|Title||Signaling Requirements for Point-to-Multipoint Traffic-Engineered
MPLS Label Switched Paths (LSPs)
|Author||S. Yasukawa, Ed.
Network Working Group S. Yasukawa, Ed.
Request for Comments: 4461 NTT
Category: Informational April 2006
Signaling Requirements for Point-to-Multipoint
Traffic-Engineered MPLS Label Switched Paths (LSPs)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The Internet Society (2006).
This document presents a set of requirements for the establishment
and maintenance of Point-to-Multipoint (P2MP) Traffic-Engineered (TE)
Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs).
There is no intent to specify solution-specific details or
application-specific requirements in this document.
The requirements presented in this document not only apply to
packet-switched networks under the control of MPLS protocols, but
also encompass the requirements of Layer Two Switching (L2SC), Time
Division Multiplexing (TDM), lambda, and port switching networks
managed by Generalized MPLS (GMPLS) protocols. Protocol solutions
developed to meet the requirements set out in this document must
attempt to be equally applicable to MPLS and GMPLS.
Table of Contents
1. Introduction ....................................................3
1.1. Non-Objectives .............................................6
2. Definitions .....................................................6
2.1. Acronyms ...................................................6
2.2. Terminology ................................................6
2.2.1. Terminology for Partial LSPs ........................8
2.3. Conventions ................................................9
3. Problem Statement ...............................................9
3.1. Motivation .................................................9
3.2. Requirements Overview ......................................9
4. Detailed Requirements for P2MP TE Extensions ...................11
4.1. P2MP LSP ..................................................11
4.2. P2MP Explicit Routing .....................................12
4.3. Explicit Path Loose Hops and Widely Scoped
Abstract Nodes ............................................13
4.4. P2MP TE LSP Establishment, Teardown, and
Modification Mechanisms ...................................14
4.5. Fragmentation .............................................14
4.6. Failure Reporting and Error Recovery ......................15
4.7. Record Route of P2MP TE LSP ...............................16
4.8. Call Admission Control (CAC) and QoS Control
Mechanism of P2MP TE LSPs .................................17
4.9. Variation of LSP Parameters ...............................17
4.10. Re-Optimization of P2MP TE LSPs ..........................18
4.11. Merging of Tree Branches .................................18
4.12. Data Duplication .........................................19
4.13. IPv4/IPv6 Support ........................................20
4.14. P2MP MPLS Label ..........................................20
4.15. Advertisement of P2MP Capability .........................20
4.16. Multi-Access LANs ........................................21
4.17. P2MP MPLS OAM ............................................21
4.18. Scalability ..............................................21
4.18.1. Absolute Limits ..................................22
4.19. Backwards Compatibility ..................................24
4.20. GMPLS ....................................................24
4.21. P2MP Crankback Routing ...................................25
5. Security Considerations ........................................25
6. Acknowledgements ...............................................26
7. References .....................................................26
7.1. Normative References ......................................26
7.2. Informative References ....................................26
Existing MPLS traffic engineering (MPLS-TE) allows for strict QoS
guarantees, resource optimization, and fast failure recovery, but it
is limited to point-to-point (P2P) LSPs. There is a desire to
support point-to-multipoint (P2MP) services using traffic-engineered
LSPs, and this clearly motivates enhancements of the base MPLS-TE
tool box in order to support P2MP MPLS-TE LSPs.
A P2MP TE LSP is a TE LSP (per [RFC2702] and [RFC3031]) that has a
single ingress LSR and one or more egress LSRs, and is
unidirectional. P2MP services (that deliver data from a single
source to one or more receivers) may be supported by any combination
of P2P and P2MP LSPs depending on the degree of optimization required
within the network, and such LSPs may be traffic-engineered again
depending on the requirements of the network. Further, multipoint-
to-multipoint (MP2MP) services (which deliver data from more than one
source to one or more receivers) may be supported by a combination of
P2P and P2MP LSPs.
[RFC2702] specifies requirements for traffic engineering over MPLS.
In Section 2, it describes traffic engineering in some detail, and
those definitions are equally applicable to traffic engineering in a
point-to-multipoint service environment. They are not repeated here,
but it is assumed that the reader is fully familiar with them.
Section 3.0 of [RFC2702] also explains how MPLS is particularly
suited to traffic engineering; it presents the following eight
1. Explicit label switched paths that are not constrained by the
destination-based forwarding paradigm can be easily created
through manual administrative action or through automated
action by the underlying protocols.
2. LSPs can potentially be maintained efficiently.
3. Traffic trunks can be instantiated and mapped onto LSPs.
4. A set of attributes can be associated with traffic trunks that
modulate their behavioral characteristics.
5. A set of attributes can be associated with resources that
constrain the placement of LSPs and traffic trunks across them.
6. MPLS allows for both traffic aggregation and disaggregation,
whereas classical destination-only-based IP forwarding permits
7. It is relatively easy to integrate a "constraint-based routing"
framework with MPLS.
8. A good implementation of MPLS can offer significantly lower
overhead than competing alternatives for traffic engineering.
These points are equally applicable to point-to-multipoint traffic
engineering. Points 1 and 7 are particularly important. Note that
point 3 implies that the concept of a point-to-multipoint traffic
trunk is defined and is supported by (or mapped onto) P2MP LSPs.
That is, the traffic flow for a point-to-multipoint LSP is not
constrained to the path or paths that it would follow during
multicast routing or shortest path destination-based routing, but it
can be explicitly controlled through manual or automated action.
