot) and % guaranteed node protected destinations (% gtd N) for RLFA
   protection in the topologies studies.  In addition, it shows the
   percentage of destinations using an RLFA repair (% PQ) together with
   the total number of unidirectional RLFA targeted LDP sessions
   established (# PQ), and the number of PQ sessions that would be



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   required for complete protection but that could not be established
   because there was no PQ node, i.e., the number of cases whether
   neither LFA or RLFA protection was possible (no PQ).  It also shows
   the 50 (p50), 90 (p90), and 100 (p100) percentiles for the number of
   individual LDP sessions terminating at an individual node (whether
   used for TX, RX, or both).

   For example, if there were LDP sessions that required A->B, A->C,
   C->A, and C->D, these would be counted as 2, 1, 2, and 1 at nodes A,
   B, C, and D respectively because:

      A has two sessions (to nodes B and C);

      B has one session (to node A);

      C has two sessions (to nodes A and D); and

      D has one session (to node D).

   In this study, remote LFA is only used when necessary, i.e., when
   there is at least one destination that is not reparable by a per
   destination LFA and a single remote LFA tunnel is used (if available)
   to repair traffic to all such destinations.  The remote LFA repair
   target points are computed using extended P-space and choosing the PQ
   node that has the lowest metric cost from the repairing node.

     +------+--------+--------+------+------+-------+-----+-----+------+
     | topo | % prot |% gtd N | % PQ | # PQ | no PQ | p50 | p90 | p100 |
     +------+--------+--------+------+------+-------+-----+-----+------+
     |    1 | 99.7   | 53.3   | 21.2 |  295 |     3 |   1 |   5 |   14 |
     |    2 | 97.5   | 52.4   | 0.2  |    7 |    40 |   0 |   0 |    2 |
     |    3 | 99.999 | 58.4   | 0.7  |   63 |     5 |   0 |   1 |    5 |
     |    4 | 99     | 74.8   | 16   | 1424 |    54 |   1 |   3 |   23 |
     |    5 | 99.5   | 59.5   | 0.5  |  151 |     7 |   0 |   2 |    7 |
     |    6 | 100    | 34.9   | 13.6 |   63 |     0 |   1 |   2 |    6 |
     |    7 | 99.999 | 40.6   | 6.1  |   16 |     2 |   0 |   2 |    4 |
     |    8 | 99.5   | 50.2   | 4.3  |  350 |    39 |   0 |   2 |   15 |
     |    9 | 99.5   | 55     | 17.3 |  428 |     5 |   1 |   2 |   67 |
     |   10 | 99.6   | 14.1   | 1    |   49 |     7 |   1 |   2 |    5 |
     |   11 | 99.9   | 24.9   | 0.3  |   85 |     1 |   0 |   2 |    8 |
     |   12 | 99.999 | 62.8   | 0.5  |  512 |     4 |   0 |   0 |    3 |
     |   13 | 97.5   | 54.6   | 5.1  | 1188 |    95 |   0 |   2 |   27 |
     |   14 | 100    | 48.6   | 0.7  |   79 |     0 |   0 |   2 |    4 |
     +------+--------+--------+------+------+-------+-----+-----+------+

   Another study [ISOCORE2010] confirms the significant coverage
   increase provided by remote LFAs.




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9.4.  Comparison of LFA and RLFA results

   The table below provides a side-by-side comparison of the LFA and the
   remote LFA results.  This shows a significant improvement in the
   percentage of protected destinations and normally a modest
   improvement in the percentage of guaranteed node protected
   destinations.

