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Versions: (draft-minei-wijnands-mpls-ldp-p2mp) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 6388

Network Working Group                                  I. Minei (Editor)
Internet-Draft                                               K. Kompella
Intended status: Standards Track                        Juniper Networks
Expires: April 27, 2010                             I. Wijnands (Editor)
                                                               B. Thomas
                                                     Cisco Systems, Inc.
                                                        October 24, 2009


   Label Distribution Protocol Extensions for Point-to-Multipoint and
             Multipoint-to-Multipoint Label Switched Paths
                      draft-ietf-mpls-ldp-p2mp-08

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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Copyright Notice




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   Copyright (c) 2009 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 in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This document describes extensions to the Label Distribution Protocol
   (LDP) for the setup of point to multi-point (P2MP) and multipoint-to-
   multipoint (MP2MP) Label Switched Paths (LSPs) in Multi-Protocol
   Label Switching (MPLS) networks.  These extensions are also referred
   to as mLDP Multicast LDP. mLDP constructs the P2MP or MP2MP LSPs
   without interacting with or relying upon any other multicast tree
   construction protocol.  Protocol elements and procedures for this
   solution are described for building such LSPs in a receiver-initiated
   manner.  There can be various applications for P2MP/MP2MP LSPs, for
   example IP multicast or support for multicast in BGP/MPLS L3VPNs.
   Specification of how such applications can use a LDP signaled P2MP/
   MP2MP LSP is outside the scope of this document.




























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Conventions used in this document  . . . . . . . . . . . .  5
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Setting up P2MP LSPs with LDP  . . . . . . . . . . . . . . . .  6
     2.1.  Support for P2MP LSP setup with LDP  . . . . . . . . . . .  7
     2.2.  The P2MP FEC Element . . . . . . . . . . . . . . . . . . .  7
     2.3.  The LDP MP Opaque Value Element  . . . . . . . . . . . . .  9
       2.3.1.  The Generic LSP Identifier . . . . . . . . . . . . . .  9
     2.4.  Using the P2MP FEC Element . . . . . . . . . . . . . . . . 10
       2.4.1.  Label Map  . . . . . . . . . . . . . . . . . . . . . . 11
       2.4.2.  Label Withdraw . . . . . . . . . . . . . . . . . . . . 12
       2.4.3.  Upstream LSR change  . . . . . . . . . . . . . . . . . 13
   3.  Shared Trees . . . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Setting up MP2MP LSPs with LDP . . . . . . . . . . . . . . . . 14
     4.1.  Support for MP2MP LSP setup with LDP . . . . . . . . . . . 15
     4.2.  The MP2MP downstream and upstream FEC Elements.  . . . . . 15
     4.3.  Using the MP2MP FEC Elements . . . . . . . . . . . . . . . 16
       4.3.1.  MP2MP Label Map  . . . . . . . . . . . . . . . . . . . 17
       4.3.2.  MP2MP Label Withdraw . . . . . . . . . . . . . . . . . 20
       4.3.3.  MP2MP Upstream LSR change  . . . . . . . . . . . . . . 21
   5.  Micro-loops in MP LSPs . . . . . . . . . . . . . . . . . . . . 21
   6.  The LDP MP Status TLV  . . . . . . . . . . . . . . . . . . . . 22
     6.1.  The LDP MP Status Value Element  . . . . . . . . . . . . . 23
     6.2.  LDP Messages containing LDP MP Status messages . . . . . . 23
       6.2.1.  LDP MP Status sent in LDP notification messages  . . . 23
       6.2.2.  LDP MP Status TLV in Label Mapping Message . . . . . . 24
   7.  Upstream label allocation on a LAN . . . . . . . . . . . . . . 25
     7.1.  LDP Multipoint-to-Multipoint on a LAN  . . . . . . . . . . 25
       7.1.1.  MP2MP downstream forwarding  . . . . . . . . . . . . . 25
       7.1.2.  MP2MP upstream forwarding  . . . . . . . . . . . . . . 25
   8.  Root node redundancy . . . . . . . . . . . . . . . . . . . . . 26
     8.1.  Root node redundancy - procedures for P2MP LSPs  . . . . . 26
     8.2.  Root node redundancy - procedures for MP2MP LSPs . . . . . 27
   9.  Make Before Break (MBB)  . . . . . . . . . . . . . . . . . . . 27
     9.1.  MBB overview . . . . . . . . . . . . . . . . . . . . . . . 28
     9.2.  The MBB Status code  . . . . . . . . . . . . . . . . . . . 29
     9.3.  The MBB capability . . . . . . . . . . . . . . . . . . . . 29
     9.4.  The MBB procedures . . . . . . . . . . . . . . . . . . . . 30
       9.4.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . 30
       9.4.2.  Accepting elements . . . . . . . . . . . . . . . . . . 31
       9.4.3.  Procedures for upstream LSR change . . . . . . . . . . 31
       9.4.4.  Receiving a Label Map with MBB status code . . . . . . 32
       9.4.5.  Receiving a Notification with MBB status code  . . . . 32
       9.4.6.  Node operation for MP2MP LSPs  . . . . . . . . . . . . 32
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 33
   11. IANA considerations  . . . . . . . . . . . . . . . . . . . . . 33



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   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 34
   13. Contributing authors . . . . . . . . . . . . . . . . . . . . . 34
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 36
     14.2. Informative References . . . . . . . . . . . . . . . . . . 36
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37













































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1.  Introduction

   The LDP protocol is described in [RFC5036].  It defines mechanisms
   for setting up point-to-point (P2P) and multipoint-to-point (MP2P)
   LSPs in the network.  This document describes extensions to LDP for
   setting up point-to-multipoint (P2MP) and multipoint-to-multipoint
   (MP2MP) LSPs.  These are collectively referred to as multipoint LSPs
   (MP LSPs).  A P2MP LSP allows traffic from a single root (or ingress)
   node to be delivered to a number of leaf (or egress) nodes.  A MP2MP
   LSP allows traffic from multiple ingress nodes to be delivered to
   multiple egress nodes.  Only a single copy of the packet will be sent
   on any link traversed by the MP LSP (see note at end of
   Section 2.4.1).  This is accomplished without the use of a multicast
   protocol in the network.  There can be several MP LSPs rooted at a
   given ingress node, each with its own identifier.

   The solution assumes that the leaf nodes of the MP LSP know the root
   node and identifier of the MP LSP to which they belong.  The
   mechanisms for the distribution of this information are outside the
   scope of this document.  The specification of how an application can
   use a MP LSP signaled by LDP is also outside the scope of this
   document.

   Interested readers may also wish to peruse the requirements draft
   [I-D.ietf-mpls-mp-ldp-reqs] and other documents [RFC4875] and
   [I-D.ietf-l3vpn-2547bis-mcast].

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].

1.2.  Terminology

   The following terminology is taken from [I-D.ietf-mpls-mp-ldp-reqs].

   P2P LSP:  An LSP that has one Ingress LSR and one Egress LSR.


   P2MP LSP:  An LSP that has one Ingress LSR and one or more Egress
      LSRs.


   MP2P LSP:  An LSP that has one or more Ingress LSRs and one unique
      Egress LSR.





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   MP2MP LSP:  An LSP that connects a set of nodes, such that traffic
      sent by any node in the LSP is delivered to all others.


   MP LSP:  A multipoint LSP, either a P2MP or an MP2MP LSP.


   Ingress LSR:  An ingress LSR for a particular LSP is an LSR that can
      send a data packet along the LSP.  MP2MP LSPs can have multiple
      ingress LSRs, P2MP LSPs have just one, and that node is often
      referred to as the "root node".


   Egress LSR:  An egress LSR for a particular LSP is an LSR that can
      remove a data packet from that LSP for further processing.  P2P
      and MP2P LSPs have only a single egress node, but P2MP and MP2MP
      LSPs can have multiple egress nodes.


   Transit LSR:  An LSR that has reachability to the root of the MP LSP
      via a directly connected upstream LSR and one or more directly
      connected downstream LSRs.