Further, the explicit paths that are used may be computed using
algorithms based on a variety of constraints to produce all manner of
tree shapes. For example, an explicit path may be cost-based
[STEINER], shortest path, or QoS-based, or it may use some fair-cost
[RFC2702] also describes the functional capabilities required to
fully support traffic engineering over MPLS in large networks.
This document presents a set of requirements for Point-to-Multipoint
(P2MP) traffic engineering (TE) extensions to Multiprotocol Label
Switching (MPLS). It specifies functional requirements for solutions
to deliver P2MP TE LSPs.
Solutions that specify procedures for P2MP TE LSP setup MUST satisfy
these requirements. There is no intent to specify solution-specific
details or application-specific requirements in this document.
The requirements presented in this document apply equally to packet-
switched networks under the control of MPLS protocols and to packet-
switched, TDM, lambda, and port-switching networks managed by
Generalized MPLS (GMPLS) protocols. Protocol solutions developed to
meet the requirements set out in this document MUST attempt to be
equally applicable to MPLS and GMPLS.
Existing MPLS TE mechanisms such as [RFC3209] do not support P2MP TE
LSPs, so new mechanisms need to be developed. This SHOULD be
achieved with maximum re-use of existing MPLS protocols.
Note that there is a separation between routing and signaling in MPLS
TE. In particular, the path of the MPLS TE LSP is determined by
performing a constraint-based computation (such as CSPF) on a traffic
engineering database (TED). The contents of the TED may be collected
through a variety of mechanisms.
This document focuses on requirements for establishing and
maintaining P2MP MPLS TE LSPs through signaling protocols; routing
protocols are out of scope. No assumptions are made about how the
TED used as the basis for path computations for P2MP LSPs is formed.
This requirements document assumes the following conditions for P2MP
MPLS TE LSP establishment and maintenance:
o A P2MP TE LSP will be set up with TE constraints and will allow
efficient packet or data replication at various branching points in
the network. Although replication is a data plane issue, it is the
responsibility of the control plane (acting in conjunction with the
path computation component) to install LSPs in the network such
that replication can be performed efficiently. Note that the
notion of "efficient" replication is relative and may have
different meanings depending on the objectives (see Section 4.2).
o P2MP TE LSP setup mechanisms must include the ability to add/remove
receivers to/from the P2MP service supported by an existing P2MP TE
o Tunnel endpoints of P2MP TE LSP will be modified by adding/removing
egress LSRs to/from an existing P2MP TE LSP. It is assumed that
the rate of change of leaves of a P2MP LSP (that is, the rate at
which new egress LSRs join, or old egress LSRs are pruned) is "not
so high" because P2MP TE LSPs are assumed to be utilized for TE
applications. This issue is discussed at greater length in Section
o A P2MP TE LSP may be protected by fast error recovery mechanisms to
minimize disconnection of a P2MP service.
o A set of attributes of the P2MP TE LSP (e.g., bandwidth, etc.) may
be modified by some mechanism (e.g., make-before-break, etc.) to
accommodate attribute changes to the P2MP service without impacting
data traffic. These issues are discussed in Sections 4.6 and 4.10.
It is not a requirement that the ingress LSR must control the
addition or removal of leaves from the P2MP tree.
It is this document's objective that a solution compliant to the
requirements set out in this document MUST operate these P2MP TE
capabilities in a scalable fashion.
For clarity, this section lists some items that are out of scope of
It is assumed that some information elements describing the P2MP TE
LSP are known to the ingress LSR prior to LSP establishment. For
example, the ingress LSRs know the IP addresses that identify the
egress LSRs of the P2MP TE LSP. The mechanisms by which the ingress
LSR obtains this information is outside the scope of P2MP TE
signaling and so is not included in this document. Other documents
may complete the description of this function by providing automated,
protocol-based ways of passing this information to the ingress LSR.
This document does not specify any requirements for the following
- Non-TE LSPs (such as per-hop, routing-based LSPs).
- Discovery of egress leaves for a P2MP LSP.
- Hierarchical P2MP LSPs.
- OAM for P2MP LSPs.
- Inter-area and inter-AS P2MP TE LSPs.
- Applicability of P2MP MPLS TE LSPs to service scenarios.
- Specific application or application requirements.
- Algorithms for computing P2MP distribution trees.
- Multipoint-to-point LSPs.
- Multipoint-to-multipoint LSPs.
- Routing protocols.
- Construction of the traffic engineering database.
- Distribution of the information used to construct the traffic
The reader is assumed to be familiar with the terminology in
[RFC3031] and [RFC3209].
The following terms are defined for use in the context of P2MP TE
The ordered set of LSRs and TE links that comprise the path of a
P2MP TE LSP from its ingress LSR to all of its egress LSRs.
The LSR that is responsible for initiating the signaling messages
that set up the P2MP TE LSP.
One of potentially many destinations of the P2MP TE LSP. Egress
LSRs may also be referred to as leaf nodes or leaves.
An LSR that is an egress LSR, but also has one or more directly
connected downstream LSRs.
An LSR that has more than one directly connected downstream LSR.
A unique identifier of a P2MP TE LSP, which is constant for the
whole LSP regardless of the number of branches and/or leaves.
The sender of traffic that is carried on a P2MP service supported
by a P2MP LSP. The sender is not necessarily the ingress LSR of
the P2MP LSP.
A recipient of traffic carried on a P2MP service supported by a
P2MP LSP. A receiver is not necessarily an egress LSR of the P2MP
LSP. Zero, one, or more receivers may receive data through a
given egress LSR.
