Routing Area Working Group                                      A. Atlas, Ed. Atlas
Internet-Draft                                                 R. Kebler                                                 C. Bowers
Intended status: Standards Track                               C. Bowers
Expires: April 17, 2016                        Juniper Networks
Expires: June 23, 2016                                         G. Enyedi
                                                              A. Csaszar
                                                             J. Tantsura
                                                                Ericsson
                                                                R. White
                                                                     VCE
                                                        October 15,
                                                       December 21, 2015

An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
                draft-ietf-rtgwg-mrt-frr-architecture-07
                draft-ietf-rtgwg-mrt-frr-architecture-08

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

   MRT removes all the 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 April 17, June 23, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Importance of 100% Coverage . . . . . . . . . . . . . . .   5
     1.2.  Partial Deployment and Backwards Compatibility  . . . . .   6
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   6
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . .   8
   5.  Maximally Redundant Trees (MRT) and Fast-Reroute  . . . . . .  10
   6.  Unicast Forwarding with MRT Fast-Reroute  . . . . . . . . . .  11
     6.1.  MRT Forwarding Mechanisms . . . . . . . . . . . . . . . .  12  11
       6.1.1.  MRT LDP labels  . . . . . . . . . . . . . . . . . . .  12
         6.1.1.1.  Topology-scoped FEC encoded using a single label
                   (Option 1A) . . . . . . . . . . . . . . . . . . .  12
         6.1.1.2.  Topology and FEC encoded using a two label stack
                   (Option 1B) . . . . . . . . . . . . . . . . . . .  13
         6.1.1.3.  Compatibility of Option 1A and 1B . . . . . . . .  13
         6.1.1.4.  Mandatory support for MRT LDP Label option 1A . .  14  13
       6.1.2.  MRT IP tunnels (Options 2A and 2B)  . . . . . . . . .  14  13
     6.2.  Forwarding LDP Unicast Traffic over MRT Paths . . . . . .  14
       6.2.1.  Forwarding LDP traffic using MRT LDP Labels (Option
               1A) . . . . . . . . . . . . . . . . . . . . . . . . .  15  14
       6.2.2.  Forwarding LDP traffic using MRT LDP Labels (Option
               1B) . . . . . . . . . . . . . . . . . . . . . . . . .  15
       6.2.3.  Other considerations for forwarding LDP traffic using
               MRT LDP Labels  . . . . . . . . . . . . . . . . . . .  15
     6.3.  Forwarding IP Unicast Traffic over MRT Paths  . . . . . .  16  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) . . . . . . . . . . . . . . . . . . . . . . .  16
         6.3.1.2.  Tunneling IP traffic using MRT LDP Labels (Option
                   1B) . . . . . . . . . . . . . . . . . . . . . . .  17  16
       6.3.2.  Tunneling IP traffic using MRT IP Tunnels . . . . . .  17
       6.3.3.  Required support  . . . . . . . . . . . . . . . . . .  17
   7.  MRT Island Formation  . . . . . . . . . . . . . . . . . . . .  17
     7.1.  IGP Area or Level . . . . . . . . . . . . . . . . . . . .  18
     7.2.  Support for a specific MRT profile  . . . . . . . . . . .  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 . . . . . . . . . . . . . . . . . . .  20
     8.2.  Router-specific MRT paramaters  . . . . . . . . . . . . .  21
     8.3.  Default MRT profile . . . . . . . . . . . . . . . . . . .  21
   9.  LDP signaling extensions and considerations . . . . . . . . .  22
   10. Inter-area Forwarding Behavior  . . . . . . . . . . . . . . .  23
     10.1.  ABR Forwarding Behavior with MRT LDP Label Option 1A . .  23
       10.1.1.  Motivation for Creating the Rainbow-FEC  . . . . . .  24
     10.2.  ABR Forwarding Behavior with IP Tunneling (option 2) . .  24
     10.3.  ABR Forwarding Behavior with LDP Label option 1B . . . .  25
   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 .  33  31
   12. Network Convergence and Preparing for the Next Failure  . . .  34  31
     12.1.  Micro-forwarding loop prevention and MRTs  . . . . . . .  34  32
     12.2.  MRT Recalculation for the Default MRT Profile  . . . . .  32
   13. Implementation Status . . . . . . . . . . . . . . . . . . . .  33
   14. Operations and Management Considerations  . . . . . . . . . .  34
   13. Implementation Status
   15. Applying Policy to Select from Multiple Possible Alternates
       for FRR . . . . . . . . . . . . . . . . . . . . . . . . . . .  35
   14.
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  37
   15.  35
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
   16.  35
   18. Security Considerations . . . . . . . . . . . . . . . . . . .  37
   17.  36
   19. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  36
   20. References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     17.1.  36
     20.1.  Normative References . . . . . . . . . . . . . . . . . .  37
     17.2.  36
     20.2.  Informative References . . . . . . . . . . . . . . . . .  37
   Appendix A.  General Issues with Area Abstraction . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41  40

1.  Introduction

   This document gives describes a complete solution for IP/LDP fast-reroute [RFC5714].
   MRT-FRR creates two alternate trees separate from the primary next-hop 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.ietf-ospf-mrt] and
   [I-D.ietf-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  |             |                             |
   |         |             |             |                             |
   |  TI-LFA |     100%    |   Possible  |    per neighbor (link) or   |
   |         |  Link/Node  |             |  neighbor's neighbor (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 [RFC7490] 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.ietf-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] [RFC6981] 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.

   TI-LFA:   Topology Independent Loop-free Alternate Fast Re-route (TI-
      LFA) [I-D.francois-spring-segment-routing-ti-lfa] [I-D.francois-rtgwg-segment-routing-ti-lfa] aims to provide
      link and node protection of node and adjacency segments within the
      Segment Routing (SR) framework.  It guarantees complete coverage.
      The TI-LFA computation for link-protection is fairly
      straightforward, while the computation for node-protection is more
      complex.  For link-protection with symmetric link costs, TI-LFA
      can provide complete coverage by pushing up to two additional
      labels.  For node protection on arbitrary topologies, the label
      stack size can grow significantly based on repair path.  Note that
      TI-LFA requires shortest path forwarding based on SR Node-SIDs, as
      opposed to LDP labels, in order to construct label stacks for
      backups paths without relying on a large number of targeted LDP
      sessions to learn remote FEC-label bindings.  It also requires the
      use of Adj-SIDs to achieve 100% coverage.

