Routing Area Working Group                                 A. Atlas, Ed.
Internet-Draft                                                 R. Kebler
Intended status: Standards Track                        Juniper Networks
Expires: September 13, 2012 August 28, 2013                                       G. Enyedi
                                                              A. Csaszar
                                                             J. Tantsura
                                                      M. Konstantynowicz
                                                                R. White
                                                           Cisco Systems
                                                                R. White
                                                                M. Shand
                                                          March 12, 2012
                                                       February 24, 2013

An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees


   As IP and LDP Fast-Reroute are increasingly deployed, the coverage
   limitations of Loop-Free Alternates are seen as a problem that
   requires a straightforward and consistent solution for IP and LDP,
   for unicast and multicast.  This draft describes an architecture
   based on redundant backup trees where a single failure can cut a
   point-of-local-repair from the destination only on one of the pair of
   redundant trees.

   One innovative algorithm to compute such topologies is maximally
   disjoint backup trees.  Each router can compute its next-hops for
   each pair of maximally disjoint trees rooted at each node in the IGP
   area with computational complexity similar to that required by

   The additional state, address and computation requirements are
   believed to be significantly less than the Not-Via architecture

Status of this Memo

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

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   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 September 13, 2012. August 28, 2013.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Goals for Extending IP Fast-Reroute coverage beyond LFA  .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Maximally Redundant Trees (MRT)  . . . . . . . . . . . . . . .  6
   4.  Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . .  8
   5.  Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . .  9
     5.1.  LDP Unicast Forwarding - Avoid Tunneling . . . . . . . . . 10
     5.2.  IP Unicast Traffic . . . . . . . . . . . . . . . . . . . . 10
   6.  Protocol Extensions and Considerations: OSPF and ISIS  . . . . 12
   7.  Protocol Extensions and considerations: LDP  . . . . . . . . . 14
   8.  Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . . . 14
   8. 15
   9.  Inter-Area and ABR Forwarding Behavior . . . . . . . . . . . . 15
   9. 16
   10. Issues with Area Abstraction . . . . . . . . . . . . . . . . . 18
   10. 19
   11. Partial Deployment and Islands of Compatible MRT FRR
       routers  . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   11. 20
   12. Network Convergence and Preparing for the Next Failure . . . . 21
     11.1. 22
     12.1. Micro-forwarding loop prevention and MRTs  . . . . . . . . 21
     11.2. 22
     12.2. MRT Recalculation  . . . . . . . . . . . . . . . . . . . . 22
   12. 23
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   13. 23
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   14. 23
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   15. 24
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     15.1. 24
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     15.2. 24
     16.2. Informative References . . . . . . . . . . . . . . . . . . 23 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 25

1.  Introduction

   There is still work required to completely provide IP and LDP Fast-
   Reroute[RFC5714] for unicast and multicast traffic.  This draft
   proposes an architecture to provide 100% coverage for unicast
   traffic.  The associated multicast architecture is described in

   Loop-free alternates (LFAs)[RFC5286] provide a useful mechanism for
   link and node protection but getting complete coverage is quite hard.
   [LFARevisited] defines sufficient conditions to determine if a
   network provides link-protecting LFAs and also proves that augmenting
   a network to provide better coverage is NP-hard.
   [I-D.ietf-rtgwg-lfa-applicability] discusses the applicability of LFA
   to different topologies with a focus on common PoP architectures.

   While Not-Via [I-D.ietf-rtgwg-ipfrr-notvia-addresses] is defined as
   an architecture, in practice, it has proved too complicated and
   stateful to spark substantial interest in implementation or
   deployment.  Academic implementations [LightweightNotVia] exist and
   have found the address management complexity high (but no
   standardization has been done to reduce this).

   A different approach is needed and that is what is described here.
   It is based on the idea of using disjoint backup topologies as
   realized by Maximally Redundant Trees (described in
   [LightweightNotVia]); the general architecture can also apply to
   future improved redundant tree algorithms.

