





Internet Engineering Task Force (IETF)                   J. Le Roux, Ed.
Request for Comments: 6348                                 T. Morin, Ed.
Category: Historic                               France Telecom - Orange
ISSN: 2070-1721                                           September 2011


            Requirements for Point-to-Multipoint Extensions
                   to the Label Distribution Protocol

Abstract

   This document lists a set of functional requirements that served as
   input to the design of Label Distribution Protocol (LDP) extensions
   for setting up point-to-multipoint (P2MP) Label Switched Paths (LSP),
   in order to deliver point-to-multipoint applications over a
   Multiprotocol Label Switching (MPLS) infrastructure.

   This work was overtaken by the protocol solution developed by the
   MPLS working group, but that solution did not closely follow the
   requirements documented here.  This document is published as a
   historic record of the ideas and requirements that shaped the
   protocol work.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for the historical record.

   This document defines a Historic Document for the Internet community.
   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6348.












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Copyright Notice

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   document authors.  All rights reserved.

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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
     1.2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Context and Motivations  . . . . . . . . . . . . . . . . .  6
     1.4.  Document Scope . . . . . . . . . . . . . . . . . . . . . .  7
   2.  Requirements Overview  . . . . . . . . . . . . . . . . . . . .  7
   3.  Application Scenario . . . . . . . . . . . . . . . . . . . . .  8
   4.  Detailed Requirements  . . . . . . . . . . . . . . . . . . . .  9
     4.1.  P2MP LSPs  . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  P2MP LSP FEC . . . . . . . . . . . . . . . . . . . . . . . 10
     4.3.  P2MP LDP Routing . . . . . . . . . . . . . . . . . . . . . 10
     4.4.  Setting Up, Tearing Down, and Modifying P2MP LSPs  . . . . 10
     4.5.  Label Advertisement  . . . . . . . . . . . . . . . . . . . 10
     4.6.  Data Duplication . . . . . . . . . . . . . . . . . . . . . 11
     4.7.  Detecting and Avoiding Loops . . . . . . . . . . . . . . . 11
     4.8.  P2MP LSP Rerouting . . . . . . . . . . . . . . . . . . . . 11
     4.9.  Support for Multi-Access Networks  . . . . . . . . . . . . 12
     4.10. Support for Encapsulation in P2P and P2MP TE Tunnels . . . 12
     4.11. Label Spaces . . . . . . . . . . . . . . . . . . . . . . . 13
     4.12. IPv4/IPv6 Support  . . . . . . . . . . . . . . . . . . . . 13
     4.13. Multi-Area/AS LSPs . . . . . . . . . . . . . . . . . . . . 13
     4.14. OAM  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.15. Graceful Restart and Fault Recovery  . . . . . . . . . . . 14
     4.16. Robustness . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.17. Scalability  . . . . . . . . . . . . . . . . . . . . . . . 14
     4.18. Backward Compatibility . . . . . . . . . . . . . . . . . . 14
   5.  Shared Trees . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.1.  Requirements for MP2MP LSPs  . . . . . . . . . . . . . . . 15
   6.  Evaluation Criteria  . . . . . . . . . . . . . . . . . . . . . 16
     6.1.  Performance  . . . . . . . . . . . . . . . . . . . . . . . 16
     6.2.  Complexity and Risks . . . . . . . . . . . . . . . . . . . 17
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 18
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 18
   Contributing Authors . . . . . . . . . . . . . . . . . . . . . . . 20












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1.  Introduction

   This document lists a set of functional requirements that served as
   input to the design of Label Distribution Protocol (LDP) extensions
   for setting up point-to-multipoint (P2MP) Label Switched Paths (LSP)
   [MLDP], in order to deliver point-to-multipoint applications over a
   Multiprotocol Label Switching (MPLS) infrastructure.

