Internet Engineering Task Force (IETF) IJ. Wijnands, Ed.

Internet Engineering Task Force (IETF) IJ. Wijnands, Ed.
Request for Comments: 6388 Cisco Systems, Inc.
Category: Standards Track I. Minei, Ed.
ISSN: 2070-1721 K. Kompella
Juniper Networks
B. Thomas
November 2011

Label Distribution Protocol Extensions for
Point-to-Multipoint and Multipoint-to-Multipoint Label Switched Paths

Abstract

This document describes extensions to the Label Distribution Protocol
(LDP) for the setup of point-to-multipoint (P2MP) and multipoint-to-
multipoint (MP2MP) Label Switched Paths (LSPs) in MPLS networks.
These extensions are also referred to as multipoint LDP. Multipoint
LDP 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 multipoint LSPs, for example IP multicast or
support for multicast in BGP/MPLS Layer 3 Virtual Private Networks
(L3VPNs). Specification of how such applications can use an LDP
signaled multipoint LSP is outside the scope of this document.

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/rfc6388.

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Copyright Notice

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document authors. All rights reserved.

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Table of Contents

1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................4
1.2. Terminology ................................................4
1.3. Manageability ..............................................5
2. Setting Up P2MP LSPs with LDP ...................................6
2.1. Support for P2MP LSP Setup with LDP ........................6
2.2. The P2MP FEC Element .......................................6
2.3. The LDP MP Opaque Value Element ............................8
2.3.1. The Generic LSP Identifier ..........................9
2.4. Using the P2MP FEC Element .................................9
2.4.1. Label Mapping ......................................10
2.4.2. Label Withdraw .....................................12
2.4.3. Upstream LSR Change ................................13
3. Setting up MP2MP LSPs with LDP .................................14
3.1. Support for MP2MP LSP Setup with LDP ......................14
3.2. The MP2MP Downstream and Upstream FEC Elements ............15
3.3. Using the MP2MP FEC Elements ..............................15
3.3.1. MP2MP Label Mapping ................................17
3.3.2. MP2MP Label Withdraw ...............................20

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3.3.3. MP2MP Upstream LSR Change ..........................21
4. Micro-Loops in MP LSPs .........................................21
5. The LDP MP Status TLV ..........................................21
5.1. The LDP MP Status Value Element ...........................22
5.2. LDP Messages Containing LDP MP Status Messages ............22
5.2.1. LDP MP Status Sent in LDP Notification Messages ....23
5.2.2. LDP MP Status TLV in Label Mapping Message .........24
6. Upstream Label Allocation on a LAN .............................24
6.1. LDP Multipoint-to-Multipoint on a LAN .....................24
6.1.1. MP2MP Downstream Forwarding ........................25
6.1.2. MP2MP Upstream Forwarding ..........................25
7. Root Node Redundancy ...........................................25
7.1. Root Node Redundancy - Procedures for P2MP LSPs ...........26
7.2. Root Node Redundancy - Procedures for MP2MP LSPs ..........26
8. Make Before Break (MBB) ........................................27
8.1. MBB Overview .............................................27
8.2. The MBB Status Code .......................................28
8.3. The MBB Capability ........................................29
8.4. The MBB Procedures ........................................29
8.4.1. Terminology ........................................29
8.4.2. Accepting Elements .................................30
8.4.3. Procedures for Upstream LSR Change .................30
8.4.4. Receiving a Label Mapping with MBB Status Code .....31
8.4.5. Receiving a Notification with MBB Status Code ......31
8.4.6. Node Operation for MP2MP LSPs ......................32
9. Typed Wildcard for mLDP FEC Element ............................32
10. Security Considerations .......................................32
11. IANA Considerations ...........................................33
12. Acknowledgments ...............................................34
13. Contributing Authors ..........................................35
14. References ....................................................37
14.1. Normative References .....................................37
14.2. Informative References ...................................37

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. An 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
to an LDP neighbor traversed by the MP LSP. This is accomplished
without the use of a multicast protocol in the network. There can be

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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 an MP LSP signaled by LDP is also outside the scope of this
document.

Related documents that may be of interest include [RFC6348],
[L3VPN-MCAST], and [RFC4875].

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].

All new fields shown as "reserved" in this document MUST be set to
zero on transmission and MUST be ignored on receipt.

1.2. Terminology

Some of the following terminology is taken from [RFC6348].

mLDP: Multipoint extensions for LDP.

