Routing Area Working Group                                 A. Atlas, Ed.
Internet-Draft                                                 R. Kebler
Intended status: Standards Track                        Juniper Networks                               C. Bowers
Expires: January 13, 2014 5, 2015                                Juniper Networks
                                                               G. Enyedi
                                                              A. Csaszar
                                                             J. Tantsura
                                                                Ericsson
                                                      M. Konstantynowicz
                                                           Cisco Systems
                                                                R. White
                                                                     VCE
                                                            July 12, 2013 4, 2014

An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
                draft-ietf-rtgwg-mrt-frr-architecture-03
                draft-ietf-rtgwg-mrt-frr-architecture-04

Abstract

   With increasing deployment of Loop-Free Alternates (LFA) [RFC5286],
   it is clear that a complete solution for IP and LDP Fast-Reroute is
   required.  This specification provides that solution.  IP/LDP Fast-
   Reroute with Maximally Redundant Trees (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.

   MRT removes all need to engineer for coverage.  MRT is also extremely
   computationally efficient.  For any router in the network, the MRT
   computation is less than the LFA computation for a node with three or
   more neighbors.

Status of This Memo

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   This Internet-Draft will expire on January 13, 2014. 5, 2015.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Importance of 100% Coverage . . . . . . . . . . . . . . .   4   5
     1.2.  Partial Deployment and Backwards Compatibility  . . . . .   5   6
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   6
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . .   7   8
   5.  Maximally Redundant Trees (MRT) and Fast-Reroute  . . . . . .   9  10
   6.  Unicast Forwarding with MRT Fast-Reroute  . . . . . . . . . .  10  11
     6.1.  LDP Unicast  MRT Forwarding - Avoid Tunneling Mechanisms . . . . . . . . . . .  10
     6.2.  IP Unicast Traffic . . . . .  11
       6.1.1.  MRT LDP labels  . . . . . . . . . . . . . . . . . . .  11
   7.  Protocol Extensions and Considerations: OSPF and ISIS
         6.1.1.1.  Topology-scoped FEC encoded using a single label
                   (Option 1A) . . . .  12
   8.  Protocol Extensions and considerations: LDP . . . . . . . . .  14
   9.  Inter-Area . . . . . .  12
         6.1.1.2.  Topology and ABR Forwarding Behavior FEC encoded using a two label stack
                   (Option 1B) . . . . . . . . . . .  15
   10. Prefixes Multiply Attached to the MRT Island . . . . . . . .  18
     10.1.  Endpoint Selection  12
         6.1.1.3.  Compatibility of Option 1A and 1B . . . . . . . .  13
         6.1.1.4.  Mandatory support for MRT LDP Label option 1A . .  13
       6.1.2.  MRT IP tunnels (Options 2A and 2B)  . . . . . . . . .  19
     10.2.  Named Proxy-Nodes  13
     6.2.  Forwarding LDP Unicast Traffic over MRT Paths . . . . . .  14
       6.2.1.  Forwarding LDP traffic using MRT LDP Labels (Option
               1A) . . . . . . . . . . . . .  21
       10.2.1.  Computing if an Island Neighbor (IN) is loop-free .  22
     10.3.  MRT Alternates for Destinations Outside the MRT Island .  23
   11. Network Convergence and Preparing for the Next Failure . . .  24
     11.1.  Micro-forwarding loop prevention and MRTs . . . . . . .  24
     11.2.  14
       6.2.2.  Forwarding LDP traffic using MRT Recalculation LDP Labels (Option
               1B) . . . . . . . . . . . . . . . . . . .  24
   12. Acknowledgements . . . . . .  15
       6.2.3.  Other considerations for forwarding LDP traffic using
               MRT LDP Labels  . . . . . . . . . . . . . . . . . . .  25
   13. IANA Considerations  15
     6.3.  Forwarding IP Unicast Traffic over MRT Paths  . . . . . .  15
       6.3.1.  Tunneling IP traffic using MRT LDP Labels . . . . . .  16
         6.3.1.1.  Tunneling IP traffic using MRT LDP Labels (Option
                   1A) . . . . . . . . . . .  25
   14. Security Considerations . . . . . . . . . . . .  16
         6.3.1.2.  Tunneling IP traffic using MRT LDP Labels (Option
                   1B) . . . . . . .  25
   15. References . . . . . . . . . . . . . . . .  16
       6.3.2.  Tunneling IP traffic using MRT IP Tunnels . . . . . .  17
       6.3.3.  Required support  . . .  25
     15.1.  Normative References . . . . . . . . . . . . . . .  17
   7.  MRT Island Formation  . . .  25
     15.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Appendix A.  General Issues with  17
     7.1.  IGP Area Abstraction or Level . . . . . . . .  27
   Authors' Addresses . . . . . . . . . . . .  17
     7.2.  Support for a specific MRT profile  . . . . . . . . . . .  28

1.  Introduction

   This  18
     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.  Example algorithm . . . . . . . . . . . . . . . . . . . .  19
   8.  MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  MRT Profile Options . . . . . . . . . . . . . . . . . . .  19
     8.2.  Router-specific MRT paramaters  . . . . . . . . . . . . .  20
     8.3.  Default MRT profile . . . . . . . . . . . . . . . . . . .  21
   9.  LDP signaling extensions and considerations . . . . . . . . .  22
   10. Inter-area Forwarding Behavior  . . . . . . . . . . . . . . .  22
     10.1.  ABR Forwarding Behavior with MRT LDP Label Option 1A . .  23
       10.1.1.  Motivation for Creating the Rainbow-FEC  . . . . . .  23
     10.2.  ABR Forwarding Behavior with IP Tunneling (option 2) . .  24
     10.3.  ABR Forwarding Behavior with LDP Label option 1B . . . .  24
   11. Prefixes Multiply Attached to the MRT Island  . . . . . . . .  26
     11.1.  Protecting Multi-Homed Prefixes using Tunnel Endpoint
            Selection  . . . . . . . . . . . . . . . . . . . . . . .  28
     11.2.  Protecting Multi-Homed Prefixes using Named Proxy-Nodes   29
       11.2.1.  Computing if an Island Neighbor (IN) is loop-free  .  31
     11.3.  MRT Alternates for Destinations Outside the MRT Island .  32
   12. Network Convergence and Preparing for the Next Failure  . . .  33
     12.1.  Micro-forwarding loop prevention and MRTs  . . . . . . .  33
     12.2.  MRT Recalculation  . . . . . . . . . . . . . . . . . . .  33
   13. Implementation Status . . . . . . . . . . . . . . . . . . . .  34
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  36
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36
   16. Security Considerations . . . . . . . . . . . . . . . . . . .  36
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  36
     17.2.  Informative References . . . . . . . . . . . . . . . . .  37
   Appendix A.  General Issues with Area Abstraction . . . . . . . .  39
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  40

1.  Introduction

   This document gives a complete solution for IP/LDP fast-reroute
   [RFC5714].  MRT-FRR creates two alternate trees separate 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.  The associated
   protocol extensions are defined in [I-D.atlas-ospf-mrt] and
   [I-D.atlas-mpls-ldp-mrt].  The exact MRT algorithm is defined in
   [I-D.ietf-rtgwg-mrt-frr-algorithm].

   IP/LDP Fast-Reroute with MRT (MRT-FRR) uses two maximally diverse
   forwarding topologies to provide alternates.  A primary next-hop
   should be on only one of the diverse forwarding topologies; thus, the
   other can be used to provide an alternate.  Once traffic has been
   moved to one of MRTs, it is not subject to further repair actions.
   Thus, the traffic will not loop even if a worse failure (e.g. node)
   occurs when protection was only available for a simpler failure (e.g.
   link).

   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 [I-D.atlas-rtgwg-mrt-mc-arch].

   Other existing or proposed solutions are partial solutions or have
   significant issues, as described below.

                 Summary Comparison of IP/LDP FRR Methods

   +---------+-------------+-------------+-----------------------------+
   |  Method |   Coverage  |  Alternate  |    Computation (in SPFs)    |
   |         |             |   Looping?  |                             |
   +---------+-------------+-------------+-----------------------------+
   | MRT-FRR |     100%    |     None    |         less than 3         |
   |         |  Link/Node  |             |                             |
   |         |             |             |                             |
   |   LFA   |   Partial   |   Possible  |         per neighbor        |
   |         |  Link/Node  |             |                             |
   |         |             |             |                             |
   |  Remote |   Partial   |   Possible  |    per neighbor (link) or   |
   |   LFA   |  Link/Node  |             |  neighbor's neighbor (node) |
   |         |             |             |                             |
   | Not-Via |     100%    |     None    |      per link and node      |
   |         |  Link/Node  |             |                             |
   +---------+-------------+-------------+-----------------------------+

                                  Table 1

   Loop-Free Alternates (LFA):   LFAs [RFC5286] provide limited
      topology-dependent coverage for link and node protection.
      Restrictions on choice of alternates can be relaxed to improve
      coverage, but this can cause forwarding loops if a worse failure
      is experienced than protected against.  Augmenting a network to
      provide better coverage is NP-hard [LFARevisited].  [RFC6571]
      discusses the applicability of LFA to different topologies with a
      focus on common PoP architectures.

   Remote LFA:   Remote LFAs [I-D.ietf-rtgwg-remote-lfa] improve
      coverage over LFAs for link protection but still cannot guarantee
      complete coverage.  The trade-off of looping traffic to improve
      coverage is still made.  Remote LFAs can provide node-protection
      [I-D.psarkar-rtgwg-rlfa-node-protection] but not guaranteed
      coverage and the computation required is quite high (an SPF for
      each PQ-node evaluated).  [I-D.bryant-ipfrr-tunnels] describes
      additional mechanisms to further improve coverage, at the cost of
      added complexity.

   Not-Via:   Not-Via [I-D.ietf-rtgwg-ipfrr-notvia-addresses] is the
      only other solution that provides 100% coverage for link and node
      failures and does not have potential looping.  However, the
      computation is very high (an SPF per failure point) and academic
      implementations [LightweightNotVia] have found the address
      management complexity to be high.