2.2.1. Terminology for Partial LSPs
It is convenient to sub-divide P2MP trees for functional and
representational reasons. A tree may be divided in two dimensions:
- A division may be made along the length of the tree. For example,
the tree may be split into two components each running from the
ingress LSR to a discrete set of egress LSRs. Upstream LSRs (for
example, the ingress LSR) may be members of both components.
- A tree may be divided at a branch LSR (or any transit LSR) to
produce a component of the tree that runs from the branch (or
transit) LSR to all egress LSRs downstream of this point.
These two methods of splitting the P2MP tree can be combined, so it
is useful to introduce some terminology to allow the partitioned
trees to be clearly described.
Use the following designations:
Source (ingress) LSR - S
Leaf (egress) LSR - L
Branch LSR - B
Transit LSR - X (any single, arbitrary LSR that is not a source,
leaf or branch)
All - A
Partial (i.e., not all) - P
Define a new term:
A segment of a P2MP TE LSP that runs from one of the LSP's LSRs
to one or more of its other LSRs.
Using these new concepts, we can define any combination or split of
the P2MP tree. For example:
The path from the source to one specific leaf.
The path from the source to a set of leaves.
The path from a branch LSR to all downstream leaves.
A component of the P2MP LSP that is a simple path that does not
Note that the S2AL sub-LSP is equivalent to the P2MP LSP.
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 [RFC2119].
3. Problem Statement
As described in Section 1, traffic engineering and constraint-based
routing (including Call Admission Control (CAC), explicit source
routing, and bandwidth reservation) are required to enable efficient
resource usage and strict QoS guarantees. Such mechanisms also make
it possible to provide services across a congested network where
conventional "shortest path first" forwarding paradigms would fail.
Existing MPLS TE mechanisms [RFC3209] and GMPLS TE mechanisms
[RFC3473] only provide support for P2P TE LSPs. While it is possible
to provide P2MP TE services using P2P TE LSPs, any such approach is
potentially suboptimal since it may result in data replication at the
ingress LSR, or in duplicate data traffic within the network.
Hence, to provide P2MP MPLS TE services in a fully efficient manner,
it is necessary to specify specific requirements. These requirements
can then be used when defining mechanisms for the use of existing
protocols and/or extensions to existing protocols and/or new
3.2. Requirements Overview
This document states basic requirements for the setup of P2MP TE
LSPs. The requirements apply to the signaling techniques only, and
no assumptions are made about which routing protocols are run within
the network, or about how the information that is used to construct
the Traffic Engineering Database (TED) is distributed. These factors
are out of the scope of this document.
A P2MP TE LSP path computation will take into account various
constraints such as bandwidth, affinities, required level of
protection and so on. The solution MUST allow for the computation of
P2MP TE LSP paths that satisfy constraints, with the objective of
supporting various optimization criteria such as delays, bandwidth
consumption in the network, or any other combinations. This is
likely to require the presence of a TED, as well as the ability to
signal the explicit path of an LSP.
A desired requirement is also to maximize the re-use of existing MPLS
TE techniques and protocols where doing so does not adversely impact
the function, simplicity, or scalability of the solution.
This document does not restrict the choice of signaling protocol used
to set up a P2MP TE LSP, but note that [RFC3468] states
...the consensus reached by the Multiprotocol
Label Switching (MPLS) Working Group within the IETF to focus its
efforts on "Resource Reservation Protocol (RSVP)-TE: Extensions to
RSVP for Label-Switched Paths (LSP) Tunnels" (RFC 3209) as the MPLS
signalling protocol for traffic engineering applications...
The P2MP TE LSP setup mechanism MUST include the ability to
add/remove egress LSRs to/from an existing P2MP TE LSP and MUST allow
for the support of all the TE LSP management procedures already
defined for P2P TE LSP. Further, when new TE LSP procedures are
developed for P2P TE LSPs, equivalent or identical procedures SHOULD
be developed for P2MP TE LSPs.
The computation of P2MP trees is implementation dependent and is
beyond the scope of the solutions that are built with this document
as a guideline.
Consider the following figure.
Source 1 (S1)
R2----E-LSR3--LSR1 LSR2---E-LSR2--Receiver 1 (R1)
Figure 1 shows a single ingress LSR (I-LSR1), and four egress LSRs
(E-LSR2, E-LSR3, E-LSR4, and E-LSR5). I-LSR1 is attached to a
traffic source that is generating traffic for a P2MP application.
Receivers R1, R2, R3, and R4 are attached to E-LSR2, E-LSR3, and
The following are the objectives of P2MP LSP establishment and use.
a) A P2MP tree that satisfies various constraints is pre-
determined, and details are supplied to I-LSR1.
Note that no assumption is made about whether the tree is
provided to I-LSR1 or computed by I-LSR1. The solution SHOULD
also allow for the support of a partial path by means of loose
Typical constraints are bandwidth requirements, resource class
affinities, fast rerouting, and preemption. There should not
be any restriction on the possibility of supporting the set of
constraints already defined for point-to-point TE LSPs. A new
constraint may specify which LSRs should be used as branch LSRs
for the P2MP LSR in order to take into account LSR capabilities
or network constraints.
b) A P2MP TE LSP is set up from I-LSR1 to E-LSR2, E-LSR3, and
E-LSR4 using the tree information.
c) In this case, the branch LSR1 should replicate incoming packets
or data and send them to E-LSR3 and E-LSR4.
d) If a new receiver (R5) expresses an interest in receiving
traffic, a new tree is determined, and a B2L sub-LSP from LSR2
to E-LSR5 is grafted onto the P2MP TE LSP. LSR2 becomes a
4. Detailed Requirements for P2MP TE Extensions
4.1. P2MP LSP
The P2MP TE extensions MUST be applicable to the signaling of LSPs
for different switching types. For example, it MUST be possible to
signal a P2MP TE LSP in any switching medium, whether it is packet or
non-packet based (including frame, cell, TDM, lambda, etc.).