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.

MP2MP LSP: An LSP with a distinguished root node 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".

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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 to in the context of a P2MP LSP. In the context of an
MP2MP LSP, a leaf is both Ingress and Egress for the same MP2MP
LSP and can also be a Bud LSR.

CRC32: This contains a Cyclic Redundancy Check value of the
uncompressed data in network byte order computed according to
CRC-32 algorithm used in the ISO 3309 standard [ISO3309] and in
Section 8.1.1.6.2 of ITU-T recommendation V.42 [ITU.V42.1994].

FEC: Forwarding Equivalence Class

1.3. Manageability

MPLS LSRs can be modeled and managed using the MIB module defined in
[RFC3813]. That MIB module is fully capable of handling the one-to-
many in-segment to out-segment relationships needed to support P2MP
LSPs, and no further changes are required.

[RFC3815] defines managed objects for LDP. The MIB module allows the
modeling and management of LDP and LDP speakers for the protocol as
defined in [RFC5036]. The protocol extensions defined in this
document to support P2MP in LDP may require an additional MIB module
or extensions to the modules defined in [RFC3815]. This is for
future study, and at the time of this writing, no interest has been
expressed in this work.

Future manageability work should pay attention to the protocol
extensions defined in this document, and specifically the
configurable and variable elements, along with reporting the new
protocol fields that identify individual P2MP LSPs.

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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.

2.1. Support for P2MP LSP Setup with LDP

Support for the setup of P2MP LSPs is advertised using LDP
capabilities as defined in [RFC5561]. 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 (0x0508) | Length (= 1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved |
+-+-+-+-+-+-+-+-+

S: As specified in [RFC5561]

The P2MP Capability TLV MUST be advertised 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 label
messages using the P2MP FEC Element SHOULD NOT 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.

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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
type, Root Node Address, Opaque Value) uniquely identifies a P2MP LSP
within the MPLS network.

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(0x06)| Address Family | Address Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Root Node Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Length | Opaque Value ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
~ ~
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type: The type of the P2MP FEC Element is 0x06.

Address Family: Two octet quantity containing a value from IANA's
"Address Family Numbers" registry 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.

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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
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 basic type 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 < 255 | Length | Value ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ ~
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type: The Type of the LDP MP opaque value element. IANA maintains a
registry of basic types (see Section 11).

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

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

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The LDP MP opaque value element extended type 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 = 255 | Extended Type | Length (high) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| Length (low) | Value |
+-+-+-+-+-+-+-+-+ |
~ ~
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type: Type = 255.

Extended Type: The Extended Type of the LDP MP opaque value element.
IANA maintains a registry of extended types (see Section 11).

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 basic
type encoded as follows:

Type: 1

Length: 4

Value: A 32-bit 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 is 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.

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2. P2MP Label Mapping <X, Y, L>: a Label Mapping 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 Mapping
message. The use of the interface label space is outside the
scope of this document.

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 that the node plays in
the P2MP LSP. Node Z knows that it is a leaf node by a discovery
process that 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 Mapping 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.

2.4.1. Label Mapping

The remainder of this section specifies the procedures for
originating P2MP Label Mapping messages and for processing received
P2MP Label Mapping 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 [RFC5332] except
those that are assigned using the procedures of Section 6.

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 an MP LSP <X, Y> determines the LDP peer U that is Z's
next-hop on the best path from Z to the root node X. If there is

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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 MUST select
one upstream LSR. The algorithm used for the LSR selection is a
local matter. If the LSR selection is done over a LAN interface and
the Section 6 procedures are applied, the following procedure SHOULD
be applied to ensure that the same upstream LSR is elected among a
set of candidate receivers on that LAN.

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

2. The following hash is performed: H = (CRC32(Opaque Value)) modulo
N, where N is the number of upstream LSRs. The 'Opaque Value' is
the field identified in the FEC Element right after 'Opaque
Length'. The 'Opaque Length' indicates the size of the opaque
value used in this calculation.

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

This procedure will ensure that there is a single forwarder over the
LAN for a particular LSP.

2.4.1.2. Determining the Forwarding Interface to an LSR

Suppose LSR U receives an MP Label Mapping 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. The
algorithm it uses to choose a particular interface and next-hop for a
particular such packet is a local matter. For completeness, the
following procedure MAY be used. LSR U may do a lookup in the
unicast routing table to find the best interface and next-hop to
reach LSR D. If the next-hop and interface are also advertised by LSR
D via the LDP session, it can be used to transmit the packet to LSR
D.