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 LFAs are available.  Similarly, after a single failure has
   happened, the topology is changed and its associated coverage.
   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 accomodate natural
   network topology changes.

   Asymmetric link costs are also a common aspect of networks.  There
   are at least three common causes for them.  First, any broadcast
   interface is represented by a pseudo-node and has asymmetric link
   costs to and from that pseudo-node.  Second, when routers come up or
   a link with LDP comes up, it is recommended in [RFC5443] and
   [RFC3137] that the link metric be raised to the maximum cost; this
   may not be symmetric and for [RFC3137] is not expected to be.  Third,
   techniques such as IGP metric tuning for traffic-engineering can
   result in asymmetric link costs.  A fast-reroute solution needs to
   handle network topologies with asymmetric link costs.

   When a network needs to use a micro-loop prevention mechanism
   [RFC5715] such as Ordered FIB[I-D.ietf-rtgwg-ordered-fib] or Farside
   Tunneling[RFC5715], then the whole IGP area needs to have alternates
   available so that the micro-loop prevention mechanism, which requires
   slower network convergence, can 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.

1.2.  Partial Deployment and Backwards Compatibility

   MRT-FRR supports partial deployment.  As with many new features, the
   protocols (OSPF, LDP, ISIS) indicate their capability 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.

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 pseudo-node representation.

   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.
      These can 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.
      Any RT is an MRT but many MRTs are not RTs.

   MRT-Red:   MRT-Red is used to describe one of the two MRTs; it is
      used to described the associated forwarding topology and 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 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 MT-ID that refers to the
      multiple MRT topologies and to the default topology.  This is
      referred to as the Rainbow MRT 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.

   Island Border Router (IBR):   A router in the MRT Island that is
      connected to a router not in the MRT Island and both routers 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.

   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.

   2-edge-connected:   A network graph where at least two links must be
      removed to partition the network.

   block:   Either a 2-connected cluster, a cut-edge, or an isolated
      vertex.

   DAG:   Directed Acyclic Graph - a graph where all links are directed
      and there are no cycles in it.

   ADAG:   Almost Directed Acyclic Graph - a graph that, if all links
      incoming to the root were removed, would be a DAG.

   GADAG:   Generalized ADAG - a graph that is the combination of the
      ADAGs of all blocks.

   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 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
   algorithm to compute MRTs is given in
   [I-D.ietf-rtgwg-mrt-frr-algorithm].  This algorithm can be computed
   in O(e + n log n); it is less than three SPFs.  Modeling results
   comparing the alternate path lengths obtained with MRT to other
   approaches are described in [I-D.ietf-rtgwg-mrt-frr-algorithm].  This
   document gives 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.

   [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 complete solution for IP/LDP fast-reroute
   [RFC5714].  MRT-FRR creates 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
   F, G or the link F<->G failed, then the network would be partitioned.
   It is clearly impossible to have two alternate trees separate link-disjoint or node-disjoint
   paths from G, I or J 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->F->D->E->R and on the Red MRT is G->F->B->A->R.

                      [E]---[D]---|
                       |     |    |     |----[I]
                       |     |    |     |     |
                      [R]---[C]  [F]---[G]    |
                       |     |    |     |     |
                       |     |    |     |----[J]
                      [A]---[B]---|

                                  (a)
                        a non-2-connected graph

       [E]<--[D]<--|                        [E]-->[D]
        |     ^    |          [I]                  |          |----[I]
        V     |    |           |                   V          V     ^
       [R]   [C]  [F]<--[G]    |            [R]<--[C]  [F]<--[G]    |
              ^    ^     ^     V             ^          |           |
              |    |     |----[J]            |          |          [J]
       [A]-->[B]---|                        [A]<--[B]<--|

                   (b)                                    (c)
            Blue MRT towards R                    Red MRT towards R

                    Figure 2: A non-2-connected network

5.  Maximally Redundant Trees (MRT) and Fast-Reroute

   In normal IGP routing, each router has its shortest-path-tree to all
   destinations.  From the
   primary next-hop forwarding used during stable operation.  These two
   trees are perspective of a particular destination, D,
   this looks like a reverse SPT (rSPT).  To use maximally diverse from redundant
   trees, in addition, each other, providing link destination D has two MRTs associated with
   it; by convention these will be called the MRT-Blue and node
   protection for 100% of paths MRT-Red.
   MRT-FRR is realized by using multi-topology forwarding.  There is a
   MRT-Blue forwarding topology and failures as long as the failure does
   not cut the network into multiple pieces.  This document defines the
   architecture for a MRT-Red forwarding topology.

   Any IP/LDP fast-reroute with MRT. technique beyond LFA requires an additional
   dataplane procedure, such as an additional forwarding mechanism.  The associated
   protocol extensions
   well-known options are defined in [I-D.atlas-ospf-mrt] multi-topology forwarding (used by MRT-FRR),
   tunneling (e.g.  [I-D.ietf-rtgwg-ipfrr-notvia-addresses] or
   [I-D.ietf-rtgwg-remote-lfa]), and
   [I-D.atlas-mpls-ldp-mrt].  The exact MRT algorithm is defined in
   [I-D.enyedi-rtgwg-mrt-frr-algorithm].

   IP/LDP Fast-Reroute with MRT (MRT-FRR) uses two maximally diverse per-interface forwarding topologies to provide alternates.  A primary next-hop
   should be on only (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 diverse forwarding topologies; thus, the
   other can be used MRTs to provide an alternate.  Once traffic has been
   moved reach the destination D.  For example, in Figure 2, C
   would normally forward traffic to one of MRTs, it R across the C<->R link.  If that
   C<->R link fails, then C could use the Blue MRT path C->D->E->R.

   As is not subject to further repair actions.
   Thus, always the traffic will case with fast-reroute technologies, forwarding does
   not loop even if a worse failure (e.g. node)
   occurs when protection was only available for change until a simpler local failure (e.g.
   link).

   In addition to supporting IP and LDP unicast fast-reroute, is detected.  Packets are forwarded
   along the
   diverse forwarding topologies and guarantee of 100% coverage permit
   fast-reroute technology shortest path.  The appropriate alternate to be applied use is pre-
   computed.  [I-D.ietf-rtgwg-mrt-frr-algorithm] describes exactly how
   to multicast traffic as
   described in [I-D.atlas-rtgwg-mrt-mc-arch].

   Other existing or proposed solutions are partial solutions or have
   significant issues, as described below.

                 Summary Comparison of IP/LDP FRR Methods

   +-----------+---------------+---------------+-----------------------+
   |   Method  |    Coverage   |   Alternate   | Computation (in SPFs) |
   |           |               |    Looping?   |                       |
   +-----------+---------------+---------------+-----------------------+
   |  MRT-FRR  |      100%     |      None     |      less than 3      |
   |           |   Link/Node   |               |                       |
   |           |               |               |                       |
   |    LFA    |    Partial    |    Possible   |      per neighbor     |
   |           |   Link/Node   |               |                       |
   |           |               |               |                       |
   |   Remote  |    Partial    |    Possible   |  per neighbor (link)  |
   |    LFA    |   Link/Node   |               | determine whether the MRT-Blue next-hops or neighbor's     |
   |           |               |               |    neighbor (node)    |
   |           |               |               |                       |
   |  Not-Via  |      100%     |      None     |   per link and node   |
   |           |   Link/Node   |               |                       |
   +-----------+---------------+---------------+-----------------------+

                                  Table 1

   Loop-Free Alternates (LFA):   LFAs [RFC5286] provide limited
      topology-dependent coverage the MRT-Red next-hops
   should be the MRT alternate next-hops for link and node protection.
      Restrictions on choice of a particular primary next-
   hop to a particular destination.

   MRT alternates can be relaxed are always available to improve
      coverage, but this use.  It is a local decision
   whether to use an MRT alternate, a Loop-Free Alternate 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 forwarding loops
   micro-looping.  Section 1.1 of [RFC5286] gives an example of link-
   protecting alternates causing a loop on node failure.  Even if a
   worse failure
      is experienced than protected against.  Augmenting a network to
      provide better coverage is NP-hard [LFARevisited].  [RFC6571]
      discusses anticipated happens, the applicability use of LFA to different topologies with a
      focus on common PoP architectures.

   Remote LFA:   Remote LFAs [I-D.ietf-rtgwg-remote-lfa] improve
      coverage over MRT alternates
   will not cause looping.  Therefore, while node-protecting LFAs for link protection but still cannot guarantee
      complete coverage.  The trade-off of may be
   preferred, the certainty that no alternate-induced looping traffic to improve
      coverage will occur
   is still made.  Remote LFAs can provide node-protection
      [I-D.litkowski-rtgwg-node-protect-remote-lfa] but not guaranteed
      coverage and an advantage of using MRT alternates when the computation required available node-
   protecting LFA is quite high (an SPF per
      neighbor's neighbor).  [I-D.bryant-ipfrr-tunnels] describes
      additional mechanisms not a downstream path.

6.  Unicast Forwarding with MRT Fast-Reroute

   As mentioned before, MRT FRR needs multi-topology forwarding.
   Unfortunately, neither IP nor LDP provides extra bits for a packet to further improve coverage, at
   indicate its topology.  Once the MRTs are computed, the two sets of
   MRTs can be used as two additional forwarding topologies.  The same
   considerations apply for forwarding along the cost MRTs as for handling
   multiple topologies.

   There are three possible types of
      added complexity.

   Not-Via:   Not-Via [I-D.ietf-rtgwg-ipfrr-notvia-addresses] is routers involved in forwarding a
   packet along an MRT path.  At the
      only other solution that provides 100% coverage for link and node
      failures and does not have potential looping.  However, MRT ingress router, the
      computation is very high (an SPF per failure point) and academic
      implementations [LightweightNotVia] have found packet
   leaves the address
      management complexity shortest path to be high.

