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Versions: (draft-atlas-rtgwg-mrt-frr-architecture) 00 01 02 03 04 05 06 07 08 09 10 RFC 7812

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


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

Abstract

   With increasing deployment of Loop-Free Alternates (LFA) [RFC5286],
   it is clear that a complete solution for IP and LDP Fast-Reroute is
   required.  This specification provides that solution.  IP/LDP Fast-
   Reroute with Maximally Redundant Trees (MRT-FRR) is a technology that
   gives link-protection and node-protection with 100% coverage in any
   network topology that is still connected after the failure.

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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




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Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Importance of 100% Coverage . . . . . . . . . . . . . . .   4
     1.2.  Partial Deployment and Backwards Compatibility  . . . . .   5
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   6
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . .   7
   5.  Maximally Redundant Trees (MRT) and Fast-Reroute  . . . . . .   9
   6.  Unicast Forwarding with MRT Fast-Reroute  . . . . . . . . . .  10
     6.1.  LDP Unicast Forwarding - Avoid Tunneling  . . . . . . . .  10
     6.2.  IP Unicast Traffic  . . . . . . . . . . . . . . . . . . .  11
   7.  Protocol Extensions and Considerations: OSPF and ISIS . . . .  12
   8.  Protocol Extensions and considerations: LDP . . . . . . . . .  14
   9.  Inter-Area and ABR Forwarding Behavior  . . . . . . . . . . .  15
   10. Prefixes Multiply Attached to the MRT Island  . . . . . . . .  18
     10.1.  Endpoint Selection . . . . . . . . . . . . . . . . . . .  19
     10.2.  Named Proxy-Nodes  . . . . . . . . . . . . . . . . . . .  21
       10.2.1.  Computing if an Island Neighbor (IN) is loop-free  .  22
     10.3.  MRT Alternates for Destinations Outside the MRT Island .  23
   11. Network Convergence and Preparing for the Next Failure  . . .  24
     11.1.  Micro-forwarding loop prevention and MRTs  . . . . . . .  24
     11.2.  MRT Recalculation  . . . . . . . . . . . . . . . . . . .  24
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  25
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  25
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     15.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Appendix A.  General Issues with Area Abstraction . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28





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1.  Introduction

   This document gives a complete solution for IP/LDP fast-reroute
   [RFC5714].  MRT-FRR creates two alternate trees separate from the
   primary next-hop forwarding used during stable operation.  These two
   trees are maximally diverse from each other, providing link and node
   protection for 100% of paths and failures as long as the failure does
   not cut the network into multiple pieces.  This document defines the
   architecture for IP/LDP fast-reroute with MRT.  The associated
   protocol extensions are defined in [I-D.atlas-ospf-mrt] and
   [I-D.atlas-mpls-ldp-mrt].  The exact MRT algorithm is defined in
   [I-D.enyedi-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




















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

                                  Table 1

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

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

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

1.1.  Importance of 100% Coverage




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   Fast-reroute is based upon the single failure assumption - that the
   time between single failures is long enough for a network to
   reconverge and start forwarding on the new shortest paths.  That does
   not imply that the network will only experience one failure or
   change.

   It is straightforward to analyze a particular network topology for
   coverage.  However, a real network does not always have the same
   topology.  For instance, maintenance events will take links or nodes
   out of use.  Simply costing out a link can have a significant effect
   on what LFAs are available.  Similarly, after a single failure has
   happened, the topology is changed and its associated coverage.
   Finally, many networks have new routers or links added and removed;
   each of those changes can have an effect on the coverage for
   topology-sensitive methods such as LFA and Remote LFA.  If fast-
   reroute is important for the network services provided, then a method
   that guarantees 100% coverage is important to accomodate natural
   network topology changes.

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

   When a network needs to use a micro-loop prevention mechanism
   [RFC5715] such as Ordered FIB[I-D.ietf-rtgwg-ordered-fib] or Farside
   Tunneling[RFC5715], then the whole IGP area needs to have alternates
   available so that the micro-loop prevention mechanism, which requires
   slower network convergence, can take the necessary time without
   impacting traffic badly.  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.