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]
   [RFC6987] that the link metric be raised to the maximum cost; this
   may not be symmetric and for [RFC3137] [RFC6987] 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] FIB[RFC6976] 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] [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 MPLS MT-
      ID.  Specifically, MRT-Red is the decreasing MRT where links in
      the GADAG are taken in the direction from a higher topologically
      ordered node to a lower one.

   MRT-Blue:   MRT-Blue is used to describe one of the two MRTs; it is
      used to described the associated forwarding topology and MPLS MT-
      ID.  Specifically, MRT-Blue is the increasing MRT where links in
      the GADAG are taken in the direction from a lower topologically
      ordered node to a higher one.

   Rainbow MRT:   It is useful to have an MPLS MT-ID that refers to the
      multiple MRT topologies and to the default topology.  This is
      referred to as the Rainbow MRT MPLS MT-ID and is used by LDP to
      reduce signalling 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 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 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 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 perspective of a particular destination, D,
   this looks like a reverse SPT (rSPT).  To use maximally redundant
   trees, in addition, each destination D has two MRTs associated with
   it; by convention these will be called the MRT-Blue and MRT-Red.
   MRT-FRR is realized by using multi-topology forwarding.  There is a
   MRT-Blue forwarding topology and a MRT-Red forwarding topology.

   Any IP/LDP fast-reroute technique beyond LFA requires an additional
   dataplane procedure, such as an additional forwarding mechanism.  The
   well-known options are multi-topology forwarding (used by MRT-FRR),
   tunneling (e.g.  [I-D.ietf-rtgwg-ipfrr-notvia-addresses]  [RFC6981] or [RFC7490]), and per-interface
   forwarding (e.g.  Loop-Free Failure Insensitive Routing in
   [EnyediThesis]).

   When there is a link or node failure affecting, but not partitioning,
   the network, each node will still have at least one path via one of
   the MRTs to reach the destination D.  For example, in Figure 2, C
   would normally forward traffic to 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 always the case with fast-reroute technologies, forwarding does
   not change until a local failure is detected.  Packets are forwarded
   along the shortest path.  The appropriate alternate to use is pre-
   computed.  [I-D.ietf-rtgwg-mrt-frr-algorithm] describes exactly how
   to determine whether the MRT-Blue next-hops or the MRT-Red next-hops
   should be the MRT alternate next-hops for a particular primary next-
   hop to a particular destination.

   MRT alternates are always available to use.  It is a local decision
   whether to use an MRT alternate, 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 of [RFC5286] gives an example of link-
   protecting alternates causing a loop on node failure.  Even if a
   worse failure than anticipated happens, the use of MRT alternates
   will not cause looping.  Therefore, while node-protecting LFAs may be
   preferred, the certainty that no alternate-induced looping will occur
   is an advantage of using MRT alternates when the available node-
   protecting LFA is 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
   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 MRTs as for handling
   multiple topologies.

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

6.1.  MRT Forwarding Mechanisms

   The following options for MRT forwarding mechanisms are considered.

   1.  MRT LDP Labels

       A.  Topology-scoped FEC encoded using a single label

       B.  Topology and FEC encoded using a two label stack

   2.  MRT IP Tunnels

       A.  MRT IPv4 Tunnels

       B.  MRT IPv6 Tunnels

6.1.1.  MRT LDP labels

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

6.1.1.1.  Topology-scoped FEC encoded using a single label (Option 1A)

   [RFC7307] provides a mechanism to distribute FEC-Label bindings
   scoped to a given MPLS topology (represented by MPLS MT-ID).  To use
   multi-topology LDP to create MRT forwarding topologies, we associate
   two MPLS MT-IDs with the MRT-Red and MRT-Blue forwarding topologies,
   in addition to the default shortest path forwarding topology with MT-
   ID=0.

   With this forwarding mechanism, a single label is distributed for
   each topology-scoped FEC.  For a given FEC in the default topology
   (call it default-FEC-A), two additional topology-scoped FECs would be
   created, corresponding to the Red and Blue MRT forwarding topologies
   (call them red-FEC-A and blue-FEC-A).  A router supporting this MRT
   transit forwarding mechanism advertises a different FEC-label binding
   for each of the three topology-scoped FECs.  When a packet is
   received with a label corresponding to red-FEC-A (for example), an
   MRT transit router will determine the next-hop for the MRT-Red
   forwarding topology for that FEC, swap the incoming label with the
   outgoing label corresponding to red-FEC-A learned from the MRT-Red
   next-hop router, and forward the packet.

   This forwarding mechanism has the useful property that the FEC
   associated with the packet is maintained in the labels at each hop
   along the MRT.  We will take advantage of this property when
   specifying how to carry LDP traffic on MRT paths using multi-topology
   LDP labels.

   This approach is very simple for hardware to support.  However, it
   reduces the label space for other uses, and it increases the memory
   needed to store the labels and the communication required by LDP to
   distribute FEC-label bindings.

   This forwarding option uses the LDP signaling extensions described in
   [RFC7307].  The MRT-specific LDP extensions required to support this
   option are described in [I-D.ietf-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 topology and the FEC of the packet.  The top label (topology-id
   label) identifies the MRT forwarding topology, while the second label
   (FEC label) identifies the FEC.  The top label would be a new FEC
   type with two values corresponding to MRT Red and Blue topologies.

   When an MRT transit router receives a packet with a topology-id
   label, the router pops the top label and uses that it to guide the
   next-hop selection in combination with the next label in the stack
   (the FEC label).  The router then swaps the FEC label, using the FEC-
   label bindings learned through normal LDP mechanisms.  The router
   then pushes the topology-id label for the next-hop.