1.1.  Importance of 100% Coverage
   Fast-reroute is based upon the single failure assumption - that destination and follows an MRT path
   to the
   time between single failures destination.  In a FRR application, the MRT ingress router is long enough for
   the PLR.  An MRT transit router takes a network to
   reconverge packet that arrives already
   associated with the particular MRT, and start forwarding forwards it on the new shortest paths.  That does
   not imply that same MRT.
   In some situations (to be discussed later), the network packet will only experience one failure or
   change.

   It is straightforward need to analyze a particular network topology for
   coverage.  However, a real network does not always have
   leave 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 LFAs are available.  Similarly, after a single failure has
   happened, MRT path and return to the topology is changed shortest path.  This takes place
   at the MRT egress router.  The MRT ingress and its associated coverage.
   Finally, many networks have new routers egress functionality
   may depend on the underlying type of packet being forwarded (LDP or links added and removed;
   each
   IP).  The MRT transit functionality is independent of those changes can have an effect on the coverage 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.  MRT Forwarding Mechanisms

   The following options for
   topology-sensitive methods such as LFA MRT forwarding mechanisms are considered.

   1.  MRT LDP Labels

       A.  Topology-scoped FEC encoded using a single label

       B.  Topology and Remote LFA.  If fast-
   reroute is important 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 network services provided, then MRT forwarding mechanisms using MRT
   LDP labels.

6.1.1.1.  Topology-scoped FEC encoded using a method
   that guarantees 100% coverage is important single label (Option 1A)

   [I-D.ietf-mpls-ldp-multi-topology] provides a mechanism to distribute
   FEC-Label bindings scoped to accomodate natural
   network topology changes.

   Asymmetric link costs are also a common aspect of networks.  There
   are at least three common causes for them.  First, any broadcast
   interface is represented given topology (represented by a pseudo-node and has asymmetric link
   costs MT-ID).
   To use multi-topology LDP to and from that pseudo-node.  Second, when routers come up or
   a link create MRT forwarding topologies, we
   associate two MT-IDs with LDP comes up, it is recommended in [RFC5443] and
   [RFC3137] that the link metric be raised MRT-Red and MRT-Blue forwarding
   topologies, in addition to the maximum cost; default shortest path forwarding
   topology with MT-ID=0.

   With this
   may not be symmetric and for [RFC3137] forwarding mechanism, a single label is not expected to be.  Third,
   techniques such as IGP metric tuning distributed for traffic-engineering can
   result
   each topology-scoped FEC.  For a given FEC in asymmetric link costs.  A fast-reroute solution needs the default topology
   (call it default-FEC-A), two additional topology-scoped FECs would be
   created, corresponding to
   handle network the Red and Blue MRT forwarding topologies with asymmetric link costs.
   (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 network needs to use packet is
   received with a micro-loop prevention mechanism
   [RFC5715] such as Ordered FIB[I-D.ietf-rtgwg-ordered-fib] or Farside
   Tunneling[RFC5715], then the whole IGP area needs label corresponding to have alternates
   available so 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 micro-loop prevention mechanism, which requires
   slower network convergence, can take incoming label with the necessary time without
   impacting traffic badly.  Without complete coverage, traffic
   outgoing label corresponding to red-FEC-A learned from the
   unprotected destinations will be dropped for significantly longer
   than with current convergence - where routers individually converge
   as fast as possible.

1.2.  Partial Deployment MRT-Red
   next-hop router, and Backwards Compatibility

   MRT-FRR supports partial deployment.  As forward the packet.

   This forwarding mechanism has the useful property that the FEC
   associated with many new features, the
   protocols (OSPF, LDP, ISIS) indicate their capability to support MRT.
   Inside packet is maintained in the MRT-capable connected group labels at each hop
   along the MRT.  We will take advantage of routers (referred this property when
   specifying how to as an carry LDP traffic on MRT Island), the MRTs are computed.  Alternates paths using multi-topology
   LDP labels.

   This approach is very simple for hardware to destinations
   outside the MRT Island are computed and depend upon the existence of
   a loop-free neighbor of support.  However, it
   reduces the MRT Island label space for that destination.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", other uses, and it increases the memory
   needed to store the labels and "OPTIONAL" the communication required by LDP to
   distribute FEC-label bindings.

   This forwarding option uses the LDP signaling extensions described in
   [I-D.ietf-mpls-ldp-multi-topology].  The MRT-specific LDP extensions
   required to support this
   document option are to be interpreted as described in [RFC2119]

3.  Terminology

   network graph:   A graph that reflects
   [I-D.atlas-mpls-ldp-mrt].

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 network topology where all
      links connect exactly two nodes and broadcast links have been
      transformed into the standard pseudo-node representation.

   Redundant Trees (RT):   A pair FEC of trees where the path from any node
      X to packet.  The top label (topology-id
   label) identifies the root R along MRT forwarding topology, while the first tree is node-disjoint 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
      path from router pops the same node X top label and uses that it to guide the root along
   next-hop selection in combination with the second tree.
      These can be computed next label in 2-connected graphs.

   Maximally Redundant Trees (MRT):   A pair of trees where the path
      from any node X to stack
   (the FEC label).  The router then swaps the root R along FEC label, using the first tree and FEC-
   label bindings learned through normal LDP mechanisms.  The router
   then pushes the path
      from topology-id label for the same node X to next-hop.

   As with Option 1A, this forwarding mechanism also has the root along useful
   property that the second tree share FEC associated with the
      minimum number packet is maintained in the
   labels at each hop along the MRT.

   This forwarding mechanism has minimal usage of nodes additional labels,
   memory and LDP communication.  It does increase the minimum number size of links.  Each
      such shared node is a cut-vertex.  Any shared links are cut-links.
      Any RT is an MRT but many MRTs are not RTs.

   MRT-Red:   MRT-Red is used to describe one packets
   and the complexity of the two MRTs; it required label operations and look-ups.

   This forwarding option is
      used to consistent with context-specific label
   spaces, as described in [RFC 5331].  However, the associated forwarding topology precise LDP
   behavior required to support this option for MRT has not been
   specified.

6.1.1.3.  Compatibility of Option 1A and MT-ID.
      Specifically, MRT-Red is the decreasing 1B

   In principle, MRT where links in the
      GADAG are taken transit forwarding mechanisms 1A and 1B can coexist
   in the direction from same network, with a higher topologically
      ordered node to packet being forwarding along a lower one.

   MRT-Blue:   MRT-Blue is used to describe one of the two MRTs; it is
      used to described single
   MRT path using the associated forwarding topology single label of option 1A for some hops and MT-ID.
      Specifically, MRT-Blue is the increasing
   two label stack of option 1B for other hops.

6.1.1.4.  Mandatory support for MRT where links in the
      GADAG are taken in the direction from LDP Label option 1A

   If a lower topologically
      ordered node to router supports a higher one.

   Rainbow MRT:   It is useful to have an MT-ID profile that refers to includes the
      multiple MRT topologies LDP Label option
   for MRT transit forwarding mechanism, then it MUST support option 1A,
   which encodes topology-scoped FECs using a single label.

6.1.2.  MRT IP tunnels (Options 2A and to the default topology.  This is
      referred to 2B)

   IP tunneling can also be used as the Rainbow an MRT MT-ID transit forwarding mechanism.
   Each router supporting this MRT transit forwarding mechanism
   announces two additional loopback addresses and is their associated MRT
   color.  Those addresses are used by LDP to reduce
      signaling as destination addresses for MRT-
   blue and permit MRT-red IP tunnels respectively.  The special loopback
   addresses allow the same label to always be advertised transit nodes to all
      peers for identify the same (MT-ID, Prefix).

   MRT Island:   From traffic as being
   forwarded along either the computing router, MRT-blue or MRT-red topology to reach the set
   tunnel destination.  Announcements of routers that
      support a particular MRT profile and are connected.

   Island Border Router (IBR):   A these two additional loopback
   addresses per router in the with their MRT Island that is
      connected to a router color requires IGP extensions,
   which have not in the MRT Island and both routers are
      in a common area been defined.

   Either IPv4 (option 2A) or level.

   Island Neighbor (IN):   A router IPv6 (option 2B) can be used as the
   tunneling mechanism.

   Note that is the two forwarding mechanisms using LDP Label options do
   not in require additional loopbacks per router, as is required by the MRT Island but IP
   tunneling mechanism.  This is
      adjacent because LDP labels are used on a hop-
   by-hop basis to an IBR identify MRT-blue and in MRT-red forwarding topologies.

6.2.  Forwarding LDP Unicast Traffic over MRT Paths

   In the same area/level as previous section, we examined several options for providing
   MRT transit forwarding functionality, which is independent of the IBR.

   cut-link:   A link whose removal partitions
   type of traffic being carried.  We now look at the network.  A cut-link MRT ingress
   functionality, which will depend on the type of traffic being carried
   (IP or LDP).  We start 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 considering LDP traffic.

   We also simplify the set of parallel links
      would partition initial discussion by assuming that the network graph.

   cut-vertex:   A vertex whose removal partitions
   consists of a single IGP area, and that all routers in the network graph.

   2-connected:   A graph
   participate in MRT.  Other deployment scenarios that has no cut-vertices.  This 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 very desirable to avoid tunneling.
   Tunneling LDP traffic to a graph
      that remote node requires two nodes knowledge of remote
   FEC-label bindings so that the LDP traffic can continue to be removed before
   forwarded properly when it leaves the network is
      partitioned.

   2-connected cluster:   A maximal set of nodes tunnel.  This requires targeted
   LDP sessions which can add management complexity.  The two MRT LDP
   Label forwarding mechanisms have the useful property that are 2-connected.

   2-edge-connected:   A network graph where the FEC
   associated with the packet is maintained in the labels at least two links must be
      removed to partition each hop
   along the network.

   block:   Either a 2-connected cluster, a cut-edge, or MRT, as long as an isolated
      vertex.

   DAG:   Directed Acyclic Graph - a graph where all links are directed
      and there are no cycles in it.