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 Mapping <X, Y, L> to U.

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2.4.1.4. Transit Node Operation

Suppose a transit node Z receives a P2MP Label Mapping <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 Mapping 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 Mapping <X, Y, L'> to LSR U. Interface I is determined
via the procedures in Section 2.4.1.2.

If Z already has state for <X, Y>, then Z does not send a Label
Mapping message for P2MP LSP <X, Y>. If LSR T is not equal to the
upstream LSR of <X, Y> and <I, L> does not already exist as
forwarding state, the forwarding state is updated. 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 LSR T is equal to the installed upstream LSR, the Label
Mapping 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 Mapping <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 next hop, where I is the interface
associated with LSR T and determined via the procedures in Section
2.4.1.2.

2.4.2. Label Withdraw

The following section 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 that it has no downstream neighbors in
that LSP, and that it has no need to be an Egress LSR for that LSP
(by means outside the scope of this document), 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

When the root node of a P2MP LSP receives a Label Withdraw message,
the procedures are the same as those 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. Z MUST
update its forwarding state as follows. It allocates a new label,
L', for <X, Y>. The forwarding state for L' is copied from the
forwarding state for L, with one exception: if U' was present in the
forwarding state of L, it MUST NOT be installed in the forwarding
state of L'. Then the forwarding state for L is deleted and the
forwarding state for L' is installed. In addition, Z MUST send a
Label Mapping <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 the procedures defined in Section
2.4.1.4, it can now be installed.

While changing the upstream LSR, the following must be taken into
consideration. If L' is added before L is removed, there is a
potential risk of packet duplication and/or the creation of a
transient data-plane forwarding loop. If L is removed before L' is
added, packet loss may result. Ideally the change from L to L' is
done atomically such that no packet loss or duplication occurs. If

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that is not possible, the RECOMMENDED default behavior is to remove L
before adding L'.

3. 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 an 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 an 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.
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 an 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 an MP2MP LSP will not traverse a link more than
once, with the possible exception of LAN links (see Section 3.3.1),
if the procedures of [RFC5331] are not provided.

3.1. Support for MP2MP LSP Setup with LDP

Support for the setup of MP2MP LSPs is advertised using LDP
capabilities as defined in [RFC5561]. 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 (0x0509) | Length (= 1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved |
+-+-+-+-+-+-+-+-+

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S: As specified in [RFC5561]

The MP2MP Capability TLV MUST be advertised 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 label
messages using the MP2MP upstream and downstream FEC Elements SHOULD
NOT be sent to the peer.

3.2. The MP2MP Downstream and Upstream FEC Elements

For the setup of an 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
described in Section 2.2. The difference is that two new FEC types
are used: MP2MP downstream type (0x08) and MP2MP upstream type
(0x07).

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 [RFC5331], the MPLS labels
used are "downstream-assigned" [RFC5332], even if they are bound to
the "upstream FEC Element".

3.3. 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>): an
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.

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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 Mapping <X, Y, L>: a Label Mapping 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 Mapping message. The use of the interface
label space is outside the scope of this document.

6. MP2MP-U Label Mapping <X, Y, Lu>: a Label Mapping message with a
FEC TLV with a single MP2MP upstream FEC Element <X, Y> and label
TLV with label Lu. Label Lu MUST be allocated from the per-
platform label space (see [RFC3031], Section 3.14) of the LSR
sending the Label Mapping message. The use of the interface
label space is outside the scope of this document.

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 that 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 an MP2MP Label Mapping
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 a
leaf of the MP2MP LSP.

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3.3.1. MP2MP Label Mapping

The remainder of this section specifies the procedures for
originating MP2MP Label Mapping messages and for processing received
MP2MP Label Mapping messages for a particular LSP. The procedures
for a particular LSR depend upon the role that the LSR plays in the
LSP (Ingress, Transit, or Egress).

All labels discussed here are downstream-assigned [RFC5332] except
those that are assigned using the procedures of Section 6.

3.3.1.1. Determining one's upstream MP2MP LSR

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

3.3.1.2. Determining One's Downstream MP2MP LSR

An LDP peer U that receives an MP2MP-D Label Mapping from an LDP peer
D will treat D as downstream MP2MP LSR.