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

   This forwarding mechanism has minimal usage of additional labels,
   memory and LDP communication.  It does increase the size of packets
   and the complexity of the required label operations and look-ups.

   This forwarding option is consistent with context-specific label
   spaces, as described in [RFC 5331]. [RFC5331].  However, the precise LDP behavior
   required to support this option for MRT has not been specified.

6.1.1.3.  Compatibility of Option 1A and 1B

   In principle, MRT transit forwarding mechanisms 1A and 1B can coexist
   in the same network, with a packet being forwarding along a single
   MRT path using the single label of option 1A for some hops and the
   two label stack of option 1B for other hops.

6.1.1.4.  Mandatory support for MRT LDP Label option 1A

   If a router supports a profile that includes the MRT 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 2B)

   IP tunneling can also be used as an MRT transit forwarding mechanism.
   Each router supporting this MRT transit forwarding mechanism
   announces two additional loopback addresses and their associated MRT
   color.  Those addresses are used as destination addresses for MRT-
   blue and MRT-red IP tunnels respectively.  The special loopback
   addresses allow the transit nodes to identify the traffic as being
   forwarded along either the MRT-blue or MRT-red topology to reach the
   tunnel destination.  Announcements of these two additional loopback
   addresses per router with their  For example, an MRT color requires IGP extensions,
   which have not been defined.

   Either IPv4 (option ingress router can cause a
   packet to be tunneled along the MRT-red path to router X by
   encapsulating the packet using the MRT-red loopback address
   advertised by router X.  Upon receiving the packet, router X would
   remove the encapsulation header and forward the packet based on the
   original destination address.

   Announcements of these two additional loopback addresses per router
   with their MRT color requires IGP extensions, which have not been
   defined.

   Either IPv4 (option 2A) or IPv6 (option 2B) can be used as the
   tunneling mechanism.

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

6.2.  Forwarding LDP Unicast Traffic over MRT Paths

   In the previous section, we examined several options for providing
   MRT transit forwarding functionality, which is independent of the
   type of traffic being carried.  We now look at the MRT ingress
   functionality, which will depend on the type of traffic being carried
   (IP or LDP).  We start by considering LDP traffic.

   We also simplify the initial discussion by assuming that the network
   consists of a single IGP area, and that all routers in the network
   participate in MRT.  Other deployment scenarios that require MRT
   egress functionality are considered later in this document.

   In principle, it is possible to carry LDP traffic in MRT IP tunnels.
   However, for LDP traffic, it is very desirable to avoid tunneling.
   Tunneling LDP traffic to a remote node requires knowledge of remote
   FEC-label bindings so that the LDP traffic can continue to be
   forwarded properly when it leaves the tunnel.  This requires targeted
   LDP sessions which can add management complexity.  The  As described
   below, the two MRT LDP
   Label forwarding mechanisms have the useful property that the FEC
   associated with the packet is maintained in the use LDP labels at each hop
   along the MRT, as long as an MRT to the originator of the FEC is
   used.  The MRT IP tunneling mechanism does do not have this useful
   property.  Therefore, this document only considers the two MRT LDP
   Label forwarding mechanisms for protecting
   require targeted LDP traffic with MRT fast-
   reroute. sessions.

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 single label as described in section
   Section 6.1.1.1.  When a PLR receives an LDP packet that needs to be
   forwarded on the Red MRT (for example), it does a label swap
   operation, replacing the usual LDP label for the FEC with the Red MRT
   label for that FEC received from the next-hop router in the Red MRT
   computed by the PLR.  When the next-hop router in the Red MRT
   receives the packet with the Red MRT label for the FEC, the MRT
   transit forwarding functionality continues as described in
   Section 6.1.1.1.  In this way the original FEC associated with the
   packet is maintained at each hop along the MRT.

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

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

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

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

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

6.3.  Forwarding IP Unicast Traffic over MRT Paths

   For IP IPv4 traffic, there is no currently practical alternative except
   tunneling to gain the bits needed to indicate the MRT-Blue or MRT-Red
   forwarding topology.  For IPv6 traffic, in principle one could define
   bits in the IPv6 options header to indicate the MRT-Blue or MRT-Red
   forwarding topology.  However, in this document, we have chosen not
   to define a solution that would work for IPv6 traffic but not for
   IPv4 traffic.

   The 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 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 IP traffic along an MRT to a tunnel
   endpoint that is not the destination of the IP traffic.  These
   situations will be discussed in detail later.  We note here that an
   IP packet with a destination in a different IGP area/level from the
   PLR should be tunneled on the MRT to the ABR/LBR on the shortest path
   to the destination.  For a destination outside of the PLR's MRT
   Island, the packet should be tunneled on the MRT to a non-proxy-node
   immediately before the named proxy-node on that particular color MRT.

6.3.1.  Tunneling IP traffic using MRT LDP Labels

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

6.3.1.1.  Tunneling IP traffic using MRT LDP 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 Red MRT to a particular tunnel endpoint, it does a
   label push operation.  The label pushed is the 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 mechanism encodes the topology
   and the FEC using a two label stack as described in Section 6.1.1.2.
   When a PLR receives an IP packet that needs to be forwarded on the
   Red MRT to a particular tunnel endpoint, the PLR pushes two labels on
   the IP packet.  The first (inner) label is the normal LDP label
   learned from the next-hop on the Red MRT, associated with a 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 potential optimization. variation that uses a
   single label as opposed to two labels.  In order to tunnel an IP
   packet over an MRT to the destination of the IP packet (as opposed to
   an arbitrary tunnel endpoint), then we could just push a topology-identification topology-
   identification label directly onto the packet.  An MRT transit router
   would need to pop the topology-id label, do an IP route lookup in the
   context of that topology-id , and push the topology-id label.