   ADAG:   Almost Directed Acyclic Graph - a graph that, if all links
      incoming MRT to the root were removed, would be a DAG.

   GADAG:   Generalized ADAG - originator of the FEC is
   used.  The MRT IP tunneling mechanism does not have this useful
   property.  Therefore, this document only considers the two MRT LDP
   Label forwarding mechanisms for protecting LDP traffic with MRT fast-
   reroute.

6.2.1.  Forwarding LDP traffic using MRT LDP Labels (Option 1A)

   The MRT LDP Label option 1A forwarding mechanism uses topology-scoped
   FECs encoded using a graph that is the combination of the
      ADAGs of all blocks.

   named proxy-node:   A proxy-node can represent single label as described in section
   Section 6.1.1.1.  When a destination prefix PLR receives an LDP packet that can be attached needs to be
   forwarded on the Red MRT Island via at least two routers.
      It is named if there is (for example), it does a way that traffic can be encapsulated to
      reach specifically that proxy node; this could be because there is
      an label swap
   operation, replacing the usual LDP FEC label for the associated prefix or because MRT-Red and MRT-
      Blue IP addresses are advertised in an undefined fashion FEC with the Red MRT
   label for that
      proxy-node.

4.  Maximally Redundant Trees (MRT)

   A pair of Maximally Redundant Trees are directed spanning trees that
   provide 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 FEC received from the trees.  The algorithm
   to compute MRTs is given next-hop router in [I-D.enyedi-rtgwg-mrt-frr-algorithm].
   This algorithm can be the Red MRT
   computed by the PLR.  When the next-hop router in O(e + n log n); it is less than
   three SPFs.  Modeling results comparing the Red MRT alternates to
   receives the optimal
   are described in [I-D.enyedi-rtgwg-mrt-frr-algorithm].  This document
   describes how packet with the MRTs can be used and not how to compute them. Red MRT provides destination-based trees label for each destination.  Each
   router stores its normal primary next-hop(s) as well the FEC, the MRT
   transit forwarding functionality continues as MRT-Blue
   next-hop(s) and MRT-Red next-hop(s) toward described in
   Section 6.1.1.1.  In this way the original FEC associated with the
   packet is maintained at each destination. hop along the MRT.

6.2.2.  Forwarding LDP traffic using MRT LDP Labels (Option 1B)

   The
   alternate will be selected between MRT LDP Label option 1B forwarding mechanism encodes the MRT-Blue topology
   and MRT-Red.

   The most important thing to understand about MRTs is that for each
   pair of destination-routed MRTs, there is the FEC using a path from every node X two label stack as described in Section 6.1.1.2.
   When a PLR receives an LDP packet that needs to
   the destination D be forwarded on the Blue MRT
   Red MRT, 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 is as disjoint as possible FEC learned from the path next-hop on
   the Red MRT.

   For example, in Figure 1, there is a network graph that is
   2-connected in (a) and  In addition, the PLR pushes the topology-identification
   label associated MRTs in (b) with the Red MRT, and (c).  One can
   consider forward the paths from B packet to R; on the Blue MRT, the paths are
   B->F->D->E->R or B->C->D->E->R.  On
   appropriate next-hop on the Red MRT, MRT.  When the path is B->A->R.
   These are clearly link and node-disjoint.  These MRTs are redundant
   trees because next-hop router in the paths are disjoint.

   [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, receives the packet with the Red MRT label for the network FEC, the
   MRT transit forwarding functionality continues as described in (a)
   Section 6.1.1.2.  As with option 1A, the original FEC associated with
   the packet is not 2-connected.  If
   F, G or maintained at each hop along the link F<->G failed, then MRT.

6.2.3.  Other considerations for forwarding LDP traffic using MRT LDP
        Labels

   Note that forwarding LDP traffic using MRT LDP Labels requires that
   an MRT to the network would originator of the FEC be partitioned.
   It is clearly impossible used.  For example, one might
   find it desirable to have two link-disjoint or node-disjoint
   paths from G, I or J to R.  The MRTs given in (b) and (c) offer paths
   that are as disjoint as possible.  For instance, the paths from B PLR use an MRT to
   R are reach the same as in Figure 1 primary
   next-next-hop for the FEC, and then continue forwarding the LDP
   packet along the shortest path tree from G the primary next-next-hop.
   However, this would require tunneling to R on the Blue
   MRT is G->F->D->E->R primary next-next-hop
   and on the Red MRT is G->F->B->A->R.

                      [E]---[D]---|
                       |     |    |     |----[I]
                       |     |    |     |     |
                      [R]---[C]  [F]---[G]    |
                       |     |    |     |     |
                       |     |    |     |----[J]
                      [A]---[B]---|

                                  (a) a non-2-connected graph

       [E]<--[D]<--|                        [E]-->[D]
        |     ^    |          [I]                  |          |----[I]
        V     |    |           |                   V          V     ^
       [R]   [C]  [F]<--[G]    |            [R]<--[C]  [F]<--[G]    |
              ^    ^     ^     V             ^          |           |
              |    |     |----[J]            |          |          [J]
       [A]-->[B]---|                        [A]<--[B]<--|

                   (b)                                    (c)
            Blue MRT towards R                    Red MRT towards R

                    Figure 2: A non-2-connected network

5.  Maximally Redundant Trees (MRT) and Fast-Reroute

   In normal IGP routing, each router has its shortest-path-tree targeted LDP session for the PLR to all
   destinations.  From learn the perspective FEC-label binding
   for primary next-next-hop to correctly forward the packet.

   For greatest hardware compatibility, routers implementing MRT fast-
   reroute of a particular destination, D,
   this looks like a reverse SPT (rSPT).  To use maximally redundant
   trees, LDP traffic MUST support Option 1A of encoding the MT-ID
   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 labels (See Section 9).

6.3.  Forwarding IP Unicast Traffic over MRT Paths

   For IP traffic, there is a no currently practical alternative except
   tunneling to gain the bits needed to indicate the MRT-Blue forwarding topology and a or 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 choice of tunnel egress MAY be 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 multi-homed
   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 MRT-FRR),
   avoiding the need for targeted LDP sessions to the next-next-hop.
   For IP, that consideration doesn't apply.  However, consistency with
   LDP is RECOMMENDED.

   Some situations require tunneling (e.g. [I-D.ietf-rtgwg-ipfrr-notvia-addresses] or
   [I-D.ietf-rtgwg-remote-lfa]), and per-interface forwarding (e.g.
   Loop-Free Failure Insensitive Routing 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 [EnyediThesis]).

   When there is a link or node failure affecting, but not partitioning, different IGP area/level from the network, each node will still have at least one
   PLR should be tunneled on the MRT to the ABR/LBR on the shortest path via one
   to the destination.  For a destination outside of the MRTs PLR's MRT
   Island, the packet should be tunneled on the MRT to reach a non-proxy-node
   immediately before the destination D.  For example, in Figure 2, C
   would normally forward 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 R across IP headers, has the C<->R link.  If the advantage that
   C<->R link fails, then C could use 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 Labels (Option 1A)

   The MRT LDP Label option 1A forwarding mechanism uses topology-scoped
   FECs encoded using a single label as described in section
   Section 6.1.1.1.  When a PLR receives an IP packet that needs to be
   forwarded on the Blue Red MRT path C->D->E->R.

   As to a particular tunnel endpoint, it does a
   label push operation.  The label pushed is always the case with fast-reroute technologies, Red MRT label for a
   FEC originated by the tunnel endpoint, learned from the next-hop on
   the Red MRT.

6.3.1.2.  Tunneling IP traffic using MRT LDP Labels (Option 1B)

   The MRT LDP Label option 1B forwarding does
   not change until mechanism encodes the topology
   and the FEC using a two label stack as described in Section 6.1.1.2.
   When a local failure is detected.  Packets are PLR receives an IP packet that needs to be forwarded
   along on the shortest path.  The appropriate alternate
   Red MRT to use a particular tunnel endpoint, the PLR pushes two labels on
   the IP packet.  The first (inner) label is pre-
   computed.  [I-D.enyedi-rtgwg-mrt-frr-algorithm] describes exactly how
   to determine whether the MRT-Blue next-hops or normal LDP label
   learned from the MRT-Red next-hops
   should be next-hop on the MRT alternate next-hops for a particular primary next-
   hop N to Red MRT, associated with a particular destination D.

   MRT alternates are always available to use.  It FEC
   originated by the tunnel endpoint.  The second (outer) label is the
   topology-identification label associated with the Red MRT.

   For completeness, we note here a local decision
   whether potential optimization.  In order to use
   tunnel an IP packet over an MRT alternate, a Loop-Free Alternate 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
   micro-looping.  Section 1.1 to the destination of [RFC5286] gives the IP packet
   (as opposed to an example of link-
   protecting alternates causing a loop on node failure.  Even if arbitrary tunnel endpoint), then we could just push
   a
   worse failure than anticipated happens, topology-identification label directly onto the use of packet.  An MRT alternates
   will not cause looping.  Therefore, while node-protecting LFAs may be
   preferred,
   transit router would need to pop the certainty that no alternate-induced looping will occur
   is topology-id label, do an advantage IP
   route lookup in the context of using MRT alternates when that topology-id , and push the available node-
   protecting LFA is not a downstream path.

6.  Unicast Forwarding with MRT Fast-Reroute

   With LFA, there is no need to tunnel unicast traffic, whether
   topology-id label.

6.3.2.  Tunneling IP or
   LDP.  The traffic is simply sent to an alternate.  As mentioned
   earlier in Section 5, using MRT needs multi-topology forwarding.
   Unfortunately, neither IP nor LDP provides extra bits for a packet Tunnels

   In order to
   indicate its topology.

   Once tunnel over the MRTs are computed, MRT to a particular tunnel endpoint, the two sets of MRTs are seen by
   PLR encapsulates the
   forwarding plane as essentially two original IP packet with an additional topologies.  The same
   considerations apply for forwarding along IP header
   using the MRTs as for handling
   multiple topologies.