3.3.1.3. Installing the Upstream Path of an MP2MP LSP

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

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

2. We install the upstream MP2MP path (to a downstream neighbor)
based on receiving an MP2MP-U Label Mapping from the upstream
neighbor. An LSR does not need to wait for the MP2MP-U Label
Mapping if it is the root of the MP2MP LSP or if it already
received an MP2MP-U Label Mapping 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 built hop by hop towards
the root. Once the root is reached, the root node will trigger an
MP2MP-U Label Mapping to the downstream neighbor(s).

For setting up the upstream path of an MP2MP LSP, ordered mode SHOULD
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 has completed.
The advantage is that when a leaf starts sending immediately after
the upstream path is installed, packets are able to reach the root

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node without being dropped due to an incomplete LSP. Method 1 is not
able to guarantee that the upstream path has completed before the
leaf starts sending.

3.3.1.4. MP2MP Leaf Node Operation

A leaf node Z of an MP2MP LSP <X, Y> determines its upstream LSR U
for <X, Y> as per Section 3.3.1.1, allocates a label L, and sends an
MP2MP-D Label Mapping <X, Y, L> to U.

Leaf node Z expects an MP2MP-U Label Mapping <X, Y, Lu> from node U
in response to the MP2MP-D Label Mapping 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.

3.3.1.5. MP2MP Transit Node Operation

Suppose node Z receives an MP2MP-D Label Mapping <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
3.3.1.1. Using this Label Mapping 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 an MP2MP-D Label Mapping
<X, Y, L'> to U. Interface I is determined 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 Mapping 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 an MP2MP-U
Label Mapping <X, Y, Lu> from LSR U (see Section 3.3.1.3). Once the
MP2MP-U Label Mapping 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 determined via the procedures in
Section 2.4.1.2. In addition, it also adds the label swap(s) from

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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 a packet originated by D to be forwarded back to
D.

Node Z determines the downstream MP2MP LSR as per Section 3.3.1.2,
and sends an MP2MP-U Label Mapping <X, Y, Lu'> to node D.

3.3.1.6. MP2MP Root Node Operation

3.3.1.6.1. Root Node Is Also a Leaf

Suppose root/leaf node Z receives an MP2MP-D Label Mapping <X, Y, L>
from node D. Z checks whether it already has forwarding state
downstream <X, Y>. If not, Z creates downstream forwarding state 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 downstream <X, Y>, then Z will add the label push for L
over interface I to it. Interface I is determined 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
swap Lu' with the label swap(s) from the forwarding state downstream
<X, Y>, except the swap 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 3.3.1.2, and sends an
MP2MP-U Label Mapping <X, Y, Lu'> to node D. Since Z is the root of
the tree, Z will not send an MP2MP-D Label Mapping and will not
receive an MP2MP-U Label Mapping.

3.3.1.6.2. Root Node is Not a Leaf

Suppose the root node Z receives an MP2MP-D Label Mapping <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
determined 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 from which it was

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received. Root node Z determines the downstream MP2MP LSR D as per
Section 3.3.1.2, and sends an MP2MP-U Label Mapping <X, Y, Lu'> to
it. Since Z is the root of the tree, Z will not send an MP2MP-D
Label Mapping and will not receive an MP2MP-U Label Mapping.

3.3.2. MP2MP Label Withdraw

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

3.3.2.1. 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 (by means outside the
scope of this document), then it SHOULD send an MP2MP-D 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>. Leaf node Z will
also send an 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.

3.3.2.2. MP2MP Transit Node Operation

If a transit node Z receives an 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 an 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 sent 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 an 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.

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3.3.2.3. MP2MP Root Node Operation

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

3.3.3. 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 3.3.1 through Section 3.3.2.3.

4. 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
neighbor to be added as a downstream LDP neighbor into the Label
Forwarding Table (LFT) for the same FEC. Micro-loops that involve
more than 2 LSRs are not prevented.

Micro-loops that involve more than 2 LSRs may create a micro-loop in
the downstream path of either an 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 and mLDP
has updated the forwarding state accordingly. Micro-loops that occur
in the upstream path of an MP2MP LSP may be detected by including LDP
path vector in the MP2MP-U Label Mapping messages. These procedures
are currently under investigation and are subjected to further study.

5. The LDP MP Status TLV

An LDP MP capable router MAY use an LDP MP Status TLV to indicate
additional status for an 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.

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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(0x096F)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
~ ~
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LDP MP Status Type: The LDP MP Status (0x096F).

