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Versions: (RFC 2547) 00 01 02 03 04 05 06 07 08 09 10 RFC 6513

Network Working Group                             Eric C. Rosen (Editor)
Internet Draft                                       Cisco Systems, Inc.
Expiration Date: November 2005
                                                 Rahul Aggarwal (Editor)
                                                        Juniper Networks

                                                                May 2005


                     Multicast in MPLS/BGP IP VPNs


                 draft-ietf-l3vpn-2547bis-mcast-00.txt

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Abstract

   In order for IP multicast traffic within a BGP/MPLS IP VPN (Virtual
   Private Network) to travel from one VPN site to another, special
   protocols and procedures must be implemented by the VPN Service
   Provider.  These protocols and procedures are specified in this
   document.







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Table of Contents

    1          Specification of requirements  ......................   4
    2          Introduction  .......................................   4
    2.1        Optimality vs Scalability  ..........................   4
    2.2        Overview  ...........................................   6
    3          Concepts and Framework  .............................   9
    3.1        PE-CE Multicast Routing  ............................   9
    3.2        P-Multicast Service Interfaces (PMSIs)  .............  10
    3.2.1      Inclusive and Selective PMSIs  ......................  11
    3.2.2      Tunnels Instantiating PMSIs  ........................  12
    3.3        Use of PMSIs for Carrying Multicast Data  ...........  13
    3.3.1      MVPNs that Use MI-PMSIs  ............................  14
    3.3.2      MVPNs That Do Not Use MI-PMSIs  .....................  14
    4          BGP-Based Autodiscovery of MVPN Membership  .........  14
    5          PE-PE Transmission of Multicast Routing  ............  17
    5.1        PIM Peering  ........................................  17
    5.1.1      Full Per-MVPN PIM Peering Across a MI-PMSI  .........  17
    5.1.2      Lightweight PIM Peering Across a MI-PMSI  ...........  18
    5.1.3      Unicasting of PIM C-Join/Prune Messages  ............  18
    5.1.4      Details of Per-MVPN PIM Peering over MI-PMSI  .......  19
    5.1.4.1    PIM C-Instance Control Packets  .....................  20
    5.1.4.2    PIM C-instance RPF Determination  ...................  20
    5.2        Use of BGP for Carrying PE-PE Multicast Routing  ....  21
    6          I-PMSI Instantiation  ...............................  22
    6.1        MVPN Membership and Egress PE Auto-Discovery  .......  23
    6.1.1      Auto-Discovery for Ingress Replication  .............  23
    6.1.2      Auto-Discovery for P-Multicast Trees  ...............  24
    6.2        MVPN Routing Information Exchange  ..................  24
    6.3        Aggregation  ........................................  24
    6.3.1      Aggregate Tree Leaf Discovery  ......................  25
    6.3.2      Aggregation Methodology  ............................  25
    6.3.3      Encapsulation of the Aggregate Tree  ................  26
    6.3.4      Demultiplexing C-multicast traffic  .................  26
    6.4        Mapping Received Packets to MVPNs  ..................  27
    6.4.1      Unicast Tunnels  ....................................  27
    6.4.2      Non-Aggregated P-Multicast Trees  ...................  28
    6.4.3      Aggregate P-Multicast Trees  ........................  29
    6.5        I-PMSI Instantiation Using Ingress Replication  .....  29
    6.6        Establishing P-Multicast Trees  .....................  30
    6.7        RSVP-TE P2MP LSPs  ..................................  31
    6.7.1      P2MP TE LSP Tunnel - MVPN Mapping  ..................  31
    6.7.2      Demultiplexing C-Multicast Data Packets  ............  31
    7          Optimizing Multicast Distribution via S-PMSIs  ......  32
    7.1        Egress PE Discovery   ...............................  32
    7.2        S-PMSI Instantiation Using Ingress Replication  .....  33