6.1.  LDP Unicast Forwarding - Avoid Tunneling

   For LDP, it is very desirable to avoid tunneling because, for at
   least node protection, tunneling requires knowledge MRT-Blue or MRT-Red loopack address of remote LDP
   label mappings and thus requires targeted LDP sessions the tunnel endpoint.

6.3.3.  Required support

   For greatest hardware compatibility and ease in removing the
   associated management complexity.  There are two different mechanisms MRT-
   topology marking at area/level boundaries, routers that can be used; Option A support MPLS
   and implement IP MRT fast-reroute MUST be supported.

   1. support tunneling of IP
   traffic using MRT LDP Labels Option A - Encode MT-ID in Labels: In addition to sending 1A (topology-scoped FEC encoded
   using a single label label).

7.  MRT Island Formation

   The purpose of communicating support for a FEC, a router would provide two additional
       labels with MRT in the MT-IDs associated with IGP is to
   indicate that the Blue MRT-Blue and MRT-Red forwarding topologies are
   created for transit traffic.  The MRT or Red architecture allows for
   different, potentially incompatible options.  In order to create
   constistent MRT forwarding topologies.  This is very simple for hardware support.
       It does reduce topologies, the label space for other uses.  It also increases 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 memory routers in
   an MRT Island all need to store use the labels same set of nodes and links within
   the communication required by
       LDP.

   2.  Option B - Create Topology-Identification Labels: Use Island when computing the label-
       stacking ability of MPLS and specify only two additional labels -
       one for each associated MRT color - forwarding topologies.  This
   section describes the information used by a new FEC type.  When
       sending a packet onto an MRT, first swap router to determine the LDP label
   nodes and then
       push links to include in a particular MRT Island.  Some of this
   information is shared among routers using the topology-identification label newly-defined IGP
   signaling extensions for that MRT color.  When
       receiving a packet with a topology-identification label, pop it
       and use it to guide the next-hop selection described in combination with
       the next label [I-D.atlas-ospf-mrt] and
   [I-D.li-isis-mrt].  Other information already exists in the stack; then swap the remaining label, if
       appropriate, IGPs and push the topology-identification label for
   can be used by MRT in Island formation, subject to the
       next-hop.  This has minimal usage of additional labels, memory interpretation
   defined here.

   Deployment scenarios using multi-topology OSPF or IS-IS, or running
   both ISIS and LDP communication.  It does increase OSPF on the size same routers is out of packets scope for this
   specification.  As with LFA, it is expected that OSPF Virtual Links
   will not be supported.

7.1.  IGP Area or Level

   All links in an MRT Island MUST be bidirectional and belong to the complexity of the required label operations
   same IGP area or level.  For ISIS, a link belonging to both level 1
   and look-ups.
       This level 2 would qualify to be in multiple MRT Islands.  A given ABR
   or LBR can use the same mechanisms as are needed for context-aware
       label spaces.

   Note that with LDP unicast forwarding, regardless of whether
   topology-identification label belong to multiple MRT Islands, corresponding to the areas
   or encoding topology levels in label is used,
   no additional loopbacks per router are required.  This which it participates.  Inter-area forwarding behavior
   is because LDP
   labels are used on discussed in Section 10.

7.2.  Support for a hop-by-hop basis to identify MRT-blue and MRT-
   red forwading topologies.

   For greatest hardware compatibility, specific MRT profile

   All routers implementing in an MRT LDP
   fast-reroute Island MUST support Option A of encoding the MT-ID in the
   labels.  The extensions to indicate an MT-ID same MRT profile.  A
   router advertises support for a FEC are described
   in Section 3.2.1 of [I-D.ietf-mpls-ldp-multi-topology].

6.2.  IP Unicast Traffic

   For IP, there is no currently practical alternative except tunneling
   to gain the bits needed to indicate given MRT profile using the MRT-Blue or MRT-Red
   forwarding topology.  The choice of tunnel egress MAY be flexible
   since any IGP
   extensions defined in [I-D.atlas-ospf-mrt] and [I-D.li-isis-mrt]
   using an 8-bit Profile ID value.  A given router closer to the destination than the next-hop can
   work.  This architecture assumes support multiple
   MRT profiles and participate in multiple MRT Islands.  The options
   that make up an MRT profile, as well as the original destination default MRT profile, are
   defined in the
   area is selected (see Section 10 8.

7.3.  Excluding additional routers and interfaces from the MRT Island

   MRT takes into account existing IGP mechanisms for handling 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 ISIS and OSPF.  In ISIS, an interface configured
   with a metric of multi-homed
   prefixes); another possible choice is the next-next-hop towards 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
   destination.  For 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 traffic, IGP synchronization [RFC5443].

   Mechanisms also exist in ISIS and OSPF to prevent transit traffic
   from using a particular router.  In ISIS, the original destination
   simplifies MRT-FRR by avoiding the need overload bit is used
   for targeted LDP sessions this purpose.  In OSPF, [RFC3137] specifies setting all outgoing
   interface metrics to
   the next-next-hop.  For IP, 0xFFFF to accomplish this.

   The following rules for MRT Island formation ensure that MRT FRR
   protection traffic does not use a link or router that consideration doesn't apply but
   consistency with LDP is RECOMMENDED.  If the tunnel egress is the
   original destination router, then the discouraged
   from carrying traffic remains by existing IGP mechanisms.

   1.  A bidirectional link MUST be excluded from an MRT Island if
       either the forward or reverse cost on the
   redundant tree link is 0xFFFFFE (for
       ISIS) or 0xFFFF for OSPF.

   2.  A router MUST be excluded from an MRT Island if it is advertised
       with sub-optimal routing.  Selection of the tunnel
   egress overload bit set (for ISIS), or it is a router-local decision.

   There are three options available for marking IP packets advertised with which
       metric values of 0xFFFF on all of its outgoing interfaces (for
       OSPF).

7.3.2.  MRT-specific exclusion mechanism

   This architecture also defines a means of excluding an otherwise
   usable link from MRT it should be forwarded in.  For greatest hardware compatibility Islands.  [I-D.atlas-ospf-mrt] and ease in removing
   [I-D.li-isis-mrt] define the MRT-topology marking at area/level
   boundaries, routers that support MPLS IGP extensions for OSPF and implement IP MRT fast-
   reroute MUST support Option ISIS used to
   advertise that a link is MRT-Ineligible.  A - using link with either
   interface advertised as MRT-Ineligible MUST be excluded from an LDP label MRT
   Island.  Note that indicates the
   destination and MT-ID.

   1.  Tunnel IP packets via an LDP LSP.  This has the advantage interface advertised as MRT-Ineligigle by a
   router is ineligible with respect to all profiles advertised by that
       more installed
   router.

7.4.  Connectivity

   All of the routers can do line-rate encapsulation and
       decapsulation.  Also, no additional IP addresses would need to in an MRT Island MUST be
       allocated or signaled.

       a.  Option A - LDP Destination-Topology Label: Use a label connected by
   bidirectional links with other routers in the MRT Island.
   Disconnected MRT Islands will operate independently of one another.

7.5.  Example algorithm

   An algorithm that
           indicates both destination and MRT.  This method allows easy
           tunneling a computing router to identify the next-next-hop as well as to routers
   and links in the IGP-area
           destination.  For a proxy-node, local MRT Island satisfying the destination above rules is given
   in section 5.1 of [I-D.ietf-rtgwg-mrt-frr-algorithm].

8.  MRT Profile

   An MRT Profile is a set of values and options related to use MRT
   behavior.  The complete set of options is designated by the
           non-proxy-node immediately before
   corresponding 8-bit Profile ID value.

8.1.  MRT Profile Options

   Below is a description of the proxy-node on values and options that define an MRT
   Profile.

   MRT Algorithm:   This identifies the particular color MRT.

       b.  Option B - LDP Topology Label: Use a Topology-Identifier
           label on top of MRT algorithm used by
      the IP packet. router for this profile.  Algorithm transitions can be managed
      by advertising multiple MRT profiles.

   MRT-Red MT-ID:   This is very simple.  If
           tunneling specifies the MT-ID to a next-next-hop is desired, then a two-deep
           label stack can be used associated with [ Topology-ID label, Next-Next-
           Hop Label ].

   2.  Tunnel IP packets the
      MRT-Red forwarding topology.  It is needed for use in IP.  Each router supporting this option
       would announce two additional loopback addresses and their
       associated LDP
      signaling.  All routers in the MRT color.  Those addresses are used as destination
       addresses for MRT-blue and MRT-red IP tunnels respectively.  They
       allow Island MUST agree on a value.

   MRT-Blue MT-ID:   This specifies the transit nodes MT-ID to identify be associated with the traffic as being
       forwarded along either MRT-blue or MRT-red tree topology
      MRT-Blue forwarding topology.  It is needed for use in LDP
      signaling.  All routers in the MRT Island MUST agree on a value.

   GADAG Root Selection Policy:   This specifes the manner in which the
      GADAG root is selected.  All routers in the MRT island need to reach use
      the tunnel destination.  Announcements of these two additional
       loopback addresses per same GADAG root in the calculations used construct the MRTs.
      A valid GADAG Root Selection Policy MUST be such that each router with their
      in the MRT color requires IGP
       extensions.

7.  Protocol Extensions and Considerations: OSPF and ISIS

   For simplicity, island chooses the approach of defining a well-known profile is
   taken same GADAG root based on information
      available to all routers in [I-D.atlas-ospf-mrt].  The purpose of communicating support
   for the MRT island.  GADAG Root Selection
      Priority values, advertised in the IGP is 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 indicate thatqq carry transit traffic along MRT paths.  A
      router which supports a specific MRT forwarding mechanism must
      program appropriate next-hops into the MRT-Blue and MRT-Red forwarding topologies plane.  The
      current options are created for transit traffic.  This section
   describes MRT LDP Labels, IPv4 Tunneling, IPv6
      Tunneling, and None.  If the various options to be selected.  The default MRT
   profile LDP Labels option is described here supported,
      then option 1A and the appropriate signaling extensions for OSPF are
   given MUST be
      supported.  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.  The None option in [I-D.atlas-ospf-mrt].