   This work was overtaken by the protocol solution developed by the
   MPLS working group and documented in [MLDP].  That solution did not
   closely follow the requirements documented here, and it was
   recognized that this document had served its purpose in driving
   discussions of how the solution should be designed.  At this point,
   no further action is planned to update this document in line with the
   protocol solution, and this document is published simply as a
   historic record of the ideas and requirements that shaped the
   protocol work.

   The document is structured as follows:

   o  Section 2 is an overview of the requirements.

   o  Section 3 illustrates an application scenario.

   o  Section 4 addresses detailed requirements for P2MP LSPs.

   o  Section 5 discusses requirements for shared trees and multipoint-
      to-multipoint (MP2MP) LSPs.

   o  Section 6 presents criteria against which a solution can be
      evaluated.

1.1.  Requirements Language

   This document is a historic requirements document.  To clarify
   statement of requirements, key words are used as follows.  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].

1.2.  Definitions

1.2.1.  Acronyms

   P2P:  Point-to-Point

   MP2P:  Multipoint-to-Point




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   P2MP:  Point-to-Multipoint

   MP2MP:  Multipoint-to-Multipoint

   LSP:  Label Switched Path

   LSR:  Label Switching Router

   PE:  Provider Edge

   P: Provider

   IGP:  Interior Gateway Protocol

   AS:  Autonomous System

1.2.2.  Terminology

   The reader is assumed to be familiar with the terminology in
   [RFC3031], [RFC5036], and [RFC4026].

   Ingress LSR:
      Router acting as a sender of an LSP

   Egress LSR:
      Router acting as a receiver of an LSP

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

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

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

   MP2MP LSP:
      An LSP that has one or more Leaf LSRs acting indifferently as
      Ingress or Egress LSR

   Leaf LSR:
      An Egress LSR of a P2MP LSP or an Ingress/Egress LSR of an MP2MP
      LSP

   Transit LSR:
      An LSR of a P2MP or MP2MP LSP that has one or more downstream LSRs





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   Branch LSR:
      An LSR of a P2MP or MP2MP LSP that has more than one downstream
      LSR

   Bud LSR:
      An LSR of a P2MP or MP2MP LSP that is an Egress but also has one
      or more directly connected downstream LSR(s)

   P2MP tree:
      The ordered set of LSRs and links that comprise the path of a P2MP
      LSP from its Ingress LSR to all of its Egress LSRs.

1.3.  Context and Motivations

   LDP [RFC5036] has been deployed for setting up point-to-point (P2P)
   and multipoint-to-point (MP2P) LSPs, in order to offer point-to-point
   services in MPLS backbones.

   There are emerging requirements for supporting delivery of point-to-
   multipoint applications in MPLS backbones, such as those defined in
   [RFC4834] and [RFC5501].

   For various reasons, including consistency with P2P applications, and
   taking full advantages of MPLS network infrastructure, it would be
   highly desirable to use MPLS LSPs for the delivery of multicast
   traffic.  This could be implemented by setting up a group of P2P or
   MP2P LSPs, but such an approach may be inefficient since it would
   result in data replication at the Ingress LSR and duplicate data
   traffic within the network.

   Hence, new mechanisms are required that would allow traffic from an
   Ingress LSR to be efficiently delivered to a number of Egress LSRs in
   an MPLS backbone on a point-to-multipoint LSP (P2MP LSP), avoiding
   duplicate copies of a packet on a given link and relying on MPLS
   traffic replication at some Branch LSRs.

   Resource Reservation Protocol - Traffic Engineering (RSVP-TE)
   extensions for setting up point-to-multipoint Traffic Engineered LSPs
   (P2MP TE LSPs) have been defined in [RFC4875].  They meet
   requirements expressed in [RFC4461].  This approach is useful in
   network environments where P2MP Traffic Engineering capabilities are
   needed (optimization, QoS, fast recovery).

   However, for operators who want to support point-to-multipoint
   traffic delivery on an MPLS backbone, without Traffic Engineering
   needs, and who have already deployed LDP for P2P traffic, an
   interesting and useful approach would be to rely on LDP extensions in
   order to set up point-to-multipoint (P2MP) LSPs.  This would bring



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   consistency with P2P MPLS applications and would ease the delivery of
   point-to-multipoint services in an MPLS backbone.

