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Versions: 00 01 02

Routing Area Working Group                                 A. Atlas, Ed.
Internet-Draft                                                 R. Kebler
Intended status: Standards Track                        Juniper Networks
Expires: January 13, 2014                                   IJ. Wijnands
                                                     Cisco Systems, Inc.
                                                              A. Csaszar
                                                               G. Enyedi
                                                                Ericsson
                                                           July 12, 2013


An Architecture for Multicast Protection Using Maximally Redundant Trees
                    draft-atlas-rtgwg-mrt-mc-arch-02

Abstract

   Failure protection is desirable for multicast traffic, whether
   signaled via PIM or mLDP.  Different mechanisms are suitable for
   different use-cases and deployment scenarios.  This document
   describes the architecture for global protection (aka multicast live-
   live) and for local protection (aka fast-reroute).

   The general methods for global protection and local protection using
   alternate-trees are dependent upon the use of Maximally Redundant
   Trees.  Local protection can also tunnel traffic in unicast tunnels
   to take advantage of the routing and fast-reroute mechanisms
   available for IP/LDP unicast destinations.

   The failures protected against are single link or node failures.
   While the basic architecture might support protection against shared
   risk group failures, algorithms to dynamically compute MRTs
   supporting this are for future study.

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."




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

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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Maximally Redundant Trees (MRTs) . . . . . . . . . . . . .  4
     1.2.  MRTs and Multicast . . . . . . . . . . . . . . . . . . . .  6
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Use-Cases and Applicability  . . . . . . . . . . . . . . . . .  8
   4.  Global Protection: Multicast Live-Live . . . . . . . . . . . .  9
     4.1.  Creation of MRMTs  . . . . . . . . . . . . . . . . . . . . 10
     4.2.  Traffic Self-Identification  . . . . . . . . . . . . . . . 11
       4.2.1.  Merging MRMTs for PIM if Traffic Doesn't
               Self-Identify  . . . . . . . . . . . . . . . . . . . . 12
     4.3.  Convergence Behavior . . . . . . . . . . . . . . . . . . . 13
     4.4.  Inter-area/level Behavior  . . . . . . . . . . . . . . . . 14
       4.4.1.  Inter-area Node Protection with 2 border routers . . . 15
       4.4.2.  Inter-area Node Protection with > 2 Border Routers . . 16
     4.5.  PIM  . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
       4.5.1.  Traffic Handling: RPF Checks . . . . . . . . . . . . . 17
     4.6.  mLDP . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   5.  Local Repair: Fast-Reroute . . . . . . . . . . . . . . . . . . 17
     5.1.  PLR-driven Unicast Tunnels . . . . . . . . . . . . . . . . 18
       5.1.1.  Learning the MPs . . . . . . . . . . . . . . . . . . . 19
       5.1.2.  Using Unicast Tunnels and Indirection  . . . . . . . . 19
       5.1.3.  MP Alternate Traffic Handling  . . . . . . . . . . . . 20
       5.1.4.  Merge Point Reconvergence  . . . . . . . . . . . . . . 21
       5.1.5.  PLR termination of alternate traffic . . . . . . . . . 21
     5.2.  MP-driven Unicast Tunnels  . . . . . . . . . . . . . . . . 21
     5.3.  MP-driven Alternate Trees  . . . . . . . . . . . . . . . . 22
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   9.  Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     9.1.  MP-driven Alternate Trees  . . . . . . . . . . . . . . . . 23
       9.1.1.  PIM details for Alternate-Trees  . . . . . . . . . . . 26
       9.1.2.  mLDP details for Alternate-Trees . . . . . . . . . . . 26
       9.1.3.  Traffic Handling by PLR  . . . . . . . . . . . . . . . 26
     9.2.  Methods Compared for PIM . . . . . . . . . . . . . . . . . 27
     9.3.  Methods Compared for mLDP  . . . . . . . . . . . . . . . . 27
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 27
     10.2. Informative References . . . . . . . . . . . . . . . . . . 28
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29









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1.  Introduction

   This document describes how the algorithms in
   [I-D.enyedi-rtgwg-mrt-frr-algorithm], which are used in
   [I-D.ietf-rtgwg-mrt-frr-architecture] for unicast IP/LDP fast-
   reroute, can be used to provide protection for multicast traffic.  It
   specifically applies to multicast state signaled by PIM[RFC4601] or
   mLDP[RFC6388].  There are additional protocols that depend upon these
   (e.g.  VPLS, mVPN, etc.) and consideration of the applicability to
   such traffic will be in a future version.

   In this document, global protection is used to refer to the method of
   having two maximally disjoint multicast trees where traffic may be
   sent on both and resolved by the receiver.  This is similar to the
   ability with RSVP-TE LSPs to have a primary and a hot standby, except
   that it can operate in 1+1 mode.  This capability is also referred to
   as multicast live-live and is a generalized form of that discussed in
   [I-D.ietf-rtgwg-mofrr].  In this document, local protection refers to
   the method of having alternate ways of reaching the pre-identified
   merge points upon detection of a local failure.  This capability is
   also referred to as fast-reroute.

   This document describes the general architecture, framework, and
   trade-offs of the different approaches to solving these general
   problems.  It will recommend how to generally provide global
   protection and local protection for mLDP and PIM traffic.  Where
   protocol extensions are necessary, they will be defined in separate
   documents as follows.

   o  Global 1+1 Protection Using PIM

   o  Global 1+1 Protection Using mLDP

   o  Local Protection Using mLDP:
      [I-D.wijnands-mpls-mldp-node-protection]This document describes
      how to provide node-protection and the necessary extensions using
      targeted LDP session.

   o  Local Protection Using PIM

1.1.  Maximally Redundant Trees (MRTs)

   Maximally Redundant Trees (MRTs) are described in
   [I-D.enyedi-rtgwg-mrt-frr-algorithm]; here we only give a brief
   description about the concept.  A pair of MRTs is a pair of directed
   spanning trees (red and blue tree) with a common root, directed so
   that each node can be reached from the root on both trees.  Moreover,
   these trees are redundant, since they are constructed so that no



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   single link or single node failure can separate any node from the
   root on both trees, unless that failed link or node is splitting the
   network into completely separated components (e.g. the link or node
   was a cut-edge or cut-vertex).