   For any MRT profile, may be useful for
      multicast global protection.

   Recalculation:   As part of what process and timing should the MRT Island is created by starting from new
      MRTs be computed on a modified topology?  Section 12.2 describes
      the
   computing router.  If minimum behavior required to support fast-reroute.

   Area/Level Border Behavior:   Should inter-area traffic on the computing router supports MRT-
      Blue or MRT-Red be put back onto the default MRT
   profile, add shortest path tree?  Should
      it be swapped from MRT-Blue or MRT-Red in one area/level to MRT-
      Red or MRT-Blue in the MRT Island.  Add a router next area/level to avoid the MRT Island if
   the router supports the default MRT profile and is connected to potential
      failure of an ABR?  (See [I-D.atlas-rtgwg-mrt-mc-arch] for use-
      case details.

   Other Profile-Specific Behavior:   Depending upon the
   MRT Island via bidirectional links eligible use-case for MRT.
      the profile, there may be additional profile-specific behavior.

   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 No Forwarding Mechanism (e.g. as might
   be done for a profile used only for multicast global protection).  A
   router MUST NOT advertise multiple MRT profiles that overlap in their
   MRT-Red MT-ID or MRT-Blue MT-ID.

   The MRT Profile also defines different behaviors such as how MRT
   recomputation is handled and how area/level boundaries are dealt
   with.

   MRT Algorithm:   MRT Lowpoint algorithm defined in
      [I-D.enyedi-rtgwg-mrt-frr-algorithm].

   MRT-Red MT-ID:   experimental 3997, final value assigned by IANA
      allocated from the LDP MT-ID space

   MRT-Blue MT-ID:   experimental 3998, final value assigned by IANA
      allocated from the LDP MT-ID space

   GADAG Root Selection Priority:   Among the routers in the MRT Island
      and with the highest priority advertised, an implementation then it
   MUST
      pick create the router with transit forwarding topologies for each of those,
   unless the highest Router ID to be profile specifies the GADAG root. None option for MRT Forwarding Mechanisms:   LDP

   Recalculation:   Recalculation of MRTs SHOULD occur as described in
      Section 11.2.  This allows the
   Mechanism.  A router MUST NOT advertise multiple MRT forwarding topologies to
      support IP/LDP fast-reroute traffic.

   Area/Level Border Behavior:   As described in Section 9, ABRs/LBRs
      SHOULD ensure profiles that traffic leaving the area also exits the
   overlap in their MRT-Red MT-ID or MRT-Blue forwarding topology.

   The following describes the aspects to be considered to define a
   profile to advertise. MT-ID.

8.2.  Router-specific MRT paramaters

   For some profiles, associated information additional router-specific MRT parameters may need
   to be distributed, such as GADAG Root Selection Priority, Red
   MRT Loopback Address, Blue MRT Loopback Address.

   MRT Algorithm:   This identifies distributed via the particular MRT algorithm used by IGP.  While the router for this profile.  Algorithm transitions can be managed set of options indicated by advertising multiple MRT profiles.

   MRT-Red MT-ID:   This specifies
   the MT-ID to MRT Profile ID must be associated with the
      MRT-Red forwarding topology.  It is needed identical for use in LDP
      signaling.  All all routers in the an MRT Island MUST agree on a value.

   MRT-Blue MT-ID:   This specifies the MT-ID to be associated with the
      MRT-Blue forwarding topology.  It is needed for use in LDP
      signaling.  All
   Island, these router-specific MRT parameters may differ between
   routers in the same MRT Island MUST agree on a value. island.  Several such parameters are
   described below.

   GADAG Root Selection Priority:   A MRT profile might specify this to
      provide the network operator with a knob to force a particular GADAG root selection.  If not specified in the MRT profile, Root Selection Policy MAY
      rely on the
      highest Router ID GADAG Root Selection Priority values advertised by
      each router in the profile's MRT Island will be elected island.  A GADAG Root Selection Policy may
      use the GADAG Root.  If a GADAG Root Selection Priority to allow network operators
      to configure a parameter to ensure that the GADAG root is specified, then selected
      from a particular subset of routers.  An example of this use of
      the MRT profile must also specify how GADAG Root Selection Priority value by the GADAG Root
      Selection Policy is elected.

   Forwarding Mechanism:   This specifies which forwarding mechanisms given in the router supports for transit traffic.  An Default MRT island must
      program appropriate next-hops into the forwarding plane.  The
      known options are IPv4, IPv6, LDP, 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.  If LDP is
      supported, then LDP support and signaling extensions MUST be
      supported. 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, of course, can be specified for both either IPv4 and IPv6.

   MRT-Blue Loopback Address:   This provides the router's loopback
      address to reach the router via the MRT-Blue forwarding topology.
      It can, of course, can be specified for both either IPv4 and IPv6.

   The extensions to OSPF and ISIS for advertising a router's GADAG Root
   Selection Priority value are defined in [I-D.atlas-ospf-mrt] and
   [I-D.li-isis-mrt].  IGP extensions for the advertising a router's
   MRT-Red and MRT-Blue Loopback Addresses have not been defined.

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
      [I-D.ietf-rtgwg-mrt-frr-algorithm].

   MRT-Red MT-ID:   TBA-MRT-ARCH-1, final value assigned by IANA
      allocated from the LDP MT-ID space (prototype experiments have
      used 3997)

   MRT-Blue MT-ID:   TBA-MRT-ARCH-2, final value assigned by IANA
      allocated from the LDP MT-ID space (prototype experiments have
      used 3998)

   GADAG Root Selection Policy:   Among the routers in the MRT Island
      and with the highest priority advertised, an implementation MUST
      pick the router with the highest Router ID to be the GADAG root.

   Forwarding Mechanisms:   MRT LDP Labels

   Recalculation:   As part   Recalculation of what process and timing should the new MRTs be computed on a modified topology? SHOULD occur as described in
      Section 11.2 describes 12.2.  This allows the minimum behavior required MRT forwarding topologies to
      support fast-reroute. IP/LDP fast-reroute traffic.

   Area/Level Border Behavior:   Should inter-area   As described in Section 10, ABRs/LBRs
      SHOULD ensure that traffic on leaving the MRT-
      Blue or MRT-Red be put back onto area also exits the shortest path tree?  Should
      it be swapped from MRT-Blue or MRT-Red in one area/level to MRT-
      Red
      or MRT-Blue in the next area/level to avoid the potential
      failure of an ABR?  (See [I-D.atlas-rtgwg-mrt-mc-arch] for use-
      case details.

   Other Profile-Specific Behavior:   Depending upon the use-case for
      the profile, there may be additional profile-specific behavior.

   As with LFA, it is expected that OSPF Virtual Links will not be
   supported.

8.  Protocol Extensions and considerations: forwarding topology.

9.  LDP signaling extensions and considerations

   The protocol extensions for LDP are defined in
   [I-D.atlas-mpls-ldp-mrt].  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 as
   well as to profiles.
   The FEC-label bindings for the default shortest-path based MT-ID 0. 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 for the
   same FEC.  Because the Rainbow MRT MT-ID is used only by ABRs/LBRs or the
   an LDP egress, egress router, it is not MRT profile specific.  The proposed experimental value is 3999
   and

   [I-D.atlas-mpls-ldp-mrt] contains the final value will be assigned by IANA and allocated from request for the
   LDP MT-ID space.  The authoritative values are given Rainbow
   MRT MT-ID.

10.  Inter-area Forwarding Behavior

   Unless otherwise specified, in
   [I-D.atlas-mpls-ldp-mrt].

9.  Inter-Area this section we will use the terms
   area and ABR Forwarding Behavior to indicate either an OSPF area and OSPF ABR or ISIS
   level and ISIS LBR.

   An ABR/LBR has two forwarding roles.  First, it forwards traffic
   inside its area.
   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 back and return
   to the
   shortest-path 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 failures that might appear in multiple areas
   (e.g.  ABR/LBR failures) to be separately identified and repaired
   around.  Third, the packet can be fast-rerouted again, if necessary,
   due to a failure in a different area.

   An ABR/LBR that receives a packet on MRT-Red or MRT-Blue towards a
   destination in another area/level Z should continue to forward the packet in the
   area/level with the best route along MRT-Red or MRT-Blue.  If
   MRT-Blue only if the
   packet came from that area/level, this correctly avoids best route to Z is in the failure.
   However, if same area as the traffic came from a different area/level,
   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.

   To avoid per-interface forwarding state for MRT-Red and MRT-Blue, the
   ABR/LBR needs to arrange that packets destined to a different area
   arrive at the ABR/LBR already not marked as MRT-Red or MRT-Blue.

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

   For LDP forwarding where the MPLS a single label specifies (MT-ID, FEC), the
   ABR/LBR is responsible for advertising the proper label to each
   neighbor.  Assume that an ABR/LBR has allocated three labels for a
   particular destination; those labels are L_primary, L_blue, and
   L_red.  When the ABR/LBR advertises label bindings to  To those routers in the same area with as the best route to the
   destination, the ABR/LBR provides 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, exactly as expected.  However, when the
   ABR/LBR advertises label bindings to routers in
   other areas, the ABR/
   LBR ABR/LBR advertises the following FEC-label bindings:
   L_primary for the default topology, and L_primary for the Rainbow MRT MT-ID, which is then
   used for
   MT-ID.  Associating L_primary with the default topology, 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/LBR installs all next-hops from 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/LBR 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/LBR 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/LBR 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/LBR 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 implmentations and
   doesn't have other obvious uses.

   Many existing LDP implmentations do however have the ability to
   filter FEC-label binding advertisements on a per-neighbor basis.  The
   Rainbow-FEC allows us to re-use 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.

