LDP for MRTs: LDP Extensions to Support Maximally Redundant Trees

Internet Engineering Task Force (IETF)                          A. Atlas
Request for Comments: 8320                               K. Tiruveedhula
Category: Standards Track                                      C. Bowers
ISSN: 2070-1721                                         Juniper Networks
                                                             J. Tantsura
                                                              Individual
                                                            IJ. Wijnands
                                                     Cisco Systems, Inc.
                                                           February 2018


          LDP Extensions to Support Maximally Redundant Trees

Abstract

   This document specifies extensions to the Label Distribution Protocol
   (LDP) to support the creation of Label Switched Paths (LSPs) for
   Maximally Redundant Trees (MRTs).  A prime use of MRTs is for unicast
   and multicast IP/LDP Fast Reroute, which we will refer to as
   "MRT-FRR".

   The sole protocol extension to LDP is simply the ability to advertise
   an MRT Capability.  This document describes that extension and the
   associated behavior expected for Label Switching Routers (LSRs) and
   Label Edge Routers (LERs) advertising the MRT Capability.

   MRT-FRR uses LDP multi-topology extensions, so three multi-topology
   IDs have been allocated from the MPLS MT-ID space.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8320.

Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Overview of LDP Signaling Extensions for MRT  . . . . . . . .   6
     4.1.  MRT Capability Advertisement  . . . . . . . . . . . . . .   6
       4.1.1.  Interaction of MRT Capability and MT Capability . . .   7
       4.1.2.  Interaction of LDP MRT Capability with IPv4 and IPv6    8
     4.2.  Use of the Rainbow MRT MT-ID  . . . . . . . . . . . . . .   8
     4.3.  MRT-Blue and MRT-Red FECs . . . . . . . . . . . . . . . .   8
     4.4.  Interaction of MRT-Related LDP Advertisements with the
           MRT Topology and Computations . . . . . . . . . . . . . .   9
   5.  LDP MRT FEC Advertisements  . . . . . . . . . . . . . . . . .  10
     5.1.  MRT-Specific Behavior . . . . . . . . . . . . . . . . . .  10
       5.1.1.  ABR Behavior and Use of the Rainbow FEC . . . . . . .  10
       5.1.2.  Proxy-Node Attachment Router Behavior . . . . . . . .  11
     5.2.  LDP Protocol Procedures in the Context of MRT Label
           Distribution  . . . . . . . . . . . . . . . . . . . . . .  12
       5.2.1.  LDP Peer in RFC 5036  . . . . . . . . . . . . . . . .  12
       5.2.2.  Next Hop in RFC 5036  . . . . . . . . . . . . . . . .  13
       5.2.3.  Egress LSR in RFC 5036  . . . . . . . . . . . . . . .  13
       5.2.4.  Use of Rainbow FEC to Satisfy Label Mapping Existence
               Requirements in RFC 5036  . . . . . . . . . . . . . .  15
       5.2.5.  Validating FECs in the Routing Table  . . . . . . . .  15
       5.2.6.  Recognizing New FECs  . . . . . . . . . . . . . . . .  15
       5.2.7.  Not Propagating Rainbow FEC Label Mappings  . . . . .  15
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   7.  Potential Restrictions on MRT-Related MT-ID Values Imposed by
       RFC 6420  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   This document describes the LDP signaling extensions and associated
   behavior necessary to support the architecture that defines how IP/
   LDP Fast Reroute can use MRTs [RFC7812].  The current document
   provides a brief description of the MRT-FRR architecture, focusing on
   the aspects most directly related to LDP signaling.  The complete
   description and specification of the MRT-FRR architecture can be
   found in [RFC7812].

   At least one common standardized algorithm (e.g., the MRT Lowpoint
   algorithm explained and fully documented in [RFC7811]) is required to
   be deployed so that the routers supporting MRT computation
   consistently compute the same MRTs.  LDP depends on an IGP for
   computation of MRTs and alternates.  Extensions to OSPF are defined
   in [OSPF-MRT].  Extensions to IS-IS are defined in [IS-IS-MRT].

   MRT can also be used to protect multicast traffic (signaled via PIM
   or Multipoint LDP (mLDP)) using either global protection or local
   protection as described in [ARCH].  An MRT path can be used to
   provide node-protection for mLDP traffic via the mechanisms described
   in [RFC7715]; an MRT path can also be used to provide link protection
   for mLDP traffic.

   For each destination, IP/LDP Fast Reroute with MRT (MRT-FRR) creates
   two alternate destination-based trees separate from the shortest-path
   forwarding used during stable operation.  LDP uses the multi-topology
   extensions [RFC7307] to signal Forwarding Equivalency Classes (FECs)
   for these two sets of forwarding trees, MRT-Blue and MRT-Red.

