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RFC 7715

Proposed STD
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Multipoint LDP (mLDP) Node Protection

 


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Internet Engineering Task Force (IETF)                 IJ. Wijnands, Ed.
Request for Comments: 7715                                       K. Raza
Category: Standards Track                            Cisco Systems, Inc.
ISSN: 2070-1721                                                 A. Atlas
                                                  Juniper Networks, Inc.
                                                             J. Tantsura
                                                                Ericsson
                                                                 Q. Zhao
                                                       Huawei Technology
                                                            January 2016


                 Multipoint LDP (mLDP) Node Protection

Abstract

   This document describes procedures to support node protection for
   Point-to-Multipoint and Multipoint-to-Multipoint Label Switched Paths
   (P2MP and MP2MP LSPs) that have been built by the Multipoint Label
   Distribution Protocol (mLDP).  In order to protect a node N, the
   Point of Local Repair (PLR) Label Switching Router (LSR) of N must
   learn the Merge Point (MPT) LSR(s) of node N such that traffic can be
   redirected to them in case node N fails.  Redirecting the traffic
   around the failed node N depends on existing Point-to-Point (P2P)
   Label Switched Paths (LSPs).  The pre-established LSPs originate from
   the PLR LSR and terminate on the MPT LSRs while bypassing LSR N.

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 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7715.

Page 2 
Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  PLR Determination  . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Transit Node Procedure . . . . . . . . . . . . . . . . . .  5
     2.2.  MP2MP Root Node Procedure  . . . . . . . . . . . . . . . .  6
     2.3.  PLR Information Encoding . . . . . . . . . . . . . . . . .  7
   3.  Using the tLDP Session . . . . . . . . . . . . . . . . . . . .  9
   4.  Link or Node Failure . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Reconvergence after Node or Link Failure . . . . . . . . . 11
       4.1.1.  Node Failure . . . . . . . . . . . . . . . . . . . . . 12
       4.1.2.  Link Failure . . . . . . . . . . . . . . . . . . . . . 12
       4.1.3.  Switching to New Primary Path  . . . . . . . . . . . . 12
   5.  mLDP Capabilities for Node Protection  . . . . . . . . . . . . 13
     5.1.  PLR Capability . . . . . . . . . . . . . . . . . . . . . . 13
     5.2.  MPT Capability . . . . . . . . . . . . . . . . . . . . . . 14
     5.3.  The Protected LSR  . . . . . . . . . . . . . . . . . . . . 14
     5.4.  The Node Protection Capability . . . . . . . . . . . . . . 15
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . . . 18
   Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19

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1.  Introduction

   This document describes procedures to support node protection for
   Point-to-Multipoint and Multipoint-to-Multipoint Label Switched Paths
   (P2MP and MP2MP LSPs) that have been built by the Multipoint Label
   Distribution Protocol (mLDP) [RFC6388].  In order to protect a node
   N, the Point of Local Repair (PLR) LSR of N must learn the Merge
   Point (MPT) LSR(s) of node N such that traffic can be redirected to
   them in case node N fails.  Redirecting the traffic around the failed
   node N depends on existing P2P LSPs.  The pre-established LSPs
   originate from the PLR LSR and terminate on the MPT LSRs while
   bypassing LSR N.  The procedures to set up these P2P LSPs are outside
   the scope of this document, but one can imagine using techniques
   based on the Resource Reservation Protocol for Traffic Engineering
   (RSVP-TE) [RFC5420] or Label Distribution Protocol (LDP) Loop-Free
   Alternate (LFA) [RFC5286] to accomplish this.

   The solution described in this document notifies the PLR(s) of the
   MPT LSR(s) via signaling using a Targeted LDP (tLDP) session
   [RFC7060].  By having a tLDP session with the PLR, no additional
   procedures need to be defined in order to support Make-Before-Break
   (MBB), Graceful Restart (GR), and Typed Wildcard Forwarding
   Equivalence Class (FEC).  All this is achieved at the expense of
   having additional tLDP sessions between each MPT and PLR LSR.