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

Value: One or more LDP MP Status Value elements.

5.1. 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 | Length | Value ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ ~
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type: The type of the LDP MP Status Value Element. IANA maintains a
registry of status value types (see Section 11).

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

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

5.2. LDP Messages Containing LDP MP Status Messages

The LDP MP Status TLV may appear either in a Label Mapping message or
an LDP Notification message.

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5.2.1. 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, as described in [RFC5036]. The
general format of the Notification message with an LDP MP Status TLV
is:

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 any 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 a Label TLV
may also be present to provide context to the LDP MP Status TLV.

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

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5.2.2. LDP MP Status TLV in Label Mapping Message

An example of the Label Mapping message defined in [RFC5036] 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6. 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 than 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
[RFC5331]. Procedures on how to distribute upstream labels using LDP
is documented in [RFC6389].

6.1. LDP Multipoint-to-Multipoint on a LAN

The procedure to allocate a context label on a LAN is defined in
[RFC5331]. 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 [RFC6389]. 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".

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6.1.1. MP2MP Downstream Forwarding

The downstream path of an MP2MP LSP is much like a normal P2MP LSP,
so we will use the same procedures as those defined in [RFC6389]. A
label request for an LSP label is sent 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 in 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.

6.1.2. MP2MP Upstream Forwarding

An 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
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.

7. Root Node Redundancy

The root node is a single point of failure for an MP LSP, whether the
MP LSP 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 will be 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.

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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.

7.1. 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.

7.2. 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 an MP2MP LSP MUST join 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

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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.

8. Make Before Break (MBB)

An LSR selects the LSR that is its next hop to the root of the LSP as
its upstream LSR for an MP LSP. When the best path to reach the root
changes, the LSR must choose a new upstream LSR. Sections 2.4.3 and
3.3.3 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
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 previous 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.

The MBB procedures are an optional extension to the MP LSP building
procedures described in this document. The procedures in this
section offer a make-before-break behavior, except in cases where the
new path is part of a transient routing loop involving more than 2
LSRs (also see Section 4).

8.1. MBB Overview

The MBB procedures 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 LSRs 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 LSR is capable of forwarding packets on the LSP. At that

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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
or it is the root node for FEC-A.

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

8.2. The MBB Status Code

As noted in Section 8.1, the procedures for establishing an MBB MP
LSP are different from those for establishing 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 an MBB
Status Code to indicate that 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 LSRs 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 5.1. 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

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Length: 1

Status Code: 1 = MBB request

2 = MBB ack

8.3. The MBB Capability

An LSR MAY advertise that it is capable of handling MBB LSPs using
the capability advertisement as defined in [RFC5561]. The LDP MP MBB
capability 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| LDP MP MBB Capability | Length = 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved |
+-+-+-+-+-+-+-+-+

LDP MP MBB Capability: The MBB Capability Parameter (0x050A)

S: As specified in [RFC5561]

If an LSR has not advertised that it is MBB capable, its LDP peers
MUST NOT send it messages that include MBB parameters. If an LSR
receives a Label Mapping message with an 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 immediately to
LSR-U (see Section 8.4). If this happens, an MBB MP LSP will not be
established, but a normal MP LSP will be the result.

8.4. The MBB Procedures

8.4.1. 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,

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the corresponding label is not stored in the label forwarding
database.

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 an
MBB Notification message to neighbor N with label L.

6. MBB Notification <X, Y, L>: An LDP notification message with an MP
LSP <X, Y>, label L, and MBB Status code 2.

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

8.4.2. Accepting Elements

An accepting element represents a specific label value L that has
been advertised to a neighbor N for an 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 replacement the active element in the label forwarding
database is pending. 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 another
inactive accepting element is pending. If that happens, the older
inactive accepting element MUST be replaced with a newer inactive
element. If an accepting element is removed, a Label Withdraw has to
be sent for label L to neighbor N for <X, Y>.

8.4.3. Procedures for Upstream LSR Change

Suppose a node Z has an 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 Mapping <X, Y, L2> to N2 and waits for
the new upstream LSR N2 to respond with an 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).