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2.  Requirements Language

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

3.  Terminology

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

   Redundant Trees (RT):   A pair of trees where the path from any node
      X to the root R along the first tree is node-disjoint with the
      path from the same node X to the root along the second tree.
      These can be computed in 2-connected graphs.

   Maximally Redundant Trees (MRT):   A pair of trees where the path
      from any node X to the root R along the first tree and the path
      from the same node X to the root along the second tree share the
      minimum number of nodes and the minimum number of links.  Each
      such shared node is a cut-vertex.  Any shared links are cut-links.
      Any RT is an MRT but many MRTs are not RTs.

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

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

   Rainbow MRT:   It is useful to have an MT-ID that refers to the
      multiple MRT topologies and to the default topology.  This is
      referred to as the Rainbow MRT MT-ID and is used by LDP to reduce
      signaling and permit the same label to always be advertised to all
      peers for the same (MT-ID, Prefix).

   MRT Island:   From the computing router, the set of routers that
      support a particular MRT profile and are connected.

   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.



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   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 are directed spanning trees that
   provide maximally disjoint paths towards their common root.  Only
   links or nodes whose failure would partition the network (i.e. cut-
   links and cut-vertices) are shared between the trees.  The algorithm
   to compute MRTs is given in [I-D.enyedi-rtgwg-mrt-frr-algorithm].
   This algorithm can be computed in O(e + n log n); it is less than



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   three SPFs.  Modeling results comparing MRT alternates to the optimal
   are described in [I-D.enyedi-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]    |
                       |     |    |     |     |



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                       |     |    |     |----[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] or
   [I-D.ietf-rtgwg-remote-lfa]), 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.enyedi-rtgwg-mrt-frr-algorithm] describes exactly how
   to determine whether the MRT-Blue next-hops or the MRT-Red next-hops



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   should be the MRT alternate next-hops for a particular primary next-
   hop N to a particular destination D.

   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

   With LFA, there is no need to tunnel unicast traffic, whether IP or
   LDP.  The traffic is simply sent to an alternate.  As mentioned
   earlier in Section 5, MRT 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 are seen by the
   forwarding plane as essentially two additional topologies.  The same
   considerations apply for forwarding along the MRTs as for handling
   multiple topologies.

6.1.  LDP Unicast Forwarding - Avoid Tunneling

   For LDP, it is very desirable to avoid tunneling because, for at
   least node protection, tunneling requires knowledge of remote LDP
   label mappings and thus requires targeted LDP sessions and the
   associated management complexity.  There are two different mechanisms
   that can be used; Option A MUST be supported.

   1.  Option A - Encode MT-ID in Labels: In addition to sending a
       single label for a FEC, a router would provide two additional
       labels with the MT-IDs associated with the Blue MRT or Red MRT
       forwarding topologies.  This is very simple for hardware support.
       It does reduce the label space for other uses.  It also increases
       the memory to store the labels and the communication required by
       LDP.

   2.  Option B - Create Topology-Identification Labels: Use the label-
       stacking ability of MPLS and specify only two additional labels -



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       one for each associated MRT color - by a new FEC type.  When
       sending a packet onto an MRT, first swap the LDP label and then
       push the topology-identification label for that MRT color.  When
       receiving a packet with a topology-identification label, pop it
       and use it to guide the next-hop selection in combination with
       the next label in the stack; then swap the remaining label, if
       appropriate, and push the topology-identification label for the
       next-hop.  This 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 can use the same mechanisms as are needed for context-aware
       label spaces.

   Note that with LDP unicast forwarding, regardless of whether
   topology-identification label or encoding topology in label is used,
   no additional loopbacks per router are required.  This is because LDP
   labels are used on a hop-by-hop basis to identify MRT-blue and MRT-
   red forwading topologies.

   For greatest hardware compatibility, routers implementing MRT LDP
   fast-reroute MUST support Option A of encoding the MT-ID in the
   labels.  The extensions to indicate an MT-ID for a FEC are described
   in Section 3.2.1 of [I-D.ietf-mpls-ldp-multi-topology].