   The use of the Rainbow-FEC by the ABR for non-best-area
   advertisements is RECOMMENDED.  An ABR MAY advertise the label for
   the default topology in separate MRT-Blue and MRT-Red advertisements.
   However, a router that supports the LDP Label MRT 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 forwarding tunneling is used, then the ABR/LBR behavior is dependent upon
   the outermost IP address.  If the outermost IP address is an MRT
   loopback address of the ABR/LBR, 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/LBR, then the packet is simply forwarded
   along the associated 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/LBR connected
   advertising the lowest cost to the external destination.

   Thus, regardless of which of for these two forwarding mechanisms are
   used, 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.

       +----[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

             ->[D]->[E]                         -<[D]<-[E]
            /          \                       /         \
       [ABR1]  Area 0 [H]-+             +-[ABR1]         [H]
                      /   |             |      \
               [F]->[G]   V             V       -<[F]<-[G]
                          |             |
                          |             |
                [p]<------+             +--------->[p]

     (d) Blue MRT in Area 0           (e) Red MRT in Area 0

                Figure 3:

10.3.  ABR Forwarding Behavior and MRTs with LDP Label option 1B

   The other MRT forwarding mechanism described in Section 6 is using
   Topology-Identification Labels. 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/LBR ABR/
   LBR would need to determine whether the ABR/LBR would forward the
   packet out of the area/level.  If so, then that router should pop off
   the topology-
   identification topology-identification label before forwarding the packet to the
   ABR/LBR.

   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-
   Identification
   Topology-Identification 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 ISIS 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 OPSF
   area ID) and the Area ID isn't announced as part of the ASBR link-
   state advertisement (LSA).  In this one case, suboptimal forwarding
   along the MRT in the other area would happen.  If that becomes a
   realistic deployment scenario, OSPF extensions could be considered.
   This is not covered in [I-D.atlas-ospf-mrt].

10. [I-D.atlas-ospf-mrt].

       +----[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

             ->[D]->[E]                         -<[D]<-[E]
            /          \                       /         \
       [ABR1]  Area 0 [H]-+             +-[ABR1]         [H]
                      /   |             |      \
               [F]->[G]   V             V       -<[F]<-[G]
                          |             |
                          |             |
                [p]<------+             +--------->[p]

     (d) Blue MRT in Area 0           (e) Red MRT 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 multi-homed 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.

   The key difference is whether the traffic, once out of the MRT
   Island, remains in the same area/level and might reenter the MRT
   Island if a loop-free exit point is not selected.

   One

   FRR using LFA has the useful property of LFAs that it is necessary to preserve is the ability able to protect
   multi-homed 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 also be
   done for backups via MRT. accomplished
   with MRT FRR as well.

   If ASBR protection is desired, this has additonal 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 multi-homed prefix could be:

   o  An out-of-area prefix announced by more than one ABR,

   o  An AS-External route announced by 2 or more ASBRs,

   o  A prefix with iBGP multipath to different ASBRs,

   o  etc.

   There are also two different approaches to protection.  The first is
   to do
   tunnel endpoint selection to pick 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,
   with routers that can be reached via an MRT alternate, attached to the
   prefix.

   The second is to use a proxy-node, that 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.

   For IP traffic, multi-homed 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, then the routers learning that FEC MUST also advertise labels
   for (MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors inside the MRT
   Island.  If the forwarding mechanism includes LDP, any  Any router receiving a FEC corresponding to a router outside
   the MRT Island or to a multi-homed prefix MUST compute and install
   the transit MRT-Blue and MRT-Red next-hops for that FEC; FEC.  The FEC-
   label bindings for the associated topology-scoped FECs ( (MT-ID ((MT-ID 0, FEC), (MRT-Red, (MRT-
   Red, FEC), and (MRT-Blue, FEC)) MUST also be provided via LDP to
   neighbors inside the MRT Island.

10.1.  Endpoint Selection

11.1.  Protecting Multi-Homed 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].

   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)

   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 end-point.
       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 that for reaching B from S
       avoiding 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 where the MPLS labels understood by since the tunnel endpoint router are can use the IP
   header of the original packet to continue forwarding the packet.
   However, tunneling will not needed.

10.2. work for LDP traffic without targeted LDP
   sesssions for learning the FEC-label binding at the tunnel endpoint.

11.2.  Protecting Multi-Homed Prefixes using Named Proxy-Nodes

   A clear advantage to using a

   Instead, the named proxy-node is 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 (LFIN) when that interface may not be a primary next-hop.  For LDP traffic where the label indicates both the
   topology and the FEC, it is necessary to either use a named proxy-
   node or deal with learning remote MPLS labels.

   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 are not
   defined in [I-D.atlas-ospf-mrt].  The details of what label-bindings
   must be originated are described in [I-D.atlas-mpls-ldp-mrt].

   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 extremely straightforward.  The details of the simple constant-time
   functions, Select_Proxy_Node_NHs() and
   Select_Alternates_Proxy_Node(), are given in
   [I-D.enyedi-rtgwg-mrt-frr-algorithm].
   [I-D.ietf-rtgwg-mrt-frr-algorithm].  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 [[RFC2328] Section 16.5] does incremental updates
   for new summary-LSAs.

   The key 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 2 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 are those routers
   that are advertising the prefix; the cost named-proxy-cost assigned to
   each such prefix-advertising router is the announced cost to the prefix.
   The second set are 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). 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
   regard 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
   for identifying loop-free Island Neighbors(LFINs) is discussed below.
   The cost named-proxy-cost assigned to each (IBR, IN) pair is the cost(IBR, IN)
   + D_opt(IN, prefix) plus Cost(IBR, IN). prefix).

   From the set of prefix-advertising routers and the IBRs, set of IBRs with
   at least one LFIN, the two
   lowest cost routers with the lowest named-proxy-cost
   are selected and ties selected.  Ties are broken based upon the lowest Router ID.  For
   ease of discussion, such selected routers are
   proxy-node attachment routers and the two selected routers will be named A
   and B. 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 default 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 interface(s) that
   caused the router to advertise the prefix; this interface might be
   out of the area/level/AS.

10.2.1.

11.2.1.  Computing if an Island Neighbor (IN) is loop-free

   As discussed, the Island Neighbor needs to be loop-free with regard
   to the whole MRT Island for the destination.  Conceptually, the cost
   of transiting the MRT Island should be regarded as 0.  This can be
   done by collapsing the MRT Island into a single node, as seen in
   Figure 4, and then computing SPFs from each Island Neighbor and from
   the MRT Island itself.

             [G]---[E]---(V)---(U)---(T)
              | \   |     |           |
              |  \  |     |           |
              |   \ |     |           |
             [H]---[F]---(R)---(S)----|

          (1) Network Graph with Partial Deployment

            [E],[F],[G],[H] :  No support for MRT
            (R),(S),(T),(U),(V):  MRT Island - supports MRT

        [G]---[E]----|                     |---(V)---(U)---(T)
         | \   |     |                     |    |           |
         |  \  |  ( MRT Island )      [ proxy ] |           |
         |   \ |     |                     |    |           |
        [H]---[F]----|                     |---(R)---(S)----|

         (2) Graph for determining    (3) Graph for MRT computation
             loop-free neighbors

   Figure 4: Computing alternates to destinations outside the MRT Island

   The simple way to do this without manipulating the topology is to
   compute the SPFs from each IN and a node in the MRT Island (e.g. the
   GADAG root), but use a link metric of 0 for all links between routers
   in the MRT Island.  The distances computed via SPF this way will be
   refered to as Dist_mrt0.

   An IN is loop-free with respect to a destination D if: Dist_mrt0(IN,
   D) < Dist_mrt0(IN, MRT Island Router) + Dist_mrt0(MRT Island Router,
   D).  Any router in the MRT Island can be used since the cost of
   transiting between MRT Island routers is 0.  The GADAG Root is
   recommended for consistency.

10.3.

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 6 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 multi-homed 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.

11.

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 re-calculation.

11.1.

12.1.  Micro-forwarding loop prevention and MRTs

   As is well known[RFC5715], micro-loops can occur during IGP
   convergence; such loops can be local to the failure or remote from
   the failure.  Managing micro-loops is an orthogonal issue to having
   alternates for local repair, such as MRT fast-reroute provides.

   There are two possible micro-loop prevention mechanisms discussed in
   [RFC5715].  The first is Ordered FIB [I-D.ietf-rtgwg-ordered-fib].
   The second is Farside Tunneling which requires tunnels or an
   alternate topology to reach routers on the farside of the failure.

   Since MRTs provide an alternate topology through which traffic can be
   sent and which can be manipulated separately from the SPT, it is
   possible that MRTs could be used used to support Farside Tunneling.
   Details of how to do so are outside the scope of this document.

   Micro-loop mitigation mechanisms can also work when combined with
   MRT.

12.2.  MRT Recalculation

   When a failure event happens, traffic is put by the PLRs onto the MRT
   topologies.  After that, each router recomputes its shortest path
   tree (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 new SPT

   4.  If the network was stable before the failure occured, 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.

   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.

13.  Implementation Status

   [RFC Editor: please remove this section prior to publication.]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC6982].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to support Farside Tunneling.
   Details be, a catalog of how to do so available implementations or their
   features.  Readers are outside advised to note that other implementations may
   exist.

   According to [RFC6982], "this will allow reviewers and working groups
   to assign due consideration to documents that have the scope benefit of this document.

   Micro-loop mitigation mechanisms can also work when combined with
   MRT.

11.2.  MRT Recalculation

   When a failure event happens, traffic is put by the PLRs onto the MRT
   topologies.  After that, each router recomputes its shortest path
   tree (SPT)
   running code, which may serve as evidence of valuable experimentation
   and moves traffic over feedback that have made the implemented protocols more mature.
   It is up to that.  Only after all the PLRs
   have switched individual working groups to use this information as
   they see fit".