   Bud LSR:  An LSR that is an egress but also has one or more directly
      connected downstream LSRs.


   Leaf node:  A Leaf node can be either an Egress or Bud LSR when
      referred in the context of a P2MP LSP.  In the context of a MP2MP
      LSP, an LSR is both Ingress and Egress for the same MP2MP LSP and
      can also be a Bud LSR.


2.  Setting up P2MP LSPs with LDP

   A P2MP LSP consists of a single root node, zero or more transit nodes
   and one or more leaf nodes.  Leaf nodes initiate P2MP LSP setup and
   tear-down.  Leaf nodes also install forwarding state to deliver the
   traffic received on a P2MP LSP to wherever it needs to go; how this
   is done is outside the scope of this document.  Transit nodes install
   MPLS forwarding state and propagate the P2MP LSP setup (and tear-
   down) toward the root.  The root node installs forwarding state to
   map traffic into the P2MP LSP; how the root node determines which
   traffic should go over the P2MP LSP is outside the scope of this
   document.





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2.1.  Support for P2MP LSP setup with LDP

   Support for the setup of P2MP LSPs is advertised using LDP
   capabilities as defined in [I-D.ietf-mpls-ldp-capabilities].  An
   implementation supporting the P2MP procedures specified in this
   document MUST implement the procedures for Capability Parameters in
   Initialization Messages.

   A new Capability Parameter TLV is defined, the P2MP Capability.
   Following is the format of the P2MP 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1|0| P2MP Capability (TBD IANA) |     Length (= 1)             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1| Reserved    |
       +-+-+-+-+-+-+-+-+

   The P2MP Capability TLV MUST be supported in the LDP Initialization
   Message.  Advertisement of the P2MP Capability indicates support of
   the procedures for P2MP LSP setup detailed in this document.  If the
   peer has not advertised the corresponding capability, then no label
   messages using the P2MP FEC Element should be sent to the peer.

2.2.  The P2MP FEC Element

   For the setup of a P2MP LSP with LDP, we define one new protocol
   entity, the P2MP FEC Element to be used as a FEC Element in the FEC
   TLV.  Note that the P2MP FEC Element does not necessarily identify
   the traffic that must be mapped to the LSP, so from that point of
   view, the use of the term FEC is a misnomer.  The description of the
   P2MP FEC Element follows.

   The P2MP FEC Element consists of the address of the root of the P2MP
   LSP and an opaque value.  The opaque value consists of one or more
   LDP MP Opaque Value Elements.  The opaque value is unique within the
   context of the root node.  The combination of (Root Node Address,
   Opaque Value) uniquely identifies a P2MP LSP within the MPLS network.












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   The P2MP FEC Element is encoded 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |P2MP Type (TBD)|        Address Family         | Address Length|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                       Root Node Address                       ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Opaque Length              |    Opaque Value ...           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      ~                                                               ~
      |                                                               |
      |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   Type:  The type of the P2MP FEC Element is to be assigned by IANA.


   Address Family:  Two octet quantity containing a value from ADDRESS
      FAMILY NUMBERS in [RFC3232] that encodes the address family for
      the Root LSR Address.


   Address Length:  Length of the Root LSR Address in octets.


   Root Node Address:  A host address encoded according to the Address
      Family field.


   Opaque Length:  The length of the Opaque Value, in octets.


   Opaque Value:  One or more MP Opaque Value elements, uniquely
      identifying the P2MP LSP in the context of the Root Node.  This is
      described in the next section.

   If the Address Family is IPv4, the Address Length MUST be 4; if the
   Address Family is IPv6, the Address Length MUST be 16.  No other
   Address Lengths are defined at present.

   If the Address Length doesn't match the defined length for the
   Address Family, the receiver SHOULD abort processing the message
   containing the FEC Element, and send an "Unknown FEC" Notification



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   message to its LDP peer signaling an error.

   If a FEC TLV contains a P2MP FEC Element, the P2MP FEC Element MUST
   be the only FEC Element in the FEC TLV.

2.3.  The LDP MP Opaque Value Element

   The LDP MP Opaque Value Element is used in the P2MP and MP2MP FEC
   Elements defined in subsequent sections.  It carries information that
   is meaningful to Ingress LSRs and Leaf LSRs, but need not be
   interpreted by Transit LSRs.

   The LDP MP Opaque Value Element is encoded 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Type(TBD)     | Length                        | Value ...     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
       ~                                                               ~
       |                                                               |
       |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type:  The type of the LDP MP Opaque Value Element is to be assigned
      by IANA.


   Length:  The length of the Value field, in octets.


   Value:  String of Length octets, to be interpreted as specified by
      the Type field.

2.3.1.  The Generic LSP Identifier

   The generic LSP identifier is a type of Opaque Value Element encoded
   as follows:

   Type:  1 (to be assigned by IANA)








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   Length:  4


   Value:  A 32bit integer, unique in the context of the root, as
      identified by the root's address.

   This type of Opaque Value Element is recommended when mapping of
   traffic to LSPs is non-algorithmic, and done by means outside LDP.

2.4.  Using the P2MP FEC Element

   This section defines the rules for the processing and propagation of
   the P2MP FEC Element.  The following notation is used in the
   processing rules:

   1.  P2MP FEC Element <X, Y>: a FEC Element with Root Node Address X
       and Opaque Value Y.


   2.  P2MP Label Map <X, Y, L>: a Label Map message with a FEC TLV with
       a single P2MP FEC Element <X, Y> and Label TLV with label L.
       Label L MUST be allocated from the per-platform label space (see
       [RFC3031] section 3.14) of the LSR sending the Label Map Message.


   3.  P2MP Label Withdraw <X, Y, L>: a Label Withdraw message with a
       FEC TLV with a single P2MP FEC Element <X, Y> and Label TLV with
       label L.


   4.  P2MP LSP <X, Y> (or simply <X, Y>): a P2MP LSP with Root Node
       Address X and Opaque Value Y.

   5.  The notation L' -> {<I1, L1> <I2, L2> ..., <In, Ln>} on LSR X
       means that on receiving a packet with label L', X makes n copies
       of the packet.  For copy i of the packet, X swaps L' with Li and
       sends it out over interface Ii.

   The procedures below are organized by the role which the node plays
   in the P2MP LSP.  Node Z knows that it is a leaf node by a discovery
   process which is outside the scope of this document.  During the
   course of protocol operation, the root node recognizes its role
   because it owns the Root Node Address.  A transit node is any node
   (other than the root node) that receives a P2MP Label Map message
   (i.e., one that has leaf nodes downstream of it).

   Note that a transit node (and indeed the root node) may also be a
   leaf node.



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2.4.1.  Label Map

   The remainder of this section specifies the procedures for
   originating P2MP Label Map messages and for processing received P2MP
   label map messages for a particular LSP.  The procedures for a
   particular LSR depend upon the role that LSR plays in the LSP
   (ingress, transit, or egress).

   All labels discussed here are downstream-assigned
   [I-D.ietf-mpls-multicast-encaps] except those which are assigned
   using the procedures of Section 7.

2.4.1.1.  Determining one's 'upstream LSR'

   Each node that is either an Leaf or Transit LSR of MP LSP needs to
   use the procedures below to select an upstream LSR.  A node Z that
   wants to join a MP LSP <X, Y> determines the LDP peer U which is Z's
   next-hop on the best path from Z to the root node X. If there is more
   than one such LDP peer, only one of them is picked.  U is Z's
   "Upstream LSR" for <X, Y>.

   When there are several candidate upstream LSRs, the LSR MAY select
   one upstream LSR using the following procedure:

   1.  The candidate upstream LSRs are numbered from lower to higher IP
       address

   2.  The following hash is performed: H = (Sum Opaque value) modulo N,
       where N is the number of candidate upstream LSRs

   3.  The selected upstream LSR U is the LSR that has the number H.

   This allows for load balancing of a set of LSPs among a set of
   candidate upstream LSRs, while ensuring that on a LAN interface a
   single upstream LSR is selected.