1.4.  Document Scope

   This document focuses on the LDP approach for setting up P2MP LSPs.
   It lists a detailed set of requirements for P2MP extensions to LDP,
   so as to deliver P2MP traffic over an LDP-enabled MPLS
   infrastructure.  The original intent was that these requirements
   should be used as guidelines when specifying LDP extensions.

   Note that generic requirements for P2MP extensions to MPLS are out of
   the scope of this document.  Rather, this document describes
   solution-specific requirements related to LDP extensions in order to
   set up P2MP LSPs.

   Note also that other mechanisms could be used for setting up P2MP
   LSPs (for instance, PIM extensions), but these are out of the scope
   of this document.  The objective is not to compare these mechanisms
   but rather to focus on the requirements for an LDP extension
   approach.

2.  Requirements Overview

   The P2MP LDP mechanism MUST support setting up P2MP LSPs, i.e., LSPs
   with one Ingress LSR and one or more Egress LSRs, with traffic
   replication at some Branch LSRs.

   The P2MP LDP mechanism MUST allow the addition or removal of leaves
   associated with a P2MP LSP.

   The P2MP LDP mechanism MUST coexist with current LDP mechanisms and
   inherit its capability sets from [RFC5036].  It is of paramount
   importance that the P2MP LDP mechanism MUST NOT impede the operation
   of existing P2P/MP2P LDP LSPs.  Also, the P2MP LDP mechanism MUST
   coexist with P2P and P2MP RSVP-TE mechanisms [RFC3209] [RFC4875].  It
   is of paramount importance that the P2MP LDP mechanism MUST NOT
   impede the operation of existing P2P and P2MP RSVP-TE LSPs.

   The P2MP LDP mechanism MAY also allow setting up multipoint-to-
   multipoint (MP2MP) LSPs connecting a group of Leaf LSRs acting
   indifferently as Ingress LSR or Egress LSR.  This may allow a
   reduction in the amount of LDP state that needs to be maintained by
   an LSR.







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3.  Application Scenario

   Figure 1 below illustrates an LDP-enabled MPLS provider network, used
   to carry both unicast and multicast traffic of VPN customers
   following, for instance, the architecture defined in [MVPN] for BGP/
   MPLS VPNs or the one defined in [VPLS-MCAST].

   In this example, a set of MP2P LDP LSPs is set up between Provider
   Edge (PE) routers to carry unicast VPN traffic within the MPLS
   backbone (not represented in Figure 1).

   In this example, a set of P2MP LDP LSPs is set up between PE routers
   acting as Ingress LSRs and PE routers acting as Egress LSRs, so as to
   support multicast VPN traffic delivery within the MPLS backbone.

   For instance, a P2MP LDP LSP is set up between Ingress LSR PE1 and
   Egress LSRs PE2, PE3, and PE4.  It is used to transport multicast
   traffic from PE1 to PE2, PE3, and PE4.  P1 is a Branch LSR; it
   replicates MPLS traffic sent by PE1 to P2, P3, and PE2.  P2 and P3
   are non-Branch Transit LSRs; they forward MPLS traffic sent by P1 to
   PE3 and PE4, respectively.

                          PE1
                          *|                *** P2MP LDP LSP
                          *|*****
                          P1-----PE2
                         */ \*
                        */   \*
                   *****/     \******
                PE3----P2      P3----PE4
                       |       |
                       |       |
                       |       |
                      PE5     PE6

               Figure 1: P2MP LSP from PE1 to PE2, PE3, PE4















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   If later there are new receivers attached to PE5 and PE6, then PE5
   and PE6 join the P2MP LDP LSP.  P2 and P3 become Branch LSRs and
   replicate traffic received from P1 to PE3 and PE5 and to PE4 and PE6,
   respectively (see Figure 2 below).