   In order to allow a node to be protected against failure, the LSRs
   providing the PLR and the MPT functionality as well as the protected
   node MUST support the functionality described in this document.  LDP
   capability negotiation [RFC5561] is used to signal the availability
   of the functionality between the participating nodes; these nodes
   MUST support capability negotiation.

1.1.  Conventions Used in This Document

   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].

   The term "node" is used to refer to an LSR; "node" and "LSR" are used
   interchangeably in this document.  The terms "PLR" and "MPT" are used
   as shorthand to refer to "PLR LSR" and "MPT LSR", respectively.

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1.2.  Terminology

   mLDP:  Multipoint LDP

   PLR:  Point of Local Repair
      The LSR that redirects the traffic to one or more Merge Point
      LSRs.

   MPT:  Merge Point
      The LSR that merges the backup LSP with the primary LSP.  Note,
      there can be multiple MPT LSRs for a single MP-LSP node
      protection.

   tLDP:  Targeted LDP

   MP LSP:  Multi-Point LSP
      Either a P2MP or MP2MP LSP.

   root node:
      The root of either a P2MP or MP2MP LSP as defined in [RFC6388].

2.  PLR Determination

   In order for an MPT to establish a tLDP session with a PLR, it first
   has to learn the PLR for a particular MP LSP.  It is the
   responsibility of the protected node N to advertise the address of
   the PLR to the MPT.  The PLR address for an MP LSP on node N is the
   address of the upstream LDP peer, but only when node N is NOT the
   root node of the MP2MP LSP.  If the upstream LDP peer is unable to
   function as PLR, the procedures in this document do not apply and are
   out of the scope.  If node N is the root node, the procedures are
   slightly different as described in Section 2.2.  The procedures that
   follow assume that all the participating nodes (N, PLRs, MPTs) are
   enabled (e.g., by a user configuration) to support and implement the
   PLR determination feature.

   The procedures as documented in this RFC require the protected node
   to be directly connected to the PLR and MPT nodes.  This is because
   mLDP depends on unicast routing to determine the upstream LSR and
   unicast routing (by default) only has information about the next hop
   and not beyond that.  Support for non-directly connected PLR and MPT
   nodes is outside the scope of this document.

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2.1.  Transit Node Procedure

   Below are the procedures for when the protected node is a transit
   node along the path to the root.

                        root
                         ^
                         |
                       (LSR1)
                      .  |  .
                     .   |   .
                    .   (N)   .
                    .   /  \  .
                     . /    \.
                   (LSR2)  (LSR3)
                      |      |

               N: The node being protected.
               ...: Backup LSPs from LSR1 to LSR2 and LSR3.

                             Figure 1

   Node N uses the root address of the MP LSP to determine the upstream
   LSR for a given MP LSP following the procedures as documented in
   Section 2.4.1.1 of [RFC6388].  The upstream LSR in Figure 1 is LSR1
   because it is the first hop along the shortest path to reach the root
   address.  After determining the upstream LSR, node N (which has the
   node protection feature enabled) MUST advertise the address of LSR1
   as the PLR address to the downstream members of the MP LSP (i.e.,
   LSR2 and LSR3) if the given downstream member has announced support
   for node protection (see Section 5 regarding capability negotiation).
   For the format and encoding of PLR address information, see Section
   2.3.

   Note, in order for the protected traffic to reach nodes LSR2 and
   LSR3, LSR1 MUST have two unidirectional LSPs to LSR2 and LSR3,
   bypassing node N.  The procedures for setting up these LSPs are
   outside the scope of this document.

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2.2.  MP2MP Root Node Procedure

   Below are the procedures for when the protected node is the root of
   an MP2MP LSP.  Consider figure 2 below.

                         |
                       (LSR1)
                      .  |  .
                     .   |   .
                    .   (N)   . root
                    .   /  \  .
                     . /    \.
                  (LSR2)....(LSR3)
                     |        |

               N: The MP2MP root node being protected.
               ...: Backup LSPs between LSR1, LSR2, and LSR3.