   In order to create MRT paths and support IP/LDP Fast Reroute, a new
   capability extension is needed for LDP.  An LDP implementation
   supporting MRT MUST also follow the rules described here for
   originating and managing FECs related to MRT, as indicated by their
   multi-topology ID.  Network reconvergence is described in [RFC7812]
   and the worst-case network convergence time can be flooded via the
   extension in [PARAM-SYNC].

   IP/LDP Fast Reroute using MRTs can provide 100% coverage for link and
   node failures in an arbitrary network topology where the failure
   doesn't partition the network.  It can also be deployed
   incrementally; an MRT Island is formed of connected supporting
   routers and the MRTs are computed inside that island.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   For ease of reading, some of the terminology defined in [RFC7812] is
   repeated here.  Please refer to Section 3 of [RFC7812] for a more
   complete list.

   Redundant Trees (RTs):  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.
      Redundant trees can always be computed in 2-connected graphs.

   Maximally Redundant Trees (MRTs):  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.
      In graphs that are not 2-connected, it is not possible to compute
      RTs.  However, it is possible to compute MRTs.  MRTs are maximally
      redundant in the sense that they are as redundant as possible
      given the constraints of the network graph.

   MRT-Red:  MRT-Red is used to describe one of the two MRTs; it is used
      to describe the associated forwarding topology and MPLS Multi-
      Topology Identifier (MT-ID).

   MRT-Blue:  MRT-Blue is used to describe one of the two MRTs; it is
      used to described the associated forwarding topology and MPLS
      MT-ID.

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

   MRT Island:  The set of routers that support a particular MRT Profile
      and the links connecting them that support MRT.

   Island Border Router (IBR):  A router in the MRT Island that is
      connected to a router not in the MRT Island, both of which are in
      a common area or level.

   Island Neighbor (IN):  A router that is not in the MRT Island but is
      adjacent to an IBR and in the same area/level as the IBR.

   There are several places in this document where the construction
   "red(blue) FEC" is used to cover the case of the red FEC and the case
   of the blue FEC, independently.  As an example, consider the sentence
   "When the ABR requires best-area behavior for a red(blue) FEC, it
   MUST withdraw any existing label mappings advertisements for the
   corresponding Rainbow FEC and advertise label mappings for the
   red(blue) FEC."  This sentence should be read as applying to red
   FECs.  Then it should be read as applying to blue FECs.

4.  Overview of LDP Signaling Extensions for MRT

   Routers need to know which of their LDP neighbors support MRT.  This
   is communicated using the MRT Capability Advertisement.  Supporting
   MRT indicates several different aspects of behavior, as listed below.

   1.  Sending and receiving multi-topology FEC elements, as defined in
       [RFC7307].

   2.  Understanding the Rainbow MRT MT-ID and applying the associated
       labels to all relevant MT-IDs.

   3.  Advertising the Rainbow MRT FEC to the appropriate neighbors for
       the appropriate prefix.

   4.  If acting as LDP egress for a prefix in the default topology,
       also acting as egress for the same prefix in MRT-Red and
       MRT-Blue.

   5.  For a FEC learned from a neighbor that does not support MRT,
       originating FECs for MRT-Red and MRT-Blue with the same prefix.
       This MRT Island egress behavior is to support an MRT Island that
       does not include all routers in the area/level.

4.1.  MRT Capability Advertisement

   A new MRT Capability Parameter TLV is defined in accordance with the
   LDP Capability definition guidelines [RFC5561].

   The LDP MRT Capability can be advertised during LDP session
   initialization or after the LDP session is established.
   Advertisement of the MRT Capability indicates support of the
   procedures for establishing the MRT-Blue and MRT-Red Label Switched
   Paths (LSPs) detailed in this document.  If the peer has not
   advertised the MRT Capability, then it indicates that LSR does not
   support MRT procedures.

   If a router advertises the LDP MRT Capability to its peer, but the
   peer has not advertised the MRT Capability, then the router MUST NOT
   advertise MRT-related FEC-label bindings to that peer.

   The following is the format of the MRT Capability Parameter.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |U|F| MRT Capability (0x050E)   |      Length (= 1)             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |S| Reserved    |
     +-+-+-+-+-+-+-+-+

                         MRT Capability TLV Format

   Where:

   U-bit:  The unknown TLV bit MUST be 1.  A router that does not
      recognize the MRT Capability TLV will silently ignore the TLV and
      process the rest of the message as if the unknown TLV did not
      exist.

   F-bit:  The forward unknown TLV bit MUST be 0 as required by
      Section 3 of [RFC5561].

   MRT Capability:  0x050E

   Length:  The length (in octets) of the TLV.  Its value is 1.