   Although for multicast, the arcs (directed links) are directed away
   from the root instead of towards the root, the same MRT computations
   are used and apply.  This is similar to how multicast uses unicast
   routing's next-hops as the upstream-hops.  Thus this definition
   slightly differs from the one presented in
   [I-D.enyedi-rtgwg-mrt-frr-algorithm], since the arcs are directed
   away and not towards the root.  When we need two paths towards a
   given destination and not two away from it (e.g. for unicast detours
   for local repair solutions), we only need to reverse the arcs from
   how they are used for the unicast routing case; thus constructing
   MRTs towards or away from the root is the same problem.  A pair of
   MRTs is depicted in Figure 1.


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

                                 (a) a network

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

            (b) Blue MRT of root R           (c) Red MRT of root R

               Figure 1: A network and two MRTs  found in it

   It is important to realize that this redundancy criterion does not
   imply that, after a failure, either of the MRTs remains intact, since
   a node failure must affect any spanning tree.  Redundancy here means
   that there will be a set of nodes, which can be reached along the
   blue MRT, and there will be another set, which remains reachable
   along the red MRT.  As an example, suppose that node F goes down;
   that would separate B and A on the blue MRT and D and E on the red



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   MRT.  Naturally, it is possible that the intersection of these two
   sets is not empty, e.g.  C, G, H and J will remain reachable on both
   MRTs.  Additionally, observe that a single link can be used in both
   of the trees in different directions, so even a link failure can cut
   both trees.  In this example, the failure of link F<->B leads to the
   same reachability sets.

   Finally, it is critical to recall that a pair of MRTs is always
   constructed together and they are not SPTs.  While it would be useful
   to have an algorithm that could find a redundant pair for a given
   tree (e.g. for the SPT), that is impossible in general.  Moreover, if
   there is a failure and at least one of the trees change, the other
   tree may need to change as well.  Therefore, even if a node still
   receives the traffic along the red tree, it cannot keep the old red
   tree, and construct a blue pair for it; there can be reconfiguration
   in cases when traditional shortest-path-based-thinking would not
   expect it.  To converge to a new pair of disjoint MRTs, it is
   generally necessary to update both the blue MRT and the red MRT.

   The two MRTs provide two separate forwarding topologies that can be
   used in addition to the default shortest-path-tree (SPT) forwarding
   topology (usually MT-ID 0).  There is a Blue MRT forwarding topology
   represented by one MT-ID; similarly there is a Red MRT forwarding
   topology represented by a different MT-ID.  Naturally, a multicast
   protocol is required to use the forwarding topologies information to
   build the desired multicast trees.  The multicast protocol can simply
   request appropriate upstream interfaces, but include the MT-ID when
   needed.

1.2.  MRTs and Multicast

   Maximally Redundant Trees (MRT) provide two advantages for protecting
   multicast traffic.  First, for global protection, MRTs are precisely
   what needs to be computed to have maximally redundant multicast
   distribution trees.  Second, for local repair, MRTs ensure that there
   will protection to the merge points; the certainty of a path from any
   merge point to the PLR that avoids the failure node allows for the
   creation of alternate trees.

   A known disadvantage of MRT, and redundant trees in general, is that
   the trees do not necessarily provide shortest detour paths.  Modeling
   is underway to investigate and compare the MRT lengths for the
   different algorithm options [I-D.enyedi-rtgwg-mrt-frr-algorithm].


2.  Terminology





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   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.

   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
      vertex.

   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.

   Maximally Redundant Multicast Trees (MRMT):   A pair of multicast
      trees built of the sub-set of MRTs that is needed to reach all
      interested receivers.

   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.



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   Merge Point (MP):   For local repair, a router at which the alternate
      traffic rejoins the primary multicast tree.  For global
      protection, a router which receives traffic on multiple trees and
      must decide which stream to forward on.

   Point of Local Repair (PLR):   The router that detects a local
      failure and decides whether and when to forward traffic on
      appropriate alternates.

   MT-ID:   Multi-topology identifier.  The default shortest-path-tree
      topology is MT-ID 0.

   MultiCast Ingress (MCI):   Multicast Ingress, the node where the
      multicast stream enters the current transport technology (MPLS-
      mLDP or IP-PIM) domain.  This maybe the router attached to the
      multicast source, the PIM Rendezvous Point (RP) or the mLDP Root
      node address.

   Upstream Multicast Hop (UMH):   Upstream Multicast Hop, a candidate
      next-hop that can be used to reach the MCI of the tree.

   Stream Selection:   The process by which a router determines which of
      the multiple primary multicast streams to accept and forward.  The
      router can decide on a packet-by-packet basis or simply per-
      stream.  This is done for global protection 1+1 and described in
      [I-D.ietf-rtgwg-mofrr].

   MultiCast Egress (MCE):   Multicast Egress, a node where the
      multicast stream exists the current transport technology (MPLS-
      mLDP or IP-PIM) domain.  This is usually a receiving router that
      may forward the multicast traffic on towards receivers based upon
      IGMP or other technology.


3.  Use-Cases and Applicability

   Protection of multicast streams has gained importance with the use of
   multicast to distribute video, including live video such as IP-TV.
   There are a number of different scenarios and uses of multicast that
   require protection.  A few preliminary examples are described below.

   o  When video is distributed via IP or MPLS for a cable application,
      it is desirable to have global protection 1+1 so that the
      customer-perceived impact is limited.  A QAM can join two
      multicast groups and determine which stream to use based upon the
      stream quality.  A network implementing this may be custom-
      engineered for this particular purpose.




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   o  In financial markets, stock ticker data is distributed via
      multicast.  The loss of data can have a significant financial
      impact.  Depending on the network, either global protection 1+1 or
      local protection can minimize the impact.

   o  Several solutions exist for updating software or firmwares of a
      large number of end-user or operator-owned networking equipment
      that are based on IP multicast.  Since IP multicast is based on
      datagram transport so taking care of lost data is cumbersome and
      decreases the advantages offered by multicast.  Solutions may rely
      on sending the updates several times: a properly protected network
      may result in that less repetitions are required.  Other solutions
      rely on the recipent asking for lost data segments explicitly on-
      demand.  A network failure could cause data loss for a significant
      number of receivers, which in turn would start requesting the the
      lost data in a burst that could overload the server.  Properly
      engineered multicast fast reroute would minimise such impacts.

   o  Some providers offer multicast VPN services to their customers.
      SLAs between the customer and provider may set low packet loss
      requirements.  In such cases interruptions longer than the outage
      timescales targeted by FRR could cause direct financial losses for
      the provider.

   Global protection 1+1 uses maximally redundant multicast trees
   (MRMTs) to simultaneously distribute a multicast stream on both
   MRMTs.  The disadvantage is the extra state and bandwidth
   requirements of always sending the traffic twice.  The advantage is
   that the latency of each MRMT can be known and the receiver can
   select the best stream.