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RFC 6388 P2MP and MP2MP LSP Setup with LDP November 2011

While waiting for the MBB Notification from upstream LSR N2, it is
possible that another transition occurs due to a routing change.
Suppose the new upstream LSR is N3. An inactive element iA(N3, L3)
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>. 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 8.4.5 are
applied as if an MBB Notification was received for the inactive
element. If a downstream LSR detects that the old upstream LSR went
down while waiting for the MBB Notification from the new upstream
LSR, the downstream LSR can immediately proceed without waiting for
the timer to expire.

8.4.4. Receiving a Label Mapping with MBB Status Code

Suppose node Z has state for an MBB LSP <X, Y> and receives an MBB
Label Mapping <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 if node Z is the root of the
MBB LSP, an MBB notification <X, Y, L2)> is sent to node N2. If node
Z has an inactive accepting element, it marks the Forwarding state as
<X, Y, F'(N2, L2)>. If the router Z upstream LSR for <X, Y> happens
to be N2, then Z MUST NOT send an MBB notification to N2 at once.
Sending the MBB notification to N2 must be done only after Z upstream
for <X, Y> stops being N2.

8.4.5. Receiving a Notification with MBB Status Code

Suppose node Z receives an 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
another active accepting element 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
sent to these downstream LSRs. If node Z receives an 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.

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RFC 6388 P2MP and MP2MP LSP Setup with LDP November 2011

8.4.6. Node Operation for MP2MP LSPs

The procedures described above apply to the downstream path of an
MP2MP LSP. The upstream path of the MP2MP is set up as normal
without including an MBB Status code. If the MBB procedures apply to
an 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.

9. Typed Wildcard for mLDP FEC Element

The format of the mLDP FEC Typed Wildcard FEC 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Typed Wcard | Type | Len = 2 | AFI ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ |
+-+-+-+-+-+-+-+-+

Typed Wcard: As specified in [RFC5918]

Type: The type of FEC Element Type. Either the P2MP FEC Element or
the MP2MP FEC Element using the values defined for those FEC
Elements when carried in the FEC TLV as defined in this document.

Len: Len FEC Type Info, two octets (=0x02).

AFI: Address Family, two-octet quantity containing a value from
IANA's "Address Family Numbers" registry.

10. Security Considerations

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

The protocol specified in this document does not provide any
authorization mechanism for controlling the set of LSRs that may join
a given MP LSP. If such authorization is desirable, additional
mechanisms, outside the scope of this document, are needed. Note
that authorization policies cannot be implemented and/or configured
solely at the root node of the LSP, because the root node does not
learn the identities of all the leaf nodes.

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RFC 6388 P2MP and MP2MP LSP Setup with LDP November 2011

11. IANA Considerations

Per this document, IANA has created 3 new registries.

1. "LDP MP Opaque Value Element basic type"

The range is 0-255, with the following values allocated in this
document:

0: Reserved

1: Generic LSP identifier

255: Extended Type field is present in the following two bytes

The allocation policy for this space is 'Standards Action with
Early Allocation'.

2. "LDP MP Opaque Value Element extended type"

The range is 0-65535, with the following allocation policies:

0-32767: Standards Action with Early Allocation

32768-65535: First Come, First Served

3. "LDP MP Status Value Element type"

The range is 0-255, with the following values allocated in this
document:

0: Reserved

1: MBB Status

The allocation policy for this space is 'Standards Action with
Early Allocation'.

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RFC 6388 P2MP and MP2MP LSP Setup with LDP November 2011

The code point values listed below have been allocated by IANA
through early allocation.

IANA allocated three new code points from the LDP registry
"Forwarding Equivalence Class (FEC) Type Name Space". The values
are:

P2MP FEC type - requested value 0x06

MP2MP-up FEC type - requested value 0x07

MP2MP-down FEC type - requested value 0x08

IANA assigned three new code points for new Capability Parameter TLVs
from the LDP registry "TLV Type Name Space", corresponding to the
advertisement of the P2MP, MP2MP, and MBB capabilities. The values
are:

P2MP Capability Parameter - 0x0508

MP2MP Capability Parameter - 0x0509

MBB Capability Parameter - 0x050A

IANA assigned an LDP Status Code to indicate that an LDP MP Status
TLV is following in the Notification message. The value assigned in
the LDP registry "LDP Status Code Name Space" is:

LDP MP status - requested value 0x00000040

IANA assigned a new code point for an LDP MP Status TLV. The value
assigned in the LDP registry "LDP TLV Type Name Space" is:

LDP MP Status TLV Type - requested value 0x096F

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, Thomas Morin
and Ben Niven-Jenkins.