                          PE1
                          *|               *** P2MP LDP LSP
                          *|*****
                          P1-----PE2
                         */ \*
                        */   \*
                   *****/     \******
                PE3----P2      P3----PE4
                      *|       |*
                      *|       |*
                      *|       |*
                      PE5     PE6

                    Figure 2: Attachment of PE5 and PE6

   The above example is provided for the sake of illustration.  Note
   that P2MP LSPs Ingress and Egress LSRs may not necessarily be PE
   routers.  Also, Branch LSRs may not necessarily be P routers.

4.  Detailed Requirements

4.1.  P2MP LSPs

   The P2MP LDP mechanism MUST support setting up P2MP LSPs.  Data plane
   aspects related to P2MP LSPs are those already defined in [RFC4461].
   That is, a P2MP LSP has one Ingress LSR and one or more Egress LSRs.
   Traffic sent by the Ingress LSR is received by all Egress LSRs.  The
   specific aspect related to P2MP LSPs is the action required at a
   Branch LSR, where data replication occurs.  Incoming labeled data is
   appropriately replicated to several outgoing interfaces, which may
   use different labels.

   An LSR SHOULD NOT send more than one copy of a packet on any given
   link of a P2MP LSP.  Exceptions to this are mentioned in Sections 4.9
   and 4.18.

   A P2MP LSP MUST be identified by a constant and unique identifier
   within the whole LDP domain, whatever the number of leaves, which may
   vary dynamically.  This identifier will be used so as to add/remove
   leaves to/from the P2MP tree.






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4.2.  P2MP LSP FEC

   As with P2P MPLS technology [RFC5036], traffic MUST be classified
   into a Forwarding Equivalence Class (FEC) in this P2MP extension.
   All packets that belong to a particular P2MP FEC and that travel from
   a particular node MUST use the same P2MP LSP.

   If existing FECs cannot be used for this purpose, a new LDP FEC that
   is suitable for P2MP forwarding MUST be specified.

4.3.  P2MP LDP Routing

   As with P2P and MP2P LDP LSPs, the P2MP LDP mechanism MUST support
   hop-by-hop LSP routing.  P2MP LDP-based routing SHOULD rely upon the
   information maintained in LSR Routing Information Bases (RIBs).

   It is RECOMMENDED that the P2MP LSP routing rely upon the unicast
   route to the Ingress LSR to build a reverse path tree.

4.4.  Setting Up, Tearing Down, and Modifying P2MP LSPs

   The P2MP LDP mechanism MUST support the establishment, maintenance,
   and teardown of P2MP LSPs in a scalable manner.  This MUST include
   both the existence of a large number of P2MP LSPs within a single
   network and a large number of Leaf LSRs for a single P2MP LSP (see
   also Section 4.17 for scalability considerations and figures).

   In order to scale well with a large number of leaves, it is
   RECOMMENDED to follow a leaf-initiated P2MP LSP setup approach.  For
   that purpose, leaves will have to be aware of the P2MP LSP
   identifier.  The ways a Leaf LSR discovers P2MP LSP identifiers rely
   on the applications that will use P2MP LSPs and are out of the scope
   of this document.

   The P2MP LDP mechanism MUST allow the dynamic addition and removal of
   leaves to and from a P2MP LSP, without any restriction (provided
   there is network connectivity).  It is RECOMMENDED that these
   operations be leaf-initiated.  These operations MUST NOT impact the
   data transfer (packet loss, duplication, delay) towards other leaves.
   It is RECOMMENDED that these operations do not cause any additional
   processing except on the path from the added/removed Leaf LSR to the
   Branch LSR.

4.5.  Label Advertisement

   The P2MP LDP mechanism MUST support downstream unsolicited label
   advertisement mode.  This is well suited to a leaf-initiated approach
   and is consistent with P2P/MP2P LDP operations.



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   Other advertisement modes MAY also be supported.