2.4.1.2.  Determining the forwarding interface to an LSR

   Suppose LSR U receives a MP Label Map message from a downstream LSR
   D, specifying label L. Suppose further that U is connected to D over
   several LDP enabled interfaces or RSVP-TE Tunnel interfaces.  If U
   needs to transmit to D a data packet whose top label is L, U is free
   to transmit the packet on any of those interfaces.  LSR U is able to
   discover the directly connected interfaces via the LDP Discovery
   messages exchanged between the LSR U and D. The algorithm it uses to
   choose a particular interface for a particular such packet is a local
   matter.




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2.4.1.3.  Leaf Operation

   A leaf node Z of P2MP LSP <X, Y> determines its upstream LSR U for
   <X, Y> as per Section 2.4.1.1, allocates a label L, and sends a P2MP
   Label Map <X, Y, L> to U.

2.4.1.4.  Transit Node operation

   Suppose a transit node Z receives a P2MP Label Map <X, Y, L> from LSR
   T. Z checks whether it already has state for <X, Y>.  If not, Z
   determines its upstream LSR U for <X, Y> as per Section 2.4.1.1.
   Using this Label Map to update the label forwarding table MUST NOT be
   done as long as LSR T is equal to LSR U. If LSR U is different from
   LSR T, Z will allocate a label L', and install state to swap L' with
   L over interface I associated with LSR T and send a P2MP Label Map
   <X, Y, L'> to LSR U. Interface I is determind via the procedures in
   Section 2.4.1.2.

   If Z already has state for <X, Y>, then Z does not send a Label Map
   message for P2MP LSP <X, Y>.  All that Z needs to do in this case is
   check that LSR T is not equal to the upstream LSR of <X, Y> and
   update its forwarding state.  Assuming its old forwarding state was
   L'-> {<I1, L1> <I2, L2> ..., <In, Ln>}, its new forwarding state
   becomes L'-> {<I1, L1> <I2, L2> ..., <In, Ln>, <I, L>}.  If the LSR T
   is equal to the installed upstream LSR, the Label Map from LSR T MUST
   be retained and MUST not update the label forwarding table.

2.4.1.5.  Root Node Operation

   Suppose the root node Z receives a P2MP Label Map <X, Y, L> from LSR
   T. Z checks whether it already has forwarding state for <X, Y>.  If
   not, Z creates forwarding state to push label L onto the traffic that
   Z wants to forward over the P2MP LSP (how this traffic is determined
   is outside the scope of this document).

   If Z already has forwarding state for <X, Y>, then Z adds "push label
   L, send over interface I" to the nexthop, where I is the interface
   associated with LSR T and determind via the procedures in
   Section 2.4.1.2.

2.4.2.  Label Withdraw

   The following lists procedures for generating and processing P2MP
   Label Withdraw messages for nodes that participate in a P2MP LSP.  An
   LSR should apply those procedures that apply to it, based on its role
   in the P2MP LSP.





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2.4.2.1.  Leaf Operation

   If a leaf node Z discovers (by means outside the scope of this
   document) that it has no downstream neighbors in that LSP, and that
   it has no need to be an egress LSR for that LSP, then it SHOULD send
   a Label Withdraw <X, Y, L> to its upstream LSR U for <X, Y>, where L
   is the label it had previously advertised to U for <X, Y>.

2.4.2.2.  Transit Node Operation

   If a transit node Z receives a Label Withdraw message <X, Y, L> from
   a node W, it deletes label L from its forwarding state, and sends a
   Label Release message with label L to W.

   If deleting L from Z's forwarding state for P2MP LSP <X, Y> results
   in no state remaining for <X, Y>, then Z propagates the Label
   Withdraw for <X, Y>, to its upstream T, by sending a Label Withdraw
   <X, Y, L1> where L1 is the label Z had previously advertised to T for
   <X, Y>.

2.4.2.3.  Root Node Operation

   The procedure when the root node of a P2MP LSP receives a Label
   Withdraw message are the same as for transit nodes, except that it
   would not propagate the Label Withdraw upstream (as it has no
   upstream).

2.4.3.  Upstream LSR change

   Suppose that for a given node Z participating in a P2MP LSP <X, Y>,
   the upstream LSR changes from U to U' as per Section 2.4.1.1.  If U'
   is present in the forwarding table of <X, Y> then it MUST be removed.
   Z MUST also update its forwarding state by deleting the state for
   label L, allocating a new label, L', for <X, Y>, and installing the
   forwarding state for L'.  Installing the forwarding state for L' MUST
   NOT be done before the forwarding state L is removed to avoid
   microloops.  In addition Z MUST send a Label Map <X, Y, L'> to U' and
   send a Label Withdraw <X, Y, L> to U. Note, if there was a downstream
   mapping from U that was not installed in the forwarding due to
   Section 2.4.1.4 it can now be installed.


3.  Shared Trees

   The mechanism described above shows how to build a tree with a single
   root and multiple leaves, i.e., a P2MP LSP.  One can use essentially
   the same mechanism to build Shared Trees with LDP.  A Shared Tree can
   be used by a group of routers that want to multicast traffic among



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   themselves, i.e., each node is both a root node (when it sources
   traffic) and a leaf node (when any other member of the group sources
   traffic).  A Shared Tree offers similar functionality to a MP2MP LSP,
   but the underlying multicasting mechanism uses a P2MP LSP.  One
   example where a Shared Tree is useful is video-conferencing.  Another
   is Virtual Private LAN Service (VPLS) [RFC4664], where for some types
   of traffic, each device participating in a VPLS must send packets to
   every other device in that VPLS.

   One way to build a Shared Tree is to build an LDP P2MP LSP rooted at
   a common point, the Shared Root (SR), and whose leaves are all the
   members of the group.  Each member of the Shared Tree unicasts
   traffic to the SR (using, for example, the MP2P LSP created by the
   unicast LDP FEC advertised by the SR); the SR then splices this
   traffic into the LDP P2MP LSP.  The SR may be (but need not be) a
   member of the multicast group.

   A major advantage of this approach is that no further protocol
   mechanisms beyond the one already described are needed to set up a
   Shared Tree.  Furthermore, a Shared Tree is very efficient in terms
   of the multicast state in the network, and is reasonably efficient in
   terms of the bandwidth required to send traffic.

   A property of this approach is that a sender will receive its own
   packets as part of the multicast; thus a sender must be prepared to
   recognize and discard packets that it itself has sent.  For a number
   of applications (for example, VPLS), this requirement is easy to
   meet.  Another consideration is the various techniques that can be
   used to splice unicast LDP MP2P LSPs to the LDP P2MP LSP; these will
   be described in a later revision.


4.  Setting up MP2MP LSPs with LDP

   An MP2MP LSP is much like a P2MP LSP in that it consists of a single
   root node, zero or more transit nodes and one or more leaf LSRs
   acting equally as Ingress or Egress LSR.  A leaf node participates in
   the setup of an MP2MP LSP by establishing both a downstream LSP,
   which is much like a P2MP LSP from the root, and an upstream LSP
   which is used to send traffic toward the root and other leaf nodes.
   Transit nodes support the setup by propagating the upstream and
   downstream LSP setup toward the root and installing the necessary
   MPLS forwarding state.  The transmission of packets from the root
   node of a MP2MP LSP to the receivers is identical to that for a P2MP
   LSP.  Traffic from a downstream node follows the upstream LSP toward
   the root node and branches downward along the downstream LSP as
   required to reach other leaf nodes.  A packet that is received from a
   downstream node MUST never be forwarded back out to that same node.



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   Mapping traffic to the MP2MP LSP may happen at any leaf node.  How
   that mapping is established is outside the scope of this document.

   Due to how a MP2MP LSP is built a leaf LSR that is sending packets on
   the MP2MP LSP does not receive its own packets.  There is also no
   additional mechanism needed on the root or transit LSR to match
   upstream traffic to the downstream forwarding state.  Packets that
   are forwarded over a MP2MP LSP will not traverse a link more than
   once, with the possible exception of LAN links (see Section 4.3.1),
   if the procedures of [I-D.ietf-mpls-upstream-label] are not provided.