1.1.  Goals for Extending IP Fast-Reroute coverage beyond LFA

   Any scheme proposed for extending IPFRR network topology coverage
   beyond LFA, apart from attaining basic IPFRR properties, should also
   aim to achieve the following usability goals:

   o  ensure maximum physically feasible link and node disjointness
      regardless of topology,

   o  automatically compute backup next-hops based on the topology
      information distributed by link-state IGP,

   o  do not require any signaling in the case of failure and use pre-
      programmed backup next-hops for forwarding,

   o  introduce minimal amount of additional addressing and state on

   o  enable gradual introduction of the new scheme and backward

   o  and do not impose requirements for external computation.

2.  Terminology

   2-connected:   A graph that has no cut-vertices.  This is a graph
      that requires two nodes to be removed before the network is

   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.

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

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

   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.

   cut-vertex:   A vertex whose removal partitions the network.

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

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

   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.

   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.

3.  Maximally Redundant Trees (MRT)

   In the last few years, there's been substantial research on how to
   compute and use redundant trees.  Redundant trees are directed
   spanning trees that provide disjoint paths towards their common root.
   These redundant trees only exist and provide link protection if the
   network is 2-edge-connected and node protection if the network is
   2-connected.  Such connectiveness may not be the case in real
   networks, either due to architecture or due to a previous failure.
   The work on maximally redundant trees has added two useful pieces
   that make them ready for use in a real network.

   o  Computable regardless of network topology: The maximally redundant
      trees are computed so that only the cut-edges or cut-vertices are
      shared between the multiple trees.

   o  Computationally practical algorithm is based on a common network
      topology database.  Algorithm variants can compute in O( e) or O(e
      + n log n), as given in [I-D.enyedi-rtgwg-mrt-frr-algorithm].

   There is, of course, significantly more in the literature related to
   redundant trees and even fast-reroute, but the formulation of the
   Maximally Redundant Trees (MRT) algorithm makes it very well suited
   to use in routers.

   A known disadvantage of MRT, and redundant trees in general, is that
   the trees do not necessarily provide shortest detour paths.  The use
   of the shortest-path-first algorithm in tree-building and including
   all links in the network as possibilities for one path or another
   should improve this.  Modeling is underway to investigate and compare
   the MRT alternates to the optimal
   [I-D.enyedi-rtgwg-mrt-frr-algorithm].  Providing shortest detour
   paths would require failure-specific detour paths to the
   destinations, but the state-reduction advantage of MRT lies in the
   detour being established per destination (root) instead of per
   destination AND per failure.

   The specific algorithms to compute MRTs as well as the logic behind
   that algorithm and alternative computational approaches are given in
   detail in [I-D.enyedi-rtgwg-mrt-frr-algorithm].  Those interested are
   highly recommended to read that document.  This document describes
   how the MRTs can be used and not how to compute them.

   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.  The two paths along the two MRTs to a
   given destination-root of a 2-connected graph are node-disjoint and
   link-disjoint, while in any non-2-connected graph, only the cut-
   vertices and cut-edges can be contained by both of the paths.

   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.

                       |     |    |     |----[I]
                       |     |    |     |     |
                      [R]---[C]  [F]---[G]    |
                       |     |    |     |     |
                       |     |    |     |----[J]

                        a non-2-connected graph

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

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

                    Figure 2: A non-2-connected network

4.  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 blue and red MRTs.

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

   MRTs are practical to maintain redundancy even after a single link or
   node failure.  If a pair of MRTs is computed rooted at each
   destination, all the destinations remain reachable along one of the
   MRTs in the case of a single link or node failure.

   When there is a link or node failure affecting the rSPT, 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 either the Blue MRT path C->D->E->R or the Red MRT path

   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 Blue MRT next-hops or the Red MRT next-hops
   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, unless the network has
   been partitioned.  It is a local decision whether to use an MRT
   alternate, a Loop-Free Alternate or some other type of alternate.
   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.

   As described in [RFC5286], when a worse failure than is anticipated
   happens, using LFAs that are not downstream neighbors can cause
   micro-looping.  An example is given of link-protecting alternates
   causing a loop on node failure.  Even if a worse failure than
   anticipated happened, the use of MRT alternates will not cause
   looping.  Therefore, while node-protecting LFAs may be prefered, an
   the certainty that no alternate-induced looping will occur is an
   advantage to of using MRT alternates when such a the available node-protecting
   LFA is not a downstream path is the certainty that no alternate-induced
   looping will occur. path.