As with P2P MPLS technology [RFC3031], traffic is classified with a
FEC in this extension. All packets that belong to a particular FEC
and that travel from a particular node MUST follow the same P2MP
In order to scale to a large number of branches, P2MP TE LSPs SHOULD
be identified by a unique identifier (the P2MP ID or P2ID) that is
constant for the whole LSP regardless of the number of branches
4.2. P2MP Explicit Routing
Various optimizations in P2MP tree formation need to be applied to
meet various QoS requirements and operational constraints.
Some P2MP applications may request a bandwidth-guaranteed P2MP tree
that satisfies end-to-end delay requirements. And some operators may
want to set up a cost-minimum P2MP tree by specifying branch LSRs
The P2MP TE solution therefore MUST provide a means of establishing
arbitrary P2MP trees under the control of an external tree
computation process, path configuration process, or dynamic tree
computation process located on the ingress LSR. Figure 2 shows two
| / \
B B C
| / \ / \
C D E F G
| / \ / \/ \ / \
D--E*-F*-G*-H*-I*-J*-K*--L H I J KL M N O
Steiner P2MP tree SPF P2MP tree
Figure 2: Examples of P2MP TE LSP topology
One example is the Steiner P2MP tree (cost-minimum P2MP tree)
[STEINER]. This P2MP tree is suitable for constructing a cost-
minimum P2MP tree so as to minimize the bandwidth consumption in the
core. To realize this P2MP tree, several intermediate LSRs must be
both MPLS data terminating LSRs and transit LSRs (LSRs E, F, G, H, I,
J, and K in Figure 2). Therefore, the P2MP TE solution MUST support
a mechanism that can set up this kind of bud LSR between an ingress
LSR and egress LSRs. Note that this includes constrained Steiner
trees that allow for the computation of a minimal cost trees with
some other constraints such as a bounded delay between the source and
Another example is a CSPF (Constraint Shortest Path First) P2MP tree.
By some metric (which can be set upon any specific criteria like the
delay, bandwidth, or a combination of those), one can calculate a
shortest-path P2MP tree. This P2MP tree is suitable for carrying
The solution MUST allow the operator to make use of any tree
computation technique. In the former case, an efficient/optimal tree
is defined as a minimal cost tree (Steiner tree), whereas in the
later case, it is defined as the tree that provides shortest path
between the source and any receiver.
To support explicit setup of any reasonable P2MP tree shape, a P2MP
TE solution MUST support some form of explicit source-based control
of the P2MP tree that can explicitly include particular LSRs as
branch LSRs. This can be used by the ingress LSR to set up the P2MP
TE LSP. For instance, a P2MP TE LSP can be represented simply as a
whole tree or by its individual branches.
4.3. Explicit Path Loose Hops and Widely Scoped Abstract Nodes
A P2MP tree is completely specified if all the required branches and
hops between a sender and leaf LSR are indicated.
A P2MP tree is partially specified if only a subset of intermediate
branches and hops is indicated. This may be achieved using loose
hops in the explicit path, or using widely scoped abstract nodes
(that is, abstract nodes that are not simple [RFC3209]) such as IPv4
prefixes shorter than 32 bits, or AS numbers. A partially specified
P2MP tree might be particularly useful in inter-area and inter-AS
situations, although P2MP requirements for inter-area and inter-AS
are beyond the scope of this document.
Protocol solutions SHOULD include a way to specify loose hops and
widely scoped abstract nodes in the explicit source-based control of
the P2MP tree as defined in the previous section. Where this support
is provided, protocol solutions MUST allow downstream LSRs to apply
further explicit control to the P2MP tree to resolve a partially
specified tree into a (more) completely specified tree.
Protocol solutions MUST allow the P2MP tree to be completely
specified at the ingress LSR where sufficient information exists to
allow the full tree to be computed and where policies along the path
(such as at domain boundaries) support full specification.
In all cases, the egress LSRs of the P2MP TE LSP must be fully
specified either individually or through some collective identifier.
Without this information, it is impossible to know where the TE LSP
should be routed to.
In case of a tree being computed by some downstream LSRs (e.g., the
case of hops specified as loose hops), the solution MUST provide
protocol mechanisms for the ingress LSR of the P2MP TE LSP to learn
the full P2MP tree. Note that this information may not always be
obtainable owing to policy considerations, but where part of the path
remains confidential, it MUST be reported through aggregation (for
example, using an AS number).
4.4. P2MP TE LSP Establishment, Teardown, and Modification Mechanisms
The P2MP TE solution MUST support establishment, maintenance, and
teardown of P2MP TE LSPs in a manner that is at least scalable in a
linear way. This MUST include both the existence of very many LSPs
at once, and the existence of very many destinations for a single
In addition to P2MP TE LSP establishment and teardown mechanisms, the
solution SHOULD support a partial P2MP tree modification mechanism.
For the purpose of adding sub-P2MP TE LSPs to an existing P2MP TE
LSP, the extensions SHOULD support a grafting mechanism. For the
purpose of deleting a sub-P2MP TE LSPs from an existing P2MP TE LSP,
the extensions SHOULD support a pruning mechanism.
