Network Working Group I. Minei
Internet-Draft K. Kompella
Expires: January 18, 2006 Juniper Networks
J-L. Le Roux
France Telecom
L. Fang
AT&T
L. Wang
Telenor
S. Amante
Level 3 Communications, LLC
July 17, 2005
Label Distribution Protocol Extensions for Point-to-Multipoint Label
Switched Paths
draft-minei-mpls-ldp-p2mp-01
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Abstract
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This document describes extensions to the Label Distribution Protocol
(LDP) for the setup of point to multi-point (P2MP) Label Switched
Paths (LSPs) in Multi-Protocol Label Switching (MPLS) networks. The
solution relies on LDP without requiring a multicast routing protocol
in the network. Protocol elements and procedures for this solution
are described. There can be various applications for P2MP LSPs such
as IP multicast. Specification of how such applications can use a
LDP signaled P2MP LSP is outside the scope of this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Conventions used in this document . . . . . . . . . . . . 3
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Operation . . . . . . . . . . . . . . . . . . . . . . 5
2.1 The P2MP FEC Element . . . . . . . . . . . . . . . . . . . 5
2.2 Using the P2MP FEC Element . . . . . . . . . . . . . . . . 6
2.2.1 Label Map . . . . . . . . . . . . . . . . . . . . . . 7
2.2.2 Label Withdraw . . . . . . . . . . . . . . . . . . . . 8
3. Shared Trees . . . . . . . . . . . . . . . . . . . . . . . . . 10
4. Security considerations . . . . . . . . . . . . . . . . . . . 11
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1 Normative References . . . . . . . . . . . . . . . . . . . 13
6.2 Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . 15
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1. Introduction
The LDP protocol is described in [1]. It defines mechanisms for
setting up point to point (P2P) and multi-point to point (MP2P) LSPs
in the network. This document describes extensions to LDP for
setting up point to multi-point (P2MP) LSPs. Specifically, this
document describes how a P2MP LSP can be set up that allows traffic
from a single root (or ingress) node to be delivered to a number of
leaf (or egress) nodes. Only a single copy of the packet will be
sent on any link traversed by the P2MP LSP (see note at end of
Section 2.2.1). This is accomplished without the use of a multicast
protocol in the network. There can be several P2MP LSPs rooted at a
given ingress node, each with its own identifier.
The solution assumes that the leaf nodes of the P2MP LSP know the
root node and identifier of the P2MP 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 P2MP LSP signaled by LDP is also outside the scope of this
document.
Interested readers may also wish to peruse the documents [4] and [6].
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 [2].
1.2 Terminology
The following terminology is taken from [4].
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: A LSP that has one or more Ingress LSRs and one unique
Egress LSR.
MP2MP LSP: A LSP that has one or more Ingress LSRs and one or more
Egress LSRs.
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Ingress LSR: Source of the P2MP LSP, also referred to as root node.
Egress LSR: One of potentially many destinations of an LSP, also
referred to as leaf node in the case of P2MP and MP2MP LSPs.
Transit LSR: An LSR that has 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.
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2. Protocol Operation
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.
For the setup of a P2MP LSP with LDP, we define one new protocol
entity, the P2MP FEC Element to be used in the FEC TLV. The
description of the P2MP FEC Element follows.
2.1 The P2MP FEC Element
The P2MP FEC Element consists of the address of the root of the P2MP
LSP and an opaque identifier. The opaque identifier is unique within
the context of the root node. The combination of (root LSR address,
opaque identifier) 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (TBD) | Address Family | Address Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Node Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Identifier Type | Opaque Identifier Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Identifier ... |
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 [3] that encodes the address family for the Root
LSR Address.
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Address Length: Length of the Root LSR Address in octets.
Root Node Address: A host address encoded according to the Address
Family field.
Opaque Identifier Type: The type of Opaque Identifier.
Length: The length of the P2MP Opaque Identifier, in octets.
Opaque Identifier: An opaque identifier of Length octets, padded at
the end with zeros so as to be 4-octet aligned.
If 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
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.
2.2 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 Identifier 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.
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.
