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rfc:rfc7812

Internet Engineering Task Force (IETF) A. Atlas Request for Comments: 7812 C. Bowers Category: Standards Track Juniper Networks ISSN: 2070-1721 G. Enyedi

                                                              Ericsson
                                                             June 2016
              An Architecture for IP/LDP Fast Reroute
             Using Maximally Redundant Trees (MRT-FRR)

Abstract

 This document defines the architecture for IP and LDP Fast Reroute
 using Maximally Redundant Trees (MRT-FRR).  MRT-FRR is a technology
 that gives link-protection and node-protection with 100% coverage in
 any network topology that is still connected after the failure.

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 7841.
 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/rfc7812.

Copyright Notice

 Copyright (c) 2016 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.

Atlas, et al. Standards Track [Page 1] RFC 7812 MRT Unicast FRR Architecture June 2016

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   1.1.  Importance of 100% Coverage . . . . . . . . . . . . . . .   4
   1.2.  Partial Deployment and Backwards Compatibility  . . . . .   5
 2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
 3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
 4.  Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . .   7
 5.  MRT and Fast Reroute  . . . . . . . . . . . . . . . . . . . .   9
 6.  Unicast Forwarding with MRT Fast Reroute  . . . . . . . . . .   9
   6.1.  Introduction to MRT Forwarding Options  . . . . . . . . .  10
     6.1.1.  MRT LDP Labels  . . . . . . . . . . . . . . . . . . .  10
       6.1.1.1.  Topology-Scoped FEC Encoded Using a Single Label
                 (Option 1A) . . . . . . . . . . . . . . . . . . .  10
       6.1.1.2.  Topology and FEC Encoded Using a Two-Label Stack
                 (Option 1B) . . . . . . . . . . . . . . . . . . .  11
       6.1.1.3.  Compatibility of MRT LDP Label Options 1A and 1B   12
       6.1.1.4.  Required Support for MRT LDP Label Options  . . .  12
     6.1.2.  MRT IP Tunnels (Options 2A and 2B)  . . . . . . . . .  12
   6.2.  Forwarding LDP Unicast Traffic over MRT Paths . . . . . .  13
     6.2.1.  Forwarding LDP Traffic Using MRT LDP Label Option 1A   13
     6.2.2.  Forwarding LDP Traffic Using MRT LDP Label Option 1B   14
     6.2.3.  Other Considerations for Forwarding LDP Traffic Using
             MRT LDP Labels  . . . . . . . . . . . . . . . . . . .  14
     6.2.4.  Required Support for LDP Traffic  . . . . . . . . . .  14
   6.3.  Forwarding IP Unicast Traffic over MRT Paths  . . . . . .  14
     6.3.1.  Tunneling IP Traffic Using MRT LDP Labels . . . . . .  15
       6.3.1.1.  Tunneling IP Traffic Using MRT LDP Label Option
                 1A  . . . . . . . . . . . . . . . . . . . . . . .  15
       6.3.1.2.  Tunneling IP Traffic Using MRT LDP Label Option
                 1B  . . . . . . . . . . . . . . . . . . . . . . .  15
     6.3.2.  Tunneling IP Traffic Using MRT IP Tunnels . . . . . .  16
     6.3.3.  Required Support for IP Traffic . . . . . . . . . . .  16
 7.  MRT Island Formation  . . . . . . . . . . . . . . . . . . . .  16
   7.1.  IGP Area or Level . . . . . . . . . . . . . . . . . . . .  17
   7.2.  Support for a Specific MRT Profile  . . . . . . . . . . .  17
   7.3.  Excluding Additional Routers and Interfaces from the MRT
         Island  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.3.1.  Existing IGP Exclusion Mechanisms . . . . . . . . . .  18
     7.3.2.  MRT-Specific Exclusion Mechanism  . . . . . . . . . .  19
   7.4.  Connectivity  . . . . . . . . . . . . . . . . . . . . . .  19
   7.5.  Algorithm for MRT Island Identification . . . . . . . . .  19
 8.  MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . .  19
   8.1.  MRT Profile Options . . . . . . . . . . . . . . . . . . .  19
   8.2.  Router-Specific MRT Parameters  . . . . . . . . . . . . .  21
   8.3.  Default MRT Profile . . . . . . . . . . . . . . . . . . .  21
 9.  LDP Signaling Extensions and Considerations . . . . . . . . .  22

Atlas, et al. Standards Track [Page 2] RFC 7812 MRT Unicast FRR Architecture June 2016

 10. Inter-area Forwarding Behavior  . . . . . . . . . . . . . . .  23
   10.1.  ABR Forwarding Behavior with MRT LDP Label Option 1A . .  23
     10.1.1.  Motivation for Creating the Rainbow-FEC  . . . . . .  24
   10.2.  ABR Forwarding Behavior with IP Tunneling (Option 2) . .  24
   10.3.  ABR Forwarding Behavior with MRT LDP Label Option 1B . .  25
 11. Prefixes Multiply Attached to the MRT Island  . . . . . . . .  26
   11.1.  Protecting Multihomed Prefixes Using Tunnel Endpoint
          Selection  . . . . . . . . . . . . . . . . . . . . . . .  28
   11.2.  Protecting Multihomed Prefixes Using Named Proxy-Nodes .  29
   11.3.  MRT Alternates for Destinations outside the MRT Island .  31
 12. Network Convergence and Preparing for the Next Failure  . . .  32
   12.1.  Micro-loop Prevention and MRTs . . . . . . . . . . . . .  32
   12.2.  MRT Recalculation for the Default MRT Profile  . . . . .  33
 13. Operational Considerations  . . . . . . . . . . . . . . . . .  34
   13.1.  Verifying Forwarding on MRT Paths  . . . . . . . . . . .  34
   13.2.  Traffic Capacity on Backup Paths . . . . . . . . . . . .  34
   13.3.  MRT IP Tunnel Loopback Address Management  . . . . . . .  36
   13.4.  MRT-FRR in a Network with Degraded Connectivity  . . . .  36
   13.5.  Partial Deployment of MRT-FRR in a Network . . . . . . .  37
 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
 15. Security Considerations . . . . . . . . . . . . . . . . . . .  38
 16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
   16.1.  Normative References . . . . . . . . . . . . . . . . . .  38
   16.2.  Informative References . . . . . . . . . . . . . . . . .  39
 Appendix A.  Inter-level Forwarding Behavior for IS-IS  . . . . .  41
 Appendix B.  General Issues with Area Abstraction . . . . . . . .  42
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  43
 Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  43
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44

1. Introduction

 This document describes a solution for IP/LDP fast reroute [RFC5714].
 MRT-FRR creates two alternate forwarding trees that are distinct from
 the primary next-hop forwarding used during stable operation.  These
 two trees are maximally diverse from each other, providing link and
 node protection for 100% of paths and failures as long as the failure
 does not cut the network into multiple pieces.  This document defines
 the architecture for IP/LDP fast reroute with MRT.
 [RFC7811] describes how to compute maximally redundant trees using a
 specific algorithm: the MRT Lowpoint algorithm.  The MRT Lowpoint
 algorithm is used by a router that supports the Default MRT Profile,
 as specified in this document.
 IP/LDP Fast Reroute using Maximally Redundant Trees (MRT-FRR) uses
 two maximally diverse forwarding topologies to provide alternates.  A
 primary next hop should be on only one of the diverse forwarding

Atlas, et al. Standards Track [Page 3] RFC 7812 MRT Unicast FRR Architecture June 2016

 topologies; thus, the other can be used to provide an alternate.
 Once traffic has been moved to one of the MRTs by one Point of Local
 Repair (PLR), that traffic is not subject to further repair actions
 by another PLR, even in the event of multiple simultaneous failures.
 Therefore, traffic repaired by MRT-FRR will not loop between
 different PLRs responding to different simultaneous failures.
 While MRT provides 100% protection for a single link or node failure,
 it may not protect traffic in the event of multiple simultaneous
 failures, nor does it take into account Shared Risk Link Groups
 (SRLGs).  Also, while the MRT Lowpoint algorithm is computationally
 efficient, it is also new.  In order for MRT-FRR to function
 properly, all of the other nodes in the network that support MRT must
 correctly compute next hops based on the same algorithm and install
 the corresponding forwarding state.  This is in contrast to other FRR
 methods where the calculation of backup paths generally involves
 repeated application of the simpler and widely deployed Shortest Path
 First (SPF) algorithm, and backup paths themselves reuse the
 forwarding state used for shortest path forwarding of normal traffic.
 Section 13 provides operational guidance related to verification of
 MRT forwarding paths.
 In addition to supporting IP and LDP unicast fast reroute, the
 diverse forwarding topologies and guarantee of 100% coverage permit
 fast-reroute technology to be applied to multicast traffic as
 described in [MRT-ARCH].  However, the current document does not
 address the multicast applications of MRTs.

1.1. Importance of 100% Coverage

 Fast reroute is based upon the single failure assumption: that the
 time between single failures is long enough for a network to
 reconverge and start forwarding on the new shortest paths.  That does
 not imply that the network will only experience one failure or
 change.
 It is straightforward to analyze a particular network topology for
 coverage.  However, a real network does not always have the same
 topology.  For instance, maintenance events will take links or nodes
 out of use.  Simply costing out a link can have a significant effect
 on what Loop-Free Alternates (LFAs) are available.  Similarly, after
 a single failure has happened, the topology is changed and its
 associated coverage has changed as well.  Finally, many networks have
 new routers or links added and removed; each of those changes can
 have an effect on the coverage for topology-sensitive methods such as
 LFA and Remote LFA.  If fast reroute is important for the network
 services provided, then a method that guarantees 100% coverage is
 important to accommodate natural network topology changes.

Atlas, et al. Standards Track [Page 4] RFC 7812 MRT Unicast FRR Architecture June 2016

 When a network needs to use Ordered FIB [RFC6976] or Nearside
 Tunneling [RFC5715] as a micro-loop prevention mechanism [RFC5715],
 then the whole IGP area needs to have alternates available.  This
 allows the micro-loop prevention mechanism, which requires slower
 network convergence, to take the necessary time without adversely
 impacting traffic.  Without complete coverage, traffic to the
 unprotected destinations will be dropped for significantly longer
 than with current convergence -- where routers individually converge
 as fast as possible.  See Section 12.1 for more discussion of micro-
 loop prevention and MRTs.

1.2. Partial Deployment and Backwards Compatibility

 MRT-FRR supports partial deployment.  Routers advertise their ability
 to support MRT.  Inside the MRT-capable connected group of routers
 (referred to as an MRT Island), the MRTs are computed.  Alternates to
 destinations outside the MRT Island are computed and depend upon the
 existence of a loop-free neighbor of the MRT Island for that
 destination.  MRT Islands are discussed in detail in Section 7, and
 partial deployment is discussed in more detail in Section 13.5.

2. 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 [RFC2119].

3. Terminology

 network graph:   A graph that reflects the network topology where all
    links connect exactly two nodes and broadcast links have been
    transformed into the standard pseudonode representation.
 cut-link:   A link whose removal partitions the network.  A cut-link
    by definition must be connected between two cut-vertices.  If
    there are multiple parallel links, then they are referred to as
    cut-links in this document if removing the set of parallel links
    would partition the network graph.
 cut-vertex:   A vertex whose removal partitions the network graph.
 2-connected:   A graph that has no cut-vertices.  This is a graph
    that requires two nodes to be removed before the network is
    partitioned.
 2-connected cluster:   A maximal set of nodes that are 2-connected.
 block:   Either a 2-connected cluster, a cut-edge, or a cut-vertex.