6.2.  IP Unicast Traffic

   For IP, there is no currently practical alternative except tunneling
   to gain the bits needed to indicate the MRT-Blue or MRT-Red
   forwarding topology.  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 10 for handling of multi-homed
   prefixes); another possible choice is the next-next-hop towards the
   destination.  For LDP traffic, using 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 but
   consistency with LDP is RECOMMENDED.  If the tunnel egress is the
   original destination router, then the traffic remains on the
   redundant tree with sub-optimal routing.  Selection of the tunnel
   egress is a router-local decision.

   There are three options available for marking IP packets with which
   MRT it should be forwarded in.  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 Option A - using an LDP label that indicates the
   destination and MT-ID.




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   1.  Tunnel IP packets via an LDP LSP.  This has the advantage that
       more installed routers can do line-rate encapsulation and
       decapsulation.  Also, no additional IP addresses would need to be
       allocated or signaled.

       a.  Option A - LDP Destination-Topology Label: Use a label that
           indicates both destination and MRT.  This method allows easy
           tunneling to the next-next-hop as well as to the IGP-area
           destination.  For a proxy-node, the destination to use is the
           non-proxy-node immediately before the proxy-node on that
           particular color MRT.

       b.  Option B - LDP Topology Label: Use a Topology-Identifier
           label on top of the IP packet.  This is very simple.  If
           tunneling to a next-next-hop is desired, then a two-deep
           label stack can be used with [ Topology-ID label, Next-Next-
           Hop Label ].

   2.  Tunnel IP packets in IP.  Each router supporting this option
       would announce 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.  They
       allow the transit nodes to identify the traffic as being
       forwarded along either MRT-blue or MRT-red tree topology to reach
       the tunnel destination.  Announcements of these two additional
       loopback addresses per router with their MRT color requires IGP
       extensions.

7.  Protocol Extensions and Considerations: OSPF and ISIS

   For simplicity, the approach of defining a well-known profile is
   taken in [I-D.atlas-ospf-mrt].  The purpose of communicating support
   for MRT in the IGP is to indicate thatqq the MRT-Blue and MRT-Red
   forwarding topologies are created for transit traffic.  This section
   describes the various options to be selected.  The default MRT
   profile is described here and the signaling extensions for OSPF are
   given in [I-D.atlas-ospf-mrt].

   For any MRT profile, the MRT Island is created by starting from the
   computing router.  If the computing router supports the default MRT
   profile, add it to the MRT Island.  Add a router to the MRT Island if
   the router supports the default MRT profile and is connected to the
   MRT Island via bidirectional links eligible for MRT.

   If a router advertises support for multiple MRT profiles, then it
   MUST create the transit forwarding topologies for each of those,
   unless the profile specifies No Forwarding Mechanism (e.g. as might
   be done for a profile used only for multicast global protection).  A



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   router MUST NOT advertise multiple MRT profiles that overlap in their
   MRT-Red MT-ID or MRT-Blue MT-ID.

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

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

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

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

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

   Forwarding Mechanisms:   LDP

   Recalculation:   Recalculation of MRTs SHOULD occur as described in
      Section 11.2.  This allows the MRT forwarding topologies to
      support IP/LDP fast-reroute traffic.

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

   The following describes the aspects to be considered to define a
   profile to advertise.  For some profiles, associated information may
   need to be distributed, such as GADAG Root Selection Priority, Red
   MRT Loopback Address, Blue MRT Loopback Address.

   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.





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   GADAG Root Selection Priority:   A MRT profile might specify this to
      provide the network operator with a knob to force a particular
      GADAG root selection.  If not specified in the MRT profile, the
      highest Router ID in the profile's MRT Island will be elected the
      GADAG Root.  If a GADAG Root Selection Priority is specified, then
      the MRT profile must also specify how the GADAG Root is elected.