5.  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 4, MRT needs multi-topology forwarding.
   Unfortunately, neither IP nor LDP provide 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.

5.1.  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.

   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

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

5.2.  IP Unicast Traffic

   For IP, there is no currently practical alternative except tunneling.
   The tunnel egress could be the original destination in the area, the
   next-next-hop, etc..  If the tunnel egress is the original
   destination router, then the traffic remains on the redundant tree
   with sub-optimal routing.  If the tunnel egress is the next-next-hop,
   then protection of multi-homed prefixes and node-failure for ABRs is
   not available.  Selection of the tunnel egress is a router-local

   There are three options available for marking IP packets with which
   MRT it should be forwarded in.

   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 multi-homed prefixes, this requires a proxy-node, the destination to use is the
           non-proxy-node immediately before the proxy-node on that
           additional labels be advertised for each proxy-node.
           particular color MRT.

       B.  Option B - LDP Topology Label: Use a Topology-Identifier
           label on top of the IP packet.  This is very simple and
           doesn't require additional labels for proxy-nodes. 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

   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 SHOULD support Option A - using an
   LDP label that indicates the destination and MT-ID.

   For proxy-nodes associated with one or more multi-homed prefixes,
   there is no router associated with the proxy-node, so its loopbacks
   can't be known or used.  Instead, the loopback addresses of the two
   routers that are attached to the proxy-node can be used.  One of
   those routers will be on the Red MRT and the other on the Blue MRT.
   The MRT-red loopback of the first router would be used to reach the
   router on the Red MRT and similarly the MRT-blue loopback of the
   second router would be used.  The routers connected to the proxy-node
   are the end of the area/level and can decapsulate the traffic and
   properly forward it into the next area.

6.  Protocol Extensions and Considerations: OSPF and ISIS

   This captures an initial understanding of

   There are two possible approaches to what may need additional information to be
   specified.  In cases of partial deployment, it
   distribute in the IGP.  The first is necessary for a
   router to determine a consistent set of routers to include allow full flexibility in the
   island of MRT support.  To facilitate this, each router can announce
   both what its capabilities are all
   information and what it requires from other
   routers to add them distribute whichever values and combinations are
   desired.  The second is to the MRT island.  Generally, there will be simply distribute flags indicating a
   set of information advertised about
   particular well-known profile is supported.  Thus the MRT support.  This
   information has only area/level-wide scope.

   MRT Island
   Creation ID:   This identifies the process that is trivial.  The profile approach is recommended,
   with the router
      uses added flexibility of being able to form an specify more specific
   information if necessary and supported.

   For example, a simple profile "metric-insensitive MRT Island.  By advertising an ID for unicast fast-
   reroute via LDP" could specify:

   MRT Island Creation:   Only include other routers advertising this

   MRT Algorithm ID:   The MRT Lowpoint algorithm defined in

   Red MRT MT-ID:   The Red MRT MT-ID is the single well-known value
      allocated by IANA from the OSPF, ISIS, LDP and PIM MT-ID spaces.

   Blue MRT MT-ID:   The Blue MRT MT-ID is the single well-known value
      allocated by IANA from the OSPF, ISIS, LDP and PIM MT-ID spaces.

   GADAG Root Election Priority:   Pick the router with the lowest
      router ID to be the GADAG root.

   Forwarding Mechanisms for IP:   Use IP-in-LDP.

   MRT Capabilities:   Computes MRTs, IP Fast-Reroute, LDP Fast-Reroute

   The following captures an initial understanding of the aspects that
   must be considered to fully form a profile to advertise.  For some
   profiles, associated information may need to be distributed, such as
   GADAG Root Election Priority, Red MRT Loopback Address, Blue MRT
   Loopback Address, or MRT Algorithm ID.