Atlas, et al. Standards Track [Page 5] RFC 7812 MRT Unicast FRR Architecture June 2016

 Redundant Trees (RT):   A pair of trees where the path from any node
    X to the root R along the first tree is node-disjoint with the
    path from the same node X to the root along the second tree.
    Redundant trees can always be computed in 2-connected graphs.
 Maximally Redundant Trees (MRT):   A pair of trees where the path
    from any node X to the root R along the first tree and the path
    from the same node X to the root along the second tree share the
    minimum number of nodes and the minimum number of links.  Each
    such shared node is a cut-vertex.  Any shared links are cut-links.
    In graphs that are not 2-connected, it is not possible to compute
    RTs.  However, it is possible to compute MRTs.  MRTs are maximally
    redundant in the sense that they are as redundant as possible
    given the constraints of the network graph.
 Directed Acyclic Graph (DAG):   A graph where all links are directed
    and there are no cycles in it.
 Almost Directed Acyclic Graph (ADAG):   A graph with one node
    designated as the root.  The graph has the property that if all
    links incoming to the root were removed, then the resulting graph
    would be a DAG.
 Generalized ADAG (GADAG):   A graph that is the combination of the
    ADAGs of all blocks.
 MRT-Red:   MRT-Red is used to describe one of the two MRTs; it is
    used to describe the associated forwarding topology and MPLS
    Multi-Topology IDentifier (MT-ID).  Specifically, MRT-Red is the
    decreasing MRT where links in the GADAG are taken in the direction
    from a higher topologically ordered node to a lower one.
 MRT-Blue:   MRT-Blue is used to describe one of the two MRTs; it is
    used to described the associated forwarding topology and MPLS
    MT-ID.  Specifically, MRT-Blue is the increasing MRT where links
    in the GADAG are taken in the direction from a lower topologically
    ordered node to a higher one.
 Rainbow MRT:   It is useful to have an MPLS MT-ID that refers to the
    multiple MRT forwarding topologies and to the default forwarding
    topology.  This is referred to as the Rainbow MRT MPLS MT-ID and
    is used by LDP to reduce signaling and permit the same label to
    always be advertised to all peers for the same (MT-ID, Prefix).
 MRT Island:   The set of routers that support a particular MRT
    profile and the links connecting them that support MRT.

Atlas, et al. Standards Track [Page 6] RFC 7812 MRT Unicast FRR Architecture June 2016

 Island Border Router (IBR):   A router in the MRT Island that is
    connected to a router not in the MRT Island, both of which are in
    a common area or level.
 Island Neighbor (IN):   A router that is not in the MRT Island but is
    adjacent to an IBR and in the same area/level as the IBR.
 named proxy-node:   A proxy-node can represent a destination prefix
    that can be attached to the MRT Island via at least two routers.
    It is named if there is a way that traffic can be encapsulated to
    reach specifically that proxy node; this could be because there is
    an LDP FEC (Forwarding Equivalence Class) for the associated
    prefix or because MRT-Red and MRT-Blue IP addresses are advertised
    in an undefined fashion for that proxy-node.

4. Maximally Redundant Trees (MRT)

 A pair of Maximally Redundant Trees is a pair of directed spanning
 trees that provides maximally disjoint paths towards their common
 root.  Only links or nodes whose failure would partition the network
 (i.e., cut-links and cut-vertices) are shared between the trees.  The
 MRT Lowpoint algorithm is given in [RFC7811].  This algorithm can be
 computed in O(e + n log n); it is less than three SPFs.  This
 document describes how the MRTs can be used and not how to compute
 them.
 MRT provides destination-based trees for each destination.  Each
 router stores its normal primary next hop(s) as well as MRT-Blue next
 hop(s) and MRT-Red next hop(s) toward each destination.  The
 alternate will be selected between the MRT-Blue and MRT-Red.
 The most important thing to understand about MRTs is that for each
 pair of destination-routed MRTs, there is a path from every node X to
 the destination D on the Blue MRT that is as disjoint as possible
 from the path on the Red MRT.
 For example, in Figure 1, there is a network graph that is
 2-connected in (a) and associated MRTs in (b) and (c).  One can
 consider the paths from B to R; on the Blue MRT, the paths are
 B->F->D->E->R or B->C->D->E->R.  On the Red MRT, the path is B->A->R.
 These are clearly link and node-disjoint.  These MRTs are redundant
 trees because the paths are disjoint.

Atlas, et al. Standards Track [Page 7] RFC 7812 MRT Unicast FRR Architecture June 2016

 [E]---[D]---|           [E]<--[D]<--|                [E]-->[D]---|
  |     |    |            |     ^    |                       |    |
  |     |    |            V     |    |                       V    V
 [R]   [F]  [C]          [R]   [F]  [C]               [R]   [F]  [C]
  |     |    |                  ^    ^                 ^     |    |
  |     |    |                  |    |                 |     V    |
 [A]---[B]---|           [A]-->[B]---|                [A]<--[B]<--|
       (a)                     (b)                         (c)
 a 2-connected graph     Blue MRT towards R          Red MRT towards R
                    Figure 1: A 2-Connected Network
 By contrast, in Figure 2, the network in (a) is not 2-connected.  If
 C, G, or the link C<->G failed, then the network would be
 partitioned.  It is clearly impossible to have two link-disjoint or
 node-disjoint paths from G, J, or H to R.  The MRTs given in (b) and
 (c) offer paths that are as disjoint as possible.  For instance, the
 paths from B to R are the same as in Figure 1 and the path from G to
 R on the Blue MRT is G->C->D->E->R and on the Red MRT is
 G->C->B->A->R.
                      [E]---[D]---|     |---[J]
                       |     |    |     |    |
                       |     |    |     |    |
                      [R]   [F]  [C]---[G]   |
                       |     |    |     |    |
                       |     |    |     |    |
                      [A]---[B]---|     |---[H]
                     (a) a graph that is not 2-connected
       [E]<--[D]<--|         [J]        [E]-->[D]---|     |---[J]
        |     ^    |          |                |    |     |    ^
        V     |    |          |                V    V     V    |
       [R]   [F]  [C]<--[G]   |         [R]   [F]  [C]<--[G]   |
              ^    ^     ^    |          ^     |    |          |
              |    |     |    V          |     V    |          |
       [A]-->[B]---|     |---[H]        [A]<--[B]<--|         [H]
        (b) Blue MRT towards R          (c) Red MRT towards R
              Figure 2: A Network That Is Not 2-Connected

Atlas, et al. Standards Track [Page 8] RFC 7812 MRT Unicast FRR Architecture June 2016

5. MRT and Fast Reroute

 In normal IGP routing, each router has its Shortest Path Tree (SPT)
 to all destinations.  From the perspective of a particular
 destination, D, this looks like a reverse SPT (rSPT).  To use MRT, in
 addition, each destination D has two MRTs associated with it; by
 convention these will be called the MRT-Blue and MRT-Red.  MRT-FRR is
 realized by using multi-topology forwarding.  There is a MRT-Blue
 forwarding topology and a MRT-Red forwarding topology.
 Any IP/LDP fast-reroute technique beyond LFA requires an additional
 dataplane procedure, such as an additional forwarding mechanism.  The
 well-known options are multi-topology forwarding (used by MRT-FRR),
 tunneling (e.g., [RFC6981] or [RFC7490]), and per-interface
 forwarding (e.g., Loop-Free Failure Insensitive Routing in
 [EnyediThesis]).
 When there is a link or node failure affecting, but not partitioning,
 the network, each node will still have at least one path via one of
 the MRTs to reach the destination D.  For example, in Figure 2, B
 would normally forward traffic to R across the path B->A->R.  If the
 B<->A link fails, then B could use the MRT-Blue path B->F->D->E->R.
 As is always the case with fast-reroute technologies, forwarding does
 not change until a local failure is detected.  Packets are forwarded
 along the shortest path.  The appropriate alternate to use is pre-
 computed.  [RFC7811] describes exactly how to determine whether the
 MRT-Blue next hops or the MRT-Red next hops should be the MRT
 alternate next hops for a particular primary next hop to a particular
 destination.
 MRT alternates are always available to use.  It is a local decision
 whether to use an MRT alternate, an LFA, or some other type of
 alternate.
 As described in [RFC5286], when a worse failure than is anticipated
 happens, using LFAs that are not downstream neighbors can cause
 looping among alternates.  Section 1.1 of [RFC5286] gives an example
 of link-protecting alternates causing a loop on node failure.  Even
 if a worse failure than anticipated happens, the use of MRT
 alternates will not cause looping.

6. Unicast Forwarding with MRT Fast Reroute

 There are three possible types of routers involved in forwarding a
 packet along an MRT path.  At the MRT ingress router, the packet
 leaves the shortest path to the destination and follows an MRT path
 to the destination.  In an FRR application, the MRT ingress router is

Atlas, et al. Standards Track [Page 9] RFC 7812 MRT Unicast FRR Architecture June 2016

 the PLR.  An MRT transit router takes a packet that arrives already
 associated with the particular MRT, and forwards it on that same MRT.
 In some situations (to be discussed later), the packet will need to
 leave the MRT path and return to the shortest path.  This takes place
 at the MRT egress router.  The MRT ingress and egress functionality
 may depend on the underlying type of packet being forwarded (LDP or
 IP).  The MRT transit functionality is independent of the type of
 packet being forwarded.  We first consider several MRT transit
 forwarding mechanisms.  Then, we look at how these forwarding
 mechanisms can be applied to carrying LDP and IP traffic.

6.1. Introduction to MRT Forwarding Options

 The following options for MRT forwarding mechanisms are considered.
 1.  MRT LDP Labels
     A.  Topology-scoped FEC encoded using a single label
     B.  Topology and FEC encoded using a two-label stack
 2.  MRT IP Tunnels
     A.  MRT IPv4 Tunnels
     B.  MRT IPv6 Tunnels

6.1.1. MRT LDP Labels

 We consider two options for the MRT forwarding mechanisms using MRT
 LDP labels.

6.1.1.1. Topology-Scoped FEC Encoded Using a Single Label (Option 1A)

 [RFC7307] provides a mechanism to distribute FEC-label bindings
 scoped to a given MPLS topology (represented by MPLS MT-ID).  To use
 multi-topology LDP to create MRT forwarding topologies, we associate
 two MPLS MT-IDs with the MRT-Red and MRT-Blue forwarding topologies,
 in addition to the default shortest path forwarding topology with
 MT-ID=0.
 With this forwarding mechanism, a single label is distributed for
 each topology-scoped FEC.  For a given FEC in the default topology
 (call it default-FEC-A), two additional topology-scoped FECs would be
 created, corresponding to the Red and Blue MRT forwarding topologies
 (call them red-FEC-A and blue-FEC-A).  A router supporting this MRT
 transit forwarding mechanism advertises a different FEC-label binding
 for each of the three topology-scoped FECs.  When a packet is

Atlas, et al. Standards Track [Page 10] RFC 7812 MRT Unicast FRR Architecture June 2016

 received with a label corresponding to red-FEC-A (for example), an
 MRT transit router will determine the next hop for the MRT-Red
 forwarding topology for that FEC, swap the incoming label with the
 outgoing label corresponding to red-FEC-A learned from the MRT-Red
 next-hop router, and forward the packet.
 This forwarding mechanism has the useful property that the FEC
 associated with the packet is maintained in the labels at each hop
 along the MRT.  We will take advantage of this property when
 specifying how to carry LDP traffic on MRT paths using multi-topology
 LDP labels.
 This approach is very simple for hardware to support.  However, it
 reduces the label space for other uses, and it increases the memory
 needed to store the labels and the communication required by LDP to
 distribute FEC-label bindings.  In general, this approach will also
 increase the time needed to install the FRR entries in the Forwarding
 Information Base (FIB) and, hence, the time needed before the next
 failure can be protected.
 This forwarding option uses the LDP signaling extensions described in
 [RFC7307].  The MRT-specific LDP extensions required to support this
 option will be described elsewhere.