It is RECOMMENDED that these grafting and pruning operations cause no
additional processing in nodes that are not along the path to the
grafting or pruning node, or that are downstream of the grafting or
pruning node toward the grafted or pruned leaves. Moreover, both
grafting and pruning operations MUST NOT disrupt traffic currently
forwarded along the P2MP tree.
There is no assumption that the explicitly routed P2MP LSP remains on
an optimal path after several grafts and prunes have occurred. In
this context, scalable refers to the signaling process for the P2MP
TE LSP. The TE nature of the LSP allows that re-optimization may
take place from time to time to restore the optimality of the LSP.
The P2MP TE solution MUST handle the situation where a single
protocol message cannot contain all the information necessary to
signal the establishment of the P2MP LSP. It MUST be possible to
establish the LSP in these circumstances.
This situation may arise in either of the following circumstances.
a. The ingress LSR cannot signal the whole tree in a single
b. The information in a message expands to be too large (or is
discovered to be too large) at some transit node. This may
occur because of some increase in the information that needs to
be signaled or because of a reduction in the size of signaling
message that is supported.
The solution to these problems SHOULD NOT rely on IP fragmentation of
protocol messages, and it is RECOMMENDED to rely on some protocol
procedures specific to the signaling solution.
In the event that fragmented IP packets containing protocol messages
are received, it is NOT RECOMMENDED that they are reassembled at the
4.6. Failure Reporting and Error Recovery
Failure events may cause egress LSRs or sub-P2MP LSPs to become
detached from the P2MP TE LSP. These events MUST be reported
upstream as for a P2P LSP.
The solution SHOULD provide recovery techniques, such as protection
and restoration, allowing recovery of any impacted sub-P2MP TE LSPs.
In particular, a solution MUST provide fast protection mechanisms
applicable to P2MP TE LSP similar to the solutions specified in
[RFC4090] for P2P TE LSPs. Note also that no assumption is made
about whether backup paths for P2MP TE LSPs should or should not be
shared with P2P TE LSPs backup paths.
Note that the functions specified in [RFC4090] are currently specific
to packet environments and do not apply to non-packet environments.
Thus, while solutions MUST provide fast protection mechanisms similar
to those specified in [RFC4090], this requirement is limited to the
subset of the solution space that applies to packet-switched networks
Note that the requirements expressed in this document are general to
all MPLS TE P2MP signaling, and any solution that meets them will
therefore be general. Specific applications may have additional
requirements or may want to relax some requirements stated in this
document. This may lead to variations in the solution.
The solution SHOULD also support the ability to meet other network
recovery requirements such as bandwidth protection and bounded
propagation delay increase along the backup path during failure.
A P2MP TE solution MUST support the P2MP fast protection mechanism to
handle P2MP applications sensitive to traffic disruption.
If the ingress LSR is informed of the failure of delivery to fewer
than all the egress LSRs, this SHOULD NOT cause automatic teardown of
the P2MP TE LSP. That is, while some egress LSRs remain connected to
the P2MP tree, it SHOULD be a matter of local policy at the ingress
LSR whether the P2MP LSP is retained.
When all egress LSRs downstream of a branch LSR have become
disconnected from the P2MP tree, and some branch LSR is unable to
restore connectivity to any of them by means of some recovery or
protection mechanisms, the branch LSR MAY remove itself from the P2MP
tree provided that it is not also an egress LSR (that is, a bud).
Since the faults that severed the various downstream egress LSRs from
the P2MP tree may be disparate, the branch LSR MUST report all such
errors to its upstream neighbor. An upstream LSR or the ingress LSR
can then decide to re-compute the path to those particular egress
LSRs around the failure point.
Solutions MAY include the facility for transit LSRs and particularly
branch LSRs to recompute sub-P2MP trees to restore them after
failures. In the event of successful repair, error notifications
SHOULD NOT be reported to upstream nodes, but the new paths are
reported if route recording is in use. Crankback requirements are
discussed in Section 4.21.
4.7. Record Route of P2MP TE LSP
Being able to identify the established topology of P2MP TE LSP is
very important for various purposes such as management and operation
of some local recovery mechanisms like Fast Reroute [RFC4090]. A
network operator uses this information to manage P2MP TE LSPs.
Therefore, the P2MP TE solution MUST support a mechanism that can
collect and update P2MP tree topology information after the P2MP LSP
establishment and modification process.
It is RECOMMENDED that the information is collected in a data format
that allows easy recognition of the P2MP tree topology.
The solution MUST support mechanisms for the recording of both
outgoing interfaces and node-ids.
The solution MUST gracefully handle scaling issues concerned with the
collection of P2MP tree information, including the case where the
collected information is too large to be carried in a single protocol
4.8. Call Admission Control (CAC) and QoS Control Mechanism of
P2MP TE LSPs
P2MP TE LSPs may share network resource with P2P TE LSPs. Therefore,
it is important to use CAC and QoS in the same way as P2P TE LSPs for
easy and scalable operation.
P2MP TE solutions MUST support both resource sharing and exclusive
resource utilization to facilitate coexistence with other LSPs to the
P2MP TE solutions MUST be applicable to DiffServ-enabled networks
that can provide consistent QoS control in P2MP LSP traffic.
Any solution SHOULD also satisfy the DS-TE requirements [RFC3564] and
interoperate smoothly with current P2P DS-TE protocol specifications.