4.6.  Data Duplication

   Data duplication refers to the receipt of multiple copies of a packet
   by any leaf.  Although this may be a marginal situation, it may also
   be detrimental for certain applications.  Hence, data duplication
   SHOULD be avoided as much as possible and limited to (hopefully rare)
   transitory conditions.

   Note, in particular, that data duplication might occur if P2MP LSP
   rerouting is being performed (see also Section 4.8).

4.7.  Detecting and Avoiding Loops

   The P2MP LDP extension MUST have a mechanism to detect routing loops.
   This MAY rely on extensions to the LDP loop detection mechanism
   defined in [RFC5036].  A loop detection mechanism MAY require
   recording the set of LSRs traversed on the P2MP tree.  The P2MP loop
   avoidance mechanism MUST NOT impact the scalability of the P2MP LDP
   solution.

   The P2MP LDP mechanism SHOULD have a mechanism to avoid routing loops
   in the data plane even during transient events.

   Furthermore, the P2MP LDP mechanism MUST avoid routing loops in the
   data plane, which may trigger unexpected non-localized exponential
   growth of traffic.

4.8.  P2MP LSP Rerouting

   The P2MP LDP mechanism MUST support the rerouting of a P2MP LSP in
   the following cases:

   o  Network failure (link or node);

   o  A better path exists (e.g., new link or metric change); and

   o  Planned maintenance.

   Given that P2MP LDP routing should rely on the RIB, the achievement
   of the following requirements relies on the underlying routing
   protocols (IGP, etc.).








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4.8.1.  Rerouting upon Network Failure

   The P2MP LDP mechanism MUST allow for rerouting of a P2MP LSP in case
   of link or node failure(s) by relying upon update of the routes in
   the RIB.  The rerouting time SHOULD be minimized as much as possible
   so as to reduce traffic disruption.

   A mechanism MUST be defined to prevent constant P2MP LSP teardown and
   rebuild, which may be caused by the instability of a specific link/
   node in the network.  This can rely on IGP dampening but may be
   completed by specific dampening at the LDP level.

4.8.2.  Rerouting on a Better Path

   The P2MP LDP mechanism MUST allow for rerouting of a P2MP LSP in case
   a better path is created in the network, for instance, as a result of
   a metric change, a link repair, or the addition of links or nodes.
   This will rely on update of the routes in the RIB.

4.8.3.  Rerouting upon Planned Maintenance

   The P2MP LDP mechanism MUST support planned maintenance operations.
   It MUST be possible to reroute a P2MP LSP before a link/node is
   deactivated for maintenance purposes.  Traffic disruption and data
   duplication SHOULD be minimized as much as possible during such
   planned maintenance.  P2MP LSP rerouting upon planned maintenance MAY
   rely on a make-before-break procedure.

4.9.  Support for Multi-Access Networks

   The P2MP LDP mechanism SHOULD provide a way for a Branch LSR to send
   a single copy of the data onto an interface to a multi-access network
   (e.g., an Ethernet LAN) and reach multiple adjacent downstream nodes.
   This requires that the same label be negotiated with all downstream
   LSRs for the LSP.

   When there are several candidate upstream LSRs on an interface to a
   multi-access LAN, the P2MP LDP mechanism SHOULD provide a way for all
   downstream LSRs of a given P2MP LSP to select the same upstream LSR,
   so as to avoid traffic replication.  In addition, the P2MP LDP
   mechanism SHOULD allow for an efficient balancing of a set of P2MP
   LSPs among a set of candidate upstream LSRs on a LAN interface.

4.10.  Support for Encapsulation in P2P and P2MP TE Tunnels

   The P2MP LDP mechanism MUST support nesting P2MP LSPs into P2P and
   P2MP TE tunnels.




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   The P2MP LDP mechanism MUST provide a way for a Branch LSR of a P2MP
   LSP, which is also a Head End LSR of a P2MP TE tunnel, to send a
   single copy of the data onto the tunnel and reach all downstream LSRs
   on the P2MP LSP, which are also Egress LSRs of the tunnel.  As with
   LAN interfaces, this requires that the same label be negotiated with
   all downstream LSRs of the P2MP LDP LSP.