6.1.1.2. Topology and FEC Encoded Using a Two-Label Stack (Option 1B)

 With this forwarding mechanism, a two-label stack is used to encode
 the topology and the FEC of the packet.  The top label (topology-id
 label) identifies the MRT forwarding topology, while the second label
 (FEC label) identifies the FEC.  The top label would be a new FEC
 type with two values corresponding to MRT Red and Blue topologies.
 When an MRT transit router receives a packet with a topology-id
 label, the router pops the top label and uses that it to guide the
 next-hop selection in combination with the next label in the stack
 (the FEC label).  The router then swaps the FEC label, using the FEC-
 label bindings learned through normal LDP mechanisms.  The router
 then pushes the topology-id label for the next hop.
 As with Option 1A, this forwarding mechanism also has the useful
 property that the FEC associated with the packet is maintained in the
 labels at each hop along the MRT.
 This forwarding mechanism has minimal usage of additional labels,
 memory and LDP communication.  It does increase the size of packets
 and the complexity of the required label operations and lookups.

Atlas, et al. Standards Track [Page 11] RFC 7812 MRT Unicast FRR Architecture June 2016

 This forwarding option is consistent with context-specific label
 spaces, as described in [RFC5331].  However, the precise LDP behavior
 required to support this option for MRT has not been specified.

6.1.1.3. Compatibility of MRT LDP Label Options 1A and 1B

 MRT transit forwarding based on MRT LDP Label options 1A and 1B can
 coexist in the same network, with a packet being forwarded along a
 single MRT path using the single label of Option 1A for some hops and
 the two-label stack of Option 1B for other hops.  However, to
 simplify the process of MRT Island formation, we require that all
 routers in the MRT Island support at least one common forwarding
 mechanism.  As an example, the Default MRT Profile requires support
 for the MRT LDP Label Option 1A forwarding mechanism.  This ensures
 that the routers in an MRT island supporting the Default MRT Profile
 will be able to establish MRT forwarding paths based on MRT LDP Label
 Option 1A.  However, an implementation supporting Option 1A may also
 support Option 1B.  If the scaling or performance characteristics for
 the two options differ in this implementation, then it may be
 desirable for a pair of adjacent routers to use Option 1B labels
 instead of the Option 1A labels.  If those routers successfully
 negotiate the use of Option 1B labels, they are free to use them.
 This can occur without any of the other routers in the MRT Island
 being made aware of it.
 Note that this document only defines the Default MRT Profile, which
 requires support for the MRT LDP Label Option 1A forwarding
 mechanism.

6.1.1.4. Required Support for MRT LDP Label Options

 If a router supports a profile that includes the MRT LDP Label Option
 1A for the MRT transit forwarding mechanism, then it MUST support
 Option 1A, which encodes topology-scoped FECs using a single label.
 The router MAY also support Option 1B.
 If a router supports a profile that includes the MRT LDP Label Option
 1B for the MRT transit forwarding mechanism, then it MUST support
 Option 1B, which encodes the topology and FEC using a two-label
 stack.  The router MAY also support Option 1A.

6.1.2. MRT IP Tunnels (Options 2A and 2B)

 IP tunneling can also be used as an MRT transit forwarding mechanism.
 Each router supporting this MRT transit forwarding mechanism
 announces two additional loopback addresses and their associated MRT
 color.  Those addresses are used as destination addresses for MRT-
 blue and MRT-red IP tunnels, respectively.  The special loopback

Atlas, et al. Standards Track [Page 12] RFC 7812 MRT Unicast FRR Architecture June 2016

 addresses allow the transit nodes to identify the traffic as being
 forwarded along either the MRT-blue or MRT-red topology to reach the
 tunnel destination.  For example, an MRT ingress router can cause a
 packet to be tunneled along the MRT-red path to router X by
 encapsulating the packet using the MRT-red loopback address
 advertised by router X.  Upon receiving the packet, router X would
 remove the encapsulation header and forward the packet based on the
 original destination address.
 Either IPv4 (Option 2A) or IPv6 (Option 2B) can be used as the
 tunneling mechanism.
 Note that the two forwarding mechanisms using LDP Label options do
 not require additional loopbacks per router, as is required by the IP
 tunneling mechanism.  This is because LDP labels are used on a hop-
 by-hop basis to identify MRT-blue and MRT-red forwarding topologies.

6.2. Forwarding LDP Unicast Traffic over MRT Paths

 In the previous section, we examined several options for providing
 MRT transit forwarding functionality, which is independent of the
 type of traffic being carried.  We now look at the MRT ingress
 functionality, which will depend on the type of traffic being carried
 (IP or LDP).  We start by considering LDP traffic.
 We also simplify the initial discussion by assuming that the network
 consists of a single IGP area, and that all routers in the network
 participate in MRT.  Other deployment scenarios that require MRT
 egress functionality are considered later in this document.
 In principle, it is possible to carry LDP traffic in MRT IP tunnels.
 However, for LDP traffic, it is desirable to avoid tunneling.
 Tunneling LDP traffic to a remote node requires knowledge of remote
 FEC-label bindings so that the LDP traffic can continue to be
 forwarded properly when it leaves the tunnel.  This requires targeted
 LDP sessions, which can add management complexity.  As described
 below, the two MRT forwarding mechanisms that use LDP labels do not
 require targeted LDP sessions.

6.2.1. Forwarding LDP Traffic Using MRT LDP Label Option 1A

 The MRT LDP Label Option 1A forwarding mechanism uses topology-scoped
 FECs encoded using a single label as described in Section 6.1.1.1.
 When a PLR receives an LDP packet that needs to be forwarded on the
 MRT-Red (for example), it does a label swap operation, replacing the
 usual LDP label for the FEC with the MRT-Red label for that FEC
 received from the next-hop router in the MRT-Red computed by the PLR.
 When the next-hop router in the MRT-Red receives the packet with the

Atlas, et al. Standards Track [Page 13] RFC 7812 MRT Unicast FRR Architecture June 2016

 MRT-Red label for the FEC, the MRT transit forwarding functionality
 continues as described in Section 6.1.1.1.  In this way, the original
 FEC associated with the packet is maintained at each hop along the
 MRT.

6.2.2. Forwarding LDP Traffic Using MRT LDP Label Option 1B

 The MRT LDP Label Option 1B forwarding mechanism encodes the topology
 and the FEC using a two-label stack as described in Section 6.1.1.2.
 When a PLR receives an LDP packet that needs to be forwarded on the
 MRT-Red, it first does a normal LDP label swap operation, replacing
 the incoming normal LDP label associated with a given FEC with the
 outgoing normal LDP label for that FEC learned from the next hop on
 the MRT-Red.  In addition, the PLR pushes the topology-id label
 associated with the MRT-Red, and forward the packet to the
 appropriate next hop on the MRT-Red.  When the next-hop router in the
 MRT-Red receives the packet with the MRT-Red label for the FEC, the
 MRT transit forwarding functionality continues as described in
 Section 6.1.1.2.  As with Option 1A, the original FEC associated with
 the packet is maintained at each hop along the MRT.

6.2.3. Other Considerations for Forwarding LDP Traffic Using MRT LDP

      Labels
 Note that forwarding LDP traffic using MRT LDP Labels can be done
 without the use of targeted LDP sessions when an MRT path to the
 destination FEC is used.  The alternates selected in [RFC7811] use
 the MRT path to the destination FEC, so targeted LDP sessions are not
 needed.  If instead one found it desirable to have the PLR use an MRT
 to reach the primary next-next-hop for the FEC, and then continue
 forwarding the LDP packet along the shortest path from the primary
 next-next-hop, this would require tunneling to the primary next-next-
 hop and a targeted LDP session for the PLR to learn the FEC-label
 binding for primary next-next-hop to correctly forward the packet.

6.2.4. Required Support for LDP Traffic

 For greatest hardware compatibility, routers implementing MRT fast
 reroute of LDP traffic MUST support Option 1A of encoding the MT-ID
 in the labels (See Section 9).

6.3. Forwarding IP Unicast Traffic over MRT Paths

 For IPv4 traffic, there is no currently practical alternative except
 tunneling to gain the bits needed to indicate the MRT-Blue or MRT-Red
 forwarding topology.  For IPv6 traffic, in principle, one could
 define bits in the IPv6 options header to indicate the MRT-Blue or
 MRT-Red forwarding topology.  However, in this document, we have

Atlas, et al. Standards Track [Page 14] RFC 7812 MRT Unicast FRR Architecture June 2016

 chosen not to define a solution that would work for IPv6 traffic but
 not for IPv4 traffic.
 The choice of tunnel egress is flexible since any router closer to
 the destination than the next hop can work.  This architecture
 assumes that the original destination in the area is selected (see
 Section 11 for handling of multihomed prefixes); another possible
 choice is the next-next-hop towards the destination.  As discussed in
 the previous section, for LDP traffic, using the MRT to the original
 destination simplifies MRT-FRR by avoiding the need for targeted LDP
 sessions to the next-next-hop.  For IP, that consideration doesn't
 apply.
 Some situations require tunneling IP traffic along an MRT to a tunnel
 endpoint that is not the destination of the IP traffic.  These
 situations will be discussed in detail later.  We note here that an
 IP packet with a destination in a different IGP area/level from the
 PLR should be tunneled on the MRT to the Area Border Router (ABR) or
 Level Border Router (LBR) on the shortest path to the destination.
 For a destination outside of the PLR's MRT Island, the packet should
 be tunneled on the MRT to a non-proxy-node immediately before the
 named proxy-node on that particular color MRT.

6.3.1. Tunneling IP Traffic Using MRT LDP Labels

 An IP packet can be tunneled along an MRT path by pushing the
 appropriate MRT LDP label(s).  Tunneling using LDP labels, as opposed
 to IP headers, has the advantage that more installed routers can do
 line-rate encapsulation and decapsulation using LDP than using IP.
 Also, no additional IP addresses would need to be allocated or
 signaled.

6.3.1.1. Tunneling IP Traffic Using MRT LDP Label Option 1A

 The MRT LDP Label Option 1A forwarding mechanism uses topology-scoped
 FECs encoded using a single label as described in Section 6.1.1.1.
 When a PLR receives an IP packet that needs to be forwarded on the
 MRT-Red to a particular tunnel endpoint, it does a label push
 operation.  The label pushed is the MRT-Red label for a FEC
 originated by the tunnel endpoint, learned from the next hop on the
 MRT-Red.

6.3.1.2. Tunneling IP Traffic Using MRT LDP Label Option 1B

 The MRT LDP Label Option 1B forwarding mechanism encodes the topology
 and the FEC using a two-label stack as described in Section 6.1.1.2.
 When a PLR receives an IP packet that needs to be forwarded on the
 MRT-Red to a particular tunnel endpoint, the PLR pushes two labels on

Atlas, et al. Standards Track [Page 15] RFC 7812 MRT Unicast FRR Architecture June 2016

 the IP packet.  The first (inner) label is the normal LDP label
 learned from the next hop on the MRT-Red, associated with a FEC
 originated by the tunnel endpoint.  The second (outer) label is the
 topology-id label associated with the MRT-Red.
 For completeness, we note here a potential variation that uses a
 single label as opposed to two labels.  In order to tunnel an IP
 packet over an MRT to the destination of the IP packet as opposed to
 an arbitrary tunnel endpoint, one could just push a topology-id label
 directly onto the packet.  An MRT transit router would need to pop
 the topology-id label, do an IP route lookup in the context of that
 topology-id label, and push the topology-id label.

6.3.2. Tunneling IP Traffic Using MRT IP Tunnels

 In order to tunnel over the MRT to a particular tunnel endpoint, the
 PLR encapsulates the original IP packet with an additional IP header
 using the MRT-Blue or MRT-Red loopback address of the tunnel
 endpoint.

6.3.3. Required Support for IP Traffic

 For greatest hardware compatibility and ease in removing the MRT-
 topology marking at area/level boundaries, routers that support MPLS
 and implement IP MRT fast reroute MUST support tunneling of IP
 traffic using MRT LDP Label Option 1A (topology-scoped FEC encoded
 using a single label).