4.1.  Support for MP2MP LSP setup with LDP

   Support for the setup of MP2MP LSPs is advertised using LDP
   capabilities as defined in [I-D.ietf-mpls-ldp-capabilities].  An
   implementation supporting the MP2MP procedures specified in this
   document MUST implement the procedures for Capability Parameters in
   Initialization Messages.

   A new Capability Parameter TLV is defined, the MP2MP Capability.
   Following is the format of the MP2MP 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1|0| MP2MP Capability (TBD IANA) |    Length (= 1)             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1| Reserved    |
       +-+-+-+-+-+-+-+-+

   The MP2MP Capability TLV MUST be supported in the LDP Initialization
   Message.  Advertisement of the MP2MP Capability indicates support of
   the procedures for MP2MP LSP setup detailed in this document.  If the
   peer has not advertised the corresponding capability, then no label
   messages using the MP2MP upstream and downstream FEC Elements should
   be sent to the peer.

4.2.  The MP2MP downstream and upstream FEC Elements.

   For the setup of a MP2MP LSP with LDP we define 2 new protocol
   entities, the MP2MP downstream FEC and upstream FEC Element.  Both
   elements will be used as FEC Elements in the FEC TLV.  Note that the
   MP2MP FEC Elements do not necessarily identify the traffic that must
   be mapped to the LSP, so from that point of view, the use of the term
   FEC is a misnomer.  The description of the MP2MP FEC Elements follow.

   The structure, encoding and error handling for the MP2MP downstream
   and upstream FEC Elements are the same as for the P2MP FEC Element



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   described in Section 2.2.  The difference is that two new FEC types
   are used: MP2MP downstream type (TBD) and MP2MP upstream type (TBD).

   If a FEC TLV contains an MP2MP FEC Element, the MP2MP FEC Element
   MUST be the only FEC Element in the FEC TLV.

   Note, except when using the procedures of
   [I-D.ietf-mpls-upstream-label], the MPLS labels used are "downstream-
   assigned" [I-D.ietf-mpls-multicast-encaps], even if they are bound to
   the "upstream FEC element".

4.3.  Using the MP2MP FEC Elements

   This section defines the rules for the processing and propagation of
   the MP2MP FEC Elements.  The following notation is used in the
   processing rules:

   1.   MP2MP downstream LSP <X, Y> (or simply downstream <X, Y>): an
        MP2MP LSP downstream path with root node address X and opaque
        value Y.


   2.   MP2MP upstream LSP <X, Y, D> (or simply upstream <X, Y, D>): a
        MP2MP LSP upstream path for downstream node D with root node
        address X and opaque value Y.


   3.   MP2MP downstream FEC Element <X, Y>: a FEC Element with root
        node address X and opaque value Y used for a downstream MP2MP
        LSP.


   4.   MP2MP upstream FEC Element <X, Y>: a FEC Element with root node
        address X and opaque value Y used for an upstream MP2MP LSP.


   5.   MP2MP-D Label Map <X, Y, L>: A Label Map message with a FEC TLV
        with a single MP2MP downstream FEC Element <X, Y> and label TLV
        with label L. Label L MUST be allocated from the per-platform
        label space (see [RFC3031] section 3.14) of the LSR sending the
        Label Map Message.


   6.   MP2MP-U Label Map <X, Y, Lu>: A Label Map message with a FEC TLV
        with a single MP2MP upstream FEC Element <X, Y> and label TLV
        with label Lu.  Label L MUST be allocated from the per-platform
        label space (see [RFC3031] section 3.14) of the LSR sending the
        Label Map Message.



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   7.   MP2MP-D Label Withdraw <X, Y, L>: a Label Withdraw message with
        a FEC TLV with a single MP2MP downstream FEC Element <X, Y> and
        label TLV with label L.


   8.   MP2MP-U Label Withdraw <X, Y, Lu>: a Label Withdraw message with
        a FEC TLV with a single MP2MP upstream FEC Element <X, Y> and
        label TLV with label Lu.


   9.   MP2MP-D Label Release <X, Y, L>: a Label Release message with a
        FEC TLV with a single MP2MP downstream FEC Element <X, Y> and
        label TLV with label L.


   10.  MP2MP-U Label Release <X, Y, Lu>: a Label Release message with a
        FEC TLV with a single MP2MP upstream FEC Element <X, Y> and
        label TLV with label Lu.

   The procedures below are organized by the role which the node plays
   in the MP2MP LSP.  Node Z knows that it is a leaf node by a discovery
   process which is outside the scope of this document.  During the
   course of the protocol operation, the root node recognizes its role
   because it owns the root node address.  A transit node is any node
   (other then the root node) that receives a MP2MP Label Map message
   (i.e., one that has leaf nodes downstream of it).

   Note that a transit node (and indeed the root node) may also be a
   leaf node and the root node does not have to be an ingress LSR or
   leaf of the MP2MP LSP.

4.3.1.  MP2MP Label Map

   The remainder of this section specifies the procedures for
   originating MP2MP Label Map messages and for processing received
   MP2MP label map messages for a particular LSP.  The procedures for a
   particular LSR depend upon the role that LSR plays in the LSP
   (ingress, transit, or egress).

   All labels discussed here are downstream-assigned
   [I-D.ietf-mpls-multicast-encaps] except those which are assigned
   using the procedures of Section 7.

4.3.1.1.  Determining one's upstream MP2MP LSR

   Determining the upstream LDP peer U for a MP2MP LSP <X, Y> follows
   the procedure for a P2MP LSP described in Section 2.4.1.1.




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4.3.1.2.  Determining one's downstream MP2MP LSR

   A LDP peer U which receives a MP2MP-D Label Map from a LDP peer D
   will treat D as downstream MP2MP LSR.

4.3.1.3.  Installing the upstream path of a MP2MP LSP

   There are two methods for installing the upstream path of a MP2MP LSP
   to a downstream neighbor.

   1.  We can install the upstream MP2MP path (to a downstream neighbor)
       based on receiving a MP2MP-D Label Map from the downstream
       neighbor.  This will install the upstream path on a per hop by
       hop bases.


   2.  We install the upstream MP2MP path (to a downstream neighbor)
       based on receiving a MP2MP-U Label Map from the upstream
       neighbor.  An LSR does not need to wait for the MP2MP-U Label Map
       if it is the root of the MP2MP LSP or already has received an
       MP2MP-U Label Map from the upstream neighbor.  We call this
       method ordered mode.  The typical result of this mode is that the
       downstream path of the MP2MP is build hop by hop towards the
       root.  Once the root is reached, the root node will trigger a
       MP2MP-U Label Map to the downstream neighbor(s).


   For setting up the upstream path of a MP2MP LSP ordered mode MUST be
   used.  Due to ordered mode the upstream path of the MP2MP LSP is
   installed at the leaf node once the path to the root is completed.
   The advantage is that when a leaf starts sending immediately after
   the upstream path is installed, packets are able to reach the root
   node without being dropped due to an incomplete LSP.  Method 1 is not
   able to guarantee that the upstream path is completed before the leaf
   starts sending.

4.3.1.4.  MP2MP leaf node operation

   A leaf node Z of a MP2MP LSP <X, Y> determines its upstream LSR U for
   <X, Y> as per Section 4.3.1.1, allocates a label L, and sends a
   MP2MP-D Label Map <X, Y, L> to U.

   Leaf node Z expects an MP2MP-U Label Map <X, Y, Lu> from node U in
   response to the MP2MP-D Label Map it sent to node U. Z checks whether
   it already has forwarding state for upstream <X, Y>.  If not, Z
   creates forwarding state to push label Lu onto the traffic that Z
   wants to forward over the MP2MP LSP.  How it determines what traffic
   to forward on this MP2MP LSP is outside the scope of this document.



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4.3.1.5.  MP2MP transit node operation

   Suppose node Z receives a MP2MP-D Label Map <X, Y, L> from LSR D. Z
   checks whether it has forwarding state for downstream <X, Y>.  If
   not, Z determines its upstream LSR U for <X, Y> as per
   Section 4.3.1.1.  Using this Label Map to update the label forwarding
   table MUST NOT be done as long as LSR D is equal to LSR U. If LSR U
   is different from LSR D, Z will allocate a label L' and install
   downstream forwarding state to swap label L' with label L over
   interface I associated with LSR D and send a MP2MP-D Label Map <X, Y,
   L'> to U. Interface I is determind via the procedures in
   Section 2.4.1.2.