Wijnands, et al. Standards Track [Page 34]

RFC 6388 P2MP and MP2MP LSP Setup with LDP November 2011

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: [email protected]

Luyuan Fang
Cisco Systems
300 Beaver Brook Road
Boxborough, MA 01719
US
EMail: [email protected]

Hitoshi Fukuda
NTT Communications Corporation
1-1-6, Uchisaiwai-cho, Chiyoda-ku
Tokyo 100-8019,
Japan
EMail: [email protected]

Yuji Kamite
NTT Communications Corporation
Tokyo Opera City Tower
3-20-2 Nishi Shinjuku, Shinjuku-ku,
Tokyo 163-1421,
Japan
EMail: [email protected]

Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
EMail: [email protected]

Wijnands, et al. Standards Track [Page 35]

RFC 6388 P2MP and MP2MP LSP Setup with LDP November 2011

Jean-Louis Le Roux
France Telecom
2, avenue Pierre-Marzin
Lannion, Cedex 22307
France
EMail: [email protected]

Ina Minei
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
EMail: [email protected]

Bob Thomas
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough, MA, 01719
EMail: [email protected]

Lei Wang
Telenor
Snaroyveien 30
Fornebu 1331
Norway
EMail: [email protected]

IJsbrand Wijnands
Cisco Systems, Inc.
De kleetlaan 6a
1831 Diegem
Belgium
EMail: [email protected]

Wijnands, et al. Standards Track [Page 36]

RFC 6388 P2MP and MP2MP LSP Setup with LDP November 2011

14. References

14.1. Normative References

[ITU.V42.1994]
International Telecommunications Union, "Error-correcting
Procedures for DCEs Using Asynchronous-to-Synchronous
Conversion", ITU-T Recommendation V.42, 1994.
http://www.itu.int/rec/T-REC-V.42-200203-I

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

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

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

[RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space", RFC
5331, August 2008.

[RFC5561] Thomas, B., Raza, K., Aggarwal, S., Aggarwal, R., and JL.
Le Roux, "LDP Capabilities", RFC 5561, July 2009.

[RFC5918] Asati, R., Minei, I., and B. Thomas, "Label Distribution
Protocol (LDP) 'Typed Wildcard' Forward Equivalence Class
(FEC)", RFC 5918, August 2010.

[RFC6389] Aggarwal, R. and JL. Le Roux, "MPLS Upstream Label
Assignment for LDP", RFC 6389, September 2011.

14.2. Informative References

[ISO3309] International Organization for Standardization, "ISO
Information Processing Systems - Data Communication -
High-Level Data Link Control Procedure - Frame Structure",
ISO 3309, 3rd Edition, October 1984.

[L3VPN-MCAST]
Rosen, E., Ed., and R. Aggarwal, Ed., "Multicast in
MPLS/BGP IP VPNs", Work in Progress, January 2010.

[RFC3813] Srinivasan, C., Viswanathan, A., and T. Nadeau,
"Multiprotocol Label Switching (MPLS) Label Switching
Router (LSR) Management Information Base (MIB)", RFC 3813,
June 2004.

Wijnands, et al. Standards Track [Page 37]

RFC 6388 P2MP and MP2MP LSP Setup with LDP November 2011

[RFC3815] Cucchiara, J., Sjostrand, H., and J. Luciani, "Definitions
of Managed Objects for the Multiprotocol Label Switching
(MPLS), Label Distribution Protocol (LDP)", RFC 3815, June
2004.

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

[RFC5332] Eckert, T., Rosen, E., Ed., Aggarwal, R., and Y. Rekhter,
"MPLS Multicast Encapsulations", RFC 5332, August 2008.

[RFC6348] Le Roux, J., Ed., and T. Morin, Ed., "Requirements for
Point-to-Multipoint Extensions to the Label Distribution
Protocol", RFC 6348, September 2011.

Wijnands, et al. Standards Track [Page 38]

RFC 6388 P2MP and MP2MP LSP Setup with LDP November 2011

Authors' Addresses

IJsbrand Wijnands (editor)
Cisco Systems, Inc.
De kleetlaan 6a
Diegem 1831
Belgium
EMail: [email protected]

Ina Minei (editor)
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
EMail: [email protected]

Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
EMail: [email protected]

Bob Thomas
300 Beaver Brook Road
Boxborough 01719
US
EMail: [email protected]

Wijnands, et al. Standards Track [Page 39]