   Juniper Networks Implementation

   o  Organization responsible for the implementation: Juniper Networks

   o  Implementation name: MRT-FRR algorithm
   o  Implementation description: The MRT-FRR algorithm using their SPTs and traffic OSPF as
      the IGP has drained from been implemented and verified.

   o  The implementation's level of maturity: prototype

   o  Protocol coverage: This implementation of the MRT topologies should each router install the recomputed MRTs into
   the FIBs.

   At each router, therefore, algorithm
      includes Island identification, GADAG root selection, Lowpoint
      algorithm, augmentation of GADAG with additional links, and
      calculation of MRT transit next-hops alternate next-hops based on
      draft "draft-ietf-rtgwg-mrt-frr-algorithm-00".  This
      implementation also includes the sequence is M-bit in OSPF based on "draft-
      atlas-ospf-mrt-01" as follows:

   1.  Receive failure notification

   2.  Recompute SPT

   3.  Install new SPT

   4.  If the network well as LDP MRT Capability based on "draft-
      atlas-mpls-ldp-mrt-00".

   o  Licensing: proprietary

   o  Implementation experience: Implementation was stable before useful for verifying
      functionality and lack of gaps.  It has also been useful for
      improving aspects of the algorithm.

   o  Contact information: akatlas@juniper.net, shraddha@juniper.net,
      kishoret@juniper.net

   Huawei Technology Implementation

   o  Organization responsible for the implementation: Huawei Technology
      Co., Ltd.

   o  Implementation name: MRT-FRR algorithm and IS-IS extensions for
      MRT.

   o  Implementation description: The MRT-FRR algorithm, IS-IS
      extensions for MRT and LDP multi-topology have been implemented
      and verified.

   o  The implementation's level of maturity: prototype

   o  Protocol coverage: This implementation of the failure occured, wait a
       configured (or advertised) period for all routers to be using
       their SPTs MRT algorithm
      includes Island identification, GADAG root selection, Lowpoint
      algorithm, augmentation of GADAG with additional links, and traffic
      calculation of MRT transit next-hops alternate next-hops based on
      draft "draft-enyedi-rtgwg-mrt-frr-algorithm-03".  This
      implementation also includes IS-IS extension for MRT based on
      "draft-li-mrt-00".

   o  Licensing: proprietary
   o  Implementation experience: It is important produce a second
      implementation to drain from the MRTs.

   5.  Recompute MRTs

   6.  Install new MRTs.

   While the recomputed MRTs are not installed in verify the FIB, protection
   coverage algorithm is lowered.  Therefore, it implemented correctly
      without looping.  It is important to recalculate verify the
   MRTs and install them quickly.

12. ISIS extensions
      work for MRT-FRR.

   o  Contact information: lizhenbin@huawei.com, eric.wu@huawei.com

14.  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, and Raveendra Torvi and Chris Bowers for their suggestions
   and review.

13.

15.  IANA Considerations

   This doument includes no request

   Please create an MRT Profile registry for the MRT Profile TLV.  The
   range is 0 to IANA.

14. 255.  The default MRT Profile has value 0.  Values
   1-200 are by Standards Action.  Values 201-220 are for
   experimentation.  Values 221-255 are for vendor private use.

   Please allocate values from the LDP Multi-Topology (MT) ID Name Space
   [I-D.ietf-mpls-ldp-multi-topology] for the following: Default Profile
   MRT-Red MT-ID (TBA-MRT-ARCH-1) and Default Profile MRT-Blue MT-ID
   (TBA-MRT-ARCH-2).  Please allocate MT-ID values less than 4096 so
   that they can also be signalled in PIM.

16.  Security Considerations

   This architecture is not currently believed to introduce new security
   concerns.

15.

17.  References

15.1.

17.1.  Normative References

   [I-D.enyedi-rtgwg-mrt-frr-algorithm]
              Atlas, A., Envedi,

   [I-D.ietf-rtgwg-mrt-frr-algorithm]
              Enyedi, G., Csaszar, A., Gopalan, Atlas, A., and C. Bowers, C., and A.
              Gopalan, "Algorithms for computing Maximally Redundant
              Trees for IP/LDP Fast- Reroute", draft-enyedi-rtgwg-mrt-
              frr-algorithm-03 Fast-Reroute", draft-rtgwg-mrt-frr-
              algorithm-01 (work in progress), July 2013. 2014.

   [RFC5286]  Atlas, A. and A. Zinin, "Basic Specification for IP Fast
              Reroute: Loop-Free Alternates", RFC 5286, September 2008.

   [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
              5714, January 2010.

15.2.

17.2.  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,
              <http://timon.tmit.bme.hu/theses/thesis_book.pdf>.

   [I-D.atlas-mpls-ldp-mrt]
              Atlas, A., Tiruveedhula, K., Tantsura, J., and IJ.
              Wijnands, "LDP Extensions to Support Maximally Redundant
              Trees", draft-atlas-mpls-ldp-mrt-00 draft-atlas-mpls-ldp-mrt-01 (work in progress),
              July 2013. 2014.

   [I-D.atlas-ospf-mrt]
              Atlas, A., Hegde, S., Chris, Bowers, C., and J. Tantsura, "OSPF
              Extensions to Support Maximally Redundant Trees", draft-
              atlas-ospf-mrt-00
              atlas-ospf-mrt-02 (work in progress), July 2013. 2014.

   [I-D.atlas-rtgwg-mrt-mc-arch]
              Atlas, A., Kebler, R., Wijnands, I., Csaszar, A., and G.
              Envedi, "An Architecture for Multicast Protection Using
              Maximally Redundant Trees", draft-atlas-rtgwg-mrt-mc-
              arch-02 (work in progress), July 2013.

   [I-D.bryant-ipfrr-tunnels]
              Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
              Fast Reroute using tunnels", draft-bryant-ipfrr-tunnels-03
              (work in progress), November 2007.

   [I-D.ietf-mpls-ldp-multi-topology]
              Zhao, Q., Fang, L., Raza, K., Zhou, C., Fang, L., Li, L., and K. Raza, D.
              King, "LDP Extensions for Multi Topology Routing", draft-ietf-mpls-
              ldp-multi-topology-08 Topology", draft-ietf-
              mpls-ldp-multi-topology-12 (work in progress), May 2013. April 2014.

   [I-D.ietf-rtgwg-ipfrr-notvia-addresses]
              Bryant, S., Previdi, S., and M. Shand, "A Framework for IP
              and MPLS Fast Reroute Using Not-via Addresses", draft-
              ietf-rtgwg-ipfrr-notvia-addresses-11 (work in progress),
              May 2013.

   [I-D.ietf-rtgwg-ordered-fib]
              Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
              Francois, P., and O. Bonaventure, "Framework for Loop-free
              convergence using oFIB", draft-ietf-rtgwg-ordered-fib-12
              (work in progress), May 2013.

   [I-D.ietf-rtgwg-remote-lfa]
              Bryant, S., Filsfils, C., Previdi, S., Shand, M., and S.
              Ning, "Remote LFA FRR", draft-ietf-rtgwg-remote-lfa-02 draft-ietf-rtgwg-remote-lfa-06
              (work in progress), May 2013.

   [I-D.litkowski-rtgwg-node-protect-remote-lfa]
              Litkowski, S., "Node protecting remote LFA", 2014.

   [I-D.li-isis-mrt]
              Li, Z., Wu, N., Zhao, Q., Atlas, A., Bowers, C., and J.
              Tantsura, "Intermediate System to Intermediate System (IS-
              IS) Extensions for Maximally Redundant Trees(MRT)", draft-
              litkowski-rtgwg-node-protect-remote-lfa-00
              li-isis-mrt-01 (work in progress), July 2014.

   [I-D.psarkar-rtgwg-rlfa-node-protection]
              psarkar@juniper.net, p., Gredler, H., Hegde, S.,
              Raghuveer, H., Bowers, C., and S. Litkowski, "Remote-LFA
              Node Protection and Manageability", draft-psarkar-rtgwg-
              rlfa-node-protection-04 (work in progress), April 2013. 2014.

   [LFARevisited]
              Retvari, G., Tapolcai, J., Enyedi, G., and A. Csaszar, "IP
              Fast ReRoute: Loop Free Alternates Revisited", Proceedings
              of IEEE INFOCOM , 2011, <http://opti.tmit.bme.hu/~tapolcai
              /papers/retvari2011lfa_infocom.pdf>.
              <http://opti.tmit.bme.hu/~tapolcai/papers/
              retvari2011lfa_infocom.pdf>.

   [LightweightNotVia]
              Enyedi, G., Retvari, G., Szilagyi, P., and A. Csaszar, "IP
              Fast ReRoute: Lightweight Not-Via without Additional
              Addresses", Proceedings of IEEE INFOCOM , 2009,
              <http://mycite.omikk.bme.hu/doc/71691.pdf>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC3137]  Retana, A., Nguyen, L., White, R., Zinin, A., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 3137,
              June 2001.

   [RFC5443]  Jork, M., Atlas, A., and L. Fang, "LDP IGP
              Synchronization", RFC 5443, March 2009.

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, January 2010.

   [RFC6571]  Filsfils, C., Francois, P., Shand, M., Decraene, B.,
              Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
              Alternate (LFA) Applicability in Service Provider (SP)
              Networks", RFC 6571, June 2012.

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982, July
              2013.

Appendix A.  General Issues with Area Abstraction

   When a multi-homed 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 5: AS external prefixes in different areas

   Consider the network in Figure 5 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.

Authors' Addresses

   Alia Atlas (editor)
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   USA

   Email: akatlas@juniper.net

   Robert Kebler
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   USA

   Email: rkebler@juniper.net

   Chris Bowers
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   USA

   Email: cbowers@juniper.net

   Gabor Sandor Enyedi
   Ericsson
   Konyves Kalman krt 11.
   Budapest  1097
   Hungary

   Email: Gabor.Sandor.Enyedi@ericsson.com

   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
   USA

   Email: jeff.tantsura@ericsson.com

   Maciek Konstantynowicz
   Cisco Systems

   Email: maciek@bgp.nu

   Russ White
   VCE

   Email: russw@riw.us