   If Z already has forwarding state for downstream <X, Y>, all that Z
   needs to do in this case is check that LSR D is not equal to the
   upstream LSR of <X, Y> and update its forwarding state.  Assuming its
   old forwarding state was L'-> {<I1, L1> <I2, L2> ..., <In, Ln>}, its
   new forwarding state becomes L'-> {<I1, L1> <I2, L2> ..., <In, Ln>,
   <I, L>}.  If the LSR D is equal to the installed upstream LSR, the
   Label Map from LSR D MUST be retained and MUST not update the label
   forwarding table.

   Node Z checks if upstream LSR U already assigned a label Lu to <X,
   Y>.  If not, transit node Z waits until it receives a MP2MP-U Label
   Map <X, Y, Lu> from LSR U. See Section 4.3.1.3.  Once the MP2MP-U
   Label Map is received from LSR U, node Z checks whether it already
   has forwarding state upstream <X, Y, D>.  If it does, then no further
   action needs to happen.  If it does not, it allocates a label Lu' and
   creates a new label swap for Lu' with Label Lu over interface Iu.
   Interface Iu is determind via the procedures in Section 2.4.1.2.  In
   addition, it also adds the label swap(s) from the forwarding state
   downstream <X, Y>, omitting the swap on interface I for node D. The
   swap on interface I for node D is omitted to prevent packet
   originated by D to be forwarded back to D.

   Node Z determines the downstream MP2MP LSR as per Section 4.3.1.2,
   and sends a MP2MP-U Label Map <X, Y, Lu'> to node D.

4.3.1.6.  MP2MP root node operation

4.3.1.6.1.  Root node is also a leaf

   Suppose root/leaf node Z receives a MP2MP-D Label Map <X, Y, L> from
   node D. Z checks whether it already has forwarding state downstream
   <X, Y>.  If not, Z creates forwarding state for downstream to push
   label L on traffic that Z wants to forward down the MP2MP LSP.  How
   it determines what traffic to forward on this MP2MP LSP is outside
   the scope of this document.  If Z already has forwarding state for



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   downstream <X, Y>, then Z will add the label push for L over
   interface I to it.  Interface I is determind via the procedures in
   Section 2.4.1.2.

   Node Z checks if it has forwarding state for upstream <X, Y, D> If
   not, Z allocates a label Lu' and creates upstream forwarding state to
   push Lu' with the label push(s) from the forwarding state downstream
   <X, Y>, except the push on interface I for node D. This allows
   upstream traffic to go down the MP2MP to other node(s), except the
   node from which the traffic was received.  Node Z determines the
   downstream MP2MP LSR as per section Section 4.3.1.2, and sends a
   MP2MP-U Label Map <X, Y, Lu'> to node D. Since Z is the root of the
   tree Z will not send a MP2MP-D Label Map and will not receive a
   MP2MP-U Label Map.

4.3.1.6.2.  Root node is not a leaf

   Suppose the root node Z receives a MP2MP-D Label Map <X, Y, L> from
   node D. Z checks whether it already has forwarding state for
   downstream <X, Y>.  If not, Z creates downstream forwarding state and
   installs a outgoing label L over interface I. Interface I is
   determind via the procedures in Section 2.4.1.2.  If Z already has
   forwarding state for downstream <X, Y>, then Z will add label L over
   interface I to the existing state.

   Node Z checks if it has forwarding state for upstream <X, Y, D>.  If
   not, Z allocates a label Lu' and creates forwarding state to swap Lu'
   with the label swap(s) from the forwarding state downstream <X, Y>,
   except the swap for node D. This allows upstream traffic to go down
   the MP2MP to other node(s), except the node is was received from.
   Root node Z determines the downstream MP2MP LSR D as per
   Section 4.3.1.2, and sends a MP2MP-U Label Map <X, Y, Lu'> to it.
   Since Z is the root of the tree Z will not send a MP2MP-D Label Map
   and will not receive a MP2MP-U Label Map.

4.3.2.  MP2MP Label Withdraw

   The following lists procedures for generating and processing MP2MP
   Label Withdraw messages for nodes that participate in a MP2MP LSP.
   An LSR should apply those procedures that apply to it, based on its
   role in the MP2MP LSP.

4.3.2.1.  MP2MP leaf operation

   If a leaf node Z discovers (by means outside the scope of this
   document) that it has no downstream neighbors in that LSP, and that
   it has no need to be an egress LSR for that LSP, then it SHOULD send
   a MP2MP-D Label Withdraw <X, Y, L> to its upstream LSR U for <X, Y>,



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   where L is the label it had previously advertised to U for <X,Y>.
   Leaf node Z will also send a unsolicited label release <X, Y, Lu> to
   U to indicate that the upstream path is no longer used and that Label
   Lu can be removed.

   Leaf node Z expects the upstream router U to respond by sending a
   downstream label release for L.

4.3.2.2.  MP2MP transit node operation

   If a transit node Z receives a MP2MP-D Label Withdraw message <X, Y,
   L> from node D, it deletes label L from its forwarding state
   downstream <X, Y> and from all its upstream states for <X, Y>.  Node
   Z sends a MP2MP-D Label Release message with label L to D. Since node
   D is no longer part of the downstream forwarding state, Z cleans up
   the forwarding state upstream <X, Y, D>.  There is no need to send an
   MP2MP-U Label Withdraw <X, Y, Lu> to D because node D already removed
   Lu and send a label release for Lu to Z.

   If deleting L from Z's forwarding state for downstream <X, Y> results
   in no state remaining for <X, Y>, then Z propagates the MP2MP-D Label
   Withdraw <X, Y, L> to its upstream node U for <X,Y> and will also
   send a unsolicited MP2MP-U Label Release <X, Y, Lu> to U to indicate
   that the upstream path is no longer used and that Label Lu can be
   removed.

4.3.2.3.  MP2MP root node operation

   The procedure when the root node of a MP2MP LSP receives a MP2MP-D
   Label Withdraw message is the same as for transit nodes, except that
   the root node would not propagate the Label Withdraw upstream (as it
   has no upstream).

4.3.3.  MP2MP Upstream LSR change

   The procedure for changing the upstream LSR is the same as documented
   in Section 2.4.3, except it is applied to MP2MP FECs, using the
   procedures described in Section 4.3.1 through Section 4.3.2.3.


5.  Micro-loops in MP LSPs

   Micro-loops created by the unicast routing protocol during
   convergence may also effect mLDP MP LSPs.  Since the tree building
   logic in mLDP is based on unicast routing, a unicast routing loop may
   also result in a micro-loop in the MP LSPs.  Micro-loops that involve
   2 directly connected routers don't create a loop in mLDP. mLDP is
   able to prevent this inconsistency by never allowing an upstream LDP



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   neighbor to be added as a downstream LDP neighbor into the LFT for
   the same FEC.  Micro-loops that involve more then 2 LSRs are not
   prevented.

   Micro-loops that involve more then 2 LSRs may create a micro-loop in
   the downstream path of either a MP2MP LSP or P2MP LSP and the
   upstream path of the MP2MP LSP.  The loops are transient and will
   disappear as soon as the unicast routing protocol converges.  Micro-
   loops that occur in the upstream path of a MP2MP LSP may be detected
   by including LDP path vector in the MP2MP-U Label Map messages.
   These procedures are currently under investigation and are subjected
   to further study.


6.  The LDP MP Status TLV

   An LDP MP capable router MAY use an LDP MP Status TLV to indicate
   additional status for a MP LSP to its remote peers.  This includes
   signaling to peers that are either upstream or downstream of the LDP
   MP capable router.  The value of the LDP MP status TLV will remain
   opaque to LDP and MAY encode one or more status elements.