   Local protection provides a patch around the fault while the
   multicast tree reconverges.  When PLR replication is used, there is
   no extra multicast state in the network, but the bandwidth
   requirements vary based upon how many potential merge-points must be
   provided.  When alternate-trees are used, there is extra multicast
   state but the bandwidth requirements on a link can be minimized to no
   more than once for the primary multicast tree traffic and once for
   the alternate-tree traffic.


4.  Global Protection: Multicast Live-Live

   In MoFRR [I-D.ietf-rtgwg-mofrr], the idea of joining both a primary
   and a secondary tree is introduced with the requirement that the
   primary and secondary trees be link and node disjoint.  This works
   well for networks where there are dual-planes, as explained in
   [I-D.ietf-rtgwg-mofrr].  For other networks, it is still desirable to



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   have two disjoint multicast trees and allow a receiver to join both
   and make its own decision about which traffic to accept.

   Using MRTs gives the ability to guarantee that the two trees are as
   disjoint as possible and dynamically recomputed whenever the topology
   changes.  The MRTs used are rooted at the MultiCast Ingress (MCI).
   One multicast tree is created using the Blue MRT forwarding topology.
   The second multicast tree is created using the Red MRT forwarding
   topology.  This can be accomplished by specifying the appropriate
   MT-ID associated with each forwarding topology.

   There are four different aspects of using MRTs for 1+1 Global
   Protection that are necessary to consider.  They are as follows.

   1.  Creation of the maximally redundant multicast trees (MRMTs) based
       upon the forwarding topologies.

   2.  Traffic Identification: How to handle traffic when the two MRMTs
       overlap due to a cut-vertex or cut-link.

   3.  Convergence: How to converge after a network change and get back
       to a protected state.

   4.  Inter-area/inter-level Behavior: How to compute and use MRMTs
       when the multicast source is outside the area/level and how to
       provide border-router protection.

4.1.  Creation of MRMTs

   The creation of the two maximally redundant multicast trees occurs as
   described below.  This assumes that the next-hops to the MCI
   associated with the Blue and Red forwarding topologies have already
   been computed and stored.

   1.  A receiving router determines that it wants to join both the Blue
       tree and the Red tree.  The details on how it does this decision
       are not covered in this document and could be based on
       configuration, additional protocols, etc.

   2.  The router selects among the Blue next-hops an Upstream Multicast
       Hop (UMH) to reach the MCI node.  The router joins the tree
       towards the selected UMH including a multi-topology id (MT-ID)
       identifying the Blue MRT.

   3.  The router selects among the Red next-hops an Upstream Multicast
       Hop (UMH) to reach the MCI node.  The router joins the tree
       towards the selected UMH including a multi-topology id (MT-ID)
       identifying the Red MRT.



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   4.  When a router receives a tree setup request specifying a
       particular MT-ID (e.g.  Color), then the router selects among the
       Color next-hops to the MCI a UMH node, creates the necessary
       multicast state, and joins the tree towards the UMH node.

4.2.  Traffic Self-Identification

   Two maximally redundant trees will share any cut-vertices and cut-
   links in the network.  In the multicast global protection 1+1 case,
   this means that the potential single failures of the other nodes and
   links in the network are still protected against.  If each cut-vertex
   cannot associate traffic to a particular MRMT, then the traffic would
   be incorrectly replicated to both MRMT resulting in complete
   duplication of traffic.  An example of such MRTs is given earlier in
   Figure 1 and repeated below in Figure 2, where there are two cut-
   vertices C and G and a cut-link C<->G.


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

                                 (a) a network

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

            (b) Blue MRT of root R           (c) Red MRT of root R

               Figure 2: A network and two MRTs  found in it

   In this example, traffic from the multicast source R to a receiver G,
   J, or H will cross link C<->G on both the Blue and Red MRMTs.  When
   this occurs, there are several different possibilities depending upon
   protocol.







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   mLDP:   Different label bindings will be created for the Blue and Red
      MRMTs.  As specified in [I-D.iwijnand-mpls-mldp-multi-topology],
      the P2MP FEC Element will use the MT IP Address Family to encode
      the Root node address and MRT T-ID.  Each MRMT will therefore have
      a different P2MP FEC Element and be assigned an independent label.

   PIM:   There are three different ways to handle IP traffic forwarded
      based upon PIM when that traffic will overlap on a link.

      A.  Different Groups: If different multicast groups are used for
          each MRMT, then the traffic clearly indicates which MRMT it
          belongs to.  In this case, traffic on the Blue MRMT would use
          multicast group G-blue and traffic on the Red MRMT would use
          multicast group G-red.

      B.  Different Source Loopbacks: Another option is to use different
          IP addresses for the source S, so S might announce S-red and
          S-blue.  In this case, traffic on the Blue MRMT would have an
          IP source of S-blue and traffic on the Red MRMT would have an
          IP source of S-red.

      C.  Stream Selection and Merging: The third option, described in
          Section 4.2.1, is to have a router that gets (S,G) Joins for
          both the Blue MT-ID and the Red MT-ID merge those into a
          single tree.  The router may need to select which upstream
          stream to use, just as if it were a receiving router.

   There are three options presented for PIM.  The most appropriate will
   depend upon deployment scenario as well as router capabilities.

4.2.1.  Merging MRMTs for PIM if Traffic Doesn't Self-Identify

   When traffic doesn't self-identify, the cut-vertices must follow
   specific rules to avoid traffic duplication.  This section describes
   that behavior which allows the same (S,G) to be used for both the
   Blue MT-ID and Red MT-ID (e.g. when the traffic doesn't self-identify
   as to its MT-ID).

   The behavior described in this section differs from the conflict
   resolution described in [RFC6420] because these rules apply to the
   Global Protection 1+1 case.  Specifically, it is not sufficient for a
   upstream router to pick only one of the two MT-IDs to join because
   that does not maximize the protection provided.

   As described in [RFC6420], a router that receives (S,G) Joins for
   both the Blue MT-ID and the Red MT-ID can merge the set of downstream
   interfaces in its forwarding entry.  Unlike the procedures defined in
   [RFC6420], the router must send a Join upstream for each MT-ID.  If a



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   router has different upstream interfaces for these MRMTs, then the
   router will need to do stream selection and forward the selected
   stream to its outgoing interfaces, just as if it were an MCE.  The
   stream selection methods of detecting failures and handle traffic
   discarding are described in [I-D.ietf-rtgwg-mofrr].