   MRT Island Creation ID:   This identifies the process that the router
      uses to form an MRT Island.  By advertising an ID for the process,
      it is possible to have different processes in the future.  It may
      be desirable to advertise a list ordered by preference to allow

   MRT Algorithm ID:   This identifies the particular MRT algorithm used
      by the router.  By having an Algorithm ID, it is possible to
      change the algorithm used or use different ones in different
      networks.  It may be desirable to advertise a list ordered by
      preference to allow transitions.

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

   Blue MRT MT-ID:   This specifies the MT-ID to be associated with the
      Blue MRT forwarding topology.  It is needed for use in signaling.
      All routers in the MRT Island MUST agree on a value.

   GADAG Root Election Priority:   This specifies the priority of the
      router for being used as the GADAG root of its island.  A GADAG
      root is elected from the set of routers with the highest priority;
      ties are broken based upon highest Router ID.  The sensitivity of
      the MRT Algorithms to GADAG root selection is still being
      evaluated.  This provides the network operator with a knob to
      force particular GADAG root selection.

   Forwarding Mechanism for IP:   This specifies which forwarding
      mechanisms the router supports for IP traffic.  An MRT island must
      support a common set of forwarding mechanisms, which may be less
      than the full set advertised.  Multiple forwarding mechanisms may
      be specified, such as IP-in-IPv4, IP-in-IPv6 or IP-in-LDP-
      Destination-Topology IP-in-LDP Label.
      None is also an option.

   Forwarding Mechanism for LDP:   This specifies which forwarding
      mechanisms the router supports for LDP traffic.  An MRT island
      must support a common set of forwarding mechanisms, which may be
      less than the full set advertised.  The expected mechanisms are
      "Encode MT-ID in Labels" or None.

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

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

   MRT Capabilities Available:   This is the set of capabilities that
      the router is configured to support.

   MRT Capabilities Required:   This is the set of capabilities that
      other routers must have available to be added into the MRT island.

   MRT Capability: Computes MRTs:   The router can compute MRTs.

   MRT Capability: IP Fast-Reroute:   The router can use the computed
      MRTs for IP fast-reroute.

   MRT Capability: LDP Fast-Reroute:   The router can use the computed
      MRTs for LDP fast-reroute.

   MRT Capability: PIM Fast-Reroute:   The router can use the computed
      MRTs for PIM fast-reroute.

   MRT Capability: mLDP Fast-Reroute:   The router can use the computed
      MRTs for mLDP fast-reroute.

   MRT Capability: PIM Global Protection:   The router can use the
      computed MRTs for PIM Global Protection 1+1.

   MRT Capability: mLDP Global Protection:   The router can use the
      computed MRTs for mLDP Global Protection 1+1.

   The assumption is that a router will form 1 an MRT island, compute MRTs
   within that island, and then use those MRTs for the purposes
   specified in the profile.  If multiple profiles are supported with
   purposes.  Including a router that, for instance, doesn't support purposes (e.g. mLDP Global Protection would mean that Protection), then the whole MRT island could not
   support it.  In router may
   use a fully deployed case, of course, the whole area/
   level would support MRT different profile and the complexities of associated MRT island formation
   would to be minimal. used for the
   purposes in that different profile.  If a router wanted to form
   multiple MRT islands for different application purposes, that could
   be done by specifying different Red MRT MT-ID and Blue MRT MT-IDs.

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

7.  Protocol Extensions and considerations: LDP

   Capability negotiation in LDP is needed to indicate support for MRT;
   having this explicit allows the use of MRT-specific signaling
   extensions.  A router also needs to indicate, via FEC advertisement,
   whether it supports LDP Destination-Topology Labels, LDP Topology
   Labels, or both.  Since the label or labels are swapped at each LSR,
   consistency across the network is not required.

   If both mechanisms are supported, then if a Destination-Topology
   label is provided for a FEC, that should be used so that an ABR/LBR
   can indicate the appropriate labels, as discussed in Section
   Section 9.

8.  Multi-homed Prefixes

   One advantage 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.

   This generalizes to any multi-homed prefix.  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.

   For each prefix, the two lowest total cost attached ABRs are selected and a proxy-node is
   created connected to those two ABRs.  If there exist multiple multi-homed
   prefixes that share the same two best
   connectivity, connectivity and costs to each of those
   ABRs, then a single proxy-node can be used to represent the set.  An
   example of this is shown in Figure 3.