   The LDP MP Status TLV is encoded 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1|0| LDP MP Status Type(TBD)   |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Value                               |
       ~                                                               ~
       |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   LDP MP Status Type:  The LDP MP Status Type to be assigned by IANA.


   Length:  Length of the LDP MP Status Value in octets.


   Value:  One or more LDP MP Status Value elements.








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6.1.  The LDP MP Status Value Element

   The LDP MP Status Value Element that is included in the LDP MP Status
   TLV Value has the following encoding.

       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(TBD)     | Length                        | Value ...     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
       ~                                                               ~
       |                                                               |
       |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type:  The type of the LDP MP Status Value Element is to be assigned
      by IANA.


   Length:  The length of the Value field, in octets.


   Value:  String of Length octets, to be interpreted as specified by
      the Type field.

6.2.  LDP Messages containing LDP MP Status messages

   The LDP MP status message may appear either in a label mapping
   message or a LDP notification message.

6.2.1.  LDP MP Status sent in LDP notification messages

   An LDP MP status TLV sent in a notification message must be
   accompanied with a Status TLV.  The general format of the
   Notification Message with an LDP MP status TLV is:













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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   Notification (0x0001)     |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Message ID                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Status TLV                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   LDP MP Status TLV                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 Optional LDP MP FEC TLV                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 Optional Label TLV                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Status TLV status code is used to indicate that LDP MP status TLV
   and an additional information follows in the Notification message's
   "optional parameter" section.  Depending on the actual contents of
   the LDP MP status TLV, an LDP P2MP or MP2MP FEC TLV and Label TLV may
   also be present to provide context to the LDP MP Status TLV.  (NOTE:
   Status Code is pending IANA assignment).

   Since the notification does not refer to any particular message, the
   Message Id and Message Type fields are set to 0.

6.2.2.  LDP MP Status TLV in Label Mapping Message

   An example of the Label Mapping Message defined in RFC3036 is shown
   below to illustrate the message with an Optional LDP MP Status TLV
   present.


      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   Label Mapping (0x0400)    |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     FEC TLV                                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Label TLV                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional LDP MP Status TLV                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Additional Optional Parameters            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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7.  Upstream label allocation on a LAN

   On a LAN, the procedures so far discussed would require the upstream
   LSR to send a copy of the packet to each receiver individually.  If
   there is more then one receiver on the LAN we don't take full benefit
   of the multi-access capability of the network.  We may optimize the
   bandwidth consumption on the LAN and replication overhead on the
   upstream LSR by using upstream label allocation
   [I-D.ietf-mpls-upstream-label].  Procedures on how to distribute
   upstream labels using LDP is documented in
   [I-D.ietf-mpls-ldp-upstream].

7.1.  LDP Multipoint-to-Multipoint on a LAN

   The procedure to allocate a context label on a LAN is defined in
   [I-D.ietf-mpls-upstream-label].  That procedure results in each LSR
   on a given LAN having a context label which, on that LAN, can be used
   to identify itself uniquely.  Each LSR advertises its context label
   as an upstream-assigned label, following the procedures of
   [I-D.ietf-mpls-ldp-upstream].  Any LSR for which the LAN is a
   downstream link on some P2MP or MP2MP LSP will allocate an upstream-
   assigned label identifying that LSP.  When the LSR forwards a packet
   downstream on one of those LSPs, the packet's top label must be the
   LSR's context label, and the packet's second label is the label
   identifying the LSP.  We will call the top label the "upstream LSR
   label" and the second label the "LSP label".

7.1.1.  MP2MP downstream forwarding

   The downstream path of a MP2MP LSP is much like a normal P2MP LSP, so
   we will use the same procedures as defined in
   [I-D.ietf-mpls-ldp-upstream].  A label request for a LSP label is
   send to the upstream LSR.  The label mapping that is received from
   the upstream LSR contains the LSP label for the MP2MP FEC and the
   upstream LSR context label.  The MP2MP downstream path (corresponding
   to the LSP label) will be installed the context specific forwarding
   table corresponding to the upstream LSR label.  Packets sent by the
   upstream router can be forwarded downstream using this forwarding
   state based on a two label lookup.

7.1.2.  MP2MP upstream forwarding

   A MP2MP LSP also has an upstream forwarding path.  Upstream packets
   need to be forwarded in the direction of the root and downstream on
   any node on the LAN that has a downstream interface for the LSP.  For
   a given MP2MP LSP on a given LAN, exactly one LSR is considered to be
   the upstream LSR.  If an LSR on the LAN receives a packet from one of
   its downstream interfaces for the LSP, and if it needs to forward the



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   packet onto the LAN, it ensures that the packet's top label is the
   context label of the upstream LSR, and that its second label is the
   LSP label that was assigned by the upstream LSR.

   Other LSRs receiving the packet will not be able to tell whether the
   packet really came from the upstream router, but that makes no
   difference in the processing of the packet.  The upstream LSR will
   see its own upstream LSR in the label, and this will enable it to
   determine that the packet is traveling upstream.


8.  Root node redundancy

   The root node is a single point of failure for an MP LSP, whether
   this is P2MP or MP2MP.  The problem is particularly severe for MP2MP
   LSPs.  In the case of MP2MP LSPs, all leaf nodes must use the same
   root node to set up the MP2MP LSP, because otherwise the traffic
   sourced by some leafs is not received by others.  Because the root
   node is the single point of failure for an MP LSP, we need a fast and
   efficient mechanism to recover from a root node failure.

   An MP LSP is uniquely identified in the network by the opaque value
   and the root node address.  It is likely that the root node for an MP
   LSP is defined statically.  The root node address may be configured
   on each leaf statically or learned using a dynamic protocol.  How
   leafs learn about the root node is out of the scope of this document.

   Suppose that for the same opaque value we define two (or more) root
   node addresses and we build a tree to each root using the same opaque
   value.  Effectively these will be treated as different MP LSPs in the
   network.  Once the trees are built, the procedures differ for P2MP
   and MP2MP LSPs.  The different procedures are explained in the
   sections below.

8.1.  Root node redundancy - procedures for P2MP LSPs

   Since all leafs have set up P2MP LSPs to all the roots, they are
   prepared to receive packets on either one of these LSPs.  However,
   only one of the roots should be forwarding traffic at any given time,
   for the following reasons: 1) to achieve bandwidth savings in the
   network and 2) to ensure that the receiving leafs don't receive
   duplicate packets (since one cannot assume that the receiving leafs
   are able to discard duplicates).  How the roots determine which one
   is the active sender is outside the scope of this document.







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8.2.  Root node redundancy - procedures for MP2MP LSPs

   Since all leafs have set up an MP2MP LSP to each one of the root
   nodes for this opaque value, a sending leaf may pick either of the
   two (or more) MP2MP LSPs to forward a packet on.  The leaf nodes
   receive the packet on one of the MP2MP LSPs.  The client of the MP2MP
   LSP does not care on which MP2MP LSP the packet is received, as long
   as they are for the same opaque value.  The sending leaf MUST only
   forward a packet on one MP2MP LSP at a given point in time.  The
   receiving leafs are unable to discard duplicate packets because they
   accept on all LSPs.  Using all the available MP2MP LSPs we can
   implement redundancy using the following procedures.

   A sending leaf selects a single root node out of the available roots
   for a given opaque value.  A good strategy MAY be to look at the
   unicast routing table and select a root that is closest in terms of
   the unicast metric.  As soon as the root address of the active root
   disappears from the unicast routing table (or becomes less
   attractive) due to root node or link failure, the leaf can select a
   new best root address and start forwarding to it directly.  If
   multiple root nodes have the same unicast metric, the highest root
   node addresses MAY be selected, or per session load balancing MAY be
   done over the root nodes.

   All leafs participating in a MP2MP LSP MUST join to all the available
   root nodes for a given opaque value.  Since the sending leaf may pick
   any MP2MP LSP, it must be prepared to receive on it.