      +------+--------+--------+---------+---------+
      | topo |  LFA   | RLFA   |  LFA    |  RLFA   |
      |      | % prot | %prot  | % gtd N | % gtd N |
      +------+--------+--------+---------+---------+
      |    1 | 78.5   | 99.7   | 36.9    | 53.3    |
      |    2 | 97.3   | 97.5   | 52.4    | 52.4    |
      |    3 | 99.3   | 99.999 | 58      | 58.4    |
      |    4 | 83.1   | 99     | 63.1    | 74.8    |
      |    5 | 99     | 99.5   | 59.1    | 59.5    |
      |    6 | 86.4   |100     | 21.4    | 34.9    |
      |    7 | 93.9   | 99.999 | 35.4    | 40.6    |
      |    8 | 95.3   | 99.5   | 48.1    | 50.2    |
      |    9 | 82.2   | 99.5   | 49.5    | 55      |
      |   10 | 98.5   | 99.6   | 14.9    | 14.1    |
      |   11 | 99.6   | 99.9   | 24.8    | 24.9    |
      |   12 | 99.5   | 99.999 | 62.4    | 62.8    |
      |   13 | 92.4   | 97.5   | 51.6    | 54.6    |
      |   14 | 99.3   |100     | 48.6    | 48.6    |
      +------+--------+--------+---------+---------+

   As shown in the table, remote LFA provides close to 100% prefix
   protection against link failure in 11 of the 14 topologies studied
   and provides a significant improvement in two of the remaining three
   cases.  Note that in an MPLS network, the tunnels to the PQ nodes are
   always present as a property of an LDP-based deployment.

   In the small number of cases where there is no intersection between
   the (extended) P-space and the Q-space, a number of solutions to
   providing a suitable path between such disjoint regions in the
   network have been discussed in the working group.  For example, an
   explicitly routed LSP between P and Q might be set up using RSVP-TE
   or using Segment Routing [SEGMENT-ROUTING].  Such extended repair
   methods are outside the scope of this document.










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10.  Management and Operational Considerations

   The management of LFA and remote LFA is the subject of ongoing work
   within the IETF [LFA-MANAGE], to which the reader is referred.
   Management considerations may lead to a preference for the use of a
   remote LFA over an available LFA.  This preference is a matter for
   the network operator and not a matter of protocol correctness.

   When the network reconverges, micro-loops [RFC5715] can form due to
   transient inconsistencies in the forwarding tables of different
   routers.  If it is determined that micro-loops are a significant
   issue in the deployment, then a suitable loop-free convergence
   method, such as one of those described in [RFC5715], [RFC6976], or
   [ULOOP-DELAY], should be implemented.

11.  Historical Note

   The basic concepts behind remote LFA were invented in 2002 and were
   later included in [IP-FRR], submitted in 2004.

   [IP-FRR] targeted a 100% protection coverage and hence included
   additional mechanisms on top of the remote LFA concept.  The addition
   of these mechanisms made the proposal very complex and
   computationally intensive, and it was therefore not pursued as a
   working group item.

   As explained in [RFC6571], the purpose of the LFA FRR technology is
   not to provide coverage at any cost.  A solution for this already
   exists with MPLS-TE FRR.  MPLS-TE FRR is a mature technology that is
   able to provide protection in any topology thanks to the explicit
   routing capability of MPLS-TE.

   The purpose of LFA FRR technology is to provide for a simple FRR
   solution when such a solution is possible.  The first step along this
   simplicity approach was "local" LFA [RFC5286].  This specification of
   "remote LFA" is a natural second step.

12.  Security Considerations

   The security considerations of [RFC5286] also apply.

   Targeted LDP sessions and MPLS tunnels are normal features of an MPLS
   network, and their use in this application raises no additional
   security concerns.

   IP repair tunnel endpoints (where used) SHOULD be assigned from a set
   of addresses that are not reachable from outside the routing domain;
   this would prevent their use as an attack vector.



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   Other than OAM traffic used to verify the correct operation of a
   repair tunnel, only traffic that is being protected as a result of a
   link failure is placed in a repair tunnel.  The repair tunnel MUST
   NOT be advertised by the routing protocol as a link that may be used
   to carry normal user traffic or routing protocol traffic.

13.  References

13.1.  Normative References

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

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

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

13.2.  Informative References

   [IP-FRR]   Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
              Fast Reroute using tunnels", Work in Progress,
              draft-bryant-ipfrr-tunnels-03, November 2007.

   [ISOCORE2010]
              So, N., Lin, T., and C. Chen, "LFA (Loop Free Alternates)
              Case Studies in Verizon's LDP Network", 13th Annual MPLS
              Conference, 2010.