   S-bit:  The State bit MUST be 1 if used in the LDP Initialization
      message.  MAY be set to 0 or 1 in the dynamic Capability message
      to advertise or withdraw the capability, respectively, as
      described in [RFC5561].

4.1.1.  Interaction of MRT Capability and MT Capability

   An LSR advertising the LDP MRT Capability MUST also advertise the LDP
   Multi-Topology (MT) Capability.  If an LSR negotiates the LDP MRT
   Capability with an LDP neighbor without also negotiating the LDP MT
   Capability, the LSR MUST behave as if the LDP MRT Capability was not
   negotiated and respond with the "MRT Capability negotiated without MT
   Capability" status code in the LDP Notification message (defined in
   the document).  The E-bit of this Notification should be set to 0 to
   indicate that this is an Advisory Notification.  The LDP session
   SHOULD NOT be terminated.

4.1.2.  Interaction of LDP MRT Capability with IPv4 and IPv6

   The MRT LDP Capability Advertisement does not distinguish between
   IPv4 and IPv6 address families.  An LSR that advertises the MRT LDP
   Capability is expected to advertise MRT-related FEC-label bindings
   for the same address families for which it advertises shortest-path
   FEC-label bindings.  Therefore, an LSR advertising MRT LDP Capability
   and shortest-path FEC-label bindings for IPv4 only (or IPv6 only)
   would be expected to advertise MRT-related FEC-label binding for IPv4
   only (or IPv6 only).  An LSR advertising the MRT LDP Capability and
   shortest-path FEC-label bindings for BOTH IPv4 and IPv6 is expected
   to advertise MRT-related FEC-label bindings for BOTH IPv4 and IPv6.
   In this scenario, advertising MRT-related FEC-label bindings only for
   IPv4 only (or only for IPv6) is not supported.

4.2.  Use of the Rainbow MRT MT-ID

   Section 10.1 of [RFC7812] describes the need for an Area Border
   Router (ABR) to have different neighbors use different MPLS labels
   when sending traffic to the ABR for the same FEC.  More detailed
   discussion of the Rainbow MRT MT-ID is provided in Section 5.1.1.

   Another use for the Rainbow MRT MT-ID is for an LSR to send the
   Rainbow MRT MT-ID with an IMPLICIT_NULL label to indicate
   penultimate-hop-popping for all three types of FECs (shortest path,
   red, and blue).  The EXPLICIT_NULL label advertised using the Rainbow
   MRT MT-ID similarly applies to all the types of FECs.  Note that the
   only scenario in which it is generally useful to advertise the
   implicit or explicit null label for all three FEC types is when the
   FEC refers to the LSR itself.  See Section 5.2.3 for more details.

   The value of the Rainbow MRT MPLS MT-ID (3945) has been assigned by
   IANA from the MPLS MT-ID space.

4.3.  MRT-Blue and MRT-Red FECs

   To provide MRT support in LDP, the MT Prefix FEC is used.  [RFC7812]
   defines the Default MRT Profile.  Section 8 specifies the values in
   the "MPLS Multi-Topology Identifiers" registry for the MRT-Red and
   MRT-Blue MPLS MT-IDs associated with the Default MRT Profile (3946
   and 3947).

   As described in Section 8.1 of [RFC7812], when a new MRT Profile is
   defined, new and unique values should be allocated from the "MPLS
   Multi-Topology Identifiers" registry, corresponding to the MRT-Red
   and MRT-Blue MT-ID values for the new MRT Profile.

   The MT Prefix FEC encoding is defined in [RFC7307] and is used
   without alteration for advertising label mappings for MRT-Blue,
   MRT-Red, and Rainbow MRT FECs.

4.4.  Interaction of MRT-Related LDP Advertisements with the MRT
      Topology and Computations

   [RFC7811] and [RFC7812] describe how the MRT topology is created
   based on information in IGP advertisements.  The MRT topology and
   computations rely on IGP advertisements.  The presence or absence of
   MRT-related LDP advertisements does not affect the MRT topology or
   the MRT-Red and MRT-Blue next hops computed for that topology.

   As an example, consider a network where all nodes are running MRT IGP
   extensions to determine the MRT topology, which is then used to
   compute MRT-Red and MRT-Blue next hops.  The network operator also
   configures the nodes in this network to exchange MRT-related LDP
   advertisements in order to distribute MPLS labels corresponding to
   those MRT next hops.  Suppose that, due to a misconfiguration on one
   particular link, the MRT-related LDP advertisements are not being
   properly exchanged for that link.  Since the MRT-related IGP
   advertisements for the link are still being distributed, the link is
   still included in the MRT topology and computations.  In this
   scenario, there will be missing MPLS forwarding entries corresponding
   to paths that use the misconfigured link.