   Forwarding Mechanism:   This specifies which forwarding mechanisms
      the router supports for transit traffic.  An MRT island must
      program appropriate next-hops into the forwarding plane.  The
      known options are IPv4, IPv6, LDP, and None.  If IPv4 is
      supported, then both MRT-Red and MRT-Blue IPv4 Loopback Addresses
      SHOULD be specified.  If IPv6 is supported, both MRT-Red and MRT-
      Blue IPv6 Loopback Addresses SHOULD be specified.  If LDP is
      supported, then LDP support and signaling extensions MUST be
      supported.

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

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

   Recalculation:   As part of what process and timing should the new
      MRTs be computed on a modified topology?  Section 11.2 describes
      the minimum behavior required to support fast-reroute.

   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.

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

8.  Protocol Extensions and considerations: LDP








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   The protocol extensions for LDP are defined in
   [I-D.atlas-mpls-ldp-mrt].  A router must indicate that it has the
   ability to support MRT; having this explicit allows the use of MRT-
   specific processing, such as special handling of FECs sent with the
   Rainbow MRT MT-ID.

   A FEC sent with the Rainbow MRT MT-ID indicates that the FEC applies
   to all the MRT-Blue and MRT-Red MT-IDs in supported MRT profiles as
   well as to the default shortest-path based MT-ID 0.  The Rainbow MRT
   MT-ID is defined to provide an easy way to handle the special
   signaling that is needed at ABRs or LBRs.  It avoids the problem of
   needing to signal different MPLS labels for the same FEC.  Because
   the Rainbow MRT MT-ID is used only by ABRs/LBRs or the LDP egress, it
   is not MRT profile specific.  The proposed experimental value is 3999
   and the final value will be assigned by IANA and allocated from the
   LDP MT-ID space.  The authoritative values are given in
   [I-D.atlas-mpls-ldp-mrt].

9.  Inter-Area and ABR Forwarding Behavior

   An ABR/LBR has two forwarding roles.  First, it forwards traffic
   inside its area.  Second, it forwards traffic from one area into
   another.  These same two roles apply for MRT transit traffic.
   Traffic on MRT-Red or MRT-Blue destined inside the area needs to stay
   on MRT-Red or MRT-Blue in that area.  However, it is desirable for
   traffic leaving the area to also exit MRT-Red or MRT-Blue back to the
   shortest-path forwarding.

   For unicast MRT-FRR, the need to stay on an MRT forwarding topology
   terminates at the ABR/LBR whose best route is via a different area/
   level.  It is highly desirable to go back to the default forwarding
   topology when leaving an area/level.  There are three basic reasons
   for this.  First, the default topology uses shortest paths; the
   packet will thus take the shortest possible route to the destination.
   Second, this allows failures that might appear in multiple areas
   (e.g. ABR/LBR failures) to be separately identified and repaired
   around.  Third, the packet can be fast-rerouted again, if necessary,
   due to a failure in a different area.

   An ABR/LBR that receives a packet on MRT-Red or MRT-Blue towards a
   destination in another area/level should forward the packet in the
   area/level with the best route along MRT-Red or MRT-Blue.  If the
   packet came from that area/level, this correctly avoids the failure.
   However, if the traffic came from a different area/level, the packet
   should be removed from MRT-Red or MRT-Blue and forwarded on the
   shortest-path default forwarding topology.





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   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.

   For LDP forwarding where the MPLS label specifies (MT-ID, FEC), the
   ABR/LBR is responsible for advertising the proper label to each
   neighbor.  Assume that an ABR/LBR has allocated three labels for a
   particular destination; those labels are L_primary, L_blue, and
   L_red.  When the ABR/LBR advertises label bindings to routers in the
   area with the best route to the destination, the ABR/LBR provides
   L_primary for the default topology, L_blue for the MRT-Blue MT-ID and
   L_red for the MRT-Red MT-ID, exactly as expected.  However, when the
   ABR/LBR advertises label bindings to routers in other areas, the ABR/
   LBR advertises L_primary for the Rainbow MRT MT-ID, which is then
   used for the default topology, for the MRT-Blue MT-ID and for the
   MRT-Red MT-ID.

   The ABR/LBR installs all next-hops from 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.