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4. P2MP LSP <X, Y> (or simply <X, Y>): a P2MP LSP with Root Node
Address X and Opaque Identifier Y.
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.
2.2.1 Label Map
The following lists procedures for generating and processing P2MP
Label Map 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.
In the current approach, if there are several receivers for a P2MP
LSP on a LAN, packets are replicated over the LAN. This may not be
optimal; optimizing this case is for further study.
2.2.1.1 Determining one's 'upstream LSR'
A node Z that is part of P2MP LSP <X, Y> determines the LDP peer U
which lies on the best path from Z to the root node X. If there are
more than one such LDP peers, only one of them is picked. U is Z's
"Upstream LSR" for <X, Y>.
2.2.1.2 Leaf Operation
A leaf node Z of P2MP LSP <X, Y> determines its upstream LSR U for
<X, Y> as per Section 2.2.1.1, allocates a label L, and sends a P2MP
Label Map <X, Y, L> to U.
2.2.1.3 Transit Node operation
Suppose a transit node Z receives a P2MP Label Map <X, Y, L> over
interface I. Z checks whether it already has state for <X, Y>. If
not, Z allocates a label L', and installs state to swap L' with L
over interface I. Z also determines its upstream LSR U for <X, Y> as
per Section 2.2.1.1, and sends a P2MP Label Map <X, Y, L'> to U.
If Z already has state for <X, Y>, then Z adds "swap L, send over
interface I" to the nexthop. Z does not send a Label Map message for
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P2MP LSP <X, Y>.
2.2.1.4 Root Node Operation
Suppose the root node Z receives a P2MP Label Map <X, Y, L> over
interface I. 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.
2.2.2 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.
2.2.2.1 Leaf Operation
If a leaf node Z discovers (by means outside the scope of this
document) that it is no longer a leaf of the P2MP LSP, 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>.
2.2.2.2 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 <X, Y, L> to its upstream for <X, Y>.
2.2.2.3 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).
2.2.2.4 Upstream LSR change
If, for a given node Z participating in a P2MP LSP <X, Y>, the
upstream LSR changes, say from U to U', then Z MUST
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1. send a Label Withdraw <X, Y, L> to U, where L is the label Z had
previously sent to U for <X, Y>;
2. delete all forwarding state for the P2MP LSP <X, Y>;
3. allocate a label L' for <X, Y>, and send a Label Map <X, Y, L'>
to U';
4. install forwarding state for label L'.
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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
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) [5], 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.
An important consideration in 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.
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4. Security considerations
The same security considerations apply as for the base LDP
specification, as described in [1].
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5. Acknowledgments
The authors would like to thank Nischal Sheth, Yakov Rekhter and
Rahul Aggarwal for their suggestions and review.
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6. References
6.1 Normative References
[1] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and B.
Thomas, "LDP Specification", RFC 3036, January 2001.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1700,
October 1994.
[4] Roux, J., "Requirements for multipoint extensions to the Label
Distribution Protocol", draft-leroux-mpls-mp-ldp-reqs-00 (work
in progress), July 2005.
6.2 Informative References
[5] Andersson, L. and E. Rosen, "Framework for Layer 2 Virtual
Private Networks (L2VPNs)", draft-ietf-l2vpn-l2-framework-05
(work in progress), June 2004.
[6] Aggarwal, R., "Extensions to RSVP-TE for Point to Multipoint TE
LSPs", draft-ietf-mpls-rsvp-te-p2mp-01 (work in progress),
January 2005.
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
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Jean-Louis Le Roux
France Telecom
2, avenue Pierre-Marzin
Lannion Cedex 22307
France
Email: jeanlouis.leroux@francetelecom.com
Luyuan Fang
AT&T
200 Laurel Avenue, Room C2-3B35
Middletown, NJ 07748
US
Email: luyuanfang@att.com
Lei Wang
Telenor
Snaroyveien 30
Fornebu 1331
Norway
Email: lei.wang@telenor.com
Shane Amante
Level 3 Communications, LLC
1025 Eldorado Blvd
Broomfield, CO 80021
US
Email: Shane.Amante@Level3.com
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