                    2    2                     2     2
                  A----B----C                A----B----C
                2 |         | 2            2 |         | 2
                  |         |                |         |
                [ABR1]    [ABR2]           [ABR1]    [ABR2]
                  |         |                |         |
                 p,10      p,15           10 |---[P]---| 15

                (a) Initial topology         (b)with proxy-node

                A<---B<---C                 A--->B--->C
                |         ^                 ^         |
                V         |                 |         V
              [ABR1]    [ABR2]            [ABR1]    [ABR2]
                |                                     |
                |-->[P]                         [P]<--|

                (c) Blue MRT                (d) Red MRT

              Figure 3: Prefixes Advertised by Multiple ABRs

   The proxy-nodes and associated links are added to the network
   topology after all real links have been assigned to a direction and
   before the actual MRTs are computed.  Proxy-nodes cannot be transited
   when computing the MRTs.  In addition to computing the pair of MRTs
   associated with each router destination D in the area, a pair of MRTs
   can be computed for each such proxy-node to fully protect against ABR

   Each ABR or attaching router must remove the MRT marking[see
   Section 5] and then forward the traffic outside of the area (or
   island of MRT-fast-reroute-supporting routers).

   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.


9.  Inter-Area and ABR Forwarding Behavior

   In regular forwarding, packets destined outside the area arrive at
   the ABR and the ABR forwards them into the other area because the
   next-hops from the area with the best route (according to tie-
   breaking rules) are used by the ABR.  The question is then what to do
   with packets marked with an MRT that are received by the ABR.

   For unicast fast-reroute, 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 marked with an MRT towards a
   destination in another area area/level should forward the MRT marked
   packet in the area area/level with the best route along its associated
   MRT.  If the packet came from that area, area/level, this correctly avoids
   the failure.

   How does an ABR/LBR ensure that MRT-marked packets do not arrive at
   the ABR/LBR?  There are two different mechanisms depending upon the
   forwarding mechanism being used.

   If the LDP label encodes the MT-ID as well as the destination, then
   the ABR/LBR is responsible for advertising a particular label to each
   neighbor.  Additionally, an LDP label is associated with an MT-ID due
   to the MT FEC that was used and not due to any intrisic particular
   value for the label.  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 Blue
   MRT MT-ID and L_red for the Red MRT 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 default
   topology, for the Blue MRT MT-ID, and for the Red MRT MT-ID.  The
   ABR/LBR installs next-hops from the best area for L_primary based on
   the default topology, for L_blue based on the Blue MRT forwarding
   topology, and for L_red based on the Red MRT forwarding topology.
   Therefore, packets from the non-best area 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-in-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 ABR/
   LBR immediately before the proxy-node on that MRT.

   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]   |
                                          |                    |
                                          | other              |

         (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
                |                                |

                  (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 4: ABR Forwarding Behavior and MRTs

   The other potential forwarding mechanisms require additional
   computation by the penultimate router along the in-local-area MRT
   immediately before the ABR/LBR is reached.  The penultimate router
   can determine that the ABR/LBR will forward the packet out of area/
   level and, in that case, the penultimate router can remove the MRT
   marking but still forward the packet along the MRT next-hop to reach
   the ABR.  For instance, in Figure 4, if node H fails, node E has to
   put traffic towards prefix p onto the red MRT.  But since node D
   knows that ABR1 will use a best from another area, it is safe for D
   to remove the MRT marking and just send the packet to ABR1 still on
   the red MRT but unmarked.  ABR1 will use the shortest path in Area

   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 this is a realistic
   deployment scenario, OSPF extensions could be considered.


10.  Issues with Area Abstraction

   MRT fast-reroute provides complete coverage in a area that is
   2-connected.  Where a failure would partition the network, of course,
   no alternate can protect against that failure.  Similarly, there are
   ways of connecting multi-homed prefixes that make it impractical to
   protect them without excessive complexity.

         |----[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.  The only real way to get it from A to ASBR Y is to explicitly
   tunnel it to ASBR Y.