6.3.2.  Tunneling IP traffic using MRT IP Tunnels

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

6.3.3.  Required support

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

7.  MRT Island Formation

   The purpose of communicating support for MRT in the IGP is to
   indicate that the MRT-Blue and MRT-Red forwarding topologies are
   created for transit traffic.  The MRT architecture allows for
   different, potentially incompatible options.  In order to create
   constistent MRT forwarding topologies, the routers participating in a
   particular MRT Island need to use the same set of options.  These
   options are grouped into MRT profiles.  In addition, the routers in
   an MRT Island all need to use the same set of nodes and links within
   the Island when computing the MRT forwarding topologies.  This
   section describes the information used by a router to determine the
   nodes and links to include in a particular MRT Island.  Some of this
   information is shared among routers using the newly-defined IGP
   signaling extensions for MRT described in [I-D.ietf-ospf-mrt] and
   [I-D.ietf-isis-mrt].  Other information already exists in the IGPs
   and can be used by MRT in Island formation, subject to the
   interpretation defined here.

   Deployment scenarios using multi-topology OSPF or IS-IS, or running
   both ISIS and OSPF on the same routers is out of 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
   same IGP area or level.  For ISIS, a link belonging to both level 1
   and level 2 would qualify to be in multiple MRT Islands.  A given ABR
   or LBR can belong to multiple MRT Islands, corresponding to the areas
   or levels in which it participates.  Inter-area forwarding behavior
   is discussed in Section 10.

7.2.  Support for a specific MRT profile

   All routers in an MRT Island MUST support the same MRT profile.  A
   router advertises support for a given MRT profile using the IGP
   extensions defined in [I-D.ietf-ospf-mrt] and [I-D.ietf-isis-mrt]
   using an 8-bit Profile ID value.  A given router can support multiple
   MRT profiles and participate in multiple MRT Islands.  The options
   that make up an MRT profile, as well as the default MRT profile, are
   defined in Section 8.

7.3.  Excluding additional routers and interfaces from the MRT Island

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

7.3.1.  Existing IGP exclusion mechanisms

   Mechanisms for discouraging traffic from using particular links
   already exist in ISIS and OSPF.  In ISIS, an interface configured
   with a metric of 2^24-2 (0xFFFFFE) will only be used as a last
   resort.  (An interface configured with a metric of 2^24-1 (0xFFFFFF)
   will not be advertised into the topology.)  In OSPF, an interface
   configured with a metric of 2^16-1 (0xFFFF) will only be used as a
   last resort.  These metrics can be configured manually to enforce
   administrative policy, or they can be set in an automated manner as
   with LDP IGP synchronization [RFC5443].

   Mechanisms also exist in ISIS and OSPF to prevent transit traffic
   from using a particular router.  In ISIS, the overload bit is used
   for this purpose.  In OSPF, [RFC3137] [RFC6987] specifies setting all outgoing
   interface metrics to 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 is discouraged
   from carrying traffic by existing IGP mechanisms.

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

   2.  A router MUST be excluded from an MRT Island if it is advertised
       with the overload bit set (for ISIS), or it is advertised with
       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 Islands.  [I-D.ietf-ospf-mrt] and
   [I-D.ietf-isis-mrt] define the IGP extensions for OSPF and ISIS used
   to advertise that a link is MRT-Ineligible.  A link with either
   interface advertised as MRT-Ineligible MUST be excluded from an MRT
   Island.  Note that an interface advertised as MRT-Ineligigle by a
   router is ineligible with respect to all profiles advertised by that
   router.

7.4.  Connectivity

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

7.5.  Example algorithm

   An algorithm that allows a computing router to identify the routers
   and links in the local MRT Island satisfying the above rules is given
   in section 5.1 5.2 of [I-D.ietf-rtgwg-mrt-frr-algorithm].

8.  MRT Profile

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

8.1.  MRT Profile Options

   Below is a description of

   This document specifies the values and options that define correspond to the
   Default MRT Profile (Profile ID = 0).  Future documents may define
   other MRT Profiles by specifying the MRT Profile Options below.

8.1.  MRT Profile Options

   Below is a description of the values and options that define an MRT
   Profile.

   MRT Algorithm:   This identifies the particular MRT algorithm used by
      the router for this profile.  Algorithm transitions can be managed
      by advertising multiple MRT profiles.

   MRT-Red MT-ID:   This specifies the MT-ID to be associated with the
      MRT-Red forwarding topology.  It is needed for use in LDP
      signaling.  All routers in the 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 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 use
      the same GADAG root in the calculations used construct the MRTs.
      A valid GADAG Root Selection Policy MUST be such that each router
      in the MRT island chooses the same GADAG root based on information
      available to all routers in the MRT island.  GADAG Root Selection
      Priority values, advertised in the IGP as router-specific MRT
      parameters, MAY be used in a GADAG Root Selection Policy.

   MRT Forwarding Mechanism:   This specifies which forwarding mechanism
      the router uses to carry transit traffic along MRT paths.  A
      router which supports a specific MRT forwarding mechanism must
      program appropriate next-hops into the forwarding plane.  The
      current options are MRT LDP Labels, IPv4 Tunneling, IPv6
      Tunneling, and None.  If the MRT LDP Labels option is supported,
      then option 1A and the appropriate signaling extensions 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 may be useful for
      multicast global protection.

   Recalculation:   As part of what   Recalculation specifies the process and timing should the by
      which new MRTs be are computed on a modified topology?  Section 12.2 describes after the minimum behavior required to support fast-reroute. topology has been modified.

   Area/Level Border Behavior:   Should inter-area traffic on the MRT-
      Blue or MRT-Red be put back onto 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.

   If a router advertises support for multiple MRT profiles, then it
   MUST create the transit forwarding topologies for each of those,
   unless the profile specifies the None option for MRT Forwarding
   Mechanism.  A router MUST NOT advertise multiple MRT profiles that
   overlap in their MRT-Red MT-ID or MRT-Blue MT-ID.

8.2.  Router-specific MRT paramaters

   For some profiles, additional router-specific MRT parameters may need
   to be distributed via the IGP.  While the set of options indicated by
   the MRT Profile ID must be identical for all routers in an MRT
   Island, these router-specific MRT parameters may differ between
   routers in the same MRT island.  Several such parameters are
   described below.

