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 Notice

Copyright (C) The Internet Society (2006).

Abstract

   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.

   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

1. Introduction

   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
   reasons.

      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
         only aggregation.
      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
   QoS algorithm.

   [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
     LSP.

   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
     4.18.1.

   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.

1.1. Non-Objectives

   For clarity, this section lists some items that are out of scope of
   this document.

   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
   functions.

   - 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
     engineering database.

2. Definitions

2.1. Acronyms

P2P: Point-to-point

P2MP: Point-to-multipoint

2.2. Terminology

   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
   LSPs only.

   P2MP tree:

      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.

   ingress LSR:

      The LSR that is responsible for initiating the signaling messages
      that set up the P2MP TE LSP.

   egress LSR:

      One of potentially many destinations of the P2MP TE LSP.  Egress
      LSRs may also be referred to as leaf nodes or leaves.

   bud LSR:

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

   branch LSR:

      An LSR that has more than one directly connected downstream LSR.

   P2MP-ID (P2ID):

      A unique identifier of a P2MP TE LSP, which is constant for the
      whole LSP regardless of the number of branches and/or leaves.

   source:

      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.

   receiver:

      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:

      Sub-LSP:
         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:

      S2L sub-LSP:
         The path from the source to one specific leaf.

      S2PL sub-LSP:
         The path from the source to a set of leaves.

      B2AL sub-LSP:
         The path from a branch LSR to all downstream leaves.

      X2X sub-LSP:
         A component of the P2MP LSP that is a simple path that does not
         branch.

      Note that the S2AL sub-LSP is equivalent to the P2MP LSP.

2.3. Conventions

   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

3.1. Motivation

   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
   protocols.

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)
                               |
                             I-LSR1
                             |   |
                             |   |
            R2----E-LSR3--LSR1   LSR2---E-LSR2--Receiver 1 (R1)
                             |   :
                  R3----E-LSR4   E-LSR5
                             |   :
                             |   :
                            R4   R5

                           Figure 1

   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
   E-LSR4.

   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
         routing.

         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
         branch LSR.

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
   tree.

   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
   and/or leaves.

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
   explicitly.

   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
   typical examples.

               A                                      A
               |                                    /   \
               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
   every receiver.

   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
   real-time traffic.

   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
   P2MP LSP.

   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.

4.5. Fragmentation

   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
         message.

      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
   receiving LSR.

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
   only.

   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
   message.

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
   same destination(s).

   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
   LSRs.

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
   resource sharing.

   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-
   routed.

   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
   supported.

   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-
   initiated.

   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
   (Section 4.10).

   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
   switches).

   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
   interface.

   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
   merge.

   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
   interfaces.

   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
     paths.

   - 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
   forwarding.

   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
   developed.

   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.

4.18. Scalability

   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
     TE LSP.
   - 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
     extensions.
   - 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
     techniques.

     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
     group.

     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
     recipients.

     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
     change.

   - 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.

4.20. GMPLS

   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
   including:

   - 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
     LSPs;
   - 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
   network.

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
     identified;
   - 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.

6. Acknowledgements

   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. References

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,
                 January 2001.

   [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
                 2003.

   [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
                 2005.

   [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.

Editor's Address

   Seisho Yasukawa
   NTT Corporation
   9-11, Midori-Cho 3-Chome
   Musashino-Shi, Tokyo 180-8585,
   Japan

   Phone: +81 422 59 4769
   EMail: yasukawa.seisho@lab.ntt.co.jp

Authors' Addresses

   Dimitri Papadimitriou
   Alcatel
   Francis Wellensplein 1,
   B-2018 Antwerpen,
   Belgium

   Phone : +32 3 240 8491
   EMail: dimitri.papadimitriou@alcatel.be

   JP Vasseur
   Cisco Systems, Inc.
   300 Beaver Brook Road

   Boxborough, MA 01719,
   USA

   EMail: jpv@cisco.com

   Yuji Kamite
   NTT Communications Corporation
   Tokyo Opera City Tower
   3-20-2 Nishi Shinjuku, Shinjuku-ku,
   Tokyo 163-1421,
   Japan

   EMail: y.kamite@ntt.com

   Rahul Aggarwal
   Juniper Networks
   1194 North Mathilda Ave.
   Sunnyvale, CA 94089

   EMail: rahul@juniper.net

   Alan Kullberg
   Motorola Computer Group
   120 Turnpike Rd.
   Southborough, MA 01772
   EMail: alan.kullberg@motorola.com

   Adrian Farrel
   Old Dog Consulting

   Phone: +44 (0) 1978 860944
   EMail: adrian@olddog.co.uk

   Markus Jork
   Quarry Technologies
   8 New England Executive Park
   Burlington, MA 01803

   EMail: mjork@quarrytech.com

   Andrew G. Malis
   Tellabs
   2730 Orchard Parkway
   San Jose, CA 95134

   Phone: +1 408 383 7223
   EMail: andy.malis@tellabs.com

   Jean-Louis Le Roux
   France Telecom
   2, avenue Pierre-Marzin
   22307 Lannion Cedex
   France

   EMail: jeanlouis.leroux@francetelecom.com

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