   Tunnelling to the backup ASBR is for future consideration.  The
   previously proposed PHP approach needs to have an exception if BGP
   policies (e.g.  BGP local preference) determines which ASBR to use.
   Consider the case in Figure 6.  If the link between A and ASBR X (the
   preferred border router) fails, A can put the packets to p onto an
   MRT alternate, even tunnel it towards ASBR Y. Node B, however, must
   not remove the MRT marking in this case, as nodes in Area 0,
   including ASBR Y itself would not know that their preferred ASBR is

                      Area 20                    BB Area 0
          p ---[ASBR X]-X-[A]---[B]---[ABR 1]---[D]---[ASBR Y]--- p

                      BGP prefers ASBR X for prefix p

          Figure 6: Failure of path towards ASBR preferred by BGP

   The fine details of how to solve multi-area external prefix cases, or
   identifying certain cases as too unlikely and too complex to protect
   is for further consideration.


11.  Partial Deployment and Islands of Compatible MRT FRR routers

   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.

   First, a computing router S must determine its local island of
   compatible MRT fast-reroute routers.  A router that has a common
   forwarding mechanisms and common algorithm
   profile flag and is connected to either to S or to another router
   already determined to be in S's local island can be added to S's
   local island.

   Destinations inside the local island can obviously use MRT
   alternates.  Destinations outside the local island can be treated
   like a multi-homed prefix with caveats to avoid looping.  For LDP
   labels including both destination and topology, the routers at the
   borders of the local island need to originate labels for the original
   FEC and the associated MRT-specific labels.  Packets sent to an LDP
   label marked as blue or red MRT to a destination outside the local
   island will have the last router in the local island swap the label
   to one for the destination and forward the packet along the outgoing
   interface on the MRT towards a router outside the local island that
   was represented by the proxy-node.

   For IP in IP encapsulations, remote destinations' loopback addresses
   for the MRTs cannot be used, even if they were available.  Instead,
   the MRT loopback address of the router attached to a proxy-node,
   which represents destinations outside the local island, can be used.
   Packets sent to the router's MRT loopback address will have their
   outer IP header removed and will need to be explicitly forwarded
   along the outgoing interface on the MRT towards a router outside the
   local island that was represented by the proxy-node.  This behavior
   requires essentially remembering the MT-ID indicated by the outer IP
   address.  An alternate option would be to advertise different
   loopback addresses to be associated with the proxy-node; the outer IP
   address would still be removed but it would indicate the outgoing
   interface to use and no lookup would be necessary on the internal IP
   address while maintaining MT-ID context.

   A key question is which routers outside the MRT island can packets be
   forwarded to so that they are not forwarded back into the MRT island.
   An example of the necessary network graph transformations are given
   in Figure 7.  There are two parts to the computation.  First, the MRT
   island is collapsed into a single node; this assumes that the cost of
   transiting the MRT island is nothing and is pessimistic but allows
   for simpler computation.  Then, for each destination (other than the
   MRT island), the routers adjacent to the MRT island are checked to
   see if they are loop-free with respect to the MRT island and the
   destination.  The two loop-free neighbors of the MRT island that are
   closest to the destination are selected.  Then, a graph of just the
   MRT island is augmented with proxy-nodes that are attached via the
   outgoing interfaces to the selected loop-free neighbors.  Finally,
   the MRTs rooted at each proxy-node are computed on that augmented MRT
   island graph.  Essentially, the MRT island must have a loop-free
   neighbor to be able to have an alternate.

               | \   |     |           |
               |  \  |     |           |
               |   \ |     |           |

           (1) Network Graph with Partial Deployment

             [E],[F],[G],[H] :  No support for MRT-FRR
             (A),(B),(C),(D),(S):  MRT Island - supports MRT-FRR

         [G]---[E]----|                     |---(B)---(C)---(D)
          | \   |     |                     |    |           |
          |  \  |  ( MRT Island )      [ proxy ] |           |
          |   \ |     |                     |    |           |
         [H]---[F]----|                     |---(A)---(S)----|

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

   Figure 7: Computing alternates to destinations outside the MRT Island

   Naturally, there are more complicated options to improve coverage,
   such as connecting multiple MRT islands across tunnels, but it is not
   clear that the additional complexity is necessary.