   [LFA-MANAGE]
              Litkowski, S., Decraene, B., Filsfils, C., Raza, K.,
              Horneffer, M., and P. Sarkar, "Operational management of
              Loop Free Alternates", Work in Progress, draft-ietf-rtgwg-
              lfa-manageability-08, March 2015.

   [NODE-PROTECTION]
              Sarkar, P., Gredler, H., Hegde, S., Bowers, C., Litkowski,
              S., and H. Raghuveer, "Remote-LFA Node Protection and
              Manageability", Work in Progress, draft-ietf-rtgwg-rlfa-
              node-protection-01, December 2014.

   [OSPF-RI]  Xu, X., Chunduri, U., and M. Bhatia, "Carrying Routable IP
              Addresses in OSPF RI LSA", Work in Progress, draft-ietf-
              ospf-routable-ip-address-02, April 2015.



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   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, December 1990,
              <http://www.rfc-editor.org/info/rfc1195>.

   [RFC1701]  Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
              Routing Encapsulation (GRE)", RFC 1701, October 1994,
              <http://www.rfc-editor.org/info/rfc1701>.

   [RFC1853]  Simpson, W., "IP in IP Tunneling", RFC 1853, October 1995,
              <http://www.rfc-editor.org/info/rfc1853>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998,
              <http://www.rfc-editor.org/info/rfc2328>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001,
              <http://www.rfc-editor.org/info/rfc3032>.

   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, October 2007,
              <http://www.rfc-editor.org/info/rfc5036>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, October 2008,
              <http://www.rfc-editor.org/info/rfc5305>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, July 2008,
              <http://www.rfc-editor.org/info/rfc5340>.

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, January 2010,
              <http://www.rfc-editor.org/info/rfc5715>.

   [RFC6119]  Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
              Engineering in IS-IS", RFC 6119, February 2011,
              <http://www.rfc-editor.org/info/rfc6119>.

   [RFC6571]  Filsfils, C., Ed., Francois, P., Ed., 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,
              <http://www.rfc-editor.org/info/rfc6571>.







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   [RFC6976]  Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
              Francois, P., and O. Bonaventure, "Framework for Loop-Free
              Convergence Using the Ordered Forwarding Information Base
              (oFIB) Approach", RFC 6976, July 2013,
              <http://www.rfc-editor.org/info/rfc6976>.

   [RFC6987]  Retana, A., Nguyen, L., Zinin, A., White, R., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 6987,
              September 2013, <http://www.rfc-editor.org/info/rfc6987>.

   [SEGMENT-ROUTING]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Shakir, R., Tantsura, J.,
              and E. Crabbe, "Segment Routing Architecture", Work in
              Progress, draft-ietf-spring-segment-routing-01, February
              2015.

   [ULOOP-DELAY]
              Litkowski, S., Decraene, B., Filsfils, C., and P.
              Francois, "Microloop prevention by introducing a local
              convergence delay", Work in Progress, draft-litkowski-
              rtgwg-uloop-delay-03, February 2014.

Acknowledgements

   The authors wish to thank Levente Csikor and Chris Bowers for their
   contribution to the cost-based algorithm text.  The authors thank
   Alia Atlas, Ross Callon, Stephane Litkowski, Bharath R, Pushpasis
   Sarkar, and Adrian Farrel for their review of this document.






















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Authors' Addresses

   Stewart Bryant
   Cisco Systems
   9-11 New Square,
   Bedfont Lakes,
   Feltham,
   Middlesex  TW14 8HA
   United Kingdom

   EMail: stbryant@cisco.com


   Clarence Filsfils
   Cisco Systems
   De Kleetlaan 6a
   1831 Diegem
   Belgium

   EMail: cfilsfil@cisco.com


   Stefano Previdi
   Cisco Systems

   EMail: sprevidi@cisco.com


   Mike Shand
   Independent Contributor

   EMail: imc.shand@gmail.com


   Ning So
   Vinci Systems

   EMail: ningso@vinci-systems.com













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