                             Figure 2

   Assume that LSR1, LSR2, and LSR3 are all members of an MP2MP LSP for
   which N is the root node.  Since N is the root of the MP2MP LSP,
   there is no upstream LSR and no 'single' PLR LSR for protecting node
   N.  In order to protect node N, all the directly connected members of
   the MP2MP must participate in protecting node N by acting both as PLR
   and MPT LSR.  An LSR will act as MPT for traffic coming from the
   other LSR(s) and it will act as PLR for traffic it is sending to the
   other LSR(s).  Since node N knows the members of the MP2MP LSP, it
   will advertise the member list to its directly connected members,
   excluding the member it is sending to.  For example, node N will
   advertise list {LSR3,LSR1} to LSR2 excluding LSR2 from it.  Instead
   of advertising a single PLR when node N is not the root, a list of
   PLRs is advertised using the procedures documented in Section 2.3.

   It should be noted that the MP2MP root node protection mechanism
   doesn't replace the Root Node Redundancy (RNR) procedures as
   described in Section 7 of [RFC6388].  The node protection procedures
   in this document will help in restoring traffic for the existing
   MP2MP LSPs after node failure, but a new root node has to be elected
   eventually in order to allow new MP2MP LSPs to be created.

   Note, in order for the protected traffic to be exchanged between
   nodes LSR1, LSR2, and LSR3, bidirectional LSPs have to exist between
   the LSRs, bypassing node N.  The procedures for setting up these LSPs
   are outside the scope of this document.

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2.3.  PLR Information Encoding

   The upstream LSR address is conveyed via an LDP Notification message
   with an MP Status TLV, where the MP Status TLV contains a new "PLR
   Status Value Element" that specifies the address of the PLR.

   The new "PLR Status Value Element" is encoded as described below.

   PLR Status Element:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type =  2     |           Length              |  Addr Family  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Addr Fam cont | Num PLR entry |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   |                         PLR entry (1 or more)                 ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where

      Type: PLR Status Value Element (Type 2).

      Length: The Length field is an unsigned integer that encodes the
      length of the Status Value following the Length field.  The
      encoded Length varies based on the Addr Family and the number of
      PLR entries.

      Addr Family: Two-octet quantity containing a value from IANA's
      "Address Family Numbers" registry [AFI] that encodes the address
      family for the PLR address encoded in the PLR entry.

      Num PLR entry: Element as an unsigned integer followed by the
      number of "PLR entry" fields in the format specified below.

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   The format of a "PLR Entry" is as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |A|        Reserved             |       PLR address             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                  PLR address (cont)                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where

      A bit: 0 = Withdraw, 1 = Add.

      Reserved: 15 bits; MUST be zero on transmit and ignored on
      receipt.

      PLR address: PLR address encoded according to the Address Family
      field encoded in the PLR Status Value Element.  Note that the
      length of the PLR address field is specific to the Address Family
      that is encoded.

   The size of a "PLR Entry" is the 2 octets ("A bit + Reserved") + PLR
   address length.  The length of the PLR address is dependent on the
   Address Family as encoded in the PLR Status Value Element.  The size
   of a "PLR entry" is 6 octets and 18 octets, respectively, for an IPv4
   PLR address and an IPv6 PLR address.

   If the PLR address on N changes for a given MP LSP, N needs to
   trigger a new PLR Status to update the MPT(s).  Node N can advertise
   or withdraw a given PLR from its PLR set by setting the A bit to 1 or
   0 respectively in the corresponding PLR entry.  Removing a PLR
   address is likely due to a link failure; see the procedures as
   documented in Section 4.1.  To remove all PLR addresses belonging to
   the encoded Address Family, an LSR N MUST encode a PLR Status Value
   Element with no PLR entry and the "Num PLR entry" field MUST be set
   to zero.

   Both the PLR Status and an MP FEC TLV [RFC5036] MUST be included in
   the LDP Notification message so that a receiver is able to associate
   the PLR Status with the MP LSP.

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3.  Using the tLDP Session

   The receipt of a PLR MP Status (with PLR addresses) for an MP LSP on
   a receiving LSR makes it an MPT for node protection.  If not already
   established, the MPT LSR MUST establish a tLDP session with all of
   the learned PLR addresses using the procedures as documented in
   [RFC7060].