   Note that the situation is analogous to the interaction of normal LDP
   advertisements and IGP advertisements for shortest-path forwarding.
   Deactivating the distribution of labels for normal shortest-path FECs
   on a link does not change the topology on which the Shortest Path
   First (SPF) algorithm is run by the IGP.

   "LDP IGP Synchronization" [RFC5443] addresses the issue of the LDP
   topology not matching the IGP topology by advertising the maximum IGP
   cost on links where LDP is not fully operational.  This makes the IGP
   topology match the LDP topology.  As described in Section 7.3.1 of
   [RFC7812], MRT is designed to be compatible with the LDP IGP
   synchronization mechanism.  When the IGP advertises the maximum cost
   on a link where LDP is not fully operational, the link is excluded
   from MRT Island formation, which prevents the MRT algorithm from
   creating any paths using that link.

5.  LDP MRT FEC Advertisements

   This sections describes how and when labels for MRT-Red and MRT-Blue
   FECs are advertised.  In order to provide protection paths that are
   immediately usable by the point of local repair in the event of a
   failure, the associated LSPs need to be created before a failure
   occurs.

   In this section, we will use the term "shortest-path FEC" to refer to
   the usual FEC associated with the shortest-path destination-based
   forwarding tree for a given prefix as determined by the IGP.  We will
   use the terms "red FEC" and "blue FEC" to refer to FECs associated
   with the MRT-Red and MRT-Blue destination-based forwarding trees for
   a given prefix as determined by a particular MRT algorithm.

   We first describe label distribution behavior specific to MRT.  Then,
   we provide the correct interpretation of several important concepts
   in [RFC5036] in the context of MRT FEC label distribution.

   [RFC5036] specifies two different Label Distribution Control Modes
   (Independent and Ordered), two different Label Retention Modes
   (Conservative and Liberal), and two different Label Advertisement
   Modes (Downstream Unsolicited and Downstream on Demand).  The current
   specification for LDP MRT requires that the same Label Distribution
   Control, Label Retention, and Label Advertisement modes be used for
   the shortest-path FECs and the MRT FECs.

5.1.  MRT-Specific Behavior

5.1.1.  ABR Behavior and Use of the Rainbow FEC

   Section 10.1 of [RFC7812] describes the need for an ABR to have
   different neighbors use different MPLS labels when sending traffic to
   the ABR for the same FEC.  The method to accomplish this using the
   Rainbow MRT MT-ID is described in detail in [RFC7812].  Here we
   provide a brief summary.  To those LDP peers in the same area as the
   best route to the destination, the ABR advertises two different
   labels corresponding to the MRT-Red and MRT-Blue forwarding trees for
   the destination.  An LDP peer receiving these advertisements forwards
   MRT traffic to the ABR using these two different labels, depending on
   the FEC of the traffic.  We refer to this as "best-area advertising
   and forwarding behavior", which is identical to normal MRT behavior.

   For all other LDP peers supporting MRT, the ABR advertises a FEC-
   label binding for the FEC, which is in the Rainbow MRT MT-ID, with
   the label that corresponds to that FEC in the default forwarding tree
   for the destination.  An LDP peer receiving this advertisement
   forwards MRT traffic to the ABR using this label, for both MRT-Red
   and MRT-Blue traffic.  We refer to this as "non-best-area advertising
   and forwarding behavior".

   The use of the Rainbow-FEC by the ABR for non-best-area
   advertisements is RECOMMENDED.  An ABR MAY advertise the label for
   the default topology in separate MRT-Blue and MRT-Red advertisements.
   An LSR advertising the MRT Capability MUST recognize the Rainbow MRT
   MT-ID and associate the advertised label with the specific prefix
   with the MRT-Red and MRT-Blue MT-IDs associated with all MRT Profiles
   that advertise LDP as the forwarding mechanism.

   Due to changes in topology or configuration, an ABR and a given LDP
   peer may need to transition from best-area advertising and forwarding
   behavior to non-best-area behavior for a given destination, and vice
   versa.  When the ABR requires best-area behavior for a red(blue) FEC,
   it MUST withdraw any existing label mappings advertisements for the
   corresponding Rainbow FEC and advertise label mappings for the
   red(blue) FEC.  When the ABR requires non-best-area behavior for a
   red(blue) FEC, it MUST withdraw any existing label mappings for both
   red and blue FECs and advertise label mappings for the corresponding
   Rainbow FEC label-binding.