   GADAG Root Selection Priority:   A GADAG Root Selection Policy MAY
      rely on the GADAG Root Selection Priority values advertised by
      each router in the MRT island.  A GADAG Root Selection Policy may
      use the GADAG Root Selection Priority to allow network operators
      to configure a parameter to ensure that the GADAG root is selected
      from a particular subset of routers.  An example of this use of
      the GADAG Root Selection Priority value by the GADAG Root
      Selection Policy is given in the Default MRT profile below.

   MRT-Red Loopback Address:   This provides the router's loopback
      address to reach the router via the MRT-Red forwarding topology.
      It can be specified for either IPv4 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 be specified for 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.ietf-ospf-mrt] and
   [I-D.ietf-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 MPLS MT-ID:   [I-D.ietf-mpls-ldp-mrt] contains the IANA
      request for allocation of this value from the MPLS Multi-Topology
      Identifiers Registry.  Prototype experiments have used a value of
      3997.

   MRT-Blue MPLS MT-ID:   [I-D.ietf-mpls-ldp-mrt] contains the IANA
      request for allocation of this value from the MPLS Multi-Topology
      Identifiers Registry.  Prototype experiments have used a value of
      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:   Recalculation of MRTs SHOULD occur as described in
      Section 12.2.  This allows the MRT forwarding topologies to
      support IP/LDP fast-reroute traffic.

   Area/Level Border Behavior:   As described in Section 10, ABRs/LBRs
      SHOULD ensure that traffic leaving the area also exits the MRT-Red
      or MRT-Blue forwarding topology.

9.  LDP signaling extensions and considerations

   The protocol extensions for LDP are defined in
   [I-D.ietf-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.
   The FEC-label bindings for the default shortest-path based MT-ID 0
   MUST still be sent (even though it could be inferred from the Rainbow
   FEC-label bindings) to ensure continuous operation of normal LDP
   forwarding.  The Rainbow MRT MT-ID is defined to provide an easy way
   to handle the special signaling that is needed at ABRs or LBRs.  It
   avoids the problem of needing to signal different MPLS labels to
   different LDP neighbors for the same FEC.  Because the Rainbow MRT
   MT-ID is used only by ABRs/LBRs or an LDP egress router, it is not
   MRT profile specific.

   [I-D.ietf-mpls-ldp-mrt] contains the IANA request for the Rainbow MRT
   MPLS MT-ID.

10.  Inter-area Forwarding Behavior

   Unless otherwise specified, in this section we will use the terms
   area and ABR 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
   within areas.  Second, it forwards traffic from one area into
   another.  These same two roles apply for MRT transit traffic.
   Traffic on MRT-Red or MRT-Blue destined inside the area needs to stay
   on MRT-Red or MRT-Blue in that area.  However, it is desirable for
   traffic leaving the area to also exit MRT-Red or MRT-Blue and return
   to shortest path forwarding.

   For unicast MRT-FRR, the need to stay on an MRT forwarding topology
   terminates at the ABR/LBR whose best route is via a different area/
   level.  It is highly desirable to go back to the default forwarding
   topology when leaving an area/level.  There are three basic reasons
   for this.  First, the default topology uses shortest paths; the
   packet will thus take the shortest possible route to the destination.
   Second, this allows 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
   destination Z should continue to forward the packet along MRT-Red or
   MRT-Blue only if the best route to Z is in the same area as the
   interface that the packet was received on.  Otherwise, the packet
   should be removed from MRT-Red or MRT-Blue and forwarded on the
   shortest-path default forwarding topology.

   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 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.  To those routers in the same area as the best route to the
   destination, the ABR/LBR advertises the following FEC-label bindings:
   L_primary for the default topology, L_blue for the MRT-Blue MT-ID and
   L_red for the MRT-Red MT-ID, as expected.  However, to routers in
   other areas, the ABR/LBR advertises the following FEC-label bindings:
   L_primary for the default topology, and L_primary for the Rainbow MRT
   MT-ID.  Associating L_primary with the Rainbow MRT MT-ID causes the
   receiving routers to use L_primary for the MRT-Blue MT-ID and for the
   MRT-Red MT-ID.

   The ABR/LBR installs all next-hops for the best area: primary next-
   hops for L_primary, MRT-Blue next-hops for L_blue, and MRT-Red next-
   hops for L_red.  Because the ABR/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 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
   advertising the lowest cost to the external destination.

   Thus, for these two MRT Forwarding Mechanisms( MRT LDP Label option
   1A and IP tunneling option 2), there is no need for additional
   computation or per-area forwarding state.

10.3.  ABR Forwarding Behavior with LDP Label option 1B

   The other MRT forwarding mechanism described in Section 6 uses two
   labels, a topology-id label, and a FEC-label.  This mechanism would
   require that any router whose MRT-Red or MRT-Blue next-hop is an ABR/
   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 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 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.ietf-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 re-enter the MRT
   Island if a loop-free exit point is not selected.

   FRR using LFA has the useful property that it is able to protect
   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 be accomplished
   with MRT FRR as well.

   If ASBR protection is desired, this has additional complexities if
   the ASBRs are in different areas.  Similarly, protecting labeled BGP
   traffic in the event of an ASBR failure has additional complexities
   due to the per-ASBR label spaces involved.

   As discussed in [RFC5286], a 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
   tunnel endpoint selection where the PLR picks a router to tunnel to
   where that router is loop-free with respect to the failure-point.
   Conceptually, the set of candidate routers to provide LFAs expands to
   all routers that can be reached via an MRT alternate, attached to the
   prefix.

   The second is to use a proxy-node, 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.  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.  The FEC-
   label bindings for the topology-scoped FECs ((MT-ID 0, FEC), (MRT-
   Red, FEC), and (MRT-Blue, FEC)) MUST also be provided via LDP to
   neighbors inside the MRT Island.