   This method does not work if the MRMTs merge on a common LAN with
   different upstream routers.  In this case, the traffic cannot be
   distinguished on the LAN and will result in duplication on the LAN.
   The normal PIM Assert procedure would stop one of the upstream
   routers from transmitting duplicates onto the LAN once it is
   detected.  This, in turn, may cause the duplicate stream to be pruned
   back to the source.  Thus, end-to-end protection in this case of the
   MRMTs converging on a single LAN with different upstream interfaces
   can only be accomplished by the methods of traffic self-
   identification.

4.3.  Convergence Behavior

   It is necessary to handle topology changes and get back to having two
   MRMTs that provide global protection.  To understand the requirements
   and what can be computed, recall the following facts.

   a.  It is not generally possible to compute a single tree that is
       maximally redundant to an existing tree.

   b.  The pair of MRTs must be computed simultaneously.

   c.  After a single link or node failure, there is one set of nodes
       that can be reached from the root on the Blue MRMT and a second
       set of nodes that can be reached from the root on the Red MRMT.
       If the failure wasn't a cut-vertex or cut-edge, all nodes will be
       in at least one of these two sets.

   To gracefully converge, it is necessary to never have a router where
   both its red MRMT and blue MRMT are broken.  There are three
   different ways in which this could be done.  These options are being
   more fully explored to see which is most practical and provides the
   best set of trade-offs.

   Ordered Convergence  When a single failure occurs, each receiver
      determines whether it was affected or unaffected.  First, the
      affected receivers identify the broken MRMT color (e.g. blue) and
      join the MRMT via their new UMH for that MRT color.  Once the
      affected receivers receive confirmation that the new MRMT has been
      successfully created back to the MCI, then the affected receivers
      switch to using that MRMT.  The affected receivers tear down the
      old broken MRMT state and join the MRMT via their new UMH for the



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      other MRT color (e.g. red).  Finally, once the affected receivers
      receive confirmation that the new MRMT has been successfully
      created back to the MCI, the affected receivers can tear down the
      old working MRMT state.  Once the affected receivers have updated
      their state, the unaffected receivers need to also do the same
      staging - first joining the MRMT via their new UMH for the Blue
      MRT, waiting for confirmation, switching to using traffic from the
      Blue MRMT, tearing down the old Blue MRMT state, joining the MRMT
      via their new UMH for the Red MRT, waiting for confirmation, and
      tearing down the old Red MRMT state.  There are complexities
      remaining, such as determining how an Unaffected Receiver decides
      that the Affected Receivers are done.  When the topology change
      isn't a failure, all receivers are unaffected and the same process
      can apply.

   Protocol Make-Before-Break  In the control plane, a router joins the
      tree on the new Blue topology but does not stop receiving traffic
      on the old Blue topology.  Once traffic is observed from the new
      Blue UMH, then the router accepts traffic on the new Blue UMH and
      removes the old Blue UMH.  This behavior can happen simultaneously
      with both Blue and Red forwarding topologies.  An advantage is
      that it works regardless of the type of topology change and
      existing traffic streams aren't broken.  Another advantage is that
      the complexity is limited and this method is well understood.  The
      disadvantage is that the number of traffic-affecting events
      depends upon the number of hops to the MCI.

   Multicast Source Make-Before-Break  On a topology change, routers
      would create new MRMTs using new MRT forwarding state and leaving
      the old MRMTs as they are.  After the new MRMTs are complete, the
      multicast source could switch from sending on the old MRMTs to
      sending on the new MRMTs.  After a time, the old MRMTs could be
      torn down.  There are a number of details to still investigate.

4.4.  Inter-area/level Behavior

   A source outside of the IGP area/level can be treated as a proxy
   node.  When the join request reaches a border router (whether ABR for
   OSPF or LBR for ISIS), that border router needs to determine whether
   to use the Blue or Red forwarding topology in the next selected area/
   level.










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                                      |-------------------|
                                      |                   |
               |---[S]---|          [BR1]-----[ X ]       |
               |         |            |         |         |
             [ A ]-----[ B ]          |         |         |
               |         |          [ Y ]-----[BR2]--(proxy for S)
               |         |
             [BR1]-----[BR2]          (b) Area 10
                                      Y's Red next-hop:  BR1
            (a) Area 0                Y's Blue next-hop: BR2
           Red Next-Hops to S
             BR1's is BR2
             BR2's is B
             B's is S

           Blue Next-Hops to S
             BR1's is A
             BR2's is BR1
             A's is S


           Figure 3: Inter-area Selection - next-hops towards S

   Achieving maximally node-disjoint trees across multiple areas is hard
   due to the information-hiding and abstraction.  If there is only one
   border router, it is trivial but protection of the border router is
   not possible.  With exactly 2 border routers, inter-area/level node
   protection is reasonably straightforward but can require that the BR
   rewrite the (S,G) for PIM.  With more than 2 border routers, inter-
   area node protection is possible at the cost of additional bandwidth
   and border router complexity.  These two solutions are described in
   the following sub-sections.

4.4.1.  Inter-area Node Protection with 2 border routers

   If there are exactly two border routers between the areas, then the
   solution and necessary computation is straightforward.  In that
   specific case, each BR knows that only the other BR must specifically
   be avoided in the second area when a forwarding topology is selected.
   As described in [I-D.enyedi-rtgwg-mrt-frr-algorithm], it is possible
   for a node X to determine whether the Red or Blue forwarding topology
   should be used to reach a node D while avoiding another node Y.

   The results of this computation and the resulting changes in MT-ID
   from Red to Blue or Blue to Red are illustrated in Figure 3.  It
   shows an example where BR1 must modify joins received from Area 10
   for the Red MT-ID to use the Blue MT-ID in Area 0.  Similarly, BR2
   must modify joins received from Area 10 for the Blue MT-ID to use the



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   Red MT-ID in Area 0.

   For mLDP, modifying the MT-ID in the control-plane is all that is
   needed.  For PIM, if the same (S,G) is used for both the Blue MT-ID
   and the Red MT-ID, then only control-plane changes are needed.
   However, for PIM, if different group IDs (e.g.  G-red and G-blue) or
   different source loopback addresses (S-red and S-blue) are used, it
   is necessary to modify the traffic to reflect the MT-ID included in
   the join message received on that interface.  An alternative could be
   to use an MPLS label that indicates the MT-ID instead of different
   group IDs or source loopback addresses.