   Using Figure 1 as the reference topology, let us assume that both
   LSR2 and LSR3 are MPTs and have established a tLDP session with the
   PLR being LSR1.  Assume that both LSR2 and LSR3 have a FEC <R,X> with
   an upstream LSR N and label Ln assigned to FEC towards N.  The MPTs
   will create a secondary upstream LSR for the FEC <R,X> (using the
   received PLR address) and assign label Lpx to it.  The MPTs will do
   that for each PLR address that was learned for the MP LSP.  In this
   example, the MPTs will have a FEC <R,X> with two local labels
   associated with it.  Label Ln that was assigned to N using the normal
   mLDP procedures, and Label Lpx that was assigned to PLR (LSR1) for
   the purpose of node protection.  Note, when the protected node is an
   MP2MP root node, there will be an upstream LSR for each PLR address
   that was advertised along with a unique Label Lpx.

   The receipt of a FEC Label Mapping alone over the tLDP session from
   MPT on a PLR conveys the label information but does not convey the
   node being protected.  The information about a protected node is
   known to the MPT LSR and needs to be communicated to the PLR as well.
   For this reason, the FEC Label Mapping (FEC <R,X> : Lpx) sent by the
   MPT over the tLDP session to the PLR MUST include a Status TLV with
   an MP Status and a new LDP MP Status Value Element called the
   "Protected Node Status Value Element".  This new value element is
   used to specify the address of the node being protected.  The
   "Protected Node Status Value Element" has the following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type = 3      |           Length              | Addr  Family  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Addr Fam cont |        Node address                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type : Protected Node Status Value Element (Type 3).

      Length: The Length field is an unsigned integer that encodes the
      length of the Status Value following the Length field.  The
      encoded Length varies based on the Address Family and is 6 octets
      for Address Family + IPv4 address and 18 octets for Address Family
      + IPv6 address.

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      Addr Family: Two-octet quantity containing a value from IANA's
      "Address Family Numbers" registry [AFI] that encodes the address
      family for the node address.

      Node address: Protected node address encoded according to the
      Address Family field.

   When a PLR receives a Label Mapping for FEC <R,X> that includes a
   Protected Node Status, it will only use that label binding once the
   Node advertised in the Status value becomes unreachable.  If the LSP
   is an MP2MP LSP, the PLR would have assigned a Label Mapping for the
   upstream MP2MP FEC Element to the MPT ([RFC6388], Section 3) for FEC
   <R,X>.  This label binding on the MPT MUST only be used once node N
   becomes unreachable.

   The procedures to determine if a node is unreachable is a local
   decision and not spelled out in this document.  Typically, link
   failure or Bidirectional Forwarding Detection (BFD) can be used to
   determine and detect node unreachability.

4.  Link or Node Failure

   Consider the following topology:


                           root
                            ^
                            |
                        . (LSR1)
                      .   / |  .
                     .  (M) |   .
                     .    \ |    .
                      .    (N)   .
                       .   /  \  .
                        . /    \.
                      (LSR2)  (LSR3)
                         |      |

               N: The node being protected.
               M: The backup node to protect link LSR1 - N.
               ...: Backup LSPs from LSR1 to LSR2 and LSR3.

                              Figure 3

   Assume that LSR1 is the PLR for protected node N and that LSR2 and
   LSR3 are MPTs for node N.  When LSR1 discovers that node N is
   unreachable, it cannot immediately determine whether it is the link
   from LSR1 to N or the actual node N that has failed.  In Figure 3,

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   the link between LSR1 and N is also protected using Fast Reroute
   (FRR) [RFC4090] link protection via node M.  LSR1 MAY simultaneously
   invoke both link protection via node M to N using redirection of the
   traffic and node protection directly to LSR1 and LSR2.  If only the
   link failed, LSR2 and LSR3 will receive the packets twice due to the
   two protection mechanisms.  To prevent duplicate packets being
   forwarded to the receivers on the tree, LSR2 and LSR3 need to
   determine from which upstream node they should accept the packets.
   This can be either from the primary upstream LSR N or from the
   secondary upstream LSR1, but never both at the same time.  The
   selection between the primary upstream LSR or (one or more) secondary
   upstream LSRs (on LSR2 and LSR3) is based on the reachability of the
   protected node N.  As long as N is reachable from an MPT, the MPT
   should accept and forward the MPLS packets from N.  Once N becomes
   unreachable, the LSPs from secondary upstream PLR LSRs (LSR1 in our
   example) are activated.  Note that detecting if N is unreachable is a
   local decision and not spelled out in this document.