11.1.  Protecting 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 for reaching B from S that
       avoid the link (S,F).

   The tunnel endpoint selected will receive a packet destined to itself
   and, being the egress, will pop that MPLS label (or have signaled
   Implicit Null) and forward based on what is underneath.  This
   suffices for IP traffic since the tunnel endpoint can use the IP
   header of the original packet to continue forwarding the packet.
   However, tunnelling of LDP traffic requires targeted LDP sessions for
   learning the FEC-label binding at the tunnel endpoint.

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

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

   A named proxy-node represents one or more destinations and, for LDP
   forwarding, has a FEC associated with it that is signalled 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.ietf-ospf-mrt].  The details of what label-bindings
   must be originated are described in [I-D.ietf-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 straightforward.  The details of the simple constant-time
   functions, Select_Proxy_Node_NHs() and
   Select_Alternates_Proxy_Node(), are given in
   [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] (see [RFC2328] Section 16.5] 16.5) does incremental
   updates for new summary-LSAs.

   The key remaining question is how to attach the named proxy-node to the
   MRT Island; all the routers in the MRT Island MUST do this
   consistently.  No more than 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 in
   the MRT Island that are advertising the prefix; the named-proxy-cost
   assigned to each prefix-advertising router is the announced cost to
   the prefix.  The second set are those routers in the MRT Island that
   are connected to routers not in the MRT Island but in the same area/level; area/
   level; such routers will be defined as Island Border Routers (IBRs).
   The routers connected to the IBRs that are not in the MRT Island and
   are in the same area/level as the MRT island are Island
   Neighbors(INs).

   Since packets sent to the named proxy-node along MRT-Red or MRT-Blue
   may come from any router inside the MRT Island, it is necessary that
   whatever router to which an IBR forwards the packet be loop-free with
   respect to the whole MRT Island for the destination.  Thus, an IBR is
   a candidate router only if it possesses at least one IN whose
   shortest path to the prefix does not enter the MRT Island.  A method
   for identifying loop-free Island Neighbors(LFINs) is discussed below. given in
   [I-D.ietf-rtgwg-mrt-frr-algorithm].  The named-proxy-cost assigned to
   each (IBR, IN) pair is cost(IBR, IN) + D_opt(IN, prefix).

   From the set of prefix-advertising routers and the set of IBRs with
   at least one LFIN, the two routers with the lowest named-proxy-cost
   are selected.  Ties are broken based upon the lowest Router ID.  For
   ease of discussion, the two selected routers will be referred to as
   proxy-node attachment routers.

   A proxy-node attachment router has a special forwarding role.  When a
   packet is received destined to (MRT-Red, prefix) or (MRT-Blue,
   prefix), if the proxy-node attachment router is an IBR, it MUST swap
   to the shortest path forwarding topology (e.g. swap to the label for
   (MT-ID 0, prefix) or remove the outer IP encapsulation) and forward
   the packet to the IN whose cost was used in the selection.  If the
   proxy-node attachment router is not an IBR, then the packet MUST be
   removed from the MRT forwarding topology and sent along the
   interface(s) that caused the router to advertise the prefix; this
   interface might be out of the area/level/AS.

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

   As discussed above,

11.3.  MRT Alternates for Destinations Outside the MRT Island Neighbor needs to be loop-free

   A natural concern with
   respect new functionality is how to the whole MRT Island for the destination.  This can have it be
   accomplished by running useful
   when it is not deployed across an entire IGP area.  In the usual SPF algorithm while keeping track case of which shortest paths have passed through the
   MRT island.  Pseudo-
   code for this is shown in Figure 4.  The Island_Marking_SPF() is run
   for each IN that needs FRR, where it provides alternates when appropriate LFAs aren't
   available, there are also deployment scenarios where it may make
   sense to be evaluated for the loop-free condition, only enable some routers in an area with the IN as the spf_root.  Whether MRT FRR.  A simple
   example of such a scenario would be a ring of 6 or not an IN more routers that
   is loop-free with
   respect 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 then be determined by evaluating
   node.PATH_HITS_ISLAND for each destination of interest.

    Island_Marking_SPF(spf_root)
        Initialize spf_heap to empty
        Initialize nodes' spf_metric to infinity and next_hops to empty
            and PATH_HITS_ISLAND to false
        spf_root.spf_metric = 0
        insert(spf_heap, spf_root)
        while (spf_heap is not empty)
            min_node = remove_lowest(spf_heap)
            foreach interface intf of min_node
                path_metric = min_node.spf_metric + intf.metric
                if path_metric < intf.remote_node.spf_metric
                    intf.remote_node.spf_metric = path_metric
                    if min_node is spf_root
                        intf.remote_node.next_hops = make_list(intf)
                    else
                        intf.remote_node.next_hops = min_node.next_hops
                    if intf.remote_node.IN_MRT_ISLAND
                        intf.remote_node.PATH_HITS_ISLAND = true
                    else
                        intf.remote_node.PATH_HITS_ISLAND =
                            min_node.PATH_HITS_ISLAND
                    insert_or_update(spf_heap, intf.remote_node)
                else if path_metric == intf.remote_node.spf_metric
                    if min_node is spf_root
                        add_to_list(intf.remote_node.next_hops, intf)
                    else
                        add_list_to_list(intf.remote_node.next_hops,
                                         min_node.next_hops)
                    if intf.remote_node.IN_MRT_ISLAND
                        intf.remote_node.PATH_HITS_ISLAND = true
                    else
                        intf.remote_node.PATH_HITS_ISLAND =
                            min_node.PATH_HITS_ISLAND

                                 Figure 4

   It is also possible that a given prefix is originated by a
   combination of non-island routers and island routers.  The results of
   the Island_Marking_SPF computation can be used to determine if the
   shortest path from an IN to reach that prefix hits the MRT island.
   The shortest path for the IN to reach prefix P is determined by the
   total cost to reach prefix P, which is the sum of the cost for the IN
   to reach a prefix-advertising node and the cost with which that node
   advertises the prefix.  The path with the minimum total cost to
   prefix P is chosen.  If the prefix-advertising node for that minimum
   total cost path has PATH_HITS_ISLAND set to True, then the IN is not
   loop-free with respect to the MRT Island for reaching prefix P.  If
   there multiple minimum total cost paths to reach prefix P, then all
   of the prefix-advertising routers involved in the minimum total cost
   paths MUST have PATH_HITS_ISLAND set to False for the IN to be
   considered loop-free to reach P.