   To summarize the necessary logic, when a BR1 receives a join from a
   neighbor in area N to a destination D in area M on the Color MT-ID,
   the BR1:

   a.  Identifies the BR2 at the other end of the proxy node in area N.

   b.  Determines which forwarding topology may avoid BR2 to reach D in
       area M. Refer to that as Color-2 MT-ID.

   c.  Uses Color-2 MT-ID to determine the next-hops to S. When a join
       is sent upstream, the MT-ID used is that for Color-2.

4.4.2.  Inter-area Node Protection with > 2 Border Routers

   If there are more than two BRs between areas, then the problem of
   ensuring inter-area node-disjointness is not solved.  Instead, once a
   request to join the multicast tree has been received by a BR from an
   area that isn't closest to the multicast source, the BR must join
   both the Red MT-ID and the Blue MT-ID in the area closest to the
   multicast source.  Regardless of what single link or node failure
   happens, each BR will receive the multicast stream.  Then, the BR can
   use the stream-selection techniques specified in
   [I-D.ietf-rtgwg-mofrr] to pick either the Blue or Red stream and
   forward it to downstream routers in the other area.  Each of the BRs
   for the other area should be attached to a proxy-node representing
   the other area.

   This approach ensures that a BR will receive the multicast stream in
   the closest area as long as the single link or node failure isn't a
   single point of failure.  Thus, each area or level is independently
   protected.  The BR is required to be able to select among the
   multicast streams and, if necessary for PIM, translate the traffic to
   contain the correct (S,G) for forwarding.






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4.5.  PIM

   Capabilities need to be exchanged to determine that a neighbor
   supports using MRT forwarding topologies with PIM.  Additional
   signaling extensions are not necessary to PIM to support Global
   Protection.  [RFC6420] already defines how to specify an MT-ID as a
   Join Attribute.

4.5.1.  Traffic Handling: RPF Checks

   For PIM, RPF checks would still be enabled by the control plane.  The
   control plane can program different forwarding entries on the G-blue
   incoming interface and on the G-red incoming interface.  The other
   interfaces would still discard both G-blue and G-red traffic.

   The receiver would still need to detect failures and handle traffic
   discarding as is specified in [I-D.ietf-rtgwg-mofrr].

4.6.  mLDP

   Capabilities need to be exchanged to determine that a neighbor
   supports using MRT forwarding topologies with mLDP.  The basic
   mechansims for mLDP to support multi-topology are already described
   in [I-D.iwijnand-mpls-mldp-multi-topology].  It may be desirable to
   extend the capability defined in this draft to indicate that MRT is
   or is not supported.


5.  Local Repair: Fast-Reroute

   Local repair for multicast traffic is different from unicast in
   several important ways.

   o  There is more than a single final destination.  The full set of
      receiving routers may not be known by the PLR and may be extremely
      large.  Therefore, it makes sense to repair to the immediate next-
      hops for link-repair and the next-next-hops for node-repair.
      These are the potential merge points (MPs).

   o  If a failure cannot be positively identified as a node-failure,
      then it is important to repair to the immediate next-hops since
      they may have receivers attached.

   o  If a failure cannot be positively identified as a link-failure and
      node protection is desired, then it is important to repair to the
      next-next-hops since they may not receive traffic from the
      immediate next-hops.




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   o  Updating multicast forwarding state may take significantly longer
      than updating unicast state, since the multicast state is updated
      tree by tree based on control-plane signaling.

   o  For tunnel-based IP/LDP approaches, neither the PLR nor the MP may
      be able to specify which interface the alternate traffic will
      arrive at the MP on.  The simplest reason is the unicast
      forwarding includes the use of ECMP and the path selection is
      based upon internal router behavior for all paths between the PLR
      and the MP.

   For multicast fast-reroute, there are three different mechanisms that
   can be used.  As long as the necessary signaling is available, these
   methods can be combined in the same network and even for the same PLR
   and failure point.

   PLR-driven Unicast Tunnels:   The PLR learns the set of MPs that need
      protection.  On a failure, the PLR replicates the traffic and
      tunnels it to each MP using the unicast route.  If desired, an
      RSVP-TE tunnel could be used instead of relying upon unicast
      routing.

   MP-driven Unicast Tunnels:   Each MP learns the identity of the PLR.
      Before failure, each MP independently signals to the PLR the
      desire for protection and other information to use.  On a failure,
      the PLR replicates the traffic and tunnels it to each MP using the
      unicast route.  If desired, an RSVP-TE tunnel could be used
      instead of relying upon unicast routing.

   MP-driven Alternate Trees:   Each MP learns the identity of the PLR
      and the failure point (node and interface) to be protected
      against.  Each MP selects an upstream interface and forwarding
      topology where the path will avoid the failure point; each MP
      signals a join towards that upstream interface to create that
      state.

   Each of these options is described in more detail in their respective
   sections.  Then the methods are compared and contrasted for PIM and
   for mLDP.

5.1.  PLR-driven Unicast Tunnels

   With PLR-driven unicast tunnels, the PLR learns the set of merge
   points (MPs) and, on a locally detected failure, uses the existing
   unicast routing to tunnel the multicast traffic to those merge
   points.  The failure being protected against may be link or node
   failure.  If unicast forwarding can provide an SRLG-protecting
   alternate, then SRLG-protection is also possible.



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   There are five aspects to making this work.

   1.  PLR needs to learn the MPs and their associated MPLS labels to
       create protection state.

   2.  Unicast routing has to offer alternates or have dedicated tunnels
       to reach the MPs.  The PLR encapsulates the multicast traffic and
       directs it to be forwarded via unicast routing.

   3.  The MP must identify alternate traffic and decide when to accept
       and forward it or drop it.

   4.  When the MP reconverges, it must move to its new UMH using make-
       before-break so that traffic loss is minimized.

   5.  The PLR must know when to stop sending traffic on the alternates.

5.1.1.  Learning the MPs

   If link-protection is all that is desired, then the PLR already knows
   the identities of the MPs.  For node-protection, this is not
   sufficient.  In the PLR-driven case, there is no direct communication
   possible between the PLR and the next-next-hops on the multicast
   tree.  (For mLDP, when targeted LDP sessions are used, this is
   considered to be MP-driven and is covered in Section 5.2.)

   In addition to learning the identities of the MPs, the PLR must also
   learn the MPLS label, if any, associated with each MP.  For mLDP, a
   different label should be supplied for the alternate traffic; this
   allows the MP to distinguish between the primary and alternate
   traffic.  For PIM, an MPLS label is used to identify that traffic is
   the alternate.  The unicast tunnel used to send traffic to the MP may
   have penultimate-hop-popping done; thus without an explicit MPLS
   label, there is no certainty that a packet could be conclusively
   identified as primary traffic or as alternate traffic.