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    7.3        Protocol for Switching to S-PMSIs  ..................  33
    7.3.1      A UDP-based Protocol for Switching to S-PMSIs  ......  33
    7.3.1.1    Binding a Stream to an S-PMSI  ......................  34
    7.3.1.2    Packet Formats and Constants  .......................  35
    7.3.2      A BGP-based Protocol for Switching to S-PMSIs  ......  35
    7.3.2.1    Advertising C-(S, G) Binding to a S-PMSI using BGP  .  35
    7.3.2.2    Switching to S-PMSI  ................................  36
    7.4        Aggregation  ........................................  37
    7.5        Instantiating the S-PMSI with a PIM Tree  ...........  37
    7.6        Instantiating S-PMSIs using RSVP-TE P2MP Tunnels  ...  38
    8          Inter-AS Procedures  ................................  38
    8.1        Tunnel Spans Multiple ASs  ..........................  39
    8.1.1      Inter-AS MVPN Auto-Discovery  .......................  39
    8.1.2      Inter-AS MVPN Routing Information Exchange  .........  39
    8.1.3      Inter-AS I-PMSI  ....................................  40
    8.1.4      Inter-AS S-PMSI  ....................................  41
    8.2        Overlay Inter-AS Tunnel  ............................  41
    8.2.1      Inter-AS MVPN Auto-Discovery  .......................  41
    8.2.2      Inter-AS MVPN Routing Information Exchange  .........  42
    8.2.3      Inter-AS I-PMSI  ....................................  43
    8.2.3.1    Support for 2547 Unicast VPN Inter-AS Methods  ......  44
    8.2.4      Inter-AS S-PMSI  ....................................  44
    9          Deployment Models  ..................................  45
    9.1        Co-locating C-RPs on a PE  ..........................  45
    9.1.1      Initial Configuration  ..............................  46
    9.1.2      Anycast RP Based on C-(*, G) Advertisements  ........  46
    9.1.2.1    Receiver(s) Within a Site  ..........................  46
    9.1.2.2    Source within a Site  ...............................  46
    9.1.2.3    Receiver Switching from Shared to Source Tree  ......  47
    9.1.2.3.1  Handling Join (S,G)  ................................  47
    9.1.2.3.2  Handling Prune (S,G, RPT)  ..........................  47
    9.1.2.3.3  Receiving information from other PEs  ...............  47
    9.1.3      Anycast RP Based on Propagating Active Sources  .....  48
    9.1.3.1    Receiver(s) Within a Site  ..........................  48
    9.1.3.2    Source Within a Site  ...............................  48
    9.1.3.3    Receiver Switching from Shared to Source Tree  ......  49
   10          BGP Advertisements  .................................  49
   10.1        Functions and Information Elements  .................  49
   10.2        Encoding  ...........................................  49
   11          Encapsulations  .....................................  49
   11.1        Encapsulations for Single PMSI per Tunnel  ..........  49
   11.1.1      Encapsulation in GRE  ...............................  49
   11.1.2      Encapsulation in IP  ................................  51
   11.1.3      Encapsulation in MPLS  ..............................  51
   11.2        Encapsulations for Multiple PMSIs per Tunnel  .......  52
   11.2.1      Encapsulation in GRE  ...............................  52
   11.2.2      Encapsulation in IP  ................................  52
   11.3        Encapsulations for Unicasting PIM Control Messages  .  53



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   11.4        General Considerations for IP and GRE Encaps  .......  53
   11.4.1      MTU  ................................................  53
   11.4.2      TTL  ................................................  53
   11.4.3      Differentiated Services  ............................  54
   11.4.4      Avoiding Conflict with Internet Multicast  ..........  54
   12          Security Considerations  ............................  54
   13          IANA Considerations  ................................  54
   14          Other Authors  ......................................  54
   15          Other Contributors  .................................  54
   16          Authors' Addresses  .................................  55
   17          Normative References  ...............................  56
   18          Informative References  .............................  57
   19          Full Copyright Statement  ...........................  57
   20          Intellectual Property  ..............................  58




1. Specification of requirements

   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].


2. Introduction

   [RFC2547bis] specifies a set of procedures which must be implemented
   for a Service Provider (SP) to provide an IP unicast VPN service.
   However [RFC2547bis] does not provide a way for a SP to offer IP
   multicast VPN (MVPN) service. In particular it does not provide
   mechanisms for IP multicast data or control traffic to travel from
   one VPN site to another. This document specifies the protocols and
   procedures that enable a SP to provide multicast service in a VPN.


2.1. Optimality vs Scalability

   In a "BGP/MPLS VPN" [RFC2547bis], unicast routing of VPN packets is
   achieved without the need to keep any per-VPN state in the core of
   the SP's network (the "P routers").  VPN routing information is
   maintained only by the PEs participating in the VPN service.  This
   allows the SP to increase the number of VPNs it supports, without
   requiring additional state to be kept in the P routers, and it still
   allows the site-to site routing to be optimal.

   However, when supporting multicast routing in a BGP/MPLS VPN, the
   optimality of the multicast must be traded off against scalability.



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   If a particular VPN is transmitting "native" multicast traffic over
   the backbone,  we refer to it as an "MVPN".  By "native" multicast
   traffic, we mean packets that a CE sends to a PE, such that the IP
   destination address of the packets is a multicast group address, or
   the packets are multicast control packets addressed to the PE router
   itself.

   We say that the backbone multicast routing for a particular multicast
   group in a particular VPN is "optimal" if and only if all of the
   following conditions hold:

     - When a PE router receives a multicast data packet of that group
       from a CE router, it transmits the packet in such a way that the
       packet is received by every other PE router which is on the path
       to a receiver of that group;

     - The packet is not received by any other PEs;

     - While in the backbone, no more than one copy of the packet ever
       traverses any link.

   Optimal routing for a particular multicast group requires that the
   backbone maintain one or more source-trees which are specific to that
   flow.  Each such tree requires that state be maintained in all the P
   routers that are in the tree.

   This would potentially require an unbounded amount of state in the P
   routers, since the SP has no control of the number of multicast
   groups in the VPNs that it supports. Nor does the SP have any control
   over the number of transmitters in each group, nor of the
   distribution of the receivers.

   The procedures defined in this document allow a SP to provide
   multicast VPN service without requiring an unbounded amount of state
   in the P routers.  The amount of state is traded off against the
   optimality of the multicast routing.  The procedures provide
   flexibility so that a given SP can make his own tradeoffs between
   scalability and optimality.  The procedures also allow for some
   multicast groups in some VPNs to receive optimal routing, while
   others do not.