7. MRT Island Formation

 The purpose of communicating support for MRT is to indicate that the
 MRT-Blue and MRT-Red forwarding topologies are created for transit
 traffic.  The MRT architecture allows for different, potentially
 incompatible options.  In order to create consistent MRT forwarding
 topologies, the routers participating in a particular MRT Island need
 to use the same set of options.  These options are grouped into MRT
 profiles.  In addition, the routers in an MRT Island all need to use
 the same set of nodes and links within the Island when computing the
 MRT forwarding topologies.  This section describes the information
 used by a router to determine the nodes and links to include in a
 particular MRT Island.  Some information already exists in the IGPs
 and can be used by MRT in Island formation, subject to the
 interpretation defined here.
 Other information needs to be communicated between routers for which
 there do not currently exist protocol extensions.  This new
 information needs to be shared among all routers in an IGP area, so

Atlas, et al. Standards Track [Page 16] RFC 7812 MRT Unicast FRR Architecture June 2016

 defining extensions to existing IGPs to carry this information makes
 sense.  These new protocol extensions will be defined elsewhere.
 Deployment scenarios using multi-topology OSPF or IS-IS, or running
 both IS-IS and OSPF on the same routers is out of scope for this
 specification.  As with LFA, MRT-FRR does not support OSPF Virtual
 Links.
 At a high level, an MRT Island is defined as the set of routers
 supporting the same MRT profile, in the same IGP area/level and with
 bidirectional links interconnecting those routers.  More detailed
 descriptions of these criteria are given below.

7.1. IGP Area or Level

 All links in an MRT Island are bidirectional and belong to the same
 IGP area or level.  For IS-IS, a link belonging to both Level-1 and
 Level-2 would qualify to be in multiple MRT Islands.  A given ABR or
 LBR can belong to multiple MRT Islands, corresponding to the areas or
 levels in which it participates.  Inter-area forwarding behavior is
 discussed in Section 10.

7.2. Support for a Specific MRT Profile

 All routers in an MRT Island support the same MRT profile.  A router
 advertises support for a given MRT profile using an 8-bit MRT Profile
 ID value.  The "MRT Profile Identifier Registry" is defined in this
 document.  The protocol extensions for advertising the MRT Profile ID
 value will be defined in a future specification.  A given router can
 support multiple MRT profiles and participate in multiple MRT
 Islands.  The options that make up an MRT Profile, as well as the
 Default MRT Profile, are defined in Section 8.
 The process of MRT Island formation takes place independently for
 each MRT profile advertised by a given router.  For example, consider
 a network with 40 connected routers in the same area advertising
 support for MRT Profile A and MRT Profile B.  Two distinct MRT
 Islands will be formed corresponding to Profile A and Profile B, with
 each island containing all 40 routers.  A complete set of maximally
 redundant trees will be computed for each island following the rules
 defined for each profile.  If we add a third MRT Profile to this
 example, with Profile C being advertised by a connected subset of 30
 routers, there will be a third MRT Island formed corresponding to
 those 30 routers, and a third set of maximally redundant trees will
 be computed.  In this example, 40 routers would compute and install
 two sets of MRT transit forwarding entries corresponding to Profiles
 A and B, while 30 routers would compute and install three sets of MRT
 transit forwarding entries corresponding to Profiles A, B, and C.

Atlas, et al. Standards Track [Page 17] RFC 7812 MRT Unicast FRR Architecture June 2016

7.3. Excluding Additional Routers and Interfaces from the MRT Island

 MRT takes into account existing IGP mechanisms for discouraging
 traffic from using particular links and routers, and it introduces an
 MRT-specific exclusion mechanism for links.

7.3.1. Existing IGP Exclusion Mechanisms

 Mechanisms for discouraging traffic from using particular links
 already exist in IS-IS and OSPF.  In IS-IS, an interface configured
 with a metric of 2^24-2 (0xFFFFFE) will only be used as a last
 resort.  (An interface configured with a metric of 2^24-1 (0xFFFFFF)
 will not be advertised into the topology.)  In OSPF, an interface
 configured with a metric of 2^16-1 (0xFFFF) will only be used as a
 last resort.  These metrics can be configured manually to enforce
 administrative policy or they can be set in an automated manner as
 with LDP IGP synchronization [RFC5443].
 Mechanisms also already exist in IS-IS and OSPF to discourage or
 prevent transit traffic from using a particular router.  In IS-IS,
 the overload bit is prevents transit traffic from using a router.
 For OSPFv2 and OSPFv3, [RFC6987] specifies setting all outgoing
 interface metrics to 0xFFFF to discourage transit traffic from using
 a router.  ([RFC6987] defines the metric value 0xFFFF as
 MaxLinkMetric, a fixed architectural value for OSPF.)  For OSPFv3,
 [RFC5340] specifies that a router be excluded from the intra-area SPT
 computation if the V6-bit or R-bit of the Link State Advertisement
 (LSA) options is not set in the Router LSA.
 The following rules for MRT Island formation ensure that MRT FRR
 protection traffic does not use a link or router that is discouraged
 or prevented from carrying traffic by existing IGP mechanisms.
 1.  A bidirectional link MUST be excluded from an MRT Island if
     either the forward or reverse cost on the link is 0xFFFFFE (for
     IS-IS) or 0xFFFF for OSPF.
 2.  A router MUST be excluded from an MRT Island if it is advertised
     with the overload bit set (for IS-IS), or it is advertised with
     metric values of 0xFFFF on all of its outgoing interfaces (for
     OSPFv2 and OSPFv3).
 3.  A router MUST be excluded from an MRT Island if it is advertised
     with either the V6-bit or R-bit of the LSA options not set in the
     Router LSA.

Atlas, et al. Standards Track [Page 18] RFC 7812 MRT Unicast FRR Architecture June 2016

7.3.2. MRT-Specific Exclusion Mechanism

 This architecture also defines a means of excluding an otherwise
 usable link from MRT Islands.  The protocol extensions for
 advertising that a link is MRT-Ineligible will be defined elsewhere.
 A link with either interface advertised as MRT-Ineligible MUST be
 excluded from an MRT Island.  Note that an interface advertised as
 MRT-Ineligible by a router is ineligible with respect to all profiles
 advertised by that router.

7.4. Connectivity

 All of the routers in an MRT Island MUST be connected by
 bidirectional links with other routers in the MRT Island.
 Disconnected MRT Islands will operate independently of one another.

7.5. Algorithm for MRT Island Identification

 An algorithm that allows a computing router to identify the routers
 and links in the local MRT Island satisfying the above rules is given
 in Section 5.2 of [RFC7811].

8. MRT Profile

 An MRT Profile is a set of values and options related to MRT
 behavior.  The complete set of options is designated by the
 corresponding 8-bit Profile ID value.
 This document specifies the values and options that correspond to the
 Default MRT Profile (Profile ID = 0).  Future documents may define
 other MRT Profiles by specifying the MRT Profile Options below.

8.1. MRT Profile Options

 Below is a description of the values and options that define an MRT
 Profile.
 MRT Algorithm:  This identifies the particular algorithm for
    computing maximally redundant trees used by the router for this
    profile.
 MRT-Red MT-ID:  This specifies the MPLS MT-ID to be associated with
    the MRT-Red forwarding topology.  It is allocated from the MPLS
    Multi-Topology Identifiers Registry.
 MRT-Blue MT-ID:  This specifies the MPLS MT-ID to be associated with
    the MRT-Blue forwarding topology.  It is allocated from the MPLS
    Multi-Topology Identifiers Registry.

Atlas, et al. Standards Track [Page 19] RFC 7812 MRT Unicast FRR Architecture June 2016

 GADAG Root Selection Policy:  This specifies the manner in which the
    GADAG root is selected.  All routers in the MRT Island need to use
    the same GADAG root in the calculations used construct the MRTs.
    A valid GADAG Root Selection Policy MUST be such that each router
    in the MRT Island chooses the same GADAG root based on information
    available to all routers in the MRT Island.  GADAG Root Selection
    Priority values, advertised as router-specific MRT parameters, MAY
    be used in a GADAG Root Selection Policy.
 MRT Forwarding Mechanism:  This specifies which forwarding mechanism
    the router uses to carry transit traffic along MRT paths.  A
    router that supports a specific MRT forwarding mechanism must
    program appropriate next hops into the forwarding plane.  The
    current options are MRT LDP Label Option 1A, MRT LDP Label Option
    1B, IPv4 Tunneling, IPv6 Tunneling, and None.  If IPv4 is
    supported, then both MRT-Red and MRT-Blue IPv4 loopback addresses
    SHOULD be specified.  If IPv6 is supported, both MRT-Red and MRT-
    Blue IPv6 loopback addresses SHOULD be specified.
 Recalculation:  Recalculation specifies the process and timing by
    which new MRTs are computed after the topology has been modified.
 Area/Level Border Behavior:  This specifies how traffic traveling on
    the MRT-Blue or MRT-Red in one area should be treated when it
    passes into another area.
 Other Profile-Specific Behavior:  Depending upon the use-case for the
    profile, there may be additional profile-specific behavior.
 When a new MRT Profile is defined, new and unique values should be
 allocated from the "MPLS Multi-Topology Identifiers Registry",
 corresponding to the MRT-Red and MRT-Blue MT-ID values for the new
 MRT Profile.
 If a router advertises support for multiple MRT profiles, then it
 MUST create the transit forwarding topologies for each of those,
 unless the profile specifies the None option for the MRT Forwarding
 Mechanism.
 The ability of MRT-FRR to support transit forwarding entries for
 multiple profiles can be used to facilitate a smooth transition from
 an existing deployed MRT Profile to a new MRT Profile.  The new
 profile can be activated in parallel with the existing profile,
 installing the transit forwarding entries for the new profile without
 affecting the transit forwarding entries for the existing profile.
 Once the new transit forwarding state has been verified, the router
 can be configured to use the alternates computed by the new profile
 in the event of a failure.

Atlas, et al. Standards Track [Page 20] RFC 7812 MRT Unicast FRR Architecture June 2016

8.2. Router-Specific MRT Parameters

 For some profiles, additional router-specific MRT parameters may need
 to be advertised.  While the set of options indicated by the MRT
 Profile ID must be identical for all routers in an MRT Island, these
 router-specific MRT parameters may differ between routers in the same
 MRT Island.  Several such parameters are described below.
 GADAG Root Selection Priority:   A GADAG Root Selection Policy MAY
    rely on the GADAG Root Selection Priority values advertised by
    each router in the MRT Island.  A GADAG Root Selection Policy may
    use the GADAG Root Selection Priority to allow network operators
    to configure a parameter to ensure that the GADAG root is selected
    from a particular subset of routers.  An example of this use of
    the GADAG Root Selection Priority value by the GADAG Root
    Selection Policy is given in the Default MRT Profile below.
 MRT-Red Loopback Address:   This provides the router's loopback
    address to reach the router via the MRT-Red forwarding topology.
    It can be specified for either IPv4 or IPv6.  Note that this
    parameter is not needed to support the Default MRT Profile.
 MRT-Blue Loopback Address:   This provides the router's loopback
    address to reach the router via the MRT-Blue forwarding topology.
    It can be specified for either IPv4 and IPv6.  Note that this
    parameter is not needed to support the Default MRT Profile.
 Protocol extensions for advertising a router's GADAG Root Selection
 Priority value will be defined in other documents.  Protocol
 extensions for the advertising a router's MRT-Red and MRT-Blue
 loopback addresses will be defined elsewhere.