   A router must tell its UMH the identity of all downstream multicast
   routers, and their associated alternate labels, on the particular
   multicast tree.  This clearly requires protocol extensions.  The
   extensions for PIM are given in [I-D.kebler-pim-mrt-protection].

5.1.2.  Using Unicast Tunnels and Indirection

   The PLR must encapsulate the multicast traffic and tunnel it towards
   each MP.  The key point is how that traffic then reaches the MP.
   There are basically two possibilities.  It is possible that a
   dedicated RSVP-TE tunnel exists and can be used to reach the MP for
   just this traffic; such an RSVP-TE tunnel would be explicitly routed



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   to avoid the failure point.  The second possibility is that the
   packet is tunneled via LDP and uses unicast routing.  The second case
   is explored here.

   It is necessary to assume that unicast LDP fast-reroute
   [I-D.ietf-rtgwg-mrt-frr-architecture][RFC5714][RFC5286] is supported
   by the PLR.  Since multicast convergence takes longer than unicast
   convergence, the PLR may have two different routes to the MP over
   time.  When the failure happens, the PLR will have an alternate,
   whether LFA or MRT, to reach the MP.  Then the unicast routing
   converges and the PLR will have a new primary route to the MP.  Once
   the routing has converged, it is important that alternate traffic is
   no longer carried on the MRT forwarding topologies.  This rule allows
   the MRT forwarding topologies to reconverge and be available for the
   next failure.  Therefore, it is also necessary for the tunneled
   multicast traffic to move from the alternate route to the new primary
   route when the PLR reconverges.  Therefore, the tunneled multicast
   traffic should use indirection to obtain the unicast routing's
   current next-hops to the MP.  If physical indirection is not
   feasible, then when the unicast LIB is updated, the associated
   multicast alternate tunnel state should be as well.

   When the PLR detects a local failure, the PLR replicates each
   multicast packet, swaps or adds the alternate MPLS label needed by
   the MP, and finally pushes the appropriate label for the MP based
   upon the outgoing interface selected by the unicast routing.

   For PIM, if no alternate labels are supplied by the MPs, then the
   multicast traffic could be tunneled in IP.  This would require
   unicast IP fast-reroute.

5.1.3.  MP Alternate Traffic Handling

   A potential Merge Point must determine when and if to accept
   alternate traffic.  There are two critical components to this
   decision.  First, the MP must know the state of all links to its UMH.
   This allows the MP to determine whether the multicast stream could be
   received from the UMH.  Second, the MP must be able to distinguish
   between a normal multicast tree packet and an alternate packet.

   The logic is similar for PIM and mLDP, but in PIM there is only one
   RPF-interface or interface of interest to the UMH.  In mLDP, all the
   directly connected interfaces to the UMH are of interest.  When the
   MP detects a local failure, if that interface was the last connected
   to the UMH and used for the multicast group, then the MP must rapidly
   switch from accepting the normal multicast tree traffic to accepting
   the alternate traffic.  This rapid change must happen within the same
   approximately 50 milliseconds that the PLR switching to send traffic



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   on the alternate takes and for the same reasons.  It does no good for
   the PLR to send alternate traffic if the MP doesn't accept it when it
   is needed.

   The MP can identify alternate traffic based upon the MPLS label.
   This will be the alternate label that the MP supplied to its UMH for
   this purpose.

5.1.4.  Merge Point Reconvergence

   After a failure, the MP will want to join the multicast tree
   according to the new topology.  It is critical that the MP does this
   in a way that minimizes the traffic disruption.  Whenever paths
   change, there is also the possibility for a traffic-affecting event
   due to different latencies.  However, traffic impact above that
   should be avoided.

   The MP must do make-before-break.  Until the MP knows that its new
   UMH is fully connected to the MCI, the MP should continue to accept
   its old alternate traffic.  The MP could learn that the new UMH is
   sufficient either via control-plane mechanisms or data-driven.  In
   the latter case, the reception of traffic from the new UMH can
   trigger the change-over.  If the data-driven approach is used, a
   time-out to force the switch should apply to handle multicast trees
   that have long quiet periods.

5.1.5.  PLR termination of alternate traffic

   The PLR sends traffic on the alternates for a configurable time-out.
   There is no clean way for the next-hop routers and/or next-next-hop
   routers to indicate that the traffic is no longer needed.

   If better control were desired, each MP could tell its UMH what the
   desired time-out is.  The UMH could forward this to the PLR as well.
   Then the PLR could send alternate traffic to different MPs based upon
   the MP's individual timer.  This would only be an advantage if some
   of the MPs were expected to have a longer multicast reconvergence
   time than others - either due to load or router capabilities.

5.2.  MP-driven Unicast Tunnels

   MP-driven unicast tunnels are only relevant for mLDP where targeted
   LDP sessions are feasible.  For PIM, there is no mechanism to
   communicate beyond a router's immediate neighbors; these techniques
   could work for link-protection, but even then there would not be a
   way of requesting that the PLR should stop sending traffic.

   There are three differences for MP-driven unicast tunnels from PLR-



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   driven unicast tunnels.

   1.  The MPs learn the identity of the PLR from their UMH.  The PLR
       does not learn the identities of the MPs.

   2.  The MPs create direct connections to the PLR and communicate
       their alternate labels.

   3.  When the MPs have converged, each explicitly tells the PLR to
       stop sending alternate traffic.

   The first means that a router communicates its UMH to all its
   downstream multicast hops.  Then each MP communicates to the PLR(s)
   (1 for link-protection and 1 for node-protection) and indicates the
   multicast tree that protection is desired for and the associated
   alternate label.

   When the PLR learns about a new MP, it adds that MP and associated
   information to the set of MPs to be protected.  On a failure, the PLR
   does the same behavior as for the PLR-driven unicast tunnels.

   After the failure, the MP reconverges using make-before-break.  Then
   the MP explicitly communicates to the PLR(s) that alternate traffic
   is no longer needed for that multicast tree.  When the node-
   protecting PLR hasn't changed for a MP, it may be necessary to
   withdraw the old alternate label, which tells the PLR to stop
   transmitting alternate traffic, and then provide a new alternate
   label.