   One supported option to carry MVPN data traffic is to setup unicast
   tunnels from the ingress PE to each of the egress PEs. The ingress PE
   replicates the multicast data packet received from a CE and sends it
   to each of the egress PEs using the unicast tunnels.  This requires
   no multicast routing state in the P routers at all, but makes no
   attempt to optimize the multicast routing.




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   Another supported option is to use a single multicast distribution
   tree in the backbone to carry all the multicast traffic from a
   specified set of one or more MVPNs.  Such a tree is referred to as an
   "Inclusive Tree". With this option, the P routers do not maintain
   state on a per-multicast-group basis, but only on a per-set-of-MVPNs
   basis. These multicast distribution trees can be set up to carry the
   traffic of a single MVPN, or to carry the traffic of multiple MVPNs.
   The term "Aggregate Inclusive Tree" will be used to specifically
   refer to such a tree when used to carry the traffic of multiple VPNs.
   The tree will include every PE that is a member of any of the MVPNs
   that are using the tree. This enables the SP to place a bound on the
   amount of multicast routing state which the P routers must have.
   However, as data from many multicast groups is aggregated together
   onto a single "Inclusive Tree", it is likely that some PEs will
   receive multicast data for which they have no need, i.e., some degree
   of optimality has been sacrificed.

   This document also provides procedures by which a single multicast
   distribution tree in the backbone can be used to carry traffic
   belonging only to a specified set of one or more multicast groups,
   from one or more MVPNs. Such a tree is referred to as a "Selective
   Tree" and more specifically as an "Aggregate Selective Tree" when the
   multicast groups belong to different MVPNs. So traffic from most
   multicast groups could be carried by an Inclusive Tree, while traffic
   from, e.g., high bandwidth groups could be carried in one of the
   "Selective Trees".  When setting up the Selective Trees, one should
   include only those PEs which need to receive multicast data from one
   or more of the groups assigned to the tree.  This provides more
   optimal routing than can be obtained by using only Inclusive Trees,
   though it requires additional state in the P routers.


2.2. Overview

   In BGP MPLS VPNs [RFC2547bis], each CE router is a unicast routing
   adjacency of a PE router, but CE routers at different sites do not
   become unicast routing adjacencies of each other. This important
   characteristic is retained for multicast routing -- a CE router
   becomes a PIM adjacency of a PE router, but CE routers at different
   sites do not become PIM adjacencies of each other. A CE router
   exchanges "ordinary" PIM control messages with the PE router to which
   it is attached. The set of PE routers attaching to a given MVPN then
   exchange MVPN control information with each other.

   In order for the PE routers attaching to a given MVPN to exchange
   MVPN control information with each other, each one needs to discover
   all the other PEs that attach to the same MVPN. One of the options in
   this document for achieving this is the use of BGP for discovering



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   and maintaining MVPN membership. In this solution the PE routers
   advertise their MVPN membership to other PE routers using BGP. A PE
   is considered to be a "member" of a particular MVPN if it contains a
   VRF (Virtual Routing and Forwarding table, see [2547bis]) which is
   configured to contain the multicast routing information of that MVPN.
   This discovery option can be used with any multicast transport
   technology.

   This document also describes an option for discovering MVPN members
   that relies on the presence of a PIM based Inclusive Tree for each PE
   router that belongs to the MVPN.

   The BGP/MPLS VPNs [RFC2547bis] specification requires a PE to
   maintain at most one BGP peering with every other PE in the network.
   This peering is used to exchange VPN routing information. The use of
   Route Reflectors further reduces the number of BGP adjacencies
   maintained by a PE to exchange VPN routing information with other
   PEs. This document describes various options for exchanging MVPN
   control information between PE routers based on the use of PIM or
   BGP. These options have different overheads with respect to the
   number of routing adjacencies that a PE router needs to maintain to
   exchange MVPN control information with other PE routers. Some of
   these options allow the retention of the unicast BGP/MPLS VPN model
   letting a PE maintain at most one routing adjacency with other PE
   routers to exchange MVPN control information.

   The option of using PIM for exchanging MVPN control information along
   with BGP to discover the MVPN members closely mimics the unicast
   BGP/MPLS PE-PE routing adjacency model. The option of using BGP for
   exchanging MVPN control information comes even closer to the unicast
   VPN model as it allows the use of Route Reflectors.

   The solution in [RFC2547bis] uses BGP to exchange VPN routing
   information between PE routers and BGP provides a reliable transport.
   This document describes various solutions for providing a reliable
   transport for exchanging MVPN control information. One option is the
   use of PIM with reliability extensions and the other is the use of
   BGP.  The use of the currently existing, "soft state" PIM standard
   [PIM-SM], is also supported.

   Like [RFC2547bis], this document decouples procedures for exchanging
   routing information from the procedures for transmitting data
   traffic. Hence a variety of transport technologies may be used in the
   backbone, including unicast PE-PE tunnels using MPLS or IP/GRE
   encapsulation, PIM trees (created by PIM-SSM, PIM-SM, or PIM-Bidir),
   and point-to-multipoint LSPs created by RSVP-TE.  (Techniques for
   aggregating the traffic of multiple MVPNs onto a single PIM-bidir
   tree are for further study.)  Selective trees are always set up with



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   PIM-SSM or with RSVP-TE.