Note that this requirement document does not make any assumption on
the type of bandwidth pool used for P2MP TE LSPs, which can either be
shared with P2P TE LSP or be dedicated for P2MP use.
4.9. Variation of LSP Parameters
Certain parameters (such as priority and bandwidth) are associated
with an LSP. The parameters are installed by the signaling exchanges
associated with establishing and maintaining the LSP.
Any solution MUST NOT allow for variance of these parameters within a
single P2MP LSP. That is:
- No attributes set and signaled by the ingress LSR of a P2MP LSP may
be varied by downstream LSRs.
- There MUST be homogeneous QoS from the root to all leaves of a
single P2MP LSP.
Changing the parameters for the whole tree MAY be supported, but the
change MUST apply to the whole tree from ingress LSR to all egress
4.10. Re-Optimization of P2MP TE LSPs
The detection of a more optimal path (for example, one with a lower
overall cost) is an example of a situation where P2MP TE LSP re-
routing may be required. While re-routing is in progress, an
important requirement is to avoid double bandwidth reservation (over
the common parts between the old and new LSP) thorough the use of
Make-before-break MUST be supported for a P2MP TE LSP to ensure that
there is minimal traffic disruption when the P2MP TE LSP is re-
Make-before-break that only applies to a sub-P2MP tree without
impacting the data on all the other parts of the P2MP tree MUST be
The solution SHOULD allow for make-before-break re-optimization of
any subdivision of the P2MP LSP (S2PL sub-LSP, S2X sub-LSP, S2L sub-
LSP, X2AL sub-LSP, B2PL sub-LSP, X2AL sub-LSP, or B2AL tree).
Further, it SHOULD do so by minimizing the signaling impact on the
rest of the P2MP LSP, and without affecting the ability of the
management plane to manage the LSP.
The solution SHOULD also provide the ability for the ingress LSR to
have strict control over the re-optimization process. The ingress
LSR SHOULD be able to limit all re-optimization to be source-
Where sub-LSP re-optimization is allowed by the ingress LSR, such
re-optimization MAY be initiated by a downstream LSR that is the root
of the sub-LSP that is to be re-optimized. Sub-LSP re-optimization
initiated by a downstream LSR MUST be carried out with the same
regard to minimizing the impact on active traffic as was described
above for other re-optimization.
4.11. Merging of Tree Branches
It is possible for a single transit LSR to receive multiple signaling
messages for the same P2MP LSP but for different sets of
destinations. These messages may be received from the same or
different upstream nodes and may need to be passed on to the same or
different downstream nodes.
This situation may arise as the result of the signaling solution
definition or implementation options within the signaling solution.
Further, it may happen during make-before-break re-optimization
It is even possible that it is necessary to construct distinct
upstream branches in order to achieve the correct label choices in
certain switching technologies managed by GMPLS (for example,
photonic cross-connects where the selection of a particular lambda
for the downstream branches is only available on different upstream
The solution MUST support the case where multiple signaling messages
for the same P2MP LSP are received at a single transit LSR and refer
to the same upstream interface. In this case, the result of the
protocol procedures SHOULD be a single data flow on the upstream
The solution SHOULD support the case where multiple signaling
messages for the same P2MP LSP are received at a single transit LSR
and refer to different upstream interfaces, and where each signaling
message results in the use of different downstream interfaces. This
case represents data flows that cross at the LSR but that do not
The solution MAY support the case where multiple signaling messages
for the same P2MP LSP are received at a single transit LSR and refer
to different upstream interfaces, and where the downstream interfaces
are shared across the received signaling messages. This case
represents the merging of data flows. A solution that supports this
case MUST ensure that data is not replicated on the downstream
An alternative to supporting this last case is for the signaling
protocol to indicate an error such that the merge may be resolved by
the upstream LSRs.
4.12. Data Duplication
Data duplication refers to the receipt by any recipient of duplicate
instances of the data. In a packet environment, this means the
receipt of duplicate packets. Although small-scale packet
duplication (that is, a few packets over a relatively short period of
time) should be a harmless (if inefficient) situation, certain
existing and deployed applications will not tolerate packet
duplication. Sustained packet duplication is, at best, a waste of
network and processing resources and, at worst, may cause congestion
and the inability to process the data correctly.
In a non-packet environment, data duplication means the duplication
of some part of the signal that may lead to the replication of data
or to the scrambling of data.
Data duplication may legitimately arise in various scenarios
including re-optimization of active LSPs as described in the previous
section, and protection of LSPs. Thus, it is impractical to regulate
against data duplication in this document.
Instead, the solution:
- SHOULD limit to bounded transitory conditions the cases where
network bandwidth is wasted by the existence of duplicate delivery
- MUST limit the cases where duplicate data is delivered to an
application to bounded transitory conditions.
4.13. IPv4/IPv6 Support
Any P2MP TE solution MUST support IPv4 and IPv6 addressing.
4.14. P2MP MPLS Label
A P2MP TE solution MUST allow the continued use of existing
techniques to establish P2P LSPs (TE and otherwise) within the same
network, and MUST allow the coexistence of P2P LSPs within the same
network as P2MP TE LSPs.
A P2MP TE solution MUST be specified in such a way that it allows
P2MP and P2P TE LSPs to be signaled on the same interface.
4.15. Advertisement of P2MP Capability
Several high-level requirements have been identified to determine the
capabilities of LSRs within a P2MP network. The aim of such
information is to facilitate the computation of P2MP trees using TE
constraints within a network that contains LSRs that do not all have
the same capability levels with respect to P2MP signaling and data
These capabilities include, but are not limited to:
- The ability of an LSR to support branching.