   The advantage of pre-building multiple MP2MP LSPs for a single opaque
   value is that convergence from a root node failure happens as fast as
   the unicast routing protocol is able to notify.  There is no need for
   an additional protocol to advertise to the leaf nodes which root node
   is the active root.  The root selection is a local leaf policy that
   does not need to be coordinated with other leafs.  The disadvantage
   of pre-building multiple MP2MP LSPs is that more label resources are
   used, depending on how many root nodes are defined.


9.  Make Before Break (MBB)

   An LSR selects as its upstream LSR for a MP LSP the LSR that is its
   next hop to the root of the LSP.  When the best path to reach the
   root changes the LSR must choose a new upstream LSR.  Sections
   Section 2.4.3 and Section 4.3.3 describe these procedures.

   When the best path to the root changes the LSP may be broken
   temporarily resulting in packet loss until the LSP "reconverges" to a
   new upstream LSR.  The goal of MBB when this happens is to keep the



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   duration of packet loss as short as possible.  In addition, there are
   scenarios where the best path from the LSR to the root changes but
   the LSP continues to forward packets to the prevous next hop to the
   root.  That may occur when a link comes up or routing metrics change.
   In such a case a new LSP should be established before the old LSP is
   removed to limit the duration of packet loss.  The procedures
   described below deal with both scenarios in a way that an LSR does
   not need to know which of the events described above caused its
   upstream router for an MBB LSP to change.

   This MBB procedures are an optional extension to the MP LSP building
   procedures described in this draft.

9.1.  MBB overview

   The MBB procedues use additional LDP signaling.

   Suppose some event causes a downstream LSR-D to select a new upstream
   LSR-U for FEC-A.  The new LSR-U may already be forwarding packets for
   FEC-A; that is, to downstream LSR's other than LSR-D.  After LSR-U
   receives a label for FEC-A from LSR-D, it will notify LSR-D when it
   knows that the LSP for FEC-A has been established from the root to
   itself.  When LSR-D receives this MBB notification it will change its
   next hop for the LSP root to LSR-U.

   The assumption is that if LSR-U has received an MBB notification from
   its upstream router for the FEC-A LSP and has installed forwarding
   state the LSP it is capable of forwarding packets on the LSP.  At
   that point LSR-U should signal LSR-D by means of an MBB notification
   that it has become part of the tree identified by FEC-A and that
   LSR-D should initiate its switchover to the LSP.

   At LSR-U the LSP for FEC-A may be in 1 of 3 states.

   1.  There is no state for FEC-A.

   2.  State for FEC-A exists and LSR-U is waiting for MBB notification
       that the LSP from the root to it exists.

   3.  State for FEC-A exists and the MBB notification has been
       received.

   After LSR-U receives LSR-D's Label Mapping message for FEC-A LSR-U
   MUST NOT reply with an MBB notification to LSR-D until its state for
   the LSP is state #3 above.  If the state of the LSP at LSR-U is state
   #1 or #2, LSR-U should remember receipt of the Label Mapping message
   from LSR-D while waiting for an MBB notification from its upstream
   LSR for the LSP.  When LSR-U receives the MBB notification from its



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   upstream LSR it transitions to LSP state #3 and sends an MBB
   notification to LSR-D.

9.2.  The MBB Status code

   As noted in Section 9.1, the procedures to establish an MBB MP LSP
   are different from those to establish normal MP LSPs.

   When a downstream LSR sends a Label Mapping message for MP LSP to its
   upstream LSR it MAY include an LDP MP Status TLV that carries a MBB
   Status Code to indicate MBB procedures apply to the LSP.  This new
   MBB Status Code MAY also appear in an LDP Notification message used
   by an upstream LSR to signal LSP state #3 to the downstream LSR; that
   is, that the upstream LSR's state for the LSP exists and that it has
   received notification from its upstream LSR that the LSP is in state
   #3.

   The MBB Status is a type of the LDP MP Status Value Element as
   described in Section 6.1.  It is encoded 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | MBB Type = 1  |      Length = 1               | Status code   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   MBB Type:  Type 1 (to be assigned by IANA)


   Length:  1


   Status code:  1 = MBB request

                 2 = MBB ack

9.3.  The MBB capability

   An LSR MAY advertise that it is capable of handling MBB LSPs using
   the capability advertisement as defined in
   [I-D.ietf-mpls-ldp-capabilities].  The LDP MP MBB capability has the
   following format:







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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1|0| LDP MP MBB Capability     |           Length = 1          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1| Reserved    |
       +-+-+-+-+-+-+-+-+


   Note:  LDP MP MBB Capability (Pending IANA assignment)



        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1|0| LDP MP MBB Capability     |           Length = 1          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1| Reserved    |
       +-+-+-+-+-+-+-+-+


   If an LSR has not advertised that it is MBB capable, its LDP peers
   MUST NOT send it messages which include MBB parameters.  If an LSR
   receives a Label Mapping message with a MBB parameter from downstream
   LSR-D and its upstream LSR-U has not advertised that it is MBB
   capable, the LSR MUST send an MBB notification immediatly to LSR-U
   (see Section 9.4).  If this happens an MBB MP LSP will not be
   established, but normal a MP LSP will be the result.

9.4.  The MBB procedures

9.4.1.  Terminology

   1.  MBB LSP <X, Y>: A P2MP or MP2MP Make Before Break (MBB) LSP entry
       with Root Node Address X and Opaque Value Y.

   2.  A(N, L): An Accepting element that consists of an upstream
       Neighbor N and Local label L. This LSR assigned label L to
       neighbor N for a specific MBB LSP.  For an active element the
       corresponding Label is stored in the label forwarding database.

   3.  iA(N, L): An inactive Accepting element that consists of an
       upstream neighbor N and local Label L. This LSR assigned label L
       to neighbor N for a specific MBB LSP.  For an inactive element
       the corresponding Label is not stored in the label forwarding
       database.




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   4.  F(N, L): A Forwarding state that consists of downstream Neighbor
       N and Label L. This LSR is sending label packets with label L to
       neighbor N for a specific FEC.

   5.  F'(N, L): A Forwarding state that has been marked for sending a
       MBB Notification message to Neighbor N with Label L.

   6.  MBB Notification <X, Y, L>: A LDP notification message with a MP
       LSP <X, Y>, Label L and MBB Status code 2.

   7.  MBB Label Map <X, Y, L>: A P2MP Label Map or MP2MP Label Map
       downstream with a FEC element <X, Y>, Label L and MBB Status code
       1.

9.4.2.  Accepting elements

   An accepting element represents a specific label value L that has
   been advertised to a neighbor N for a MBB LSP <X, Y> and is a
   candidate for accepting labels switched packets on.  An LSR can have
   two accepting elements for a specific MBB LSP <X, Y> LSP, only one of
   them MUST be active.  An active element is the element for which the
   label value has been installed in the label forwarding database.  An
   inactive accepting element is created after a new upstream LSR is
   chosen and is pending to replace the active element in the label
   forwarding database.  Inactive elements only exist temporarily while
   switching to a new upstream LSR.  Once the switch has been completed
   only one active element remains.  During network convergence it is
   possible that an inactive accepting element is created while an other
   inactive accepting element is pending.  If that happens the older
   inactive accepting element MUST be replaced with an newer inactive
   element.  If an accepting element is removed a Label Withdraw has to
   be send for label L to neighbor N for <X, Y>.

9.4.3.  Procedures for upstream LSR change

   Suppose a node Z has a MBB LSP <X, Y> with an active accepting
   element A(N1, L1).  Due to a routing change it detects a new best
   path for root X and selects a new upstream LSR N2.  Node Z allocates
   a new local label L2 and creates an inactive accepting element iA(N2,
   L2).  Node Z sends MBB Label Map <X, Y, L2>to N2 and waits for the
   new upstream LSR N2 to respond with a MBB Notification for <X, Y,
   L2>.  During this transition phase there are two accepting elements,
   the element A(N1, L1) still accepting packets from N1 over label L1
   and the new inactive element iA(N2, L2).

   While waiting for the MBB Notification from upstream LSR N2, it is
   possible that an other transition occurs due to a routing change.
   Suppose the new upstream LSR is N3.  An inactive element iA(N3, L3)



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   is created and the old inactive element iA(N2, L2) MUST be removed.
   A label withdraw MUST be sent to N2 for <X, Y, L2&gt.  The MBB
   Notification for <X, Y, L2> from N2 will be ignored because the
   inactive element is removed.