   In order to aggregate traffic from multiple MVPNs onto a single
   multicast distribution tree, the root of the tree must be able to
   discover the MVPN membership of all the PEs and/or the set of
   multicast groups in which each PE has receivers. This document
   describes the procedures for achieving this. Aggregation also
   requires a mechanism which enables the egresses of the tree to
   demultiplex the multicast traffic received over the tree.  This
   document specifies a mechanism whereby upstream label allocation
   [MPLS-UPSTREAM-LABEL] is used by the root of the tree to assign a
   label to each flow.  This label is used by the receivers to perform
   the demultiplexing. This document also describes procedures based on
   BGP that are used by the root of an Aggregate Tree to advertise the
   Inclusive and/or Selective binding and the demultiplexing information
   to the leaves of the tree

   This document also describes the data plane encapsulation for
   supporting the various SP multicast transport options.

   [RFC2547bis] describes different options for supporting Inter-AS
   BGP/MPLS unicast VPNs. This document describes how Inter-AS MVPNs can
   be supported for each of the unicast BGP/MPLS VPN Inter-AS options.
   This document introduces a model where Inter-AS MVPN service can be
   offered without requiring a SP multicast tree to span multiple ASs.
   Each AS can have a different multicast tree, possibly with a
   different MVPN transport technology in each AS.  It is also possible
   to support Inter-AS MVPNs with source trees that extend across AS
   boundaries.

   The document also discusses different deployment models for MVPNs and
   any protocol extensions that are required for supporting the specific
   models.

   This document assumes that when SP multicast trees are used, traffic
   for a particular multicast group can be transmitted by a PE on only
   one SP multicast tree. The use of multiple SP multicast trees for
   transmitting traffic belonging to a particular multicast group is for
   further study.












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3. Concepts and Framework

3.1. PE-CE Multicast Routing

   Support of multicast in BGP/MPLS IP VPNs is modeled closely after
   support of unicast in BGP/MPLS IP VPNs. That is, a multicast routing
   protocol will be run on the PE-CE interfaces, such that PE and CE are
   multicast routing adjacencies on that interface.  CEs at different
   sites do not become multicast routing adjacencies of each other.

   If a PE attaches to n VPNs for which multicast support is provided
   (i.e., to n "MVPNs"), the PE will run n independent instances of a
   multicast routing protocol.  We will refer to these multicast routing
   instances as "VPN-specific multicast routing instances", or more
   briefly as "multicast C-instances". The notion of a "VRF", defined in
   [RFC2547bis], is extended to include multicast routing entries as
   well as unicast routing entries. Each multicast routing entry is thus
   associated with a particular VRF.

   Whether a particular VRF belongs to an MVPN  or not is determined by
   configuration.

   In this document, we will not attempt to provide support for every
   possible multicast routing protocol.  Rather, we consider multicast
   C-instances only for the following multicast routing protocols:

     - PIM Sparse Mode (PIM-SM)

     - PIM Single Source Mode (PIM-SSM)

     - PIM Bidirectional Mode (PIM-Bidir)

     - PIM Dense Mode (PIM-DM)

   As the document only support PIM-based C-instances, we will generally
   use the term "PIM C-instances" to refer to the multicast C-instances.

   A PE router may also be running a "provider-wide" instance of PIM, (a
   "PIM P-instance"), in which it has a PIM adjacency with, e.g., each
   of its IGP neighbors (i.e., with P routers), but NOT with any CE
   routers, and not with other PE routers (unless another PE router
   happens to be an IGP adjacency).  In this case, P routers would also
   run the P-instance of PIM, but NOT a C-instance.  If there is a PIM
   P-instance, it may or may not have a role to play in support of VPN
   multicast; this is discussed in later sections.  However, in no case
   will the PIM P-instance contain VPN-specific multicast routing
   information.




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   In order to help clarify when we are speaking of the PIM P-instance
   and when we are speaking of a PIM C-instance, we will also apply the
   prefixes "P-" and "C-" respectively to control messages, addresses,
   etc.  Thus a P-Join would be a PIM Join which is processed by the PIM
   P-instance, and a C-Join would be a PIM Join which is processed by a
   C-instance.  A P-group address would be a group address in the SP's
   address space, and a C-group address would be a group address in a
   VPN's address space.


3.2. P-Multicast Service Interfaces (PMSIs)

   Multicast data packets received by a PE over a PE-CE interface must
   be forwarded to one or more of the other PEs in the same MVPN for
   delivery to one or more other CEs.

   We define the notion of a "P-Multicast Service Interface" (PMSI).  If
   a particular MVPN is supported by a particular set of PE routers,
   then there will be a PMSI connecting those PE routers.  A PMSI is an
   "overlay" on the P network with the following property: a PE in a
   given MVPN can give a packet to the PMSI, and the packet will be
   delivered to some or all of the other PEs in the MVPN, such that any
   PE receiving such a packet will be able to tell which MVPN the packet
   belongs to.

   As we discuss below, a PMSI may be instantiated by a number of
   different transport mechanisms, depending on the particular
   requirements of the MVPN and of the SP.  We will refer to these
   transport mechanisms as "tunnels".

   For each MVPN, there are one or more PMSIs that are used for
   transmitting the MVPN's multicast data from one PE to others.  We
   will use the term "PMSI" such that a single PMSI belongs to a single
   MVPN.  However, the transport mechanism which is used to instantiate
   a PMSI may allow a single "tunnel" to carry the data of multiple
   PMSIs.