   In this transition, an ABR should never advertise a red(blue) FEC
   before withdrawing the corresponding Rainbow FEC (or vice versa).
   However, should this situation occur, the expected behavior of an LSR
   receiving these conflicting advertisements is defined as follows:

   -  If an LSR receives a label mapping advertisement for a Rainbow FEC
      from an MRT LDP peer while it still retains a label mapping for
      the corresponding red or blue FEC, the LSR MUST continue to use
      the label mapping for the red or blue FEC, and it MUST send a
      Label Release message corresponding to the Rainbow FEC label
      advertisement.

   -  If an LSR receives a label mapping advertisement for a red or blue
      FEC while it still retains a label mapping for the corresponding
      Rainbow FEC, the LSR MUST continue to use the label mapping for
      the Rainbow FEC, and it MUST send a Label Release message
      corresponding to the red or blue FEC label advertisement.

5.1.2.  Proxy-Node Attachment Router Behavior

   Section 11.2 of [RFC7812] describes how MRT provides FRR protection
   for multi-homed prefixes using calculations involving a named proxy-
   node.  This covers the scenario where a prefix is originated by a
   router in the same area as the MRT Island, but outside of the MRT
   Island.  It also covers the scenario of a prefix being advertised by
   multiple routers in the MRT Island.

   In the named proxy-node calculation, each multi-homed prefix is
   represented by a conceptual proxy-node that is attached to two real
   proxy-node attachment routers.  (A single proxy-node attachment
   router is allowed in the case of a prefix advertised by a same area
   router outside of the MRT Island, which is singly connected to the
   MRT Island.)  All routers in the MRT Island perform the same
   calculations to determine the same two proxy-node attachment routers
   for each multi-homed prefix.  Section 5.9 of [RFC7811] describes the
   procedure for identifying one proxy-node attachment router as "red"
   and one as "blue" with respect to the multi-homed prefix, and
   computing the MRT red and blue next hops to reach those red and blue
   proxy-node attachment routers.

   In terms of LDP behavior, a red proxy-node attachment router for a
   given prefix MUST originate a label mapping for the red FEC for that
   prefix, while the blue proxy-node attachment router for a given
   prefix MUST originate a label mapping for the blue FEC for that
   prefix.  If the red(blue) proxy-node attachment router is an Island
   Border Router (IBR), then when it receives a packet with the label
   corresponding to the red(blue) FEC for a prefix, it MUST forward the
   packet to the Island Neighbor (IN) whose cost was used in the
   selection of the IBR as a proxy-node attachment router.  The IBR MUST
   swap the incoming label for the outgoing label corresponding to the
   shortest-path FEC for the prefix advertised by the IN.  In the case
   where the IN does not support LDP, the IBR MUST pop the incoming
   label and forward the packet to the IN.

   If the proxy-node attachment router is not an IBR, then the packet
   MUST be removed from the MRT forwarding topology and sent along the
   interface(s) that caused the router to advertise the prefix.  This
   interface might be out of the area/level/AS.

5.2.  LDP Protocol Procedures in the Context of MRT Label Distribution

   [RFC5036] specifies the LDP label distribution procedures for
   shortest-path FECs.  In general, the same procedures can be applied
   to the distribution of label mappings for red and blue FECs, provided
   that the procedures are interpreted in the context of MRT FEC label
   distribution.  The correct interpretation of several important
   concepts in [RFC5036] in the context of MRT FEC label distribution is
   provided below.

5.2.1.  LDP Peer in RFC 5036

   In the context of distributing label mappings for red and blue FECs,
   we restrict the LDP peer in [RFC5036] to mean LDP peers for which the
   LDP MRT Capability has been negotiated.  In order to make this
   distinction clear, in this document we will use the term "MRT LDP
   peer" to refer to an LDP peer for which the LDP MRT Capability has
   been negotiated.

5.2.2.  Next Hop in RFC 5036

   Several procedures in [RFC5036] use the next hop of a (shortest-path)
   FEC to determine behavior.  The next hop of the shortest-path FEC is
   based on the shortest-path forwarding tree to the prefix associated
   with the FEC.  When the procedures of [RFC5036] are used to
   distribute label mapping for red and blue FECs, the next hop for the
   red(blue) FEC is based on the MRT-Red(Blue) forwarding tree to the
   prefix associated with the FEC.

   For example, Appendix A.1.7 of [RFC5036] specifies the response by an
   LSR to a change in the next hop for a FEC.  For a shortest-path FEC,
   the next hop may change as the result of the LSR running a shortest-
   path computation on a modified IGP topology database.  For the red
   and blue FECs, the red and blue next hops may change as the result of
   the LSR running a particular MRT algorithm on a modified IGP topology
   database.