- The ability of an LSR to act as an egress LSR and a branch LSR for
the same LSP.
- The ability of an LSR to support P2MP MPLS-TE signaling.
4.16. Multi-Access LANs
P2MP MPLS TE may be used to traverse network segments that are
provided by multi-access media such as Ethernet. In these cases, it
is also possible that the entry point to the network segment is a
branch LSR of the P2MP LSP.
Two options clearly exist:
- the branch LSR replicates the data and transmits multiple copies
onto the segment.
- the branch LSR sends a single copy of the data to the segment and
relies on the exit points to determine whether to receive and
forward the data.
The first option has a significant data plane scaling issue since all
replicated data must be sent through the same port and carried on the
same segment. Thus, a solution SHOULD provide a mechanism for a
branch LSR to send a single copy of the data onto a multi-access
network to reach multiple (adjacent) downstream nodes. The second
option may have control plane scaling issues.
4.17. P2MP MPLS OAM
The MPLS and GMPLS MIB modules MUST be enhanced to provide P2MP TE
LSP management in line with whatever signaling solutions are
In order to facilitate correct management, P2MP TE LSPs MUST have
unique identifiers, since otherwise it is impossible to determine
which LSP is being managed.
Further discussions of OAM are out of scope for this document. See
[P2MP-OAM] for more details.
Scalability is a key requirement in P2MP MPLS systems. Solutions
MUST be designed to scale well with an increase in the number of any
of the following:
- the number of recipients
- the number of egress LSRs
- the number of branch LSRs
- the number of branches
Both scalability of control plane operation (setup, maintenance,
modification, and teardown) MUST be considered.
Key considerations MUST include:
- the amount of refresh processing associated with maintaining a P2MP
- the amount of protocol state that must be maintained by ingress and
transit LSRs along a P2MP tree.
- the number of protocol messages required to set up or tear down a
P2MP LSP as a function of the number of egress LSRs.
- the number of protocol messages required to repair a P2MP LSP after
failure or to perform make-before-break.
- the amount of protocol information transmitted to manage a P2MP TE
LSP (i.e., the message size).
- the amount of additional data distributed in potential routing
- the amount of additional control plane processing required in the
network to detect whether an add/delete of a new branch is
required, and in particular, the amount of processing in steady
state when no add/delete is requested
- the amount of control plane processing required by the ingress,
transit, and egress LSRs to add/delete a branch LSP to/from an
existing P2MP LSP.
It is expected that the applicability of each solution will be
evaluated with regards to the aforementioned scalability criteria.
4.18.1. Absolute Limits
In order to achieve the best solution for the problem space, it is
helpful to clarify the boundaries for P2MP TE LSPs.
- Number of egress LSRs.
A scaling bound is placed on the solution mechanism such that a
P2MP TE LSP MUST reduce to similar scaling properties as a P2P LSP
when the number of egress LSRs reduces to one. That is,
establishing a P2MP TE LSP to a single egress LSR should cost
approximately as much as establishing a P2P LSP.
It is important to classify the issues of scaling within the
context of traffic engineering. It is anticipated that the initial
deployments of P2MP TE LSPs will be limited to a maximum of around
a hundred egress LSRs, but that within five years deployments may
increase this to several hundred, and that future deployments may
require significantly larger numbers.
An acceptable upper bound for a solution, therefore, is one that
scales linearly with the number of egress LSRs. It is expected
that solutions will scale better than linearly.
Solutions that scale worse than linearly (that is, exponentially or
polynomially) are not acceptable whatever the number of egress LSRs
they could support.
- Number of branch LSRs.
Solutions MUST support all possibilities from one extreme of a
single branch LSR that forks to all leaves on a separate branch, to
the greatest number of branch LSRs which is (n-1) for n egress
LSRs. Assumptions MUST NOT be made in the solution regarding which
topology is more common, and the solution MUST be designed to
ensure scalability in all topologies.
- Dynamics of P2MP tree.
Recall that the mechanisms for determining which egress LSRs should
be added to an LSP and for adding and removing egress LSRs from
that group are out of the scope of this document. Nevertheless, it
is useful to understand the expected rates of arrival and departure
of egress LSRs, since this can impact the selection of solution
Again, this document is limited to traffic engineering, and in this
model the rate of change of LSP egress LSRs may be expected to be
lower than the rate of change of recipients in an IP multicast
Although the absolute number of egress LSRs coming and going is the
important element for determining the scalability of a solution,
note that a percentage may be a more comprehensible measure, but
that this is not as significant for LSPs with a small number of
A working figure for an established P2MP TE LSP is less than 10%
churn per day; that is, a relatively slow rate of churn.
We could say that a P2MP LSP would be shared by multiple multicast
groups, so the dynamics of the P2MP LSP would be relatively small.
Solutions MUST optimize for such relatively low rates of change and
are not required to optimize for significantly higher rates of
- Rate of change within the network.
It is also important to understand the scaling with regard to
changes within the network. That is, one of the features of a P2MP
TE LSP is that it can be robust or protected against network
failures, and it can be re-optimized to take advantage of newly
available network resources.
It is more important that a solution be optimized for scaling with
respect to recovery and re-optimization of the LSP than for change
in the egress LSRs, because P2MP is used as a TE tool.
The solution MUST follow this distinction and optimize accordingly.
4.19. Backwards Compatibility
It SHOULD be an aim of any P2MP solution to offer as much backward
compatibility as possible. An ideal that is probably impossible to
achieve would be to offer P2MP services across legacy MPLS networks
without any change to any LSR in the network.