4.11.  Label Spaces

   Labels for P2MP LSPs and P2P/MP2P LSPs MAY be assigned from shared or
   dedicated label spaces.

   Note that dedicated label spaces will require the establishment of
   separate P2P and P2MP LDP sessions.

4.12.  IPv4/IPv6 Support

   The P2MP LDP mechanism MUST support the establishment of LDP sessions
   over both IPv4 and IPv6 control planes.

4.13.  Multi-Area/AS LSPs

   The P2MP LDP mechanism MUST support the establishment of multi-area
   P2MP LSPs, i.e., LSPs whose leaves do not all reside in the same IGP
   area as the Ingress LSR.  This SHOULD be possible without requiring
   the advertisement of Ingress LSRs' addresses across IGP areas.

   The P2MP LDP mechanism MUST also support the establishment of
   inter-AS P2MP LSPs, i.e., LSPs whose leaves do not all reside in the
   same AS as the Ingress LSR.  This SHOULD be possible without
   requiring the advertisement of Ingress LSRs' addresses across ASes.

4.14.  OAM

   LDP management tools ([RFC3815], etc.) will have to be enhanced to
   support P2MP LDP extensions.  This may yield a new MIB module, which
   may possibly be inherited from the LDP MIB.

   Built-in diagnostic tools MUST be defined to check the connectivity,
   trace the path, and ensure fast detection of data plane failures on
   P2MP LDP LSPs.

   Further and precise requirements and mechanisms for P2MP MPLS
   Operations, Administration, and Maintenance (OAM) purposes are out of
   the scope of this document and are addressed in [RFC4687].






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4.15.  Graceful Restart and Fault Recovery

   LDP Graceful Restart mechanisms [RFC3478] and Fault Recovery
   mechanisms [RFC3479] SHOULD be enhanced to support P2MP LDP LSPs.

4.16.  Robustness

   A solution MUST be designed to re-establish connectivity for P2MP and
   MP2MP LSPs in the event of failures, provided there exists network
   connectivity between ingress and egress nodes (i.e., designed without
   introducing single points of failure).

4.17.  Scalability

   Scalability is a key requirement for the P2MP LDP mechanism.  It MUST
   be designed to scale well with an increase in the number of any of
   the following:

   o  Number of Leaf LSRs per P2MP LSP;

   o  Number of downstream LSRs per Branch LSR; and

   o  Number of P2MP LSPs per LSR.

   In order to scale well with an increase in the number of leaves, it
   is RECOMMENDED that the size of a P2MP LSP state on an LSR, for one
   particular LSP, depend only on the number of adjacent LSRs on the
   LSP.

4.17.1.  Orders of Magnitude Expected in Operational Networks

   Typical orders of magnitude that we expect should be supported are:

   o  Tens of thousands of P2MP trees spread out across core network
      routers; and

   o  Hundreds, or a few thousands, of leaves per tree.

   See also Section 4.2 of [RFC4834].

4.18.  Backward Compatibility

   In order to allow for a smooth migration, the P2MP LDP mechanism
   SHOULD offer as much backward compatibility as possible.  In
   particular, the solution SHOULD allow the setup of a P2MP LSP along
   non-Branch Transit LSRs that do not support P2MP LDP extensions.





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   Also, the P2MP LDP solution MUST coexist with current LDP mechanisms
   and inherit its capability sets from [RFC5036].  The P2MP LDP
   solution MUST NOT impede the operation of P2P/MP2P LSPs.  A P2MP LDP
   solution MUST be designed in such a way that it allows P2P/MP2P and
   P2MP LSPs to be signaled on the same interface.

5.  Shared Trees

   For traffic delivery between a group of N LSRs that act as egress
   and/or egress nodes on different P2MP flows, it may be useful to set
   up a shared tree connecting all these LSRs instead of having N P2MP
   LSPs.  This would reduce the amount of control and forwarding state
   that has to be maintained on a given LSR.