   In this document we are making a clear distinction between the
   multicast service (the PMSI) and its instantiation.  This allows us
   to separate the discussion of different services from the discussion
   of different instantiations of each service.  The term "tunnel" is
   used to refer only to the transport mechanism that instantiates a
   service.

   [This is a significant change from previous drafts on the topic of
   MVPN, which have used the term "Multicast Tunnel" to refer both to
   the multicast service (what we call here the PMSI) and to its
   instantiation.]



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3.2.1. Inclusive and Selective PMSIs

   We will distinguish between three different kinds of PMSI:

     - "Multidirectional Inclusive" PMSI (MI-PMSI)

       A Multidirectional Inclusive PMSI is one which enables ANY PE
       attaching to a particular MVPN to transmit a message such that it
       will be received by EVERY other PE attaching to that MVPN.

       There is at most one MI-PMSI per MVPN.  (Though the tunnel which
       instantiates an MI-PMSI may actually carry the data of more than
       one PMSI.)

       An MI-PMSI can be thought of as an overlay broadcast network
       connecting the set of PEs supporting a particular MVPN.

       [The "Default MDTs" of rosen-08 provide the transport service of
       MI-PMSIs, in this terminology.]

     - "Unidirectional Inclusive" PMSI (UI-PMSI)

       A Unidirectional Inclusive PMSI is one which enables a particular
       PE, attached to a particular MVPN, to transmit a message such
       that it will be received by all the other PEs attaching to that
       MVPN.  There is at most one UI-PMSI per PE per MVPN, though the
       "tunnel" which instantiates a UI-PMSI may in fact carry the data
       of more than one PMSI.

     - "Selective" PMSI (S-PMSI).

       A Selective PMSI is one which provides a mechanism wherein a
       particular PE in an MVPN can multicast messages so that they will
       be received by a subset of the other PEs of that MVPN.  There may
       be an arbitrary number of S-PMSIs per PE per MVPN.  Again, the
       "tunnel" which instantiates a given S-PMSI may carry data from
       multiple S-PMSIs.

       [The "Data MDTs" of earlier drafts provide the transport service
       of "Selective PMSIs" in the terminology of this draft.]

   We will see in later sections the role played by these different
   kinds of PMSI.  We will use the term "I-PMSI" when we are not
   distinguishing between "MI-PMSIs" and "UI-PMSIs".







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3.2.2. Tunnels Instantiating PMSIs

   A number of different tunneling techniques can be used to instantiate
   PMSIs.  Among these are:

     - PIM Trees.

       A PMSI can be instantiated as (a set of) Multicast Distribution
       Trees created by the PIM P-instance ("P-trees").

       PIM-SSM, PIM-Bidir, or PIM-SM can be used to create P-trees.

       A single MI-PMSI can be instantiated by a single PIM-SM shared
       tree or a PIM-Bidir tree.  Using PIM-SSM to instantiate an MI-
       PMSI requires one P-tree for each of the MI-PMSI's PEs. This P-
       tree may be shared across multiple MVPNs.

       Selective PMSIs are most naturally created with PIM-SSM, since by
       definition only one PE is the source of the multicast data on a
       Selective PMSI.

       [The "Default MDTs" of [rosen-08] are MI-PMSIs instantiated as
       PIM trees.  The "data MDTs" of [rosen-08] are S-PMSIs
       instantiated as PIM trees.]

     - RSVP-TE Point-to-Multipoint LSP.

       A PMSI may be instantiated as one or more RSVP-TE Point-to-
       Multipoint (P2MP) LSPs.  A Selective PMSI or a Unidirectional
       Inclusive PMSI would be instantiated as a single RSVP-TE P2MP
       LSP, whereas a Multidirectional Inclusive PMSI would be
       instantiated as a set of such LSPs, one for each PE in the MVPN.
       RSVP-TE P2MP LSPs can be shared across multiple MVPNs.

     - Meshed Unicast Tunnels.

       If a PMSI is implemented as a mesh of unicast tunnels, a PE
       wishing to transmit a packet through the PMSI would replicate the
       packet, and send a copy to each of the other PEs.

       An MI-PMSI for a given MVPN can be instantiated as a full mesh of
       unicast tunnels among that MVPN's PEs.  A UI-PMSI or an S-PMSI
       can be instantiated as a partial mesh.

     - Unicast Tunnels to the Root of a P-Tree.

       Any type of PMSI can be instantiated through a method in which
       there is a single P-tree (created, for example, via PIM-SSM or



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       via RSVP-TE), and a PE transmits a packet to the PMSI by sending
       it in a unicast tunnel to the root of that P-tree.  All PEs in
       the given MVPN would need to be leaves of the tree.

   It can be seen that each method of implementing PMSIs has its own
   area of applicability.  This specification therefore allows for the
   use of any of these methods.  Further discussion of the applicability
   of these methods is outside the scope of this document.


3.3. Use of PMSIs for Carrying Multicast Data

   We presuppose that each PE supporting a particular MVPN has a way of
   discovering:

     - The set of other PEs supporting that same MVPN

     - For each of the other PEs in the MVPN, whether they require a
       MI-PMSI.