8.3. Default MRT Profile

 The following set of options defines the Default MRT Profile.  The
 Default MRT Profile is indicated by the MRT Profile ID value of 0.
 MRT Algorithm:   MRT Lowpoint algorithm defined in [RFC7811].
 MRT-Red MPLS MT-ID:   This temporary registration has been allocated
    from the "MPLS Multi-Topology Identifiers" registry.  The
    registration request appears in [LDP-MRT].
 MRT-Blue MPLS MT-ID:   This temporary registration has been allocated
    from the "MPLS Multi-Topology Identifiers" registry.  The
    registration request appears in [LDP-MRT].

Atlas, et al. Standards Track [Page 21] RFC 7812 MRT Unicast FRR Architecture June 2016

 GADAG Root Selection Policy:   Among the routers in the MRT Island
    with the lowest numerical value advertised for GADAG Root
    Selection Priority, an implementation MUST pick the router with
    the highest Router ID to be the GADAG root.  Note that a lower
    numerical value for GADAG Root Selection Priority indicates a
    higher preference for selection.
 Forwarding Mechanisms:   MRT LDP Label Option 1A
 Recalculation:   Recalculation of MRTs SHOULD occur as described in
    Section 12.2.  This allows the MRT forwarding topologies to
    support IP/LDP fast-reroute traffic.
 Area/Level Border Behavior:   As described in Section 10, ABRs/LBRs
    SHOULD ensure that traffic leaving the area also exits the MRT-Red
    or MRT-Blue forwarding topology.

9. LDP Signaling Extensions and Considerations

 The protocol extensions for LDP will be defined in another document.
 A router must indicate that it has the ability to support MRT; having
 this explicit allows the use of MRT-specific processing, such as
 special handling of FECs sent with the Rainbow MRT MT-ID.
 A FEC sent with the Rainbow MRT MT-ID indicates that the FEC applies
 to all the MRT-Blue and MRT-Red MT-IDs in supported MRT profiles.
 The FEC-label bindings for the default shortest-path-based MT-ID 0
 MUST still be sent (even though it could be inferred from the Rainbow
 FEC-label bindings) to ensure continuous operation of normal LDP
 forwarding.  The Rainbow MRT MT-ID is defined to provide an easy way
 to handle the special signaling that is needed at ABRs or LBRs.  It
 avoids the problem of needing to signal different MPLS labels to
 different LDP neighbors for the same FEC.  Because the Rainbow MRT
 MT-ID is used only by ABRs/LBRs or an LDP egress router, it is not
 MRT profile specific.
 The value of the Rainbow MRT MPLS MT-ID has been temporarily
 allocated from the "MPLS Multi-Topology Identifiers" registry.  The
 registration request appears in [LDP-MRT].

Atlas, et al. Standards Track [Page 22] RFC 7812 MRT Unicast FRR Architecture June 2016

10. Inter-area Forwarding Behavior

 An ABR/LBR has two forwarding roles.  First, it forwards traffic
 within areas.  Second, it forwards traffic from one area into
 another.  These same two roles apply for MRT transit traffic.
 Traffic on MRT-Red or MRT-Blue destined inside the area needs to stay
 on MRT-Red or MRT-Blue in that area.  However, it is desirable for
 traffic leaving the area to also exit MRT-Red or MRT-Blue and return
 to shortest path forwarding.
 For unicast MRT-FRR, the need to stay on an MRT forwarding topology
 terminates at the ABR/LBR whose best route is via a different area/
 level.  It is highly desirable to go back to the default forwarding
 topology when leaving an area/level.  There are three basic reasons
 for this.  First, the default topology uses shortest paths; the
 packet will thus take the shortest possible route to the destination.
 Second, this allows a single router failure that manifests itself in
 multiple areas (as would be the case with an ABR/LBR failure) to be
 separately identified and repaired around.  Third, the packet can be
 fast-rerouted again, if necessary, due to a second distinct failure
 in a different area.
 In OSPF, an ABR that receives a packet on MRT-Red or MRT-Blue towards
 destination Z should continue to forward the packet along MRT-Red or
 MRT-Blue only if the best route to Z is in the same OSPF area as the
 interface that the packet was received on.  Otherwise, the packet
 should be removed from MRT-Red or MRT-Blue and forwarded on the
 shortest-path default forwarding topology.
 The above description applies to OSPF.  The same essential behavior
 also applies to IS-IS if one substitutes IS-IS level for OSPF area.
 However, the analogy with OSPF is not exact.  An interface in OSPF
 can only be in one area, whereas an interface in IS-IS can be in both
 Level-1 and Level-2.  Therefore, to avoid confusion and address this
 difference, we explicitly describe the behavior for IS-IS in
 Appendix A.  In the following sections, only the OSPF terminology is
 used.

10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A

 For LDP forwarding where a single label specifies (MT-ID, FEC), the
 ABR is responsible for advertising the proper label to each neighbor.
 Assume that an ABR has allocated three labels for a particular
 destination: L_primary, L_blue, and L_red.  To those routers in the
 same area as the best route to the destination, the ABR advertises
 the following FEC-label bindings: L_primary for the default topology,
 L_blue for the MRT-Blue MT-ID, and L_red for the MRT-Red MT-ID, as
 expected.  However, to routers in other areas, the ABR advertises the

Atlas, et al. Standards Track [Page 23] RFC 7812 MRT Unicast FRR Architecture June 2016

 following FEC-label bindings: L_primary for the default topology and
 L_primary for the Rainbow MRT MT-ID.  Associating L_primary with the
 Rainbow MRT MT-ID causes the receiving routers to use L_primary for
 the MRT-Blue MT-ID and for the MRT-Red MT-ID.
 The ABR installs all next hops for the best area: primary next hops
 for L_primary, MRT-Blue next hops for L_blue, and MRT-Red next hops
 for L_red.  Because the ABR advertised (Rainbow MRT MT-ID, FEC) with
 L_primary to neighbors not in the best area, packets from those
 neighbors will arrive at the ABR with a label L_primary and will be
 forwarded into the best area along the default topology.  By
 controlling what labels are advertised, the ABR can thus enforce that
 packets exiting the area do so on the shortest-path default topology.

10.1.1. Motivation for Creating the Rainbow-FEC

 The desired forwarding behavior could be achieved in the above
 example without using the Rainbow-FEC.  This could be done by having
 the ABR advertise the following FEC-label bindings to neighbors not
 in the best area: L1_primary for the default topology, L1_primary for
 the MRT-Blue MT-ID, and L1_primary for the MRT-Red MT-ID.  Doing this
 would require machinery to spoof the labels used in FEC-label binding
 advertisements on a per-neighbor basis.  Such label-spoofing
 machinery does not currently exist in most LDP implementations and
 doesn't have other obvious uses.
 Many existing LDP implementations do however have the ability to
 filter FEC-label binding advertisements on a per-neighbor basis.  The
 Rainbow-FEC allows us to reuse the existing per-neighbor FEC
 filtering machinery to achieve the desired result.  By introducing
 the Rainbow FEC, we can use per-neighbor FEC-filtering machinery to
 advertise the FEC-label binding for the Rainbow-FEC (and filter those
 for MRT-Blue and MRT-Red) to non-best-area neighbors of the ABR.
 An ABR may choose to either distribute the Rainbow-FEC or distribute
 separate MRT-Blue and MRT-Red advertisements.  This is a local
 choice.  A router that supports the MRT LDP Label Option 1A
 forwarding mechanism MUST be able to receive and correctly interpret
 the Rainbow-FEC.

10.2. ABR Forwarding Behavior with IP Tunneling (Option 2)

 If IP tunneling is used, then the ABR behavior is dependent upon the
 outermost IP address.  If the outermost IP address is an MRT loopback
 address of the ABR, then the packet is decapsulated and forwarded
 based upon the inner IP address, which should go on the default SPT
 topology.  If the outermost IP address is not an MRT loopback address
 of the ABR, then the packet is simply forwarded along the associated

Atlas, et al. Standards Track [Page 24] RFC 7812 MRT Unicast FRR Architecture June 2016

 forwarding topology.  A PLR sending traffic to a destination outside
 its local area/level will pick the MRT and use the associated MRT
 loopback address of the selected ABR advertising the lowest cost to
 the external destination.
 Thus, for these two MRT forwarding mechanisms (MRT LDP Label Option
 1A and IP tunneling Option 2), there is no need for additional
 computation or per-area forwarding state.

10.3. ABR Forwarding Behavior with MRT LDP Label Option 1B

 The other MRT forwarding mechanism described in Section 6 uses two
 labels: a topology-id label and a FEC-label.  This mechanism would
 require that any router whose MRT-Red or MRT-Blue next hop is an ABR
 would need to determine whether the ABR would forward the packet out
 of the area/level.  If so, then that router should pop off the
 topology-id label before forwarding the packet to the ABR.
 For example, in Figure 3, if node H fails, node E has to put traffic
 towards prefix p onto MRT-Red.  But since node D knows that ABR1 will
 use a best route from another area, it is safe for D to pop the
 topology-id label and just forward the packet to ABR1 along the MRT-
 Red next hop.  ABR1 will use the shortest path in Area 10.
 In all cases for IS-IS and most cases for OSPF, the penultimate
 router can determine what decision the adjacent ABR will make.  The
 one case where it can't be determined is when two ASBRs are in
 different non-backbone areas attached to the same ABR, then the
 ASBR's Area ID may be needed for tie-breaking (prefer the route with
 the largest OSPF area ID), and the Area ID isn't announced as part of
 the ASBR LSA.  In this one case, suboptimal forwarding along the MRT
 in the other area would happen.  If that becomes a realistic
 deployment scenario, protocol extensions could be developed to
 address this issue.

Atlas, et al. Standards Track [Page 25] RFC 7812 MRT Unicast FRR Architecture June 2016

     +----[C]----     --[D]--[E]                --[D]--[E]
     |           \   /         \               /         \
 p--[A] Area 10 [ABR1]  Area 0 [H]--p   +-[ABR1]  Area 0 [H]-+
     |           /   \         /        |      \         /   |
     +----[B]----     --[F]--[G]        |       --[F]--[G]   |
                                        |                    |
                                        | other              |
                                        +----------[p]-------+
                                          area
       (a) Example topology        (b) Proxy node view in Area 0 nodes
                 +----[C]<---       [D]->[E]
                 V           \             \
              +-[A] Area 10 [ABR1]  Area 0 [H]-+
              |  ^           /             /   |
              |  +----[B]<---       [F]->[G]   V
              |                                |
              +------------->[p]<--------------+
                (c) rSPT towards destination p
  1. >[D]→[E] -<[D]←[E]

/ \ / \

     [ABR1]  Area 0 [H]-+             +-[ABR1]         [H]
                    /   |             |      \
             [F]->[G]   V             V       -<[F]<-[G]
                        |             |
                        |             |
              [p]<------+             +--------->[p]
   (d) MRT-Blue in Area 0           (e) MRT-Red in Area 0
              Figure 3: ABR Forwarding Behavior and MRTs

11. Prefixes Multiply Attached to the MRT Island

 How a computing router S determines its local MRT Island for each
 supported MRT profile is already discussed in Section 7.
 There are two types of prefixes or FECs that may be multiply attached
 to an MRT Island.  The first type are multihomed prefixes that
 usually connect at a domain or protocol boundary.  The second type
 represent routers that do not support the profile for the MRT Island.