   Typically, link failure or BFD can be used to determine and detect
   node unreachability.

4.1.  Reconvergence after Node or Link Failure

   Consider the following topology:

                           root
                            ^
                         _  |
                      /.  (LSR1)
                     /.   /. |  .\
                    /.  (M). |   .\
                  (P).    \. |    .\
                    \.     ( N )   .(Q)
                     \.   /     \   ./
                      \. /       \ ./
                     (LSR2)     (LSR3)
                        |          |

               N: The node being protected.
               M: The backup node to protect link 'LSR1 - N'.
               P and Q: The nodes on the new primary path after
                  failure of node N.
               ...: P2P backup LSPs.

                               Figure 4

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   Assume that LSR1 has detected that node N is unreachable and invoked
   both the link protection and node protection procedures as described
   in this example.  LSR1 is acting as PLR and sending traffic over both
   the backup P2P LSP to node N (via M) and the P2P LSPs directly to
   LSR2 and LSR3, acting as MPT LSRs.  The sequence of events is
   dependent on whether the link from LSR1 to N has failed or node N
   itself has failed.  The nodes downstream from the protected node (and
   participating in node protection) MUST have the capability to
   determine that the protected node has become unreachable.  Otherwise,
   the procedures below cannot be applied.

4.1.1.  Node Failure

   If node N failed, both LSR2 and LSR3 will have changed the primary
   upstream LSR to the secondary upstream LSR (LSR1) due to node N being
   unreachable.  With that, the label bindings previously assigned to
   LSR1 will be activated on the MPTs (LSR2 and LSR3) and the label
   binding to N will be disabled.  Traffic is now switched over to the
   label bindings that were installed for node protection.

4.1.2.  Link Failure

   If the link 'LSR1 - N' has failed, both LSR2 and LSR3 will not change
   the primary upstream LSR because node N is still reachable.  LSR2 and
   LSR3 will receive traffic over two different bindings, the primary
   label binding assigned to node N (due to link protection via node M)
   as well as over the binding assigned to LSR1 for the node protection.
   Since the secondary upstream LSRs have not been activated, the
   traffic received due to node protection will be dropped.  Node N will
   reconverge and update LSR2 and LSR3 (Section 2.3) with the
   information that the PLR address (LSR1) is no longer applicable and
   must be removed.  In response, LSR2 and LSR3 MUST send a Label
   Withdraw to LSR1 to withdraw the label binding.  This will stop the
   traffic being forwarded over the backup P2P LSPs for node protection.
   LSR1 will respond back with a Label Release as soon as the binding
   has been removed.

4.1.3.  Switching to New Primary Path

   The network will eventually reconverge and a new best path to the
   root will be found by LSR2 and LSR3.  LSR2 will find that P is its
   new primary upstream LSR to reach the root and LSR3 will find Q.
   Note that although the current active upstream LSR can either be node
   N or LSR1 (depending on link or node failure), it does not matter for
   the following procedures.  Both LSR2 and LSR3 SHOULD use the Make-
   Before-Break (MBB) procedures as described in Section 8 of [RFC6388]
   to switch to the new primary upstream node.  As soon as the new
   primary upstream LSRs P and Q are activated, a Label Withdraw message

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   MUST be sent to the old upstream LSR.  Note that an upstream LSR
   switchover from a tLDP neighbor to a directly connected LDP neighbor
   is no different compared to switching between two directly connected
   neighbors.  After the Label Withdraw message has been received by
   LSR1 or node N, forwarding will stop and a Label Release will be
   sent.

   When it is determined that after reconvergence there is no more
   interest in the tLDP session between the MPT and the PLR, the tLDP
   session MAY be taken down.  It is possible that having no more
   interest in the tLDP session is temporarily due to link flapping.  In
   order to avoid the tLDP session from flapping, it is RECOMMENDED to
   apply a delay before tearing down the session.  Determining the delay
   is a local implementation matter.  If the operator is not concerned
   with the tLDP session flapping and/or other procedures are in place
   to avoid this altogether, there is no need to apply the delay.

