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Versions: (draft-atlas-mpls-ldp-mrt) 00 01 02 03 04 05 06

MPLS Working Group                                              A. Atlas
Internet-Draft                                           K. Tiruveedhula
Intended status: Standards Track                               C. Bowers
Expires: February 18, 2018                              Juniper Networks
                                                             J. Tantsura
                                                              Individual
                                                            IJ. Wijnands
                                                     Cisco Systems, Inc.
                                                         August 17, 2017


          LDP Extensions to Support Maximally Redundant Trees
                       draft-ietf-mpls-ldp-mrt-06

Abstract

   This document specifies extensions to the Label Distribution
   Protocol(LDP) to support the creation of label-switched paths for
   Maximally Redundant Trees (MRT).  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 LSRs (Label Switching Routers) and
   LERs (Label Edge Routers) advertising the MRT Capability.

   MRT-FRR uses LDP multi-topology extensions and requires three
   different multi-topology IDs to be allocated from the MPLS MT-ID
   space.

Status of This Memo

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

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

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

   This Internet-Draft will expire on February 18, 2018.





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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Overview of LDP Signaling Extensions for MRT  . . . . . . . .   5
     4.1.  MRT Capability Advertisement  . . . . . . . . . . . . . .   5
       4.1.1.  Interaction of MRT Capability and MT Capability . . .   6
       4.1.2.  Interaction of LDP MRT Capability with IPv4 and IPv6    6
     4.2.  Use of the Rainbow MRT MT-ID  . . . . . . . . . . . . . .   7
     4.3.  MRT-Blue and MRT-Red FECs . . . . . . . . . . . . . . . .   7
     4.4.  Interaction of MRT-related LDP advertisements with the
           MRT topology and computations . . . . . . . . . . . . . .   8
   5.  LDP MRT FEC Advertisements  . . . . . . . . . . . . . . . . .   8
     5.1.  MRT-specific behavior . . . . . . . . . . . . . . . . . .   9
       5.1.1.  ABR behavior and use of the Rainbow FEC . . . . . . .   9
       5.1.2.  Proxy-node attachment router behavior . . . . . . . .  10
     5.2.  LDP protocol procedures in the context of MRT label
           distribution  . . . . . . . . . . . . . . . . . . . . . .  11
       5.2.1.  LDP peer in RFC5036 . . . . . . . . . . . . . . . . .  11
       5.2.2.  Next hop in RFC5036 . . . . . . . . . . . . . . . . .  11
       5.2.3.  Egress LSR in RFC5036 . . . . . . . . . . . . . . . .  12
       5.2.4.  Use of Rainbow FEC to satisfy label mapping existence
               requirements in RFC5036 . . . . . . . . . . . . . . .  13
       5.2.5.  Validating FECs in routing table  . . . . . . . . . .  14
       5.2.6.  Recognizing new FECs  . . . . . . . . . . . . . . . .  14
       5.2.7.  Not propagating Rainbow FEC label mappings  . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  Potential restrictions on MRT-related MT-ID values  imposed
       by RFC6420  . . . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17



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     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   This document describes the LDP signaling extension and associated
   behavior necessary to support the architecture that defines how IP/
   LDP Fast-Reroute can use MRTs [RFC7812].  It is necessary to be
   familiar with the architecture in [RFC7812] to understand how and why
   the LDP extensions for behavior are needed.

   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 [I-D.ietf-ospf-mrt].  Extension to IS-IS are defined in
   [I-D.ietf-isis-mrt].

   MRT can also be used to protect multicast traffic (signalled via PIM
   or mLDP) using either global protection or local protection as
   described in [I-D.atlas-rtgwg-mrt-mc-arch].  An MRT path can be used
   to provide node-protection for mLDP traffic via the mechanisms
   described in [I-D.wijnands-mpls-mldp-node-protection]; 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 Section 7 of [I-D.ietf-ospf-mrt].

   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.





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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119[RFC2119]

3.  Terminology

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

   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.
      Redundant trees can always be computed in 2-connected graphs.

   Maximally Redundant Trees (MRT):   A pair of trees where the path
      from any node X to the root R along the first tree and the path
      from the same node X to the root along the second tree share the
      minimum number of nodes and the minimum number of links.  Each
      such shared node is a cut-vertex.  Any shared links are cut-links.
      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 that is not in the MRT Island
      but is adjacent to an IBR and in the same area/level as the IBR.





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

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 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 LSP paths
   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.






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      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 (IANA)     |      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:   TBD-MRT-LDP-1 (To Be Allocated by IANA)

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

   S-bit:   The State bit MUST be 1 if used in LDP "Initialization"
      message.  MAY be set to 0 or 1 in 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 LDP MRT
   Capability with an LDP neighbor without also negotiating the LDP MT
   Capability, the LSR MUST behave as if LDP MRT Capability has not been
   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 which 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



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   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 of
   the current document.

   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 (TBD-MRT-LDP-3) will be
   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 of the current document
   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 (TBD-MRT-LDP-4 and TBD-MRT-LDP-5).

   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.







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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 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 SPF algorithm is
   run by the IGP.

5.  LDP MRT FEC Advertisements

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

   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.