   As another example, Section 2.6.1.2 of [RFC5036] specifies that when
   an LSR is using LSP Ordered Control, it may initiate the transmission
   of a label mapping only for a (shortest-path) FEC for which it has a
   label mapping for the FEC next hop, or for which the LSR is the
   egress.  The FEC next hop for a shortest-path FEC is based on the
   shortest-path forwarding tree to the prefix associated with the FEC.
   In the context of distributing MRT LDP labels, this procedure is
   understood to mean the following.  When an LSR is using LSP Ordered
   Control, it may initiate the transmission of a label mapping only for
   a red(blue) FEC for which it has a label mapping for the red(blue)
   FEC next hop, or for which the LSR is the egress.  The red or blue
   FEC next hop is based on the MRT-Red or Blue forwarding tree to the
   prefix associated with the FEC.

5.2.3.  Egress LSR in RFC 5036

   Procedures in [RFC5036] related to Ordered Control label distribution
   mode rely on whether or not an LSR may act as an egress LSR for a
   particular FEC in order to determine whether or not the LSR may
   originate a label mapping for that FEC.  The status of being an
   egress LSR for a particular FEC is also used in the loop detection
   procedures described in [RFC5036].  Section 2.6.1.2 of [RFC5036]
   specifies the conditions under which an LSR may act as an egress LSR
   with respect to a particular (shortest-path) FEC:

   1.  The (shortest-path) FEC refers to the LSR itself (including one
       of its directly attached interfaces).

   2.  The next hop router for the (shortest-path) FEC is outside of the
       Label Switching Network.

   3.  (Shortest-path) FEC elements are reachable by crossing a routing
       domain boundary.

   The conditions for determining an egress LSR with respect to a red or
   blue FEC need to be modified.  An LSR may act as an egress LSR with
   respect to a particular red(blue) FEC under any of the following
   conditions:

   1.  The prefix associated with the red(blue) FEC refers to the LSR
       itself (including one of its directly attached interfaces).

   2.  The LSR is the red(blue) proxy-node attachment router with
       respect to the multi-homed prefix associated with the red(blue)
       FEC.  This includes the degenerate case of a single red and blue
       proxy-node attachment router for a single-homed prefix.

   3.  The LSR is an ABR AND the MRT LDP peer requires non-best-area
       advertising and forwarding behavior for the prefix associated
       with the FEC.

   Note that condition 3 scopes an LSR's status as an egress LSR with
   respect to a particular FEC to a particular MRT LDP peer.  Therefore,
   the condition "Is LSR egress for FEC?" that occurs in several
   procedures in [RFC5036] needs to be interpreted as "Is LSR egress for
   FEC with respect to Peer?"

   Also note that there is no explicit condition that allows an LSR to
   be classified as an egress LSR with respect to a red or blue FEC
   based only on the primary next hop for the shortest-path FEC not
   supporting LDP or not supporting LDP MRT Capability.  These
   situations are covered by the proxy-node attachment router and ABR
   conditions (conditions 2 and 3).  In particular, an Island Border
   Router is not the egress LSR for a red(blue) FEC unless it is also
   the red(blue) proxy-node attachment router for that FEC.

   Also note that, in general, a proxy-node attachment router for a
   given prefix should not advertise an implicit or explicit null label
   for the corresponding red or blue FEC, even though it may be an
   egress LSR for the shortest-path FEC.  In general, the proxy-node
   attachment router needs to forward red or blue traffic for that
   prefix to a particular loop-free island neighbor, which may be
   different from the shortest-path next hop.  The proxy-node attachment
   router needs to receive the red or blue traffic with a non-null label
   to correctly forward it.

5.2.4.  Use of Rainbow FEC to Satisfy Label Mapping Existence
        Requirements in RFC 5036

   Several procedures in [RFC5036] require the LSR to determine if it
   has previously received and retained a label mapping for a FEC from
   the next hop.  In the case of an LSR that has received and retained a
   label mapping for a Rainbow FEC from an ABR, the label mapping for
   the Rainbow FEC satisfies the label mapping existence requirement for
   the corresponding red and blue FECs.  Label mapping existence
   requirements in the context of MRT LDP label distribution are
   modified as: "Has LSR previously received and retained a label
   mapping for the red(blue) FEC (or the corresponding Rainbow FEC) from
   the red(blue) next hop?"

   As an example, this behavior allows an LSR that has received and
   retained a label mapping for the Rainbow FEC to advertise label
   mappings for the corresponding red and blue FECs when operating in
   Ordered Control label distribution mode.

5.2.5.  Validating FECs in the Routing Table

   In [RFC5036], an LSR uses its routing table to validate prefixes
   associated with shortest-path FECs.  For example, Section 3.5.7.1 of
   [RFC5036] specifies that "an LSR receiving a Label Mapping message
   from a downstream LSR for a Prefix SHOULD NOT use the label for
   forwarding unless its routing table contains an entry that exactly
   matches the FEC Element."  In the context of MRT FECs, a red or blue
   FEC element matches a routing table entry if the corresponding
   shortest-path FEC element matches a routing table entry.