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 mechanism 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 of this document.


12.2.  MRT Recalculation

   When a failure event happens, traffic is put by the PLRs onto the MRT
   topologies.  After that, each router recomputes its shortest path
   tree (SPT) and moves traffic over to that.  Only after all the PLRs
   have switched to using their SPTs and traffic has drained from the
   MRT topologies should each router install the recomputed MRTs into
   the FIBs.

   At each router, therefore, the sequence is as follows:

   1.  Receive failure notification

   2.  Recompute SPT

   3.  Install new SPT

   4.  Recompute MRTs

   5.  Wait configured period for all routers to be using their SPTs and
       traffic to drain from the 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 as quickly as possible.

   The installation of the MRTs can be staged such that the affected or
   broken MRTs are updated first and then the unbroken.

12. quickly.

13.  Acknowledgements

   The authors would like to thank Hannes Gredler, Jeff Tantsura, Ted
   Qian, Kishore Tiruveedhula, Santosh Esale, Nitin Bahadur, Harish
   Sitaraman and Raveendra Torvi for their suggestions and review.


14.  IANA Considerations

   This doument includes no request to IANA.


15.  Security Considerations

   This architecture is not currently believed to introduce new security


16.  References


16.1.  Normative References

              Enyedi, G.,
              Atlas, A. A., Envedi, G., Csaszar, A., and A. Csaszar, Gopalan,
              "Algorithms for computing Maximally Redundant Trees for
              IP/LDP Fast- Reroute", draft-enyedi-rtgwg-mrt-frr-algorithm-01
              draft-enyedi-rtgwg-mrt-frr-algorithm-02 (work in
              progress), March October 2012.

   [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.


16.2.  Informative References

              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,

              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-00 (work in progress),
              March 2012.

              Zhao, Q., Fang, L., Zhou, C., Li, L., and N. So, K. Raza, "LDP
              Extensions for Multi Topology Routing",
              draft-ietf-mpls-ldp-multi-topology-06 (work in progress),
              December 2012.

              Bryant, S., Previdi, S., and M. Shand, "IP "A Framework for IP
              and MPLS Fast Reroute Using Not-via Addresses",
              draft-ietf-rtgwg-ipfrr-notvia-addresses-10 (work in
              progress), December 2011. 2012.

              Filsfils, C. and P. Francois, "LFA applicability in SP
              networks", draft-ietf-rtgwg-lfa-applicability-06 (work in
              progress), January 2012.

              Shand, M., Bryant, S., Previdi, S., and C. Filsfils,
              "Loop-free C.,
              Francois, P., and O. Bonaventure, "Framework for Loop-free
              convergence using oFIB",
              draft-ietf-rtgwg-ordered-fib-05 draft-ietf-rtgwg-ordered-fib-09
              (work in progress), January 2013.

              Bryant, S., Filsfils, C., Previdi, S., Shand, M., and S.
              Ning, "Remote LFA FRR", draft-ietf-rtgwg-remote-lfa-01
              (work in progress),
              April 2011. December 2012.

              Retvari, G., Tapolcai, J., Enyedi, G., and A. Csaszar, "IP
              Fast ReRoute: Loop Free Alternates Revisited", Proceedings
              of IEEE INFOCOM , 2011, <

              Enyedi, G., Retvari, G., Szilagyi, P., and A. Csaszar, "IP
              Fast ReRoute: Lightweight Not-Via without Additional
              Addresses", Proceedings of IEEE INFOCOM , 2009,

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

Authors' Addresses

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

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


   Gabor Sandor Enyedi
   Konyves Kalman krt 11.
   Budapest  1097


   Andras Csaszar
   Konyves Kalman krt 11
   Budapest  1097


   Jeff Tantsura
   300 Holger Way
   San Jose, CA  95134


   Maciek Konstantynowicz
   Cisco Systems

   Russ White
   Cisco Systems
   12061 Bluemont Way
   Reston, VA  20190


   Mike Shand