   If IP forwarding is used, then the ABR/LBR behavior is dependent upon
   the outermost IP address.  If the outermost IP address is an MRT
   loopback address of the ABR/LBR, then the packet is decapsulated and
   forwarded based upon the inner IP address, which should go on the
   default SPT topology.  If the outermost IP address is not an MRT
   loopback address of the ABR/LBR, then the packet is simply forwarded
   along the associated forwarding topology.  A PLR sending traffic to a
   destination outside its local area/level will pick the MRT and use
   the associated MRT loopback address of the selected ABR/LBR connected
   to the external destination.

   Thus, regardless of which of these two forwarding mechanisms are
   used, there is no need for additional computation or per-area
   forwarding state.


       +----[C]----     --[D]--[E]                --[D]--[E]
       |           \   /         \               /         \
   p--[A] Area 10 [ABR1]  Area 0 [H]--p   +-[ABR1]  Area 0 [H]-+
       |           /   \         /        |      \         /   |
       +----[B]----     --[F]--[G]        |       --[F]--[G]   |



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                                          |                    |
                                          | 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

   The other forwarding mechanism described in Section 6 is using
   Topology-Identification Labels.  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 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.





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   In all cases for ISIS and most cases for OSPF, the penultimate router
   can determine what decision the adjacent ABR will make.  The one case
   where it can't be determined is when two ASBRs are in different non-
   backbone areas attached to the same ABR, then the ASBR's Area ID may
   be needed for tie-breaking (prefer the route with the largest OPSF
   area ID) and the Area ID isn't announced as part of the ASBR link-
   state advertisement (LSA).  In this one case, suboptimal forwarding
   along the MRT in the other area would happen.  If that becomes a
   realistic deployment scenario, OSPF extensions could be considered.
   This is not covered in [I-D.atlas-ospf-mrt].

10.  Prefixes Multiply Attached to the MRT Island

   How a computing router S determines its local MRT Island for each
   supported MRT profile is already discussed in Section 7.

   There are two types of prefixes or FECs that may be multiply attached
   to an MRT Island.  The first type are multi-homed prefixes that
   usually connect at a domain or protocol boundary.  The second type
   represent routers that do not support the profile for the MRT Island.
   The key difference is whether the traffic, once out of the MRT
   Island, remains in the same area/level and might reenter the MRT
   Island if a loop-free exit point is not selected.

   One property of LFAs that is necessary to preserve is the ability to
   protect multi-homed prefixes against ABR failure.  For instance, if a
   prefix from the backbone is available via both ABR A and ABR B, if A
   fails, then the traffic should be redirected to B. This can also be
   done for backups via MRT.

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

   As discussed in [RFC5286], a multi-homed prefix could be:

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

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

   o  A prefix with iBGP multipath to different ASBRs,

   o  etc.

   There are also two different approaches to protection.  The first is
   to do endpoint selection to pick a router to tunnel to where that
   router is loop-free with respect to the failure-point.  Conceptually,



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   the set of candidate routers to provide LFAs expands to all routers,
   with 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 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
   endpoint.

   If a FEC is advertised from outside the MRT Island into the MRT
   Island and the forwarding mechanism specified in the profile includes
   LDP, then the routers learning that FEC MUST also advertise labels
   for (MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors inside the MRT
   Island.  If the forwarding mechanism includes LDP, any 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 associated FECs ( (MT-ID 0,
   FEC), (MRT-Red, FEC), and (MRT-Blue, FEC)) MUST also be provided via
   LDP to neighbors inside the MRT Island.

10.1.  Endpoint Selection

   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.



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   The candidates for endpoint selection are those to which the
   destination prefix is attached in the area/level.  For a particular
   candidate B, it is necessary to determine if B is loop-free to reach
   p with respect to S and F for node-protection or at least with
   respect to S and the link (S, F) for link-protection.  If B will
   always prefer to send traffic to p via a different area/level, then
   this is definitional.  Otherwise, distance-based computations are
   necessary and an SPF from B's perspective may be necessary.  The
   following equations give the checks needed; the rationale is similar
   to that given in [RFC5286].

   Loop-Free for S: D_opt(B, p) < D_opt(B, S) + D_opt(S, p)

   Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(F, p)

   The latter is equivalent to the following, which avoids the need to
   compute the shortest path from F to p.

   Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(S, p) - D_opt(S,
   F)

   Finally, the rules for Endpoint selection are given below.  The basic
   idea is to repair to the prefix-advertising router selected for the
   shortest-path and only to select and tunnel to a different endpoint
   if necessary (e.g. A=F or F is a cut-vertex or the link (S,F) is a
   cut-link).

   1.  Does S have a node-protecting alternate to A?  If so, select
       that.  Tunnel the packet to A along that alternate.  For example,
       if LDP is the forwarding mechanism, then push the label (MRT-Red,
       A) or (MRT-Blue, A) onto the packet.

   2.  If not, then is there a router B that is loop-free to reach p
       while avoiding both F and S?  If so, select B as the end-point.
       Determine the MRT alternate to reach B while avoiding F.  Tunnel
       the packet to B along that alternate.  For example, with LDP,
       push the label (MRT-Red, B) or (MRT-Blue, B) onto the packet.

   3.  If not, then does S have a link-protecting alternate to A?  If
       so, select that.

   4.  If not, then is there a router B that is loop-free to reach p
       while avoiding S and the link from S to F?  If so, select B as
       the endpoint and the MRT alternate that for reaching B from S
       avoiding the link (S,F).

   The endpoint selected will receive a packet destined to itself and,
   being the egress, will pop that MPLS label (or have signaled Implicit



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   Null) and forward based on what is underneath.  This suffices for IP
   traffic where the MPLS labels understood by the endpoint router are
   not needed.

10.2.  Named Proxy-Nodes

   A clear advantage to using a named proxy-node is that it is possible
   to explicitly forward from the MRT Island along an interface to a
   loop-free island neighbor (LFIN) when that interface may not be a
   primary next-hop.  For LDP traffic where the label indicates both the
   topology and the FEC, it is necessary to either use a named proxy-
   node or deal with learning remote MPLS labels.

   A named proxy-node represents one or more destinations and, for LDP
   forwarding, has a FEC associated with it that is signaled into the
   MRT Island.  Therefore, it is possible to explicitly label packets to
   go to (MRT-Red, FEC) or (MRT-Blue, FEC); at the border of the MRT
   Island, the label will swap to meaning (MT-ID 0, FEC).  It would be
   possible to have named proxy-nodes for IP forwarding, but this would
   require extensions to signal two IP addresses to be associated with
   MRT-Red and MRT-Blue for the proxy-node.  A named proxy-node can be
   uniquely represented by the two routers in the MRT Island to which it
   is connected.  The extensions to signal such IP addresses are not
   defined in [I-D.atlas-ospf-mrt].  The details of what label-bindings
   must be originated are described in [I-D.atlas-mpls-ldp-mrt].

   Computing the MRT next-hops to a named proxy-node and the MRT
   alternate for the computing router S to avoid a particular failure
   node F is extremely straightforward.  The details of the simple
   constant-time functions, Select_Proxy_Node_NHs() and
   Select_Alternates_Proxy_Node(), are given in
   [I-D.enyedi-rtgwg-mrt-frr-algorithm].  A key point is that computing
   these MRT next-hops and alternates can be done as new named proxy-
   nodes are added or removed without requiring a new MRT computation or
   impacting other existing MRT paths.  This maps very well to, for
   example, how OSPFv2 [[RFC2328] Section 16.5] does incremental updates
   for new summary-LSAs.

   The key question is how to attach the named proxy-node to the MRT
   Island; all the routers in the MRT Island MUST do this consistently.
   No more than 2 routers in the MRT Island can be selected; one should
   only be selected if there are no others that meet the necessary
   criteria.  The named proxy-node is logically part of the area/level.