Atlas, et al. Standards Track [Page 26] RFC 7812 MRT Unicast FRR Architecture June 2016

 The key difference is whether the traffic, once out of the MRT
 Island, might re-enter the MRT Island if a loop-free exit point is
 not selected.
 FRR using LFA has the useful property that it is able to protect
 multihomed prefixes against ABR failure.  For instance, if a prefix
 from the backbone is available via both ABR A and ABR B, if A fails,
 then the traffic should be redirected to B.  This can be accomplished
 with MRT FRR as well.
 If ASBR protection is desired, this has additional complexities if
 the ASBRs are in different areas.  Similarly, protecting labeled BGP
 traffic in the event of an ASBR failure has additional complexities
 due to the per-ASBR label spaces involved.
 As discussed in [RFC5286], a multihomed prefix could be:
 o  An out-of-area prefix announced by more than one ABR,
 o  An AS-External route announced by two or more ASBRs,
 o  A prefix with iBGP multipath to different ASBRs,
 o  etc.
 See Appendix B for a discussion of a general issue with multihomed
 prefixes connected in two different areas.
 There are also two different approaches to protection.  The first is
 tunnel endpoint selection where the PLR picks a router to tunnel to
 where that router is loop-free with respect to the failure-point.
 Conceptually, the set of candidate routers to provide LFAs expands to
 all routers that can be reached via an MRT alternate, attached to the
 prefix.
 The second is to use a proxy-node, which can be named via MPLS label
 or IP address, and pick the appropriate label or IP address to reach
 it on either MRT-Blue or MRT-Red as appropriate to avoid the failure
 point.  A proxy-node can represent a destination prefix that can be
 attached to the MRT Island via at least two routers.  It is termed a
 named proxy-node if there is a way that traffic can be encapsulated
 to reach specifically that proxy-node; this could be because there is
 an LDP FEC for the associated prefix or because MRT-Red and MRT-Blue
 IP addresses are advertised (in an as-yet undefined fashion) for that
 proxy-node.  Traffic to a named proxy-node may take a different path
 than traffic to the attaching router; traffic is also explicitly
 forwarded from the attaching router along a predetermined interface
 towards the relevant prefixes.

Atlas, et al. Standards Track [Page 27] RFC 7812 MRT Unicast FRR Architecture June 2016

 For IP traffic, multihomed prefixes can use tunnel endpoint
 selection.  For IP traffic that is destined to a router outside the
 MRT Island, if that router is the egress for a FEC advertised into
 the MRT Island, then the named proxy-node approach can be used.
 For LDP traffic, there is always a FEC advertised into the MRT
 Island.  The named proxy-node approach should be used, unless the
 computing router S knows the label for the FEC at the selected tunnel
 endpoint.
 If a FEC is advertised from outside the MRT Island into the MRT
 Island and the forwarding mechanism specified in the profile includes
 LDP Label Option 1A, then the routers learning that FEC MUST also
 advertise labels for (MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors
 inside the MRT Island.  Any router receiving a FEC corresponding to a
 router outside the MRT Island or to a multihomed prefix MUST compute
 and install the transit MRT-Blue and MRT-Red next hops for that FEC.
 The FEC-label bindings for the topology-scoped FECs ((MT-ID 0, FEC),
 (MRT-Red, FEC), and (MRT-Blue, FEC)) MUST also be provided via LDP to
 neighbors inside the MRT Island.

11.1. Protecting Multihomed Prefixes Using Tunnel Endpoint Selection

 Tunnel endpoint selection is a local matter for a router in the MRT
 Island since it pertains to selecting and using an alternate and does
 not affect the transit MRT-Red and MRT-Blue forwarding topologies.
 Let the computing router be S and the next hop F be the node whose
 failure is to be avoided.  Let the destination be prefix p.  Have A
 be the router to which the prefix p is attached for S's shortest path
 to p.
 The candidates for tunnel endpoint selection are those to which the
 destination prefix is attached in the area/level.  For a particular
 candidate B, it is necessary to determine if B is loop-free to reach
 p with respect to S and F for node-protection or at least with
 respect to S and the link (S, F) for link-protection.  If B will
 always prefer to send traffic to p via a different area/level, then
 this is definitional.  Otherwise, distance-based computations are
 necessary and an SPF from B's perspective may be necessary.  The
 following equations give the checks needed; the rationale is similar
 to that given in [RFC5286].  In the inequalities below, D_opt(X,Y)
 means the shortest distance from node X to node Y, and D_opt(X,p)
 means the shortest distance from node X to prefix p.
 Loop-Free for S: D_opt(B, p) < D_opt(B, S) + D_opt(S, p)
 Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(F, p)

Atlas, et al. Standards Track [Page 28] RFC 7812 MRT Unicast FRR Architecture June 2016

 The latter is equivalent to the following, which avoids the need to
 compute the shortest path from F to p.
Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(S, p) - D_opt(S, F)
 Finally, the rules for Endpoint selection are given below.  The basic
 idea is to repair to the prefix-advertising router selected for the
 shortest-path and only to select and tunnel to a different endpoint
 if necessary (e.g., A=F or F is a cut-vertex or the link (S,F) is a
 cut-link).
 1.  Does S have a node-protecting alternate to A?  If so, select
     that.  Tunnel the packet to A along that alternate.  For example,
     if LDP is the forwarding mechanism, then push the label (MRT-Red,
     A) or (MRT-Blue, A) onto the packet.
 2.  If not, then is there a router B that is loop-free to reach p
     while avoiding both F and S?  If so, select B as the endpoint.
     Determine the MRT alternate to reach B while avoiding F.  Tunnel
     the packet to B along that alternate.  For example, with LDP,
     push the label (MRT-Red, B) or (MRT-Blue, B) onto the packet.
 3.  If not, then does S have a link-protecting alternate to A?  If
     so, select that.
 4.  If not, then is there a router B that is loop-free to reach p
     while avoiding S and the link from S to F?  If so, select B as
     the endpoint and the MRT alternate for reaching B from S that
     avoid the link (S,F).
 The tunnel endpoint selected will receive a packet destined to itself
 and, being the egress, will pop that MPLS label (or have signaled
 Implicit Null) and forward based on what is underneath.  This
 suffices for IP traffic since the tunnel endpoint can use the IP
 header of the original packet to continue forwarding the packet.
 However, tunneling of LDP traffic requires targeted LDP sessions for
 learning the FEC-label binding at the tunnel endpoint.

11.2. Protecting Multihomed Prefixes Using Named Proxy-Nodes

 Instead, the named proxy-node method works with LDP traffic without
 the need for targeted LDP sessions.  It also has a clear advantage
 over tunnel endpoint selection, in that it is possible to explicitly
 forward from the MRT Island along an interface to a loop-free island
 neighbor when that interface may not be a primary next hop.

Atlas, et al. Standards Track [Page 29] RFC 7812 MRT Unicast FRR Architecture June 2016

 A named proxy-node represents one or more destinations and, for LDP
 forwarding, has a FEC associated with it that is signaled into the
 MRT Island.  Therefore, it is possible to explicitly label packets to
 go to (MRT-Red, FEC) or (MRT-Blue, FEC); at the border of the MRT
 Island, the label will swap to meaning (MT-ID 0, FEC).  It would be
 possible to have named proxy-nodes for IP forwarding, but this would
 require extensions to signal two IP addresses to be associated with
 MRT-Red and MRT-Blue for the proxy-node.  A named proxy-node can be
 uniquely represented by the two routers in the MRT Island to which it
 is connected.  The extensions to signal such IP addresses will be
 defined elsewhere.  The details of what label-bindings must be
 originated will be described in another document.
 Computing the MRT next hops to a named proxy-node and the MRT
 alternate for the computing router S to avoid a particular failure
 node F is straightforward.  The details of the simple constant-time
 functions, Select_Proxy_Node_NHs() and
 Select_Alternates_Proxy_Node(), are given in [RFC7811].  A key point
 is that computing these MRT next hops and alternates can be done as
 new named proxy-nodes are added or removed without requiring a new
 MRT computation or impacting other existing MRT paths.  This maps
 very well to, for example, how OSPFv2 (see [RFC2328], Section 16.5)
 does incremental updates for new summary-LSAs.
 The remaining question is how to attach the named proxy-node to the
 MRT Island; all the routers in the MRT Island MUST do this
 consistently.  No more than two routers in the MRT Island can be
 selected; one should only be selected if there are no others that
 meet the necessary criteria.  The named proxy-node is logically part
 of the area/level.
 There are two sources for candidate routers in the MRT Island to
 connect to the named proxy-node.  The first set is made up of those
 routers in the MRT Island that are advertising the prefix; the named-
 proxy-cost assigned to each prefix-advertising router is the
 announced cost to the prefix.  The second set is made up of those
 routers in the MRT Island that are connected to routers not in the
 MRT Island but in the same area/level; such routers will be defined
 as Island Border Routers (IBRs).  The routers connected to the IBRs
 that are not in the MRT Island and are in the same area/level as the
 MRT Island are Island Neighbors (INs).
 Since packets sent to the named proxy-node along MRT-Red or MRT-Blue
 may come from any router inside the MRT Island, it is necessary that
 whatever router to which an IBR forwards the packet be loop-free with
 respect to the whole MRT Island for the destination.  Thus, an IBR is
 a candidate router only if it possesses at least one IN whose
 shortest path to the prefix does not enter the MRT Island.  A method

Atlas, et al. Standards Track [Page 30] RFC 7812 MRT Unicast FRR Architecture June 2016

 for identifying Loop-Free Island Neighbors (LFINs) is given in
 [RFC7811].  The named-proxy-cost assigned to each (IBR, IN) pair is
 cost(IBR, IN) + D_opt(IN, prefix).
 From the set of prefix-advertising routers and the set of IBRs with
 at least one LFIN, the two routers with the lowest named-proxy-cost
 are selected.  Ties are broken based upon the lowest Router ID.  For
 ease of discussion, the two selected routers will be referred to as
 proxy-node attachment routers.
 A proxy-node attachment router has a special forwarding role.  When a
 packet is received destined to (MRT-Red, prefix) or (MRT-Blue,
 prefix), if the proxy-node attachment router is an IBR, it MUST swap
 to the shortest path forwarding topology (e.g., swap to the label for
 (MT-ID 0, prefix) or remove the outer IP encapsulation) and forward
 the packet to the IN whose cost was used in the selection.  If the
 proxy-node attachment router is not an IBR, then the packet MUST be
 removed from the MRT forwarding topology and sent along the
 interface(s) that caused the router to advertise the prefix; this
 interface might be out of the area/level/AS.

11.3. MRT Alternates for Destinations outside the MRT Island

 A natural concern with new functionality is how to have it be useful
 when it is not deployed across an entire IGP area.  In the case of
 MRT FRR, where it provides alternates when appropriate LFAs aren't
 available, there are also deployment scenarios where it may make
 sense to only enable some routers in an area with MRT FRR.  A simple
 example of such a scenario would be a ring of six or more routers
 that is connected via two routers to the rest of the area.
 Destinations inside the local island can obviously use MRT
 alternates.  Destinations outside the local island can be treated
 like a multihomed prefix and either endpoint selection or Named
 Proxy-Nodes can be used.  Named proxy-nodes MUST be supported when
 LDP forwarding is supported and a label-binding for the destination
 is sent to an IBR.
 Naturally, there are more-complicated options to improve coverage,
 such as connecting multiple MRT Islands across tunnels, but the need
 for the additional complexity has not been justified.

Atlas, et al. Standards Track [Page 31] RFC 7812 MRT Unicast FRR Architecture June 2016

12. Network Convergence and Preparing for the Next Failure

 After a failure, MRT detours ensure that packets reach their intended
 destination while the IGP has not reconverged onto the new topology.
 As link-state updates reach the routers, the IGP process calculates
 the new shortest paths.  Two things need attention: micro-loop
 prevention and MRT recalculation.