If this ideal cannot be achieved, the aim SHOULD be to use legacy
nodes as both transit non-branch LSRs and egress LSRs.
It is a further requirement for the solution that any LSR that
implements the solution SHALL NOT be prohibited by that act from
supporting P2P TE LSPs using existing signaling mechanisms. That is,
unless doing so is administratively prohibited, P2P TE LSPs MUST be
supported through a P2MP network.
Also, it is a requirement that P2MP TE LSPs MUST be able to coexist
with IP unicast and IP multicast networks.
The requirement for P2MP services for non-packet switch interfaces is
similar to that for Packet-Switch Capable (PSC) interfaces.
Therefore, it is a requirement that reasonable attempts must be made
to make all the features/mechanisms (and protocol extensions) that
will be defined to provide MPLS P2MP TE LSPs equally applicable to
P2MP PSC and non-PSC TE-LSPs. If the requirements of non-PSC
networks over-complicate the PSC solution a decision may be taken to
separate the solutions.
Solutions for MPLS P2MP TE-LSPs, when applied to GMPLS P2MP PSC or
non-PSC TE-LSPs, MUST be compatible with the other features of GMPLS
- control and data plane separation;
- full support of numbered and unnumbered TE links;
- use of the arbitrary labels and labels for specific technologies,
as well as negotiation of labels, where necessary, to support
limited label processing and swapping capabilities;
- the ability to apply external control to the labels selected on
each hop of the LSP, and to control the next hop
label/port/interface for data after it reaches the egress LSR;
- support for graceful and alarm-free enablement and termination of
- full support for protection including link-level protection,
end-to-end protection, and segment protection;
- the ability to teardown an LSP from a downstream LSR, in
particular, from the egress LSR;
- handling of Graceful Deletion procedures; and
- support for failure and restart or reconnection of the control
plane without any disruption of the data plane.
In addition, since non-PSC TE-LSPs may have to be processed in
environments where the "P2MP capability" could be limited, specific
constraints may also apply during the P2MP TE Path computation.
Being technology specific, these constraints are outside the scope of
this document. However, technology-independent constraints (i.e.,
constraints that are applicable independently of the LSP class)
SHOULD be allowed during P2MP TE LSP message processing. It has to
be emphasized that path computation and management techniques shall
be as close as possible to those being used for PSC P2P TE LSPs and
P2MP TE LSPs.
4.21. P2MP Crankback Routing
P2MP solutions SHOULD support crankback requirements as defined in
[CRANKBACK]. In particular, they SHOULD provide sufficient
information to a branch LSR from downstream LSRs to allow the branch
LSR to re-route a sub-LSP around any failures or problems in the
5. Security Considerations
This requirements document does not define any protocol extensions
and does not, therefore, make any changes to any security models.
It is a requirement that any P2MP solution developed to meet some or
all of the requirements expressed in this document MUST include
mechanisms to enable the secure establishment and management of P2MP
MPLS-TE LSPs. This includes, but is not limited to:
- mechanisms to ensure that the ingress LSR of a P2MP LSP is
- mechanisms to ensure that communicating signaling entities can
verify each other's identities;
- mechanisms to ensure that control plane messages are protected
against spoofing and tampering;
- mechanisms to ensure that unauthorized leaves or branches are not
added to the P2MP LSP; and
- mechanisms to protect signaling messages from snooping.
Note that P2MP signaling mechanisms built on P2P RSVP-TE signaling
are likely to inherit all the security techniques and problems
associated with RSVP-TE. These problems may be exacerbated in P2MP
situations where security relationships may need to maintained
between an ingress LSR and multiple egress LSRs. Such issues are
similar to security issues for IP multicast.
It is a requirement that documents offering solutions for P2MP LSPs
MUST have detailed security sections.
The authors would like to thank George Swallow, Ichiro Inoue, Dean
Cheng, Lou Berger, and Eric Rosen for their review and suggestions.
Thanks to Loa Andersson for his help resolving the final issues in
this document and to Harald Alvestrand for a thorough GenArt review.
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and
J. McManus, "Requirements for Traffic Engineering Over
MPLS", RFC 2702, September 1999.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon,
"Multiprotocol Label Switching Architecture", RFC 3031,
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
V., and G. Swallow, "RSVP-TE: Extensions to RSVP for
LSP Tunnels", RFC 3209, December 2001.
7.2. Informative References
[RFC3468] Andersson, L. and G. Swallow, "The Multiprotocol Label
Switching (MPLS) Working Group decision on MPLS
signaling protocols", RFC 3468, February 2003.
[RFC3473] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) Extensions", RFC 3473, January
[RFC3564] Le Faucheur, F. and W. Lai, "Requirements for Support
of Differentiated Services-aware MPLS Traffic
Engineering", RFC 3564, July 2003.
[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May
[STEINER] H. Salama, et al., "Evaluation of Multicast Routing
Algorithm for Real-Time Communication on High-Speed
Networks," IEEE Journal on Selected Area in
Communications, pp.332-345, 1997.
[CRANKBACK] A. Farrel, A. Satyanarayana, A. Iwata, N. Fujita, G.
Ash, S. Marshall, "Crankback Signaling Extensions for
MPLS Signaling", Work in Progress, May 2005.
[P2MP-OAM] S. Yasukawa, A. Farrel, D. King, and T. Nadeau, "OAM
Requirements for Point-to-Multipoint MPLS Networks",
Work in Progress, February 2006.
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