5.  mLDP Capabilities for Node Protection

   In order to describe the capabilities of the participating LSRs, this
   document is organizing it per role in the network, i.e., Point of
   Local Repair (PLR), Merge Point (MPT), and protected node (as
   depicted in Figure 1).

5.1.  PLR Capability

   A PLR node should handle the following conditions:

   1. Accept an incoming tLDP session from the MPT LSR.

   2. Support the receipt of a "Protected Node Status Value Element"
      status in an MP Status TLV over tLDP session.

   3. Upon node failure detection, capable of switching traffic towards
      one or more MPT(s) over a P2P LSP (bypassing N) using the labels
      previously advertised for MP LSPs over the tLDP session.

   An LSR capable of performing these actions will advertise itself as
   PLR capable in the Node Protection Capability (see Section 5.4).
   This is a unidirectional capability announced from PLR to the
   protected LSR.

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5.2.  MPT Capability

   An MPT node should handle the following conditions;

   1. Support the receipt of "PLR Status Value Element" in an MP Status
      TLV from a protected node N.

   2. Support to transmit "Protected Node Status Value Element" in an MP
      Status TLV to a PLR.

   An LSR capable of performing these actions will advertise itself as
   MPT capable in the Node Protection Capability (see Section 5.4).
   This is a unidirectional capability from MPT to the protected LSR.

5.3.  The Protected LSR

   A protected node should handle the following conditions:

   1. Determine the PLR and MPT capability for directly connected
      upstream and downstream LSRs for a given MP FEC.

   2. Support transmitting of "PLR Status Value Element" in an MP Status
      TLV to one or more downstream MPT LSRs.

   The protected LSR does not advertise any capability for mLDP Node
   Protection because it does not need to receive any of the defined MP
   Status values as described above.  However, the protected node does
   play an important role in the signaling and setup of the node
   protection.  For a given FEC, the protected node can only send PLR
   information to a downstream LSR if the PLR has signaled the PLR
   capability and the downstream LSR has signaled the MPT capability.
   When the downstream LSR (acting as MPT) receives the PLR Status, it
   can implicitly infer that the advertised LSR(s) are PLR capable.  The
   MPT LSR can now proceed with setting up a tLDP session with the
   PLR(s) and MP LSP node protection signaling.

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5.4.  The Node Protection Capability

   We define a single capability "MP Node Protection Capability" to
   announce the PLR and MPT capability.

   The format of the capability parameter TLV is as follows:

    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| Type = 0x0972             |           Length = 2          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S| Reserved    |P|M| Reserved  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where

      U/F bits: MUST be set to 1 and 0, respectively (as per [RFC5561]).

      Type: MP Node Protection Capability (Type = 0x0972).

      Length: Unsigned integer; MUST be set to 2.

      S bit: Set to 1 to announce and 0 to withdraw the capability (as
      per [RFC5561]).

      P bit: Set to 1 to indicate the PLR is capable of MP LSP node
      protection.

      M bit: Set to 1 to indicate the MPT is capable of MP LSP node
      protection.

      Reserved: MUST be zero on transmit and ignored on receipt.

   The above capability can be sent in an LDP Initialization message to
   announce capability at the session establishment time, or it can be
   sent in an LDP Capability message to dynamically update (announce or
   withdraw) its capability towards its peer using procedures specified
   in [RFC5561].

   An LSR that supports the PLR functionality LSR MAY send this
   capability to its downstream MP peers with P bit set; whereas, an LSR
   that supports the MPT functionality MAY send this capability to its
   upstream peer with M bit set.  Moreover, an LSR that supports both
   the PLR and MPT functionality MAY sent this capability to its peers
   with both P and M bit set.