12.1. Micro-loop Prevention and MRTs

 A micro-loop is a transient packet-forwarding loop among two or more
 routers that can occur during convergence of IGP forwarding state.
 [RFC5715] discusses several techniques for preventing micro-loops.
 This section discusses how MRT-FRR relates to two of the micro-loop
 prevention techniques discussed in [RFC5715]: Nearside and Farside
 Tunneling.
 In Nearside Tunneling, a router (PLR) adjacent to a failure performs
 local repair and informs remote routers of the failure.  The remote
 routers initially tunnel affected traffic to the nearest PLR, using
 tunnels that are unaffected by the failure.  Once the forwarding
 state for normal shortest path routing has converged, the remote
 routers return the traffic to shortest path forwarding.  MRT-FRR is
 relevant for Nearside Tunneling for the following reason.  The
 process of tunneling traffic to the PLRs and waiting a sufficient
 amount of time for IGP forwarding state convergence with Nearside
 Tunneling means that traffic will generally rely on the local repair
 at the PLR for longer than it would in the absence of Nearside
 Tunneling.  Since MRT-FRR provides 100% coverage for single link and
 node failure, it may be an attractive option to provide the local
 repair paths when Nearside Tunneling is deployed.
 MRT-FRR is also relevant for the Farside Tunneling micro-loop
 prevention technique.  In Farside Tunneling, remote routers tunnel
 traffic affected by a failure to a node downstream of the failure
 with respect to traffic destination.  This node can be viewed as
 being on the farside of the failure with respect to the node
 initiating the tunnel.  Note that the discussion of Farside Tunneling
 in [RFC5715] focuses on the case where the farside node is
 immediately adjacent to a failed link or node.  However, the farside
 node may be any node downstream of the failure with respect to
 traffic destination, including the destination itself.  The tunneling
 mechanism used to reach the farside node must be unaffected by the
 failure.  The alternative forwarding paths created by MRT-FRR have
 the potential to be used to forward traffic from the remote routers
 upstream of the failure all the way to the destination.  In the event
 of failure, either the MRT-Red or MRT-Blue path from the remote
 upstream router to the destination is guaranteed to avoid a link

Atlas, et al. Standards Track [Page 32] RFC 7812 MRT Unicast FRR Architecture June 2016

 failure or inferred node failure.  The MRT forwarding paths are also
 guaranteed to not be subject to micro-loops because they are locked
 to the topology before the failure.
 We note that the computations in [RFC7811] address the case of a PLR
 adjacent to a failure determining which choice of MRT-Red or MRT-Blue
 will avoid a failed link or node.  More computation may be required
 for an arbitrary remote upstream router to determine whether to
 choose MRT-Red or MRT-Blue for a given destination and failure.

12.2. MRT Recalculation for the Default MRT Profile

 This section describes how the MRT recalculation SHOULD be performed
 for the Default MRT Profile.  This is intended to support FRR
 applications.  Other approaches are possible, but they are not
 specified in this document.
 When a failure event happens, traffic is put by the PLRs onto the MRT
 topologies.  After that, each router recomputes its SPT and moves
 traffic over to that.  Only after all the PLRs have switched to using
 their SPTs and traffic has drained from the MRT topologies should
 each router install the recomputed MRTs into the FIBs.
 At each router, therefore, the sequence is as follows:
 1.  Receive failure notification
 2.  Recompute SPT.
 3.  Install the new SPT in the FIB.
 4.  If the network was stable before the failure occurred, wait a
     configured (or advertised) period for all routers to be using
     their SPTs and traffic to drain from the MRTs.
 5.  Recompute MRTs.
 6.  Install new MRTs in the FIB.
 While the recomputed MRTs are not installed in the FIB, protection
 coverage is lowered.  Therefore, it is important to recalculate the
 MRTs and install them quickly.
 New protocol extensions for advertising the time needed to recompute
 shortest path routes and install them in the FIB will be defined
 elsewhere.

Atlas, et al. Standards Track [Page 33] RFC 7812 MRT Unicast FRR Architecture June 2016

13. Operational Considerations

 The following aspects of MRT-FRR are useful to consider when
 deploying the technology in different operational environments and
 network topologies.

13.1. Verifying Forwarding on MRT Paths

 The forwarding paths created by MRT-FRR are not used by normal (non-
 FRR) traffic.  They are only used to carry FRR traffic for a short
 period of time after a failure has been detected.  It is RECOMMENDED
 that an operator proactively monitor the MRT forwarding paths in
 order to be certain that the paths will be able to carry FRR traffic
 when needed.  Therefore, an implementation SHOULD provide an operator
 with the ability to test MRT paths with Operations, Administration,
 and Maintenance (OAM) traffic.  For example, when MRT paths are
 realized using LDP labels distributed for topology-scoped FECs, an
 implementation can use the MPLS ping and traceroute as defined in
 [RFC4379] and extended in [RFC7307] for topology-scoped FECs.

13.2. Traffic Capacity on Backup Paths

 During a fast-reroute event initiated by a PLR in response to a
 network failure, the flow of traffic in the network will generally
 not be identical to the flow of traffic after the IGP forwarding
 state has converged, taking the failure into account.  Therefore,
 even if a network has been engineered to have enough capacity on the
 appropriate links to carry all traffic after the IGP has converged
 after the failure, the network may still not have enough capacity on
 the appropriate links to carry the flow of traffic during a fast-
 reroute event.  This can result in more traffic loss during the fast-
 reroute event than might otherwise be expected.
 Note that there are two somewhat distinct aspects to this phenomenon.
 The first is that the path from the PLR to the destination during the
 fast-reroute event may be different from the path after the IGP
 converges.  In this case, any traffic for the destination that
 reaches the PLR during the fast-reroute event will follow a different
 path from the PLR to the destination than will be followed after IGP
 convergence.
 The second aspect is that the amount of traffic arriving at the PLR
 for affected destinations during the fast-reroute event may be larger
 than the amount of traffic arriving at the PLR for affected
 destinations after IGP convergence.  Immediately after a failure, any
 non-PLR routers that were sending traffic to the PLR before the
 failure will continue sending traffic to the PLR, and that traffic
 will be carried over backup paths from the PLR to the destinations.

Atlas, et al. Standards Track [Page 34] RFC 7812 MRT Unicast FRR Architecture June 2016

 After IGP convergence, upstream non-PLR routers may direct some
 traffic away from the PLR.
 In order to reduce or eliminate the potential for transient traffic
 loss due to inadequate capacity during fast-reroute events, an
 operator can model the amount of traffic taking different paths
 during a fast-reroute event.  If it is determined that there is not
 enough capacity to support a given fast-reroute event, the operator
 can address the issue either by augmenting capacity on certain links
 or modifying the backup paths themselves.
 The MRT Lowpoint algorithm produces a pair of diverse paths to each
 destination.  These paths are generated by following the directed
 links on a common GADAG.  The decision process for constructing the
 GADAG in the MRT Lowpoint algorithm takes into account individual IGP
 link metrics.  At any given node, links are explored in order from
 lowest IGP metric to highest IGP metric.  Additionally, the process
 for constructing the MRT-Red and Blue trees uses SPF traversals of
 the GADAG.  Therefore, the IGP link metric values affect the computed
 backup paths.  However, adjusting the IGP link metrics is not a
 generally applicable tool for modifying the MRT backup paths.
 Achieving a desired set of MRT backup paths by adjusting IGP metrics
 while at the same time maintaining the desired flow of traffic along
 the shortest paths is not possible in general.
 MRT-FRR allows an operator to exclude a link from the MRT Island, and
 thus the GADAG, by advertising it as MRT-Ineligible.  Such a link
 will not be used on the MRT forwarding path for any destination.
 Advertising links as MRT-Ineligible is the main tool provided by MRT-
 FRR for keeping backup traffic off of lower bandwidth links during
 fast-reroute events.
 Note that all of the backup paths produced by the MRT Lowpoint
 algorithm are closely tied to the common GADAG computed as part of
 that algorithm.  Therefore, it is generally not possible to modify a
 subset of paths without affecting other paths.  This precludes more
 fine-grained modification of individual backup paths when using only
 paths computed by the MRT Lowpoint algorithm.
 However, it may be desirable to allow an operator to use MRT-FRR
 alternates together with alternates provided by other FRR
 technologies.  A policy-based alternate selection process can allow
 an operator to select the best alternate from those provided by MRT
 and other FRR technologies.  As a concrete example, it may be
 desirable to implement a policy where a downstream LFA (if it exists
 for a given failure mode and destination) is preferred over a given
 MRT alternate.  This combination gives the operator the ability to
 affect where traffic flows during a fast-reroute event, while still

Atlas, et al. Standards Track [Page 35] RFC 7812 MRT Unicast FRR Architecture June 2016

 producing backup paths that use no additional labels for LDP traffic
 and will not loop under multiple failures.  This and other choices of
 alternate selection policy can be evaluated in the context of their
 effect on fast-reroute traffic flow and available capacity, as well
 as other deployment considerations.
 Note that future documents may define MRT profiles in addition to the
 default profile defined here.  Different MRT profiles will generally
 produce alternate paths with different properties.  An implementation
 may allow an operator to use different MRT profiles instead of or in
 addition to the default profile.

13.3. MRT IP Tunnel Loopback Address Management

 As described in Section 6.1.2, if an implementation uses IP tunneling
 as the mechanism to realize MRT forwarding paths, each node must
 advertise an MRT-Red and an MRT-Blue loopback address.  These IP
 addresses must be unique within the routing domain to the extent that
 they do not overlap with each other or with any other routing table
 entries.  It is expected that operators will use existing tools and
 processes for managing infrastructure IP addresses to manage these
 additional MRT-related loopback addresses.

13.4. MRT-FRR in a Network with Degraded Connectivity

 Ideally, routers in a service provider network using MRT-FRR will be
 initially deployed in a 2-connected topology, allowing MRT-FRR to
 find completely diverse paths to all destinations.  However, a
 network can differ from an ideal 2-connected topology for many
 possible reasons, including network failures and planned maintenance
 events.
 MRT-FRR is designed to continue to function properly when network
 connectivity is degraded.  When a network contains cut-vertices or
 cut-links dividing the network into different 2-connected blocks,
 MRT-FRR will continue to provide completely diverse paths for
 destinations within the same block as the PLR.  For a destination in
 a different block from the PLR, the redundant paths created by MRT-
 FRR will be link and node diverse within each block, and the paths
 will only share links and nodes that are cut-links or cut-vertices in
 the topology.
 If a network becomes partitioned with one set of routers having no
 connectivity to another set of routers, MRT-FRR will function
 independently in each set of connected routers, providing redundant
 paths to destinations in same set of connected routers as a given
 PLR.

Atlas, et al. Standards Track [Page 36] RFC 7812 MRT Unicast FRR Architecture June 2016

13.5. Partial Deployment of MRT-FRR in a Network

 A network operator may choose to deploy MRT-FRR only on a subset of
 routers in an IGP area.  MRT-FRR is designed to accommodate this
 partial deployment scenario.  Only routers that advertise support for
 a given MRT profile will be included in a given MRT Island.  For a
 PLR within the MRT Island, MRT-FRR will create redundant forwarding
 paths to all destinations with the MRT Island using maximally
 redundant trees all the way to those destinations.  For destinations
 outside of the MRT Island, MRT-FRR creates paths to the destination
 that use forwarding state created by MRT-FRR within the MRT Island
 and shortest path forwarding state outside of the MRT Island.  The
 paths created by MRT-FRR to non-Island destinations are guaranteed to
 be diverse within the MRT Island (if topologically possible).
 However, the part of the paths outside of the MRT Island may not be
 diverse.

14. IANA Considerations

 IANA has created the "MRT Profile Identifier Registry".  The range is
 0 to 255.  The Default MRT Profile defined in this document has value
 0.  Values 1-200 are allocated by Standards Action.  Values 201-220
 are for Experimental Use.  Values 221-254 are for Private Use.  Value
 255 is reserved for future registry extension.  (The allocation and
 use policies are described in [RFC5226].)
 The initial registry is shown below.
    Value    Description                               Reference
    -------  ----------------------------------------  ------------
    0        Default MRT Profile                       RFC 7812
    1-200    Unassigned
    201-220  Experimental Use
    221-254  Private Use
    255      Reserved (for future registry extension)
 The "MRT Profile Identifier Registry" is a new registry in the IANA
 Matrix.  Following existing conventions, http://www.iana.org/
 protocols displays a new header: "Maximally Redundant Tree (MRT)
 Parameters".  Under that header, there is an entry for "MRT Profile
 Identifier Registry", which links to the registry itself at
 http://www.iana.org/assignments/mrt-parameters.