   There are two main options for supporting such shared trees:

   o  Relying on the applications' protocols that use LDP LSPs.  A
      shared tree could consist of the combination of an MP2P LDP LSP
      from Leaf LSRs to a given root node with a P2MP LSP from this root
      to Leaf LSRs.  For instance, with Multicast L3 VPN applications,
      it would be possible to build a shared tree by combining (see
      [MVPN]):

      *  An MP2P unicast LDP LSP, from each PE router of the group to a
         particular root PE router acting as tree root and

      *  A P2MP LDP LSP from this root PE router to each PE router of
         the group.

   o  Relying on a specific LDP mechanism allowing the setup of
      multipoint-to-multipoint MPLS LSPs (MP2MP LSPs).

   The former approach (combination of MP2P and P2MP LSPs at the
   application level) is out of the scope of this document while the
   latter (MP2MP LSPs) is within the scope of this document.
   Requirements for the setup of MP2MP LSPs are listed below.

5.1.  Requirements for MP2MP LSPs

   A multipoint-to-multipoint (MP2MP) LSP is an LSP connecting a group
   of Leaf LSRs acting as Egress and/or Ingress LSRs.  Traffic sent by
   any Leaf LSR is received by all other Leaf LSRs of the group.

   Procedures for setting up MP2MP LSPs with LDP SHOULD be specified.
   An implementation that supports P2MP LDP LSPs MAY also support MP2MP
   LDP LSPs.

   The MP2MP LDP procedures MUST NOT impede the operations of P2MP LSP.



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   Requirements for P2MP LSPs, set forth in Section 4, apply equally to
   MP2MP LSPs.  Particular attention should be given to the requirements
   below:

   o  The solution MUST support recovery upon link and transit node
      failure and be designed to re-establish connectivity for MP2MP
      LSPs in the event of failures, provided network connectivity
      exists between ingress and egress nodes (i.e., designed without
      introducing single points of failure).

   o  The size of MP2MP state on an LSR, for one particular MP2MP LSP,
      SHOULD only depend on the number of adjacent LSRs on the LSP.

   o  Furthermore, the MP2MP LDP mechanism MUST avoid routing loops that
      may trigger exponential growth of traffic.  Note that this
      requirement is more challenging with MP2MP LSPs as an LSR may need
      to receive traffic for a given LSP on multiple interfaces.

   There are additional requirements specific to MP2MP LSPs:

   o  It is RECOMMENDED that an MP2MP MPLS LSP is built based on the
      unicast route to a specific LSR called root LSR.

   o  It is RECOMMENDED to define several root LSRs (e.g., a primary and
      a backup) to ensure redundancy upon root LSR failure.

   o  The receiver SHOULD NOT receive back a packet it has sent on the
      MP2MP LSP.

   o  The solution SHOULD avoid that all traffic between any pair of
      leaves is traversing a root LSR (similarly to PIM-Bidir trees) and
      SHOULD provide the operator with means to minimize the delay
      between two leaves.

   o  It MUST be possible to check connectivity of an MP2MP LSP in both
      directions.

6.  Evaluation Criteria

6.1.  Performance

   The solution will be evaluated with respect to the following
   criteria:

   (1)  Efficiency of network resource usage;

   (2)  Time to add or remove a Leaf LSR;




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   (3)  Time to repair a P2MP LSP in case of link or node failure; and

   (4)  Scalability (state size, number of messages, message size).

   Particularly, the P2MP LDP mechanism SHOULD be designed with the key
   objective of minimizing the additional amount of state and additional
   processing required in the network.

   Also, the P2MP LDP mechanism SHOULD be designed so that convergence
   times in case of link or node failure are minimized, in order to
   limit traffic disruption.

6.2.  Complexity and Risks

   The proposed solution SHOULD NOT introduce complexity to the current
   LDP operations to such a degree that it would affect the stability
   and diminish the benefits of deploying such solution.