   Note that there are other computations that could be used to
   determine if paths from a given IN _might_ pass through the MRT
   Island for a given prefix or destination.  For example, a previous
   version of this draft specified running the SPF algorithm on modified
   topology which treats the MRT island as a single node (with intra-
   island links set to zero cost) in order to provide input to
   computations to determine if the path from IN to non-island
   destination hits the MRT island in this modified topology.  This
   computation is enough to guarantee that a path will not hit the MRT
   island in the original topology.  However, it is possible that a path
   which is disqualified for hitting the MRT island in the modified
   topology will not actually hit the MRT Island in the original
   topology.  The algorithm described in Island_Marking_SPF() above does
   not modify the original topology, and will only disqualify a path if
   the actual path does in fact hit the MRT island.

   Since all routers need to come to the same conclusion about which
   routers qualify as LFINs, this specification requires that all
   routers computing LFINs MUST use an algorithm whose result is
   identical to that of the Island_Marking_SPF() in Figure 4.

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 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.

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.

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]. [RFC6976].  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 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 for the Default MRT Profile

   This section describes how the MRT recalculation SHOULD be performed
   for the Default MRT Profile.  This is intended to support FRR
   applications.  Other approaches are possible, but they are not
   specified in this document.

   When a failure event happens, traffic is put by the PLRs onto the MRT
   topologies.  After that, each router recomputes its 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 be, a catalog of available implementations or their
   features.  Readers are 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 benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the 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 OSPF as
      the IGP has been implemented and verified.

   o  The implementation's level of maturity: prototype

   o  Protocol coverage: This implementation of the MRT 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 M-bit in OSPF based on "draft-
      atlas-ospf-mrt-01" as well as LDP MRT Capability based on "draft-
      atlas-mpls-ldp-mrt-00".

   o  Licensing: proprietary

   o  Implementation experience: Implementation was 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 verified.

   o  The implementation's level of maturity: prototype

   o  Protocol coverage: This implementation of the MRT 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-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 verify the algorithm is implemented correctly
      without looping.  It is important to verify the ISIS extensions
      work for MRT-FRR.

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

14.  Operations and Management Considerations

   An implementation SHOULD provide an operator with the ability to test
   MRT paths with OAM traffic.  For example, when MRT paths are realized
   using LDP labels distributed for topology-scoped FECs, an
   implementation can use the MPLS ping and traceroute as defined in
   [RFC4379] and extended in [RFC7307] for topology-scoped FECs.

15.  Applying Policy to Select from Multiple Possible Alternates for FRR

   For a given topology, GADAG root, and profile, MRT will provide a
   node-protecting alternate path from each PLR to each destination for
   any single link or node failure, if such a path exists.  Therefore,
   an implementation may choose to only use the alternates determined by
   MRT to provide 100% FRR coverage.

   However, it may be desirable to allow an operator to use MRT
   alternates together with alternates provided by other FRR
   technologies.  A policy-based alternate selection process can allow
   an operator to select the best alternate from those provided by MRT
   and other FRR technologies.  As an example, it may be desirable to
   implement a policy where a node-protecting LFA (if it exists for a
   given failure mode and destination) is preferred over a given MRT algorithm
      includes Island identification, GADAG root selection, Lowpoint
      algorithm, augmentation of GADAG with additional links, and
      calculation
   alternate.  [I-D.ietf-rtgwg-lfa-manageability] discusses many 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 the
   potential criteria that one might take into account when evaluating
   different alternates for selection.

   Note that future documents may define MRT based on
      "draft-li-mrt-00".

   o  Licensing: proprietary

   o  Implementation experience: It is important profiles in addition to the
   default profile defined here.  Different MRT profiles will generally
   produce a second alternate paths with different properties.  An implementation
   may allow an operator to verify the algorithm is implemented correctly
      without looping.  It is important use different MRT profiles instead of or in
   addition to verify the ISIS extensions
      work for MRT-FRR.

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

14. default profile.

16.  Acknowledgements

   The authors would like to thank Mike Shand for his valuable review
   and contributions.

   The authors would like to thank Joel Halpern, Hannes Gredler, Ted
   Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin
   Bahadur, Harish Sitaraman, Raveendra Torvi, Anil Kumar SN, Bruno
   Decraene, Eric Wu, and Janos Farkas for their suggestions and review.

15.

17.  IANA Considerations

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

16.

18.  Security Considerations

   This architecture is

   In general, MRT forwarding paths do not currently believed follow shortest paths.  The
   transit forwarding state corresponding to introduce new the MRT paths is created
   during normal operations (before a failure occurs).  Therefore, a
   malicious packet with an appropriate header injected into the network
   from a compromised location would be forwarded to a destination along
   a non-shortest path.  When this technology is deployed, a network
   security
   concerns.

17. design should not rely on assumptions about potentially
   malicious traffic only following shortest paths.

   It should be noted that the creation of non-shortest forwarding paths
   is not unique to MRT.

19.  Contributors

      Robert Kebler
      Juniper Networks
      10 Technology Park Drive
      Westford, MA  01886
      USA
      Email: rkebler@juniper.net

      Andras Csaszar
      Ericsson
      Konyves Kalman krt 11
      Budapest  1097
      Hungary
      Email: Andras.Csaszar@ericsson.com

      Jeff Tantsura
      Ericsson
      300 Holger Way
      San Jose, CA  95134
      USA
      Email: jeff.tantsura@ericsson.com

      Russ White
      VCE
      Email: russw@riw.us

20.  References

17.1.