   There are two sources for candidate routers in the MRT Island to
   connect to the named proxy-node.  The first set are those routers
   that are advertising the prefix; the cost assigned to each such
   router is the announced cost to the prefix.  The second set are those



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   routers in the MRT Island that are connected to routers not in the
   MRT Island but in the same area/level; such routers will be defined
   as Island Border Routers (IBRs).  The routers connected to the IBRs
   that are not in the MRT Island and are in the same area/level 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
   regard to the whole MRT Island for the destination.  Thus, an IBR is
   a candidate router only if it possesses at least one IN whose path to
   the prefix does not enter the MRT Island.  The cost assigned to each
   (IBR, IN) pair is the D_opt(IN, prefix) plus Cost(IBR, IN).

   From the set of prefix-advertising routers and the IBRs, the two
   lowest cost routers are selected and ties are broken based upon the
   lowest Router ID.  For ease of discussion, such selected routers are
   proxy-node attachment routers and the two selected will be named A
   and B.

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

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

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


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

      (1) Network Graph with Partial Deployment



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


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

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


   Figure 4: Computing alternates to destinations outside the MRT Island

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

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

10.3.  MRT Alternates for Destinations Outside the MRT Island

   A natural concern with new functionality is how to have it be useful
   when it is not deployed across an entire IGP area.  In the case of
   MRT FRR, where it provides alternates when appropriate LFAs aren't
   available, there are also deployment scenarios where it may make
   sense to only enable some routers in an area with MRT FRR.  A simple
   example of such a scenario would be a ring of 6 or more routers that
   is connected via two routers to the rest of the area.

   Destinations inside the local island can obviously use MRT
   alternates.  Destinations outside the local island can be treated
   like a multi-homed prefix and either Endpoint Selection or Named
   Proxy-Nodes can be used.  Named Proxy-Nodes MUST be supported when
   LDP forwarding is supported and a label-binding for the destination
   is sent to an IBR.

   Naturally, there are more complicated options to improve coverage,
   such as connecting multiple MRT islands across tunnels, but the need
   for the additional complexity has not been justified.



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11.  Network Convergence and Preparing for the Next Failure

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

11.1.  Micro-forwarding loop prevention and MRTs

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

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

   Since MRTs provide an alternate topology through which traffic can be
   sent and which can be manipulated separately from the SPT, it is
   possible that MRTs could be used 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.

11.2.  MRT Recalculation

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



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   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.

12.  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 and Chris Bowers for their
   suggestions and review.

13.  IANA Considerations

   This doument includes no request to IANA.

14.  Security Considerations

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

15.  References

15.1.  Normative References

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

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

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









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15.2.  Informative References

   [EnyediThesis]
              Enyedi, G., "Novel Algorithms for IP Fast Reroute",
              Department of Telecommunications and Media Informatics,
              Budapest University of Technology and Economics Ph.D.
              Thesis, February 2011,
              <http://timon.tmit.bme.hu/theses/thesis_book.pdf>.

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

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

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

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

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

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

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




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

   [I-D.litkowski-rtgwg-node-protect-remote-lfa]
              Litkowski, S., "Node protecting remote LFA", draft-
              litkowski-rtgwg-node-protect-remote-lfa-00 (work in
              progress), April 2013.

   [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, March 1997.

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

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

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

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

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

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.




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          50
        |----[ASBR Y]---[B]---[ABR 2]---[C]      Backbone Area 0:
        |                                |           ABR 1, ABR 2, C, D
        |                                |
        |                                |       Area 20:  A, ASBR X
        |                                |
        p ---[ASBR X]---[A]---[ABR 1]---[D]      Area 10: B, ASBR Y
           5                                  p is a Type 1 AS-external


             Figure 5: AS external prefixes in different areas

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

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

Authors' Addresses

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

   Email: akatlas@juniper.net


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

   Email: rkebler@juniper.net





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

   Email: Gabor.Sandor.Enyedi@ericsson.com


   Andras Csaszar
   Ericsson
   Konyves Kalman krt 11
   Budapest  1097
   Hungary

   Email: Andras.Csaszar@ericsson.com


   Jeff Tantsura
   Ericsson
   300 Holger Way
   San Jose, CA  95134
   USA

   Email: jeff.tantsura@ericsson.com


   Maciek Konstantynowicz
   Cisco Systems

   Email: maciek@bgp.nu


   Russ White
   VCE

   Email: russw@riw.us














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