5.3.  MP-driven Alternate Trees

   In this document we have defined different solutions to achieve fast
   convergence for multicast link and node protection based on MRTs.  At
   a high level these solutions can be separated in Local and Global
   protections.  Alternate Trees, which is a Local protection schema,
   initially looked like an attractive solution for Multicast node
   protection since it avoids replicating the packet by the PLR to each
   of the receivers of the protected node and waisting bandwidth.
   However, this comes at the expense of extra multicast state and
   complexity.  In order to mitigate the extra multicast state its
   possible to aggregate the Alternate Trees by creating an Alternate
   Tree per protected node and reuse it for all the multicast trees
   going through this node.  This further complicates the procedures and
   upstream assigned labels are required to de-aggregate the trees.
   With aggregation we are also introducing an unwanted side effect.
   The receiver population of the aggregated trees will very likely not
   be the same.  That means multicast packets will be forwarded on the
   Alternate Tree to node(s) that may not have a receiver(s) for the



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   protected tree.  The more protected trees are aggregated, the higher
   the risk of forwarding unwanted multicast packets, this leads again
   to waisting bandwidth.

   Considering the complexity of this solution and the unwanted side-
   effects, the authors of this document believe its better to solve
   Multicast node protection using a Global protection schema, as
   documented in Section 4.  The solution previously defined in this
   section has been move to Appendix A (Section 9).


6.  Acknowledgements

   The authors would like to thank Kishore Tiruveedhula, Santosh Esale,
   and Maciek Konstantynowicz for their suggestions and review.


7.  IANA Considerations

   This doument includes no request to IANA.


8.  Security Considerations

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


9.  Appendix A

9.1.  MP-driven Alternate Trees

   For some networks, it is highly desirable not to have the PLR perform
   replication to each MP.  PLR replication can cause substantial
   congestion on links used by alternates to different MPs.  At the same
   time, it is also desirable to have minimal extra state created in the
   network.  This can be resolved by creating alternate-trees that can
   protect multiple multicast groups as a bypass-alternate-tree.  An
   alternate-tree can also be created per multicast group, PLR and
   failure point.

   It is not possible to merge alternate-trees for different PLRs or for
   different neighbors.  This is shown in Figure 4 where G can't select
   an acceptable upstream node on the alternate tree that doesn't
   violate either the need to avoid C (for PLR A) or D (for PLR B).






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           |-------[S]--------|           Alternate from A must avoid C
           V                  V           Alternate from B ust avoid D
          [A]------[E]-------[B]
           |        |         |
           V        |         V
       |--[C]------[F]-------[D]---|
       |   |                  |    |
       |   |-------[G]--------|    |
       |            |              |
       |            |              |
       |->[R1]-----[H]-------[R2]<-|

          (a) Multicast tree from S
        S->A->C->R1  and  S->B->D->R2


        Figure 4: Alternate Trees from PLR A and B can't be merged

   A MP that joins an alternate-tree for a particular multicast stream
   should not expect or request PLR-replicated tunneled alternate
   traffic for that same multicast stream.

   Each alternate-tree is identified by the PLR which sources the
   traffic and the failure point (node and link) (FP) to be avoided.
   Different multicast groups with the same PLR and FP may have
   different sets of MPs - but they are all at most going to include the
   FP (for link protection) and the neighbors of FP except for the PLR.
   For a bypass-alternate-tree to work, it must be acceptable to
   temporarily send a multicast group's traffic to FP's neighbors that
   do not need it.  This is the trade-off required to reduce alternate-
   tree state and use bypass-alternate-trees.  As discussed in
   Section 5.1.3, a potential MP can determine whether to accept
   alternate traffic based upon the state of its normal upstream links.
   Alternate traffic for a group the MP hasn't joined can just be
   discarded.


                             [S]......[PLR]--[ A ]
                                       | |     |
                                      1| |2    |
                                      [ FP]--[MP3]
                                        |  \   |
                                        |   \  |
                                     [MP1]--[MP2]


                     Figure 5: Alternate Tree Scenario




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   For any router, knowing the PLR and the FP to avoid will force
   selection of either the Blue MRT or the Red MRT.  It is possible that
   the FP doesn't actually appear in either MRT path, but the FP will
   always be in either the set of nodes that might be used for the Blue
   MRT path or the set of nodes that might be used for the Red MRT path.
   The FP's membership in one of the sets is a function of the partial
   ordering and topological ordering created by the MRT algorithm and is
   consistent between routers in the network graph.

   To create an alternate-tree, the following must happen:

   1.  For node-protection, the MP learns from its upstream (the FP) the
       node-id of its upstream (the PLR) and, optionally, a link
       identifier for the link used to the PLR.  The link-id is only
       needed for traffic handling in PIM, since mLDP can have targeted
       sessions between the MP and the PLR.

   2.  For link-protection, the MP needs to know the node-id of its
       upstream (the PLR) and, optionally, its identifier for the link
       used to the PLR.

   3.  The MP determines whether to use the Blue or Red forwarding
       topology to reach the PLR while avoiding the FP and associated
       interface.  This gives the MP its alternate-tree upstream
       interface.

   4.  The MP signals a backup-join to its alternate-tree upstream
       interface.  The backup-join specifies the PLR, FP and, for PIM,
       the FP-PLR link identifier.  If the alternate-tree is not to be
       used as a bypass-alternate-tree, then the multicast group (e.g.
       (S,G) or Opaque-Value) must be specified.

   5.  A router that receives a backup-join and is not the PLR needs to
       create multicast state and send a backup-join towards the PLR on
       the appropriate Blue or Red forwarding topology as is locally
       determined to avoid the FP and FP-PLR link.

   6.  Backup-joins for the same (PLR, FP, PLR-FP link-id) that
       reference the same multicast group can be merged into a single
       alternate-tree.  Similarly, backup-joins for the same (PLR, FP,
       PLR-FP link-id) that reference no multicast group can be merged
       into a single alternate-tree.

   7.  When the PLR receives the backup-join, it associates either the
       specified multicast group with that alternate-tree, if such is
       given, or associates all multicast groups that go to the FP via
       the specified FP-PLR link with the alternate-tree.




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   For an example, look at Figure 5.  FP would send a backup-join to MP3
   indicating (PLR, FP, PLR-FP link-1).  MP3 sends a backup-join to A.
   MP1 sends a backup-join to MP2 and MP2 sends a backup-join to MP3.

   It is necessary that traffic on each alternate-tree self-identify as
   to which alternate-tree it is part of.  This is because an alternate-
   tree for a multicast-group and a particular (PLR, FP, PLR-FP link-id)
   can easily overlap with an alternate-tree for the same multicast
   group and a different (PLR, FP, PLR-FP link-id).  The best way of
   doing this depends upon whether PIM or mLDP is being used.