5.2.6.  Recognizing New FECs

   Appendix A.1.6 of [RFC5036] describes the response of an LSR to the
   "Recognize New FEC" event, which occurs when an LSR learns a new
   (shortest-path) FEC via the routing table.  In the context of MRT
   FECs, if the MRT LDP Capability has been enabled, then when an LSR
   learns a new shortest-path FEC, the LSR should generate "Recognize
   New FEC" events for the corresponding Red and Blue FECS in addition
   to the normally generated "Recognize New FEC" event for the shortest-
   path FEC

5.2.7.  Not Propagating Rainbow FEC Label Mappings

   A label mapping for the Rainbow FEC should only be originated by an
   ABR under the conditions described in Section 5.1.1.  A neighbor of
   the ABR that receives a label mapping for the Rainbow FEC MUST NOT
   propagate a label mapping for that Rainbow FEC.

6.  Security Considerations

   The labels distributed by the extensions in this document create
   additional forwarding paths that do not follow shortest-path routes.
   The transit label swapping operations defining these alternative
   forwarding paths are created during normal operations (before a
   failure occurs).  Therefore, a malicious packet with an appropriate
   label injected into the network from a compromised location would be
   forwarded to a destination along a non-shortest path.  When this
   technology is deployed, a network security design should not rely on
   assumptions about potentially malicious traffic only following
   shortest paths.

   It should be noted that the creation of non-shortest forwarding paths
   is not unique to MRT.  For example, RSVP-TE [RFC3209] can be used to
   construct forwarding paths that do not follow the shortest path.

7.  Potential Restrictions on MRT-Related MT-ID Values Imposed by
    RFC 6420

   As discussed in the introduction, in addition to unicast-forwarding
   applications, MRT can be used to provide disjoint trees for multicast
   traffic distribution.  In the case of PIM, this is accomplished by
   using the MRT red and blue next hops as the PIM Reverse Path
   Forwarding (RPF) topology, the collection of routes used by PIM to
   perform the RPF operation when building source trees.  The PIM Multi-
   Topology ID (MT-ID) Join Attribute defined in Section 5.2 of
   [RFC6420] can be used to establish MRT-based multicast distribution
   trees.  [RFC6420] limits the values of the PIM MT-ID from 1 through
   4095.

   For the purpose of reducing management overhead and simplifying
   troubleshooting, it is desirable to be able to use the same numerical
   value for the PIM MT-ID as for the MPLS MT-ID for multicast and
   unicast applications using MRT routes constructed using the same MRT
   Profile.  In order to enable this simplification, the MPLS MT-ID
   values assigned in this document fall in the range 1 through 4095.
   The "MPLS Multi-Topology Identifiers" registry reflects this by
   listing the values from 3948 through 3995 as for MRT-related MPLS
   MT-ID values.  This allows for 51 MRT-related MPLS MT-ID values that
   can be directly mapped to PIM MT-ID values, which accommodates 25 MRT
   Profiles with red and blue MT-ID pairs, with one extra for the
   Rainbow MPLS MT-ID value.  [RFC7307] designates the MT-ID range
   6-3995 as "Unassigned for future IGP topologies".  As shown in the
   IANA Considerations, the guidance for the range 3948-3995 has been
   changed to "Unassigned (for future MRT-related values)".

8.  IANA Considerations

   IANA has allocated a value for the new LDP Capability TLV from the
   "Label Distribution Protocol (LDP) Parameters" registry under "TLV
   Type Name Space": MRT Capability TLV (0x050E).

    Value          Description         Reference     Notes / Reg. Date
    -------------  ------------------  ------------  -----------------
    0x050E         MRT Capability TLV  RFC 8320

   IANA has allocated a value for the new LDP Status Code from the
   "Label Distribution Protocol (LDP) Parameters" registry under "Status
   Code Name Space": MRT Capability negotiated without MT Capability
   (0x00000034).  The Status Code E-bit is set to 0.