   It is possible that the MBB Notification from upstream LSR is never
   received due to link or node failure.  To prevent waiting
   indefinitely for the MBB Notification a timeout SHOULD be applied.
   As soon as the timer expires, the procedures in Section 9.4.5 are
   applied as if a MBB Notification was received for the inactive
   element.

9.4.4.  Receiving a Label Map with MBB status code

   Suppose node Z has state for a MBB LSP <X, Y> and receives a MBB
   Label Map <X, Y, L2> from N2.  A new forwarding state F(N2, L2) will
   be added to the MP LSP if it did not already exist.  If this MBB LSP
   has an active accepting element or node Z is the root of the MBB LSP
   a MBB notification <X, Y, L2)> is send to node N2.  If node Z has an
   inactive accepting element it marks the Forwarding state as <X, Y,
   F'(N2, L2)>.

9.4.5.  Receiving a Notification with MBB status code

   Suppose node Z receives a MBB Notification <X, Y, L> from N. If node
   Z has state for MBB LSP <X, Y> and an inactive accepting element
   iA(N, L) that matches with N and L, we activate this accepting
   element and install label L in the label forwarding database.  If an
   other active accepting was present it will be removed from the label
   forwarding database.

   If this MBB LSP <X, Y> also has Forwarding states marked for sending
   MBB Notifications, like <X, Y, F'(N2, L2)>, MBB Notifications are
   send to these downstream LSRs.  If node Z receives a MBB Notification
   for an accepting element that is not inactive or does not match the
   Label value and Neighbor address, the MBB notification is ignored.

9.4.6.  Node operation for MP2MP LSPs

   The procedures described above apply to the downstream path of a
   MP2MP LSP.  The upstream path of the MP2MP is setup as normal without
   including a MBB Status code.  If the MBB procedures apply to a MP2MP
   downstream FEC element, the upstream path to a node N is only
   installed in the label forwarding database if node N is part of the
   active accepting element.  If node N is part of an inactive accepting
   element, the upstream path is installed when this inactive accepting
   element is activated.




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10.  Security Considerations

   The same security considerations apply as for the base LDP
   specification, as described in [RFC5036].


11.  IANA considerations

   This document creates a new name space (the LDP MP Opaque Value
   Element type) that is to be managed by IANA, and the allocation of
   the value 1 for the type of Generic LSP Identifier.

   This document requires allocation of three new LDP FEC Element types:

   1.  the P2MP FEC type - requested value 0x06

   2.  the MP2MP-up FEC type - requested value 0x07

   3.  the MP2MP-down FEC type - requested value 0x08

   This document requires the assignment of three new code points for
   three new Capability Parameter TLVs, corresponding to the
   advertisement of the P2MP, MP2MP and MBB capabilities.  The values
   requested are:

      P2MP Capability Parameter - requested value 0x0508

      MP2MP Capability Parameter - requested value 0x0509

      MBB Capability Parameter - requested value 0x050A

   This document requires the assignment of a LDP Status Code to
   indicate a LDP MP Status TLV is following in the Notification
   message.  The value requested from the LDP Status Code Name Space:

      LDP MP status - requested value 0x00000040

   This document requires the assigment of a new code point for a LDP MP
   Status TLV.  The value requested from the LDP TLV Type Name Space:

      LDP MP Status TLV Type - requested value 0x096F

   This document creates a new name space (the LDP MP Status Value
   Element type) that is to be managed by IANA, and the allocation of
   the value 1 for the type of MBB Status.






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12.  Acknowledgments

   The authors would like to thank the following individuals for their
   review and contribution: Nischal Sheth, Yakov Rekhter, Rahul
   Aggarwal, Arjen Boers, Eric Rosen, Nidhi Bhaskar, Toerless Eckert,
   George Swallow, Jin Lizhong, Vanson Lim, Adrian Farrel and Thomas
   Morin.


13.  Contributing authors

   Below is a list of the contributing authors in alphabetical order:

   Shane Amante
   Level 3 Communications, LLC
   1025 Eldorado Blvd
   Broomfield, CO 80021
   US
   Email: Shane.Amante@Level3.com


   Luyuan Fang
   Cisco Systems
   300 Beaver Brook Road
   Boxborough, MA 01719
   US
   Email: lufang@cisco.com


   Hitoshi Fukuda
   NTT Communications Corporation
   1-1-6, Uchisaiwai-cho, Chiyoda-ku
   Tokyo 100-8019,
   Japan
   Email: hitoshi.fukuda@ntt.com


   Yuji Kamite
   NTT Communications Corporation
   Tokyo Opera City Tower
   3-20-2 Nishi Shinjuku, Shinjuku-ku,
   Tokyo 163-1421,
   Japan
   Email: y.kamite@ntt.com







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   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA 94089
   US
   Email: kireeti@juniper.net


   Ina Minei
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US
   Email: ina@juniper.net


   Jean-Louis Le Roux
   France Telecom
   2, avenue Pierre-Marzin
   Lannion, Cedex 22307
   France
   Email: jeanlouis.leroux@francetelecom.com


   Bob Thomas
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough, MA, 01719
   E-mail: bobthomas@alum.mit.edu


   Lei Wang
   Telenor
   Snaroyveien 30
   Fornebu 1331
   Norway
   Email: lei.wang@telenor.com


   IJsbrand Wijnands
   Cisco Systems, Inc.
   De kleetlaan 6a
   1831 Diegem
   Belgium
   E-mail: ice@cisco.com


14.  References



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14.1.  Normative References

   [RFC5036]  Andersson, L., Minei, I., and B. Thomas, "LDP
              Specification", RFC 5036, October 2007.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3232]  Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
              an On-line Database", RFC 3232, January 2002.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [I-D.ietf-mpls-upstream-label]
              Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              draft-ietf-mpls-upstream-label-05 (work in progress),
              April 2008.

   [I-D.ietf-mpls-ldp-upstream]
              Aggarwal, R. and J. Roux, "MPLS Upstream Label Assignment
              for LDP", draft-ietf-mpls-ldp-upstream-02 (work in
              progress), November 2007.

   [I-D.ietf-mpls-ldp-capabilities]
              Thomas, B., "LDP Capabilities",
              draft-ietf-mpls-ldp-capabilities-02 (work in progress),
              March 2008.

14.2.  Informative References

   [RFC4664]  Andersson, L. and E. Rosen, "Framework for Layer 2 Virtual
              Private Networks (L2VPNs)", RFC 4664, September 2006.

   [RFC4875]  Aggarwal, R., Papadimitriou, D., and S. Yasukawa,
              "Extensions to Resource Reservation Protocol - Traffic
              Engineering (RSVP-TE) for Point-to-Multipoint TE Label
              Switched Paths (LSPs)", RFC 4875, May 2007.

   [I-D.ietf-mpls-mp-ldp-reqs]
              Roux, J., "Requirements for Point-To-Multipoint Extensions
              to the Label Distribution  Protocol",
              draft-ietf-mpls-mp-ldp-reqs-04 (work in progress),
              February 2008.

   [I-D.ietf-l3vpn-2547bis-mcast]
              Aggarwal, R., Bandi, S., Cai, Y., Morin, T., Rekhter, Y.,



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              Rosen, E., Wijnands, I., and S. Yasukawa, "Multicast in
              MPLS/BGP IP VPNs", draft-ietf-l3vpn-2547bis-mcast-06 (work
              in progress), January 2008.

   [I-D.ietf-mpls-multicast-encaps]
              Eckert, T., Rosen, E., Aggarwal, R., and Y. Rekhter, "MPLS
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Authors' Addresses

   Ina Minei
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: ina@juniper.net


   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: kireeti@juniper.net


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

   Email: ice@cisco.com


   Bob Thomas
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough  01719
   US

   Email: bobthomas@alum.mit.edu




Minei (Editor), et al.   Expires April 27, 2010                [Page 37]


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