     - The set of UI-PMSIs that the MVPN requires, if any; each UI-PMSI
       would be identified by its transmitting PE

     - The tunneling technique, announced by each PE, to be used for
       instantiating the I-PMSIs

     - A  "tunnel identifier", which identifies the particular tunnel or
       tunnels to be used to instantiate the PMSIs; this will be
       specific to the particular tunneling technology used;

     - Whether a PE supports Aggregation for that MVPN.

     - Any demultiplexing information which is needed in order to enable
       a specified PMSI to be carried in the specified tunnel (this is
       needed if a given tunnel is capable of carrying multiple PMSIs).

   In some cases this information is provided by means of the BGP-based
   auto-discovery procedures detailed in section 4.  In other cases,
   this information is provided after discovery is complete, by means of
   a separate set of BGP-based tunnel binding procedures.  In either
   case, the information which is provided must be sufficient to enable
   the PMSI to be bound to the identified tunnel, to enable the tunnel
   to be created if it does not already exist, and to enable the
   different PMSIs which may travel on the same tunnel to be properly
   demultiplexed.

   If an MVPN requires the use of one or more I-PMSIs, then I-PMSIs for
   that MVPN will be "created" as soon as the necessary information has



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   been discovered.  Creating a PMSI means creating the tunnel which
   carries it (unless that tunnel already exists), as well as binding
   the PMSI to the tunnel.


3.3.1. MVPNs that Use MI-PMSIs

   If an MVPN uses an MI-PMSI, then the MI-PMSI for that MVPN will be
   created as soon as the necessary information has been obtained.  The
   MI-PMSI for that MVPN is then used as the default method of
   transmitting multicast data packets for that MVPN.  In effect, all
   the multicast streams for the MVPN are, by default, aggregated onto
   the MI-MVPN.

   If a particular multicast stream from a particular source PE has
   certain characteristics, it can be desirable to migrate it from the
   MI-PMSI to an S-PMSI.  Procedures for migrating a stream from an MI-
   PMSI to an S-PMSI are discussed in later sections.



3.3.2. MVPNs That Do Not Use MI-PMSIs

   If a particular MVPN does not use MI-PMSIs, then its multicast data
   may be sent by default on a UI-PMSI.

   It is also possible to send all the multicast data on an S-PMSI,
   omitting any usage of I-PMSIs.  This prevents PEs from receiving data
   which they don't need, but requires that the transmitting PE knows
   which PEs need to receive which multicast streams.


4. BGP-Based Autodiscovery of MVPN Membership

   BGP-based autodiscovery is done by means of a new address family.
   The encoding details are to specified in a separate document.  Any PE
   which attaches to an MVPN must issue a BGP update message containing
   NLRI for this address family, as well as specific attributes.  In
   this document, we specify the information which must be contained in
   these BGP updates.

   Each such BGP update must contain the following information:

     - IPv4 address of the originating PE







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     - An RD which can be prepended to that IPv4 address to form a
       globally unique VPN-IPv4 address of the PE.

     - One or more Route Target attributes.  If any other PE has one of
       these Route Targets configured for a VRF, it treats the
       advertising PE as a member in the MVPN to which the VRF belongs.
       This allows each PE to discover the PEs that belong to a given
       MVPN.

     - An "RPF Attribute".  This is an IP address of the PE which
       originally generated the update.

     - I-PMSI attribute.  This attribute specifies:

         * Whether an MI-PMSI is to be used to support the MVPN,

         * whether the tunnel instantiating the the MVPN should be set
           up and/or joined immediately

         * whether the I-PMSI is instantiated by

             + A PIM-Bidir tree,

             + a set of PIM-SSM trees,

             + a set of PIM-SM trees (in this case the RP must be
               identified)

             + a set of RSVP-TE point-to-multipoint LSPs

             + a set of unicast tunnels

             + a set of unicast tunnels to the root of a shared tree (in
               this case the root must be identified)

         * If the PE wishes to setup a default tunnel to instantiate the
           I-PMSI, a unique identifier for the tunnel used to
           instantiate the I-PMSI.  If the tunnel is a PIM-SSM tree,
           this identifier is a group P-address.  Together with the RPF
           attribute, this provides sufficient information to enable the
           tree to be created.  If the tunnel is a set of RSVP-TE
           point-to-multipoint LSPs, this will be the tunnel identifier
           used in the RSVP-TE messages used to create the tunnel from
           the originating PE.

           Note that a default tunnel can be identified at discovery
           time only if the tunnel already exists (e.g., it was
           constructed by means of configuration), or if it can be



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           constructed without each PE knowing the the identities of all
           the others (e.g., it is constructed by a receiver-initiated
           join technique such as PIM).

           If a default tunnel is not identified at discovery time, the
           PE uses the tunnel to MVPN binding advertisements to signal
           the tunnel identifier.

         * The type of encapsulation used on the tunnel, e.g. GRE or
           MPLS.

         * Whether the tunnel used to instantiate the I-PMSI for this
           MVPN is aggregating I-PMSIs from multiple MVPNs.  This will
           affect the encapsulation used.  If aggregation is to be used,
           a demultiplexor value to be carried by packets for this
           particular MVPN must also be specified. The demultiplexing
           mechanism and signaling procedures are described in section
           6.

         * Whether multicast control information is to be sent on the
           MI-PMSI, whether it is to be unicast, or whether it is to be
           transmitted via BGP.  Procedures for transmitting multicast
           control information are specified in section 5.