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   [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 area border
   router (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 Rainbow MRT MT-ID scoped FEC with the label
   corresponding to 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



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   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 foll 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 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
   a multiple routers in the MRT Island.

   In the named proxy-node calculation, each multi-homed prefix is
   represented by a conceptual proxy-node which 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 a 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



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   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 RFC5036

   In the context of distributing label mappings for red and blue FECs,
   we restrict 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 RFC5036

   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.




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   As another example, Section 2.6.1.2 of [RFC5036] specifies how 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 RFC5036

   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 loop detection
   procedures 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.





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   3.  The LSR is an area border router (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 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 RFC5036

   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 which 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.




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5.2.5.  Validating FECs in 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

   Section 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, when MRT LDP capability has been enabled, when an LSR learns a
   new shortest path FEC, it should generate "Recognize New FEC" events
   for the corresponding red and blue FECs, in addition to the
   "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.







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7.  Potential restrictions on MRT-related MT-ID values imposed by
    RFC6420

   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 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 application using MRT routes constructed using the same MRT
   profile.  In order to enable this simplification, the MPLS MT-ID
   values assigned in this document need to fall in the range 1 through
   4095.  The IANA request below reflects this by requesting that the
   MPLS MT-ID values from 3945 through 3995 be used for MRT-related MPLS
   MT-ID values.  This allows for 51 MRT-related MPLS MT-ID values which
   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 MPLS MT-ID range
   6-3995 as "Unassigned(for future IGP topologies)".  The IANA request
   below changes the guidance for MT-ID range 3948-3995 to "Unassigned
   (for future MRT-related values)".

8.  IANA Considerations

   IANA is requested to allocate a value for the new LDP Capability TLV
   (the first free value in the range 0x0500 to 0x05FF) from the Label
   Distribution Protocol (LDP) Parameters registry "TLV Type Name
   Space": MRT Capability TLV (TBD-MRT-LDP-1).

    Value          Description         Reference     Notes / Reg. Date
    -------------  ------------------  ------------  -----------------
    TBD-MRT-LDP-1  MRT Capability TLV  [This draft]

   IANA is requested to allocate a value for the new LDP Status Code
   (the first free value in the range 0x00000032-0x00000036) from the
   Label Distribution Protocol (LDP) Parameters registry "Status Code
   Name Space": "MRT Capability negotiated without MT Capability" (TBD-
   MRT-LDP-2).  The Status Code E-bit is set to 0.






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  Value          E  Description         Reference      Notes / Reg. Date
  -------------- -  ------------------  ------------   -----------------
  TBD-MRT-LDP-2  0  MRT Capability      [This draft]
                    negotiated without
                    MT Capability

   IANA is requested to allocate three values from the MPLS Multi-
   Topology Identifiers Registry [RFC7307].

      Rainbow MRT MPLS MT-ID (TBD-MRT-LDP-3) with suggested value: 3945

      Default Profile MRT-Red MPLS MT-ID (TBD-MRT-LDP-4) with suggested
      value: 3946

      Default Profile MRT-Blue MPLS MT-ID (TBD-MRT-LDP-5) with suggested
      value: 3947

   IANA is also requested to change the purpose field of the MPLS Multi-
   Topology Identifiers Registry for MT-ID range 3948-3995 to
   "Unassigned (for future MRT-related values)", assuming the above
   suggested values are assigned.  The Registration procedure for the
   entire registry remains "Standards Action".  The entire registry
   after implementing the above requests 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)
  TBD-MRT-LDP-3  Rainbow MRT MPLS MT-ID                     [This draft]
  TBD-MRT-LDP-4  Default Profile MRT-Red MPLS MT-ID         [This draft]
  TBD-MRT-LDP-5  Default Profile MRT-Blue MPLS MT-ID        [This draft]
  3948-3995      Unassigned (for future MRT-related values) [This draft]
  3996-4095      Reserved for Experimental Use              [RFC7307]
  4096-65534     Unassigned (for MPLS topologies)
  65535          Wildcard Topology                          [RFC7307]

9.  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.





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10.  References

10.1.  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,
              <http://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>.

10.2.  Informative References

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






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   [I-D.ietf-isis-mrt]
              Li, Z., Wu, N., <>, Q., Atlas, A., Bowers, C., and J.
              Tantsura, "Intermediate System to Intermediate System (IS-
              IS) Extensions for Maximally Redundant Trees (MRT)",
              draft-ietf-isis-mrt-02 (work in progress), May 2016.

   [I-D.ietf-ospf-mrt]
              Atlas, A., Hegde, S., Bowers, C., Tantsura, J., and Z. Li,
              "OSPF Extensions to Support Maximally Redundant Trees",
              draft-ietf-ospf-mrt-02 (work in progress), May 2016.

   [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.

   [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,
              <http://www.rfc-editor.org/info/rfc3209>.

Authors' Addresses

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

   Email: akatlas@juniper.net


   Kishore Tiruveedhula
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   USA

   Email: kishoret@juniper.net











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   Chris Bowers
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   USA

   Email: cbowers@juniper.net


   Jeff Tantsura
   Individual
   USA

   Email: jefftant.ietf@gmail.com


   IJsbrand Wijnands
   Cisco Systems, Inc.

   Email: ice@cisco.com































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