9.1.1.  PIM details for Alternate-Trees

   For PIM, the (S,G) of the IP packet is a globally unique identifier
   and is understood.  To identify the alternate-tree, the most
   straightforward way is to use MPLS labels distributed in the PIM
   backup-join messages.  A MP can use the incoming label to indicate
   the set of RPF-interfaces for which the traffic may be an alternate.
   If the alternate-tree isn't a bypass-alternate-tree, then only one
   RPF interface is referenced.  If the alternate-tree is a bypass-
   alternate-tree, then multiple RPF-interfaces (parallel links to FP)
   might be intended.  Alternate-tree traffic may cross an interface
   multiple times - either because the interface is a broadcast
   interface and different downstream-assigned labels are provided
   and/or because a MP may provide different labels.

9.1.2.  mLDP details for Alternate-Trees

   For mLDP, if bypass-alternate-trees are used, then the PLR must
   provide upstream-assigned labels to each multicast stream.  The MP
   provides the label for the alternate-tree; if the alternate-tree is
   not a bypass-alternate-tree, this label also describes the multicast
   stream.  If the alternate-tree is a bypass-alternate-tree, then it
   provides the context for the PLR-assigned labels for each multicast
   stream.  If there are targeted LDP sessions between the PLR and the
   MPs, then the PLR could provide the necessary upstream-assigned
   labels.

9.1.3.  Traffic Handling by PLR

   An issue with traffic is how long should the PLR continue to send
   alternate traffic out.  With an alternate-tree, the PLR can know to
   stop forwarding alternate traffic on the alternate-tree when that
   alternate-tree's state is torn down.  This provides a clear signal
   that alternate traffic is no longer needed.






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9.2.  Methods Compared for PIM

   The two approaches that are feasible for PIM are PLR-driven Unicast
   Tunnels and MP-driven Alternate-Trees.

   +-------------------------+-------------------+---------------------+
   |          Aspect         |     PLR-driven    |      MP-driven      |
   |                         |  Unicast Tunnels  |   Alternate-Trees   |
   +-------------------------+-------------------+---------------------+
   |    Worst-case Traffic   | 1 + number of MPs |          2          |
   |   Replication Per Link  |                   |                     |
   |  PLR alternate-traffic  |    timer-based    |    control-plane    |
   |                         |                   |      terminated     |
   |  Extra multicast state  |        none       |  per (PLR,FP,S) for |
   |                         |                   |     bypass mode     |
   +-------------------------+-------------------+---------------------+

   Which approach is prefered may be network-dependent.  It should also
   be possible to use both in the same network.

9.3.  Methods Compared for mLDP

   All three approaches are feasible for mLDP.  Below is a brief
   comparison of various aspects of each.

   +-------------------+---------------+-------------+-----------------+
   |       Aspect      |   MP-driven   |  PLR-driven |    MP-driven    |
   |                   |    Unicast    |   Unicast   | Alternate-Trees |
   |                   |    Tunnels    |   Tunnels   |                 |
   +-------------------+---------------+-------------+-----------------+
   |     Worst-case    | 1 + number of |  1 + number |        2        |
   |      Traffic      |      MPs      |    of MPs   |                 |
   |  Replication Per  |               |             |                 |
   |        Link       |               |             |                 |
   |        PLR        | control-plane | timer-based |  control-plane  |
   | alternate-traffic |   terminated  |             |    terminated   |
   |  Extra multicast  |      none     |     none    |  per (PLR,FP,S) |
   |       state       |               |             | for bypass mode |
   +-------------------+---------------+-------------+-----------------+


10.  References

10.1.  Normative References

   [I-D.enyedi-rtgwg-mrt-frr-algorithm]
              Atlas, A., Envedi, G., Csaszar, A., and A. Gopalan,
              "Algorithms for computing Maximally Redundant Trees for



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              IP/LDP Fast- Reroute",
              draft-enyedi-rtgwg-mrt-frr-algorithm-03 (work in
              progress), July 2013.

   [I-D.ietf-rtgwg-mrt-frr-architecture]
              Atlas, A., Kebler, R., Envedi, G., Csaszar, A., Tantsura,
              J., Konstantynowicz, M., White, R., and M. Shand, "An
              Architecture for IP/LDP Fast-Reroute Using Maximally
              Redundant Trees", draft-ietf-rtgwg-mrt-frr-architecture-03
              (work in progress), July 2013.

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.

   [RFC6388]  Wijnands, IJ., Minei, I., Kompella, K., and B. Thomas,
              "Label Distribution Protocol Extensions for Point-to-
              Multipoint and Multipoint-to-Multipoint Label Switched
              Paths", RFC 6388, November 2011.

   [RFC6420]  Cai, Y. and H. Ou, "PIM Multi-Topology ID (MT-ID) Join
              Attribute", RFC 6420, November 2011.

10.2.  Informative References

   [I-D.ietf-rtgwg-mofrr]
              Karan, A., Filsfils, C., Farinacci, D., Wijnands, I.,
              Decraene, B., Joorde, U., and W. Henderickx, "Multicast
              only Fast Re-Route", draft-ietf-rtgwg-mofrr-02 (work in
              progress), June 2013.

   [I-D.iwijnand-mpls-mldp-multi-topology]
              Wijnands, I. and K. Raza, "mLDP Extensions for Multi
              Topology Routing",
              draft-iwijnand-mpls-mldp-multi-topology-03 (work in
              progress), June 2013.

   [I-D.kebler-pim-mrt-protection]
              Kebler, R., Atlas, A., Wijnands, IJ., and G. Enyedi, "PIM
              Extensions for Protection Using Maximally Redundant
              Trees",  draft-kebler-pim-mrt-protection-00 (work in
              progress), March 2012.

   [I-D.wijnands-mpls-mldp-node-protection]
              Wijnands, I., Rosen, E., Raza, K., Tantsura, J., Atlas,
              A., and Q. Zhao, "mLDP Node Protection",
              draft-wijnands-mpls-mldp-node-protection-04 (work in
              progress), June 2013.



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   [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.


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


   IJsbrand Wijnands
   Cisco Systems, Inc.

   Email: ice@cisco.com


   Andras Csaszar
   Ericsson
   Konyves Kalman krt 11
   Budapest  1097
   Hungary

   Email: Andras.Csaszar@ericsson.com











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

   Email: Gabor.Sandor.Enyedi@ericsson.com












































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