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Versions: (draft-leroux-mpls-mp-ldp-reqs) 00 01 02 03 04 05 06 07 08 RFC 6348

MPLS                                                     J. Le Roux, Ed.
Internet-Draft                                             T. Morin, Ed.
Intended status: Historic                        France Telecom - Orange
Expires: November 27, 2011                                  May 26, 2011


      Requirements for Point-To-Multipoint Extensions to the Label
                         Distribution Protocol
                     draft-ietf-mpls-mp-ldp-reqs-08

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 Multi
   Protocol Label Switching (MPLS) infrastructure.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 27, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as



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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   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.  Contributing Authors . . . . . . . . . . . . . . . . . . .  4
     1.2.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
     1.3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.4.  Context and Motivations  . . . . . . . . . . . . . . . . .  6
     1.5.  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 . . . . . . . . . . . . . . . . . . . . . . .  9
     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 . . . . . . . . . . . . . . . . . . . . . 10
     4.7.  Detecting and Avoiding Loops . . . . . . . . . . . . . . . 11
     4.8.  P2MP LSP Re-routing  . . . . . . . . . . . . . . . . . . . 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  . . . . . . . . . . . 13
     4.16. Robustness . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.17. Scalability  . . . . . . . . . . . . . . . . . . . . . . . 14
     4.18. Backward Compatibility . . . . . . . . . . . . . . . . . . 14
   5.  Shared Trees . . . . . . . . . . . . . . . . . . . . . . . . . 14
     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
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     10.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19











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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)[I-D.ietf-mpls-ldp-p2mp], in order to deliver point-to-
   multipoint applications over a Multi-Protocol Label Switching (MPLS)
   infrastructure.  It is published with Historic status with the
   perspective of documenting the work done.

   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 finally discusses requirements for shared trees and
      MultiPoint-to-MultiPoint (MP2MP) LSPs.

1.1.  Contributing Authors

   The following people contributed to the text and content of this
   document:

   o  Shane Amante, Level 3 Communications, LLC

   o  Luyuan Fang, Cisco Systems

   o  Yuji Kamite, NTT Communications Corporation

   o  Jean-Louis Le Roux, France Telecom - Orange

   o  Thomas Morin, France Telecom - Orange

   o  Vincent Parfait, France Telecom - Orange, Orange Business Services

   o  Lei Wang, Telenor

1.2.  Requirements Language

   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].






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1.3.  Definitions

1.3.1.  Acronyms

   P2P:  Point-To-Point

   MP2P:  Multipoint-to-Point

   P2MP:  Point-To-MultiPoint

   MP2MP:  MultiPoint-To-Multipoint

   LSP:  Label Switched Path

   LSR:  Label Switching Router

   PE:  Provider Edge router

   P: Provider router

   IGP:  Interior Gateway Protocol

   AS:  Autonomous System

1.3.2.  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 as one or more Leaf LSRs acting indifferently as
      Ingress or Egress LSR




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

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

   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.

1.4.  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 with MPLS traffic replication at some Branch LSRs.

   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



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   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 setup
   point-to-multipoint (P2MP) LSPs.  This would bring consistency with
   P2P MPLS applications and would ease the delivery of point-to-
   multipoint services in an MPLS backbone.

1.5.  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 a 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, such as 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 co-exist 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 co-
   exist 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 a



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   LSR.


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
   [I-D.ietf-l3vpn-2547bis-mcast] for BGP/MPLS VPNs, or the one defined
   in [I-D.ietf-l2vpn-vpls-mcast].

   In this example, a set of MP2P LDP LSPs are setup between Provider
   Edge (PE) routers to carry unicast VPN traffic within the MPLS
   backbone.

   And in this example a set of P2MP LDP LSPs are setup 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 setup 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.

   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).





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                          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 aspects related to P2MP LSPs is the action required at a
   Branch LSR, where data replication occurs.  Incoming labelled 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 Section 4.9
   and Section 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.

4.2.  P2MP LSP FEC

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



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   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 (RIB).

   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 amount of P2MP LSPs within a single
   network and a large amount 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 LSPs 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.

   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



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   be detrimental for certain applications.  Hence, data duplication
   SHOULD as much as possible be avoided, 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, that may trigger unexpected non-localized exponential
   growth of traffic.

4.8.  P2MP LSP Re-routing

   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, metric change);

   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.).

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



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   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.

   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.







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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 OAM
   purpose are out of the scope of this document and are addressed in
   [RFC4687].

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



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   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;

   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 a 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;

   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.

   Also, the P2MP LDP solution MUST co-exist 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 signalled on the same interface.


5.  Shared Trees

   For traffic delivery between a group of N LSRs that act as egress



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   and/or egress nodes on different P2MP flows, it may be useful to
   setup 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 actually two main options for supporting such shared trees:

   o  This could rely on the applications protocols that use LDP LSPs.
      A shared tree could consist of the combination of a MP2P LDP LSP
      from Leafs 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 [I-D.ietf-l3vpn-2547bis-mcast]):

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

      *  with a P2MP LDP LSP from this root PE to each PE of the group.

   o  Or this could rely on a specific LDP mechanism allowing to setup
      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) belong to the scope of this document.
   Requirements for the set up 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 support P2MP LDP LSPs MAY also support MP2MP
   LDP LSP.

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

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

   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 there exists network
      connectivity between ingress and egress nodes (i.e. designed
      without introducing single points of failure);



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   o  The size of MP2MP state on a 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 a 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 a 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 a 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;

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

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

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

   Also, the P2MP LDP mechanism SHOULD be designed so that convergence



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   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 issue 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.  IANA Considerations

   This informational document makes no requests to IANA for action.


9.  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 also like to thank Adrian
   Farrel for reviewing this document before publication.

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


10.  References

10.1.  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.



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   [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.

10.2.  Informative References

   [I-D.ietf-l2vpn-vpls-mcast]
              Aggarwal, R., Kamite, Y., Fang, L., and Y. Rekhter,
              "Multicast in VPLS", draft-ietf-l2vpn-vpls-mcast-08 (work
              in progress), October 2010.

   [I-D.ietf-l3vpn-2547bis-mcast]
              Aggarwal, R., Bandi, S., Cai, Y., Morin, T., Rekhter, Y.,
              Rosen, E., Wijnands, I., and S. Yasukawa, "Multicast in
              MPLS/BGP IP VPNs", draft-ietf-l3vpn-2547bis-mcast-10 (work
              in progress), January 2010.

   [I-D.ietf-mpls-ldp-p2mp]
              Minei, I., Wijnands, I., Kompella, K., and B. Thomas,
              "Label Distribution Protocol Extensions for Point-to-
              Multipoint and Multipoint-to-Multipoint Label Switched
              Paths", draft-ietf-mpls-ldp-p2mp-13 (work in progress),
              April 2011.

   [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.

   [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-



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              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.


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


   Vincent Parfait
   France Telecom - Orange, Orange Business Services

   Email: vincent.parfait@orange-ftgroup.com


   Luyuan Fang
   Cisco Systems, Inc.

   Email: lufang@cisco.com








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   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



































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