Top      ToC       Page 16 
6.  Security Considerations

   The procedures in this document add two new TLVs to existing LDP
   messages.  Those TLVs can be protected by the mechanisms that are
   used to protect LDP messages as described in [RFC6388] and [RFC5920].
   If it were possible to attack the mechanisms described in this
   document, an LSR (a PLR or a MPT) could be induced to support a large
   number of tLDP sessions and set up an even larger number of LSPs.
   The security mechanisms described in [RFC6388] and [RFC5920] are
   believed to be adequate, but an implementation could provide
   additional protection by counting such protection sessions and LSPs
   and producing a log message to the operator if a threshold is
   crossed.

7.  IANA Considerations

   IANA has allocated the following two new code points from the "LDP MP
   Status Value Element type" registry within the "Label Distribution
   Protocol (LDP) Parameters" registry.

      Value | Name                                   | Reference
      ------+----------------------------------------+-----------
         2  | PLR Status Value Element               | this doc
      ------+----------------------------------------+-----------
         3  | Protected Node Status Value Element    | this doc

   IANA has assigned the following new code point for a new Capability
   Parameter TLV.  The code point has been assigned from the IETF
   Consensus range of the "TLV Type Name Space" registry within the
   "Label Distribution Protocol (LDP) Parameters" registry.

      Value | Description                   | Reference | Notes/Reg Date
      ------+-------------------------------+-----------+---------------
      0x0972| MP Node Protection Capability | this doc  |

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8.  References

8.1.  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, <http://www.rfc-editor.org/info/rfc5036>.

   [RFC6388]  Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
              Thomas, "Label Distribution Protocol Extensions for Point-
              to-Multipoint and Multipoint-to-Multipoint Label Switched
              Paths", RFC 6388, DOI 10.17487/RFC6388, November 2011,
              <http://www.rfc-editor.org/info/rfc6388>.

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

   [RFC7060]  Napierala, M., Rosen, E., and IJ. Wijnands, "Using LDP
              Multipoint Extensions on Targeted LDP Sessions", RFC 7060,
              DOI 10.17487/RFC7060, November 2013, <http://www.rfc-
              editor.org/info/rfc7060>.

   [AFI]      IANA, "Address Family Numbers",
              <http://www.iana.org/assignments/address-family-numbers>.

8.2.  Informative References

   [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
              Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              DOI 10.17487/RFC4090, May 2005,
              <http://www.rfc-editor.org/info/rfc4090>.

   [RFC5286]  Atlas, A., Ed., and A. Zinin, Ed., "Basic Specification
              for IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <http://www.rfc-editor.org/info/rfc5286>.

Top      ToC       Page 18 
   [RFC5420]  Farrel, A., Ed., Papadimitriou, D., Vasseur, JP., and A.
              Ayyangarps, "Encoding of Attributes for MPLS LSP
              Establishment Using Resource Reservation Protocol Traffic
              Engineering (RSVP-TE)", RFC 5420, DOI 10.17487/RFC5420,
              February 2009, <http://www.rfc-editor.org/info/rfc5420>.

   [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
              <http://www.rfc-editor.org/info/rfc5920>.

Acknowledgments

   The authors thank Nagendra Kumar, Duan Hong, Martin Vigoureux, Kenji
   Fujihira, Loa Andersson, and Ben Campbell for their comments on this
   document.  Also, many thanks to Elwyn Davies and Adrian Farrel for
   the detailed review and contribution to this document.

Contributors

   The following individual contributed to this document:

   Eric Rosen
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, MA  01886
   United States
   Email: erosen@juniper.net

Top      ToC       Page 19 
Authors' Addresses

   IJsbrand Wijnands (editor)
   Cisco Systems, Inc.
   De kleetlaan 6a
   Diegem  1831
   Belgium

   Email: ice@cisco.com


   Kamran Raza
   Cisco Systems, Inc.
   2000 Innovation Drive
   Ottawa, Ontario K2K-3E8
   Canada

   Email: skraza@cisco.com


   Alia Atlas
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, MA  01886
   United States

   Email: akatlas@juniper.net


   Jeff Tantsura
   Ericsson
   300 Holger Way
   San Jose, CA  95134
   United States

   Email: jeff.tantsura@ericsson.com


   Quintin Zhao
   Huawei Technology
   125 Nagog Technology Park
   Acton, MA  01719
   United States

   Email: quintin.zhao@huawei.com