   Value         E  Description         Reference      Notes / Reg. Date
   ------------- -  ------------------  ------------   -----------------
   0x00000034    0  MRT Capability      RFC 8320
                    negotiated without
                    MT Capability

   IANA has allocated three values from the "MPLS Multi-Topology
   Identifiers" registry [RFC7307]:

      3945  Rainbow MRT MPLS MT-ID

      3946  Default Profile MRT-Red MPLS MT-ID

      3947  Default Profile MRT-Blue MPLS MT-ID
   Also, IANA has changed the Purpose field of the "MPLS Multi-Topology
   Identifiers" registry for MT-ID range 3948-3995 to "Unassigned (for
   future MRT-related values)".  The registration procedure for the
   entire registry remains Standards Action [RFC8126].  The current
   registry is shown below:

   Value         Purpose                                    Reference
   ------------  ----------------------                     ------------
   0             Default/standard topology                  [RFC7307]
   1             IPv4 in-band management                    [RFC7307]
   2             IPv6 routing topology                      [RFC7307]
   3             IPv4 multicast topology                    [RFC7307]
   4             IPv6 multicast topology                    [RFC7307]
   5             IPv6 in-band management                    [RFC7307]
   6-3944        Unassigned (for future IGP topologies)
   3945          Rainbow MRT MPLS MT-ID                      RFC 8320
   3946          Default Profile MRT-Red MPLS MT-ID          RFC 8320
   3947          Default Profile MRT-Blue MPLS MT-ID         RFC 8320
   3948-3995     Unassigned (for future MRT-related values)  RFC 8320
   3996-4095     Reserved for Experimental Use              [RFC7307]
   4096-65534    Unassigned (for MPLS topologies)
   65535         Wildcard Topology                          [RFC7307]

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
              October 2007, <https://www.rfc-editor.org/info/rfc5036>.

   [RFC5561]  Thomas, B., Raza, K., Aggarwal, S., Aggarwal, R., and
              JL. Le Roux, "LDP Capabilities", RFC 5561,
              DOI 10.17487/RFC5561, July 2009,
              <https://www.rfc-editor.org/info/rfc5561>.

   [RFC6420]  Cai, Y. and H. Ou, "PIM Multi-Topology ID (MT-ID) Join
              Attribute", RFC 6420, DOI 10.17487/RFC6420, November 2011,
              <https://www.rfc-editor.org/info/rfc6420>.

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

   [RFC7811]  Enyedi, G., Csaszar, A., Atlas, A., Bowers, C., and
              A. Gopalan, "An Algorithm for Computing IP/LDP Fast
              Reroute Using Maximally Redundant Trees (MRT-FRR)",
              RFC 7811, DOI 10.17487/RFC7811, June 2016,
              <https://www.rfc-editor.org/info/rfc7811>.

   [RFC7812]  Atlas, A., Bowers, C., and G. Enyedi, "An Architecture for
              IP/LDP Fast Reroute Using Maximally Redundant Trees
              (MRT-FRR)", RFC 7812, DOI 10.17487/RFC7812, June 2016,
              <https://www.rfc-editor.org/info/rfc7812>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [ARCH]     Atlas, A., Kebler, R., Wijnands, IJ., Csaszar, A., and G.
              Envedi, "An Architecture for Multicast Protection Using
              Maximally Redundant Trees", Work in Progress,
              draft-atlas-rtgwg-mrt-mc-arch-02, July 2013.

   [IS-IS-MRT]
              Li, Z., Wu, N., Zhao, Q., Atlas, A., Bowers, C., and
              J. Tantsura, "Intermediate System to Intermediate System
              (IS-IS) Extensions for Maximally Redundant Trees (MRTs)",
              Work in Progress, draft-ietf-isis-mrt-03, June 2017.

   [OSPF-MRT] Atlas, A., Hegde, S., Bowers, C., Tantsura, J., and Z. Li,
              "OSPF Extensions to Support Maximally Redundant Trees",
              Work in Progress, draft-ietf-ospf-mrt-03, June 2017.

   [PARAM-SYNC]
              Bryant, S., Atlas, A., and C. Bowers, "Routing Timer
              Parameter Synchronization", Work in Progress,
              draft-ietf-rtgwg-routing-timer-param-sync-00, October
              2017.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

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

   [RFC7715]  Wijnands, IJ., Ed., Raza, K., Atlas, A., Tantsura, J., and
              Q. Zhao, "Multipoint LDP (mLDP) Node Protection",
              RFC 7715, DOI 10.17487/RFC7715, January 2016,
              <https://www.rfc-editor.org/info/rfc7715>.

Acknowledgements

   The authors would like to thank Ross Callon, Loa Andersson, Stewart
   Bryant, Mach Chen, Greg Mirsky, Uma Chunduri, and Tony Przygienda for
   their comments and suggestions.

Authors' Addresses

   Alia Atlas
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   United States of America

   Email: akatlas@juniper.net


   Kishore Tiruveedhula
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   United States of America

   Email: kishoret@juniper.net


   Chris Bowers
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   United States of America

   Email: cbowers@juniper.net


   Jeff Tantsura
   Individual
   United States of America

   Email: jefftant.ietf@gmail.com


   IJsbrand Wijnands
   Cisco Systems, Inc.

   Email: ice@cisco.com