             + If multicast control information is to be sent on the
               MI-PMSI, whether this PE requires to receive periodic PIM
               Hello messages or not.

             + If multicast control information is to be sent on the
               MI-PMSI, or by using unicast PIM, whether this PE can
               perform PIM Refresh Reduction procedures.

             + If multicast control information is to be unicast, rather
               than sent on an MI-PMSI, then a demultiplexor value must
               be carried by the control messages in order to identify
               the particular MVPN to which the control message belongs.
               The form of this demultiplexor is not yet agreed upon,
               but one possibility is a downstream-assigned MPLS label.
               If this procedure is adopted, this label would be carried
               in the BGP update message.  The PE originating the BGP
               update message would specify the label which it expects
               to see on received control packets for the specified
               MVPN.

   The information specified here should be sufficient to enable a PE
   attached to a given MVPN:





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     - to discover the identities of all the other PEs attached to that
       same MVPN,

     - to learn the procedure used to transmit multicast control
       messages,

     - to learn the default procedure used to transmit multicast data
       messages, and

     - if an I-PMSI is to be used, to identify any default tunnels, if
       possible.  The cases when a default tunnel can be identified are
       described above and also discussed in further detail in section
       6.

   Further details of the use of this information are provided in
   subsequent sections.



5. PE-PE Transmission of Multicast Routing

   As a PE attached to a given MVPN receives C-Join/Prune messages from
   its CEs in that MVPN, it must convey the information contained in
   those messages to other PEs that are attached to the same MVPN.

   There are several different methods for doing this. As these methods
   are not interoperable, the method to be used for a particular MVPN
   must either be configured, or discovered as part of the BGP-based
   auto-discovery process.


5.1. PIM Peering

5.1.1. Full Per-MVPN PIM Peering Across a MI-PMSI

   If the set of PEs attached to a given MVPN are connected via a MI-
   PMSI, the PEs can form "normal" PIM adjacencies with each other.
   Since the MI-PMSI functions as a broadcast network, the standard PIM
   procedures for forming and maintaining adjacencies over a LAN can be
   applied.

   As a result, the C-Join/Prune messages which a PE receives from a CE
   can be multicast to all the other PEs of the MVPN.  PIM "join
   suppression" can be enabled and the PEs can send Asserts as needed.

   [This is the procedure specified in [rosen-08].]





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5.1.2. Lightweight PIM Peering Across a MI-PMSI

   The procedure of the previous section has the following
   disadvantages:

     - Periodic Hello messages must be sent by all PEs.

       Standard PIM procedures require that each PE in a particular MVPN
       periodically multicast a Hello to all the other PEs in that MVPN.
       If the number of MVPNs becomes very large, sending and receiving
       these Hellos can become a substantial overhead for the PE
       routers.

     - Periodic retransmission of C-Join/Prune messages.

       PIM is a "soft-state" protocol, in which reliability is assured
       through frequent retransmissions (refresh) of control messages.
       This too can begin to impose a large overhead on the PE routers
       as the number of MVPNs grows.

   The first of these disadvantages is easily remedied.  The reason for
   the periodic PIM Hellos is to ensure that each PIM speaker on a LAN
   knows who all the other PIM speakers on the LAN are.  However, in the
   context of MVPN, PEs in a given MVPN can learn the identities of all
   the other PEs in the MVPN by means of a BGP-based auto-discovery
   procedure.  So in MVPN the periodic Hellos serve no function, and can
   simply be eliminated.  (Of course, this does imply a change to the
   standard PIM procedures.)

   When Hellos are suppressed, we may speak of "lightweight PIM
   peering".

   The periodic refresh of the C-Join/Prunes is not as simple to
   eliminate.  The L3VPN WG has asked the PIM WG to specify "refresh
   reduction" procedures for PIM, so as to eliminate the need for the
   periodic refreshes.  If and when such procedures have been specified,
   it will be very useful to incorporate them, so as to make the
   lightweight PIM peering procedures even more lightweight.


5.1.3. Unicasting of PIM C-Join/Prune Messages

   PIM does not require that the C-Join/Prune messages which a PE
   receives from a CE to be multicast to all the other PEs; it allows
   them to be unicast to a single PE, the one which is upstream on the
   path to the root of the multicast tree mentioned in the Join/Prune
   message. Note that when the C-Join/Prune messages are unicast, there
   is no such thing as "join suppression".  Therefore PIM Refresh



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   Reduction may be considered to be a pre-requisite for the procedure
   of unicasting the C-Join/Prune messages.



   When the C-Join/Prunes are unicast, they are not transmitted on a
   PMSI at all.  Note that the procedure of unicasting the C-Join/Prunes
   is different than the procedure of transmitting the C-Join/Prunes on
   an MI-PMSI which is instantiated as a mesh of unicast tunnels.

   If there are multiple PEs that can be used to reach a given C-source,
   all PEs in the MVPN must send the C-Join/Prune for the given C-Source
   to the same PE. This is achieved using the following procedure:

   If the next-hop interface on a PE's route to C-S, that is installed
   in the VRF, is a VRF interface than the PE should use that route to
   reach C-S. Else: from all the VPN-IPv4 routes that could be imported
   into the VRF and have exactly the same IPv4 prefix as the route in
   the VRF the PE uses the one with the highest next-hop address to
   determine the upstream PE to send C-Joins/C-Prunes for C-S.