20.1.  Normative References

   [I-D.ietf-rtgwg-mrt-frr-algorithm]
              Envedi, G., Csaszar, A., Atlas, A., Bowers, C., and A.
              Gopalan, "Algorithms for computing Maximally Redundant
              Trees for IP/LDP Fast- Reroute", draft-ietf-rtgwg-mrt-frr-
              algorithm-05
              algorithm-06 (work in progress), July October 2015.

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

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

17.2.

20.2.  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-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.francois-spring-segment-routing-ti-lfa]

   [I-D.francois-rtgwg-segment-routing-ti-lfa]
              Francois, P., Filsfils, C., Bashandy, A., and B. Decraene,
              "Topology Independent Fast Reroute using Segment Routing",
              draft-francois-spring-segment-routing-ti-lfa-01
              draft-francois-rtgwg-segment-routing-ti-lfa-00 (work in
              progress), October 2014. August 2015.

   [I-D.ietf-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-ietf-isis-mrt-00
              draft-ietf-isis-mrt-01 (work in progress), February October 2015.

   [I-D.ietf-mpls-ldp-mrt]
              Atlas, A., Tiruveedhula, K., Bowers, C., Tantsura, J., and
              I. Wijnands, "LDP Extensions to Support Maximally
              Redundant Trees", draft-ietf-mpls-ldp-mrt-00 draft-ietf-mpls-ldp-mrt-02 (work in
              progress), January September 2015.

   [I-D.ietf-ospf-mrt]
              Atlas, A., Hegde, S., Bowers, C., Tantsura, J., and Z. Li,
              "OSPF Extensions to Support Maximally Redundant Trees",
              draft-ietf-ospf-mrt-00
              draft-ietf-ospf-mrt-01 (work in progress), January October 2015.

   [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,

   [I-D.ietf-rtgwg-lfa-manageability]
              Litkowski, S., Decraene, B., Filsfils, C.,
              Francois, P., Raza, K.,
              Horneffer, M., and O. Bonaventure, "Framework for Loop-free
              convergence using oFIB", draft-ietf-rtgwg-ordered-fib-12 P. Sarkar, "Operational management of
              Loop Free Alternates", draft-ietf-rtgwg-lfa-
              manageability-11 (work in progress), May 2013. June 2015.

   [I-D.ietf-rtgwg-rlfa-node-protection]
              Sarkar, P., Gredler, H., Hegde, S., Bowers, C., Litkowski,
              S., Gredler, H., and H. Raghuveer, S.
              Litkowski, "Remote-LFA Node Protection and Manageability", draft-ietf-rtgwg-rlfa-node-protection-02
              draft-ietf-rtgwg-rlfa-node-protection-05 (work in
              progress), June December 2015.

   [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>.

   [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,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <http://www.rfc-editor.org/info/rfc2328>.

   [RFC3137]  Retana, A., Nguyen, L., White,

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              DOI 10.17487/RFC4379, February 2006,
              <http://www.rfc-editor.org/info/rfc4379>.

   [RFC5331]  Aggarwal, R., Zinin, A., Rekhter, Y., and D.
              McPherson, "OSPF Stub Router Advertisement", E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              RFC 3137, 5331, DOI 10.17487/RFC3137, June 2001,
              <http://www.rfc-editor.org/info/rfc3137>. 10.17487/RFC5331, August 2008,
              <http://www.rfc-editor.org/info/rfc5331>.

   [RFC5443]  Jork, M., Atlas, A., and L. Fang, "LDP IGP
              Synchronization", RFC 5443, DOI 10.17487/RFC5443, March
              2009, <http://www.rfc-editor.org/info/rfc5443>.

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

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, DOI 10.17487/RFC5715, January
              2010, <http://www.rfc-editor.org/info/rfc5715>.

   [RFC6571]  Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
              B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
              Alternate (LFA) Applicability in Service Provider (SP)
              Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
              <http://www.rfc-editor.org/info/rfc6571>.

   [RFC6976]  Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
              Francois, P., and O. Bonaventure, "Framework for Loop-Free
              Convergence Using the Ordered Forwarding Information Base
              (oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
              2013, <http://www.rfc-editor.org/info/rfc6976>.

   [RFC6981]  Bryant, S., Previdi, S., and M. Shand, "A Framework for IP
              and MPLS Fast Reroute Using Not-Via Addresses", RFC 6981,
              DOI 10.17487/RFC6981, August 2013,
              <http://www.rfc-editor.org/info/rfc6981>.

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982,
              DOI 10.17487/RFC6982, July 2013,
              <http://www.rfc-editor.org/info/rfc6982>.

   [RFC6987]  Retana, A., Nguyen, L., Zinin, A., White, R., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 6987,
              DOI 10.17487/RFC6987, September 2013,
              <http://www.rfc-editor.org/info/rfc6987>.

   [RFC7307]  Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
              King, "LDP Extensions for Multi-Topology", RFC 7307,
              DOI 10.17487/RFC7307, July 2014,
              <http://www.rfc-editor.org/info/rfc7307>.

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,
              <http://www.rfc-editor.org/info/rfc7490>.

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: 4: AS external prefixes in different areas

   Consider the network in Figure 5 4 and assume there is a richer
   connective topology that isn't shown, where the same prefix is
   announced by ASBR X and ASBR Y which are in different non-backbone
   areas.  If the link from A to ASBR X fails, then an MRT alternate
   could forward the packet to ABR 1 and ABR 1 could forward it to D,
   but then D would find the shortest route is back via ABR 1 to Area
   20.  This problem occurs because the routers, including the ABR, in
   one area are not yet aware of the failure in a different area.

   The only way to get it from A to ASBR Y is to explicitly tunnel it to
   ASBR Y.  If the traffic is unlabeled or the appropriate MPLS labels
   are known, then explicit tunneling MAY be used as long as the
   shortest-path of the tunnel avoids the failure point.  In that case,
   A must determine that it should use an explicit tunnel instead of an
   MRT alternate.

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

   Russ White
   VCE

   Email: russw@riw.us