Atlas, et al. Standards Track [Page 37] RFC 7812 MRT Unicast FRR Architecture June 2016

15. Security Considerations

 In general, MRT forwarding paths do not follow shortest paths.  The
 transit forwarding state corresponding to the MRT paths is created
 during normal operations (before a failure occurs).  Therefore, a
 malicious packet with an appropriate header injected into the network
 from a compromised location would be forwarded to a destination along
 a non-shortest path.  When this technology is deployed, a network
 security design should not rely on assumptions about potentially
 malicious traffic only following shortest paths.
 It should be noted that the creation of non-shortest forwarding paths
 is not unique to MRT.
 MRT-FRR requires that routers advertise information used in the
 formation of MRT backup paths.  While this document does not specify
 the protocol extensions used to advertise this information, we
 discuss security considerations related to the information itself.
 Injecting false MRT-related information could be used to direct some
 MRT backup paths over compromised transmission links.  Combined with
 the ability to generate network failures, this could be used to send
 traffic over compromised transmission links during a fast-reroute
 event.  In order to prevent this potential exploit, a receiving
 router needs to be able to authenticate MRT-related information that
 claims to have been advertised by another router.

16. References

16.1. 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,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            DOI 10.17487/RFC5226, May 2008,
            <http://www.rfc-editor.org/info/rfc5226>.
 [RFC7307]  Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
            King, "LDP Extensions for Multi-Topology", RFC 7307,
            DOI 10.17487/RFC7307, July 2014,
            <http://www.rfc-editor.org/info/rfc7307>.

Atlas, et al. Standards Track [Page 38] RFC 7812 MRT Unicast FRR Architecture June 2016

 [RFC7811]  Enyedi, G., Ed., Csaszar, A., Atlas, A., Ed., Bowers, C.,
            and A. Gopalan, "An Algorithm for Computing IP/LDP Fast
            Reroute Using Maximally Redundant Trees (MRT-FRR)",
            RFC 7811, DOI 10.17487/RFC7811, June 2016,
            <http://www.rfc-editor.org/info/rfc7811>.

16.2. Informative References

 [EnyediThesis]
            Enyedi, G., "Novel Algorithms for IP Fast Reroute",
            Department of Telecommunications and Media Informatics,
            Budapest University of Technology and Economics Ph.D.
            Thesis, February 2011,
            <https://repozitorium.omikk.bme.hu/bitstream/
            handle/10890/1040/ertekezes.pdf>.
 [LDP-MRT]  Atlas, A., Tiruveedhula, K., Bowers, C., Tantsura, J., and
            IJ. Wijnands, "LDP Extensions to Support Maximally
            Redundant Trees", Work in Progress, draft-ietf-mpls-ldp-
            mrt-03, May 2016.
 [MRT-ARCH]
            Atlas, A., Kebler, R., Wijnands, IJ., Csaszar, A., and G.
            Enyedi, "An Architecture for Multicast Protection Using
            Maximally Redundant Trees", Work in Progress, draft-atlas-
            rtgwg-mrt-mc-arch-02, July 2013.
 [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
            DOI 10.17487/RFC2328, April 1998,
            <http://www.rfc-editor.org/info/rfc2328>.
 [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
            Label Switched (MPLS) Data Plane Failures", RFC 4379,
            DOI 10.17487/RFC4379, February 2006,
            <http://www.rfc-editor.org/info/rfc4379>.
 [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,
            <http://www.rfc-editor.org/info/rfc5286>.
 [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
            Label Assignment and Context-Specific Label Space",
            RFC 5331, DOI 10.17487/RFC5331, August 2008,
            <http://www.rfc-editor.org/info/rfc5331>.

Atlas, et al. Standards Track [Page 39] RFC 7812 MRT Unicast FRR Architecture June 2016

 [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
            for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
            <http://www.rfc-editor.org/info/rfc5340>.
 [RFC5443]  Jork, M., Atlas, A., and L. Fang, "LDP IGP
            Synchronization", RFC 5443, DOI 10.17487/RFC5443, March
            2009, <http://www.rfc-editor.org/info/rfc5443>.
 [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework",
            RFC 5714, DOI 10.17487/RFC5714, January 2010,
            <http://www.rfc-editor.org/info/rfc5714>.
 [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
            Convergence", RFC 5715, DOI 10.17487/RFC5715, January
            2010, <http://www.rfc-editor.org/info/rfc5715>.
 [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, DOI 10.17487/RFC6976, July
            2013, <http://www.rfc-editor.org/info/rfc6976>.
 [RFC6981]  Bryant, S., Previdi, S., and M. Shand, "A Framework for IP
            and MPLS Fast Reroute Using Not-Via Addresses", RFC 6981,
            DOI 10.17487/RFC6981, August 2013,
            <http://www.rfc-editor.org/info/rfc6981>.
 [RFC6987]  Retana, A., Nguyen, L., Zinin, A., White, R., and D.
            McPherson, "OSPF Stub Router Advertisement", RFC 6987,
            DOI 10.17487/RFC6987, September 2013,
            <http://www.rfc-editor.org/info/rfc6987>.
 [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
            So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
            RFC 7490, DOI 10.17487/RFC7490, April 2015,
            <http://www.rfc-editor.org/info/rfc7490>.

Atlas, et al. Standards Track [Page 40] RFC 7812 MRT Unicast FRR Architecture June 2016

Appendix A. Inter-level Forwarding Behavior for IS-IS

 In the description below, we use the terms "Level-1-only interface",
 "Level-2-only interface", and "Level-1-and-Level-2 interface" to mean
 an interface that has formed only a Level-1 adjacency, only a Level-2
 adjacency, or both Level-1 and Level-2 adjacencies.  Note that IS-IS
 also defines the concept of areas.  A router is configured with an
 IS-IS area identifier, and a given router may be configured with
 multiple IS-IS area identifiers.  For an IS-IS Level-1 adjacency to
 form between two routers, at least one IS-IS area identifier must
 match.  IS-IS Level-2 adjacencies do not require any area identifiers
 to match.  The behavior described below does not explicitly refer to
 IS-IS area identifiers.  However, IS-IS area identifiers will
 indirectly affect the behavior by affecting the formation of Level-1
 adjacencies.
 First, consider a packet destined to Z on MRT-Red or MRT-Blue
 received on a Level-1-only interface.  If the best shortest path
 route to Z was learned from a Level-1 advertisement, then the packet
 should continue to be forwarded along MRT-Red or MRT-Blue.  If,
 instead, the best route was learned from a Level-2 advertisement,
 then the packet should be removed from MRT-Red or MRT-Blue and
 forwarded on the shortest-path default forwarding topology.
 Now consider a packet destined to Z on MRT-Red or MRT-Blue received
 on a Level-2-only interface.  If the best route to Z was learned from
 a Level-2 advertisement, then the packet should continue to be
 forwarded along MRT-Red or MRT-Blue.  If, instead, the best route was
 learned from a Level-1 advertisement, then the packet should be
 removed from MRT-Red or MRT-Blue and forwarded on the shortest-path
 default forwarding topology.
 Finally, consider a packet destined to Z on MRT-Red or MRT-Blue
 received on a Level-1-and-Level-2 interface.  This packet should
 continue to be forwarded along MRT-Red or MRT-Blue, regardless of
 which level the route was learned from.
 An implementation may simplify the decision-making process above by
 using the interface of the next hop for the route to Z to determine
 the level from which the best route to Z was learned.  If the next
 hop points out a Level-1-only interface, then the route was learned
 from a Level-1 advertisement.  If the next hop points out a Level-
 2-only interface, then the route was learned from a Level-2
 advertisement.  A next hop that points out a Level-1-and-Level-2
 interface does not provide enough information to determine the source
 of the best route.  With this simplification, an implementation would
 need to continue forwarding along MRT-Red or MRT-Blue when the next-
 hop points out a Level-1-and-Level-2 interface.  Therefore, a packet

Atlas, et al. Standards Track [Page 41] RFC 7812 MRT Unicast FRR Architecture June 2016

 on MRT-Red or MRT-Blue going from Level-1 to Level-2 (or vice versa)
 that traverses a Level-1-and-Level-2 interface in the process will
 remain on MRT-Red or MRT-Blue.  This simplification may not always
 produce the optimal forwarding behavior, but it does not introduce
 interoperability problems.  The packet will stay on an MRT backup
 path longer than necessary, but it will still reach its destination.

Appendix B. General Issues with Area Abstraction

 When a multihomed prefix is connected in two different areas, it may
 be impractical to protect them without adding the complexity of
 explicit tunneling.  This is also a problem for LFA and Remote-LFA.
        50
      |----[ASBR Y]---[B]---[ABR 2]---[C]      Backbone Area 0:
      |                                |           ABR 1, ABR 2, C, D
      |                                |
      |                                |       Area 20:  A, ASBR X
      |                                |
      p ---[ASBR X]---[A]---[ABR 1]---[D]      Area 10: B, ASBR Y
         5                                  p is a Type 1 AS-external
           Figure 4: AS External Prefixes in Different Areas
 Consider the network in Figure 4 and assume there is a richer
 connective topology that isn't shown, where the same prefix is
 announced by ASBR X and ASBR Y, which are in different non-backbone
 areas.  If the link from A to ASBR X fails, then an MRT alternate
 could forward the packet to ABR 1 and ABR 1 could forward it to D,
 but then D would find the shortest route is back via ABR 1 to Area
 20.  This problem occurs because the routers, including the ABR, in
 one area are not yet aware of the failure in a different area.
 The only way to get it from A to ASBR Y is to explicitly tunnel it to
 ASBR Y.  If the traffic is unlabeled or the appropriate MPLS labels
 are known, then explicit tunneling MAY be used as long as the
 shortest path of the tunnel avoids the failure point.  In that case,
 A must determine that it should use an explicit tunnel instead of an
 MRT alternate.

Atlas, et al. Standards Track [Page 42] RFC 7812 MRT Unicast FRR Architecture June 2016

Acknowledgements

 The authors would like to thank Mike Shand for his valuable review
 and contributions.
 The authors would like to thank Joel Halpern, Hannes Gredler, Ted
 Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin
 Bahadur, Harish Sitaraman, Raveendra Torvi, Anil Kumar SN, Bruno
 Decraene, Eric Wu, Janos Farkas, Rob Shakir, Stewart Bryant, and
 Alvaro Retana for their suggestions and review.

Contributors

 Robert Kebler
 Juniper Networks
 10 Technology Park Drive
 Westford, MA  01886
 United States
 Email: rkebler@juniper.net
 Andras Csaszar
 Ericsson
 Konyves Kalman krt 11
 Budapest  1097
 Hungary
 Email: Andras.Csaszar@ericsson.com
 Jeff Tantsura
 Ericsson
 300 Holger Way
 San Jose, CA  95134
 United States
 Email: jeff.tantsura@ericsson.com
 Russ White
 VCE
 Email: russw@riw.us

Atlas, et al. Standards Track [Page 43] RFC 7812 MRT Unicast FRR Architecture June 2016

Authors' Addresses

 Alia Atlas
 Juniper Networks
 10 Technology Park Drive
 Westford, MA  01886
 United States
 Email: akatlas@juniper.net
 Chris Bowers
 Juniper Networks
 1194 N. Mathilda Ave.
 Sunnyvale, CA  94089
 United States
 Email: cbowers@juniper.net
 Gabor Sandor Enyedi
 Ericsson
 Konyves Kalman krt 11.
 Budapest  1097
 Hungary
 Email: Gabor.Sandor.Enyedi@ericsson.com

Atlas, et al. Standards Track [Page 44]

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