7.  Security Considerations

   It is expected that addressing the requirements defined in this
   document should not introduce any new security issues beyond those
   inherent to LDP and that a P2MP LDP solution will rely on the
   security mechanisms defined in [RFC5036] (e.g., TCP MD5 Signature).

   An evaluation of the security features for MPLS networks may be found
   in [RFC5920], and where issues or further work is identified by that
   document, new security features or procedures for the MPLS protocols
   will need to be developed.

8.  Acknowledgments

   We would like to thank Christian Jacquenet, Hitoshi Fukuda, Ina
   Minei, Dean Cheng, and Benjamin Niven-Jenkins for their highly useful
   comments and suggestions.  We would like to thank Adrian Farrel for
   reviewing this document before publication.

   We would also like to thank the authors of [RFC4461], which inspired
   some of the text in this document.












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9.  References

9.1.  Normative References

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

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

   [RFC3478]     Leelanivas, M., Rekhter, Y., and R. Aggarwal, "Graceful
                 Restart Mechanism for Label Distribution Protocol",
                 RFC 3478, February 2003.

   [RFC3479]     Farrel, A., "Fault Tolerance for the Label Distribution
                 Protocol (LDP)", RFC 3479, February 2003.

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

   [RFC4461]     Yasukawa, S., "Signaling Requirements for Point-to-
                 Multipoint Traffic-Engineered MPLS Label Switched Paths
                 (LSPs)", RFC 4461, April 2006.

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

9.2.  Informative References

   [MLDP]        Minei, I., Wijnands, I., Kompella, K., and B. Thomas,
                 "Label Distribution Protocol Extensions for Point-to-
                 Multipoint and Multipoint-to-Multipoint Label Switched
                 Paths", Work in Progress, August 2011.

   [MVPN]        Aggarwal, R., Bandi, S., Cai, Y., Morin, T., Rekhter,
                 Y., Rosen, E., Wijnands, I., and S. Yasukawa,
                 "Multicast in MPLS/BGP IP VPNs", Work in Progress,
                 January 2010.

   [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.






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   [RFC4026]     Andersson, L. and T. Madsen, "Provider Provisioned
                 Virtual Private Network (VPN) Terminology", RFC 4026,
                 March 2005.

   [RFC4687]     Yasukawa, S., Farrel, A., King, D., and T. Nadeau,
                 "Operations and Management (OAM) Requirements for
                 Point-to-Multipoint MPLS Networks", RFC 4687,
                 September 2006.

   [RFC4834]     Morin, T., Ed., "Requirements for Multicast in Layer 3
                 Provider-Provisioned Virtual Private Networks
                 (PPVPNs)", RFC 4834, April 2007.

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

   [RFC5501]     Kamite, Y., Wada, Y., Serbest, Y., Morin, T., and L.
                 Fang, "Requirements for Multicast Support in Virtual
                 Private LAN Services", RFC 5501, March 2009.

   [RFC5920]     Fang, L., "Security Framework for MPLS and GMPLS
                 Networks", RFC 5920, July 2010.

   [VPLS-MCAST]  Aggarwal, R., Kamite, Y., Fang, L., and Y. Rekhter,
                 "Multicast in VPLS", Work in Progress, July 2011.
























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Contributing Authors

   Vincent Parfait
   France Telecom - Orange, Orange Business Services

   EMail: vincent.parfait@orange-ftgroup.com


   Luyuan Fang
   Cisco Systems, Inc.

   EMail: lufang@cisco.com


   Lei Wang
   Telenor

   EMail: lei.wang@telenor.com


   Yuji Kamite
   NTT Communications Corporation

   EMail: y.kamite@ntt.com


   Shane Amante
   Level 3 Communications, LLC

   EMail: shane@level3.net

Authors' Addresses

   Jean-Louis Le Roux (editor)
   France Telecom - Orange

   EMail: jeanlouis.leroux@orange-ftgroup.com


   Thomas Morin (editor)
   France Telecom - Orange

   EMail: thomas.morin@orange-ftgroup.com








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