5.1.4. Details of Per-MVPN PIM Peering over MI-PMSI

   When an MVPN uses an MI-PMSI, the C-instances of that MVPN can treat
   the MI-PMSI as a LAN interface, and form either full PIM adjacencies
   or lightweight PIM adjacencies with each other over that "LAN
   interface".

   To form a full PIM adjacency, the PEs execute the PIM LAN procedures,
   including the generation and processing of PIM Hello, Join/Prune,
   Assert, DF election and other PIM control packets.  These are
   executed independently for each C-instance.  PIM "join suppression"
   MAY be enabled.

   If it is known that all C-instances of a particular MVPN can support
   lightweight adjacencies, then lightweight adjacencies MUST be used.
   If it is not known that all such C-instances support lightweight
   instances, then full adjacencies MUST be used.  Whether all the C-
   instances support lightweight adjacencies is known by virtue of the
   BGP-based auto-discovery procedures (combined with configuration).
   This knowledge might change over time, so the PEs must be able to
   switch in real time between the use of full adjacencies and
   lightweight adjacencies.

   The difference between a lightweight adjacency and a full adjacency
   is that no PIM Hellos are sent or received on a lightweight
   adjacency.  The function which Hellos usually provide in PIM can be



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   provided in MVPN by the BGP-based auto-discovery procedures, so the
   Hellos become superfluous.

   Whether or not Hellos are sent, if PIM Refresh Reduction procedures
   are available, and all the PEs supporting the  MVPN are known to
   support these procedures, then the refresh reduction procedures MUST
   be used.


5.1.4.1. PIM C-Instance Control Packets

   All PIM C-Instance control packets of a particular MVPN are addressed
   to the ALL-PIM-ROUTERS (224.0.0.13) IP destination address, and
   transmitted over the MI-PMSI of that MVPN.  While in transit in the
   P-network, the packets are encapsulated as required for the
   particular kind of tunnel that is being used to instantiate the MI-
   PMSI.  Thus the C-instance control packets are not processed by the P
   routers, and MVPN-specific PIM routes can be extended from site to
   site without appearing in the P routers.


5.1.4.2. PIM C-instance RPF Determination

   Although the MI-PMSI is treated by PIM as a LAN interface, unicast
   routing is NOT run over it, and there are no unicast routing
   adjacencies over it.  It is therefore necessary to specify special
   procedures for determining when the MI-PMSI is to be regarded as the
   "RPF Interface" for a particular C-address.

   When a PE needs to determine the RPF interface of a particular C-
   address, it looks up the C-address in the VRF. If the route matching
   it (call this the "RPF route") is not a VPN-IP route learned from
   MP-BGP as described in [RFC2547bis], or if that route's outgoing
   interface is one of the interfaces associated with the VRF, then
   ordinary PIM procedures for determining the RPF interface apply.

   However, if the RPF route is a VPN-IP route whose outgoing interface
   is not one of the interfaces associated with the VRF, then PIM will
   consider the outgoing interface to be the MI-PMSI associated with the
   VPN-specific PIM instance.

   Once PIM has determined that the RPF interface for a particular C-
   address is the MI-PMSI, it is necessary for PIM to determine the RPF
   neighbor for that C-address.  This will be one of the other PEs that
   is a PIM adjacency over the MI-PMSI.

   When a PE distributes a given VPN-IP route via BGP, the PE must
   determine whether that route might possibly be regarded, by another



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   PE, as an RPF route. (If a given VRF is part of an MVPN, it may be
   simplest to regard every route exported from that VRF to be a
   potential RPF route.)  If the given VPN-IP route is a potential RPF
   route, then when the VPN-IP route is distributed by BGP, it SHOULD be
   accompanied by an "RPF attribute".

   The value of the RPF attribute is an IP address which the PE will use
   as its Source IP address in any PIM control messages which it
   transmits to other PEs in the same MVPN.

   When a PE has determined that the RPF interface for a particular C-
   address is the MI-PMSI, it must look up the RPF attribute that was
   distributed along with the VPN-IPv4 address corresponding to that C-
   address.  The value of this RPF attribute will be considered to be
   the IP address of the RPF adjacency for the C-address.

   If the RPF attribute is not present, but the "BGP Next Hop" for the
   C-address is one of the PEs that is a PIM adjacency over the MI-PMSI,
   then that PE should be treated as the RPF adjacency for that C-
   address.  However, if the MVPN spans multiple Autonomous Systems, the
   BGP Next Hop might not be a PIM adjacency, and if that is the case
   the RPF check will not succeed unless the RPF attribute is used.


5.2. Use of BGP for Carrying PE-PE Multicast Routing

   It may be possible to use BGP to carry multicast routing information
   from PE to PE, dispensing entirely with the transmission of C-
   Join/Prune messages from PE to PE.

   A new SAFI is used for this purpose. The following information is
   required in BGP to advertise the MVPN routing information:

      1. The RD configured, for the VPN, on the PE that is advertising
         the information.  This is required to uniquely identify the
         <C-Source, C-Group> as the addresses could overlap between
         different MVPNs.

      2. The address of the advertising PE. This is the same address
         that is used by the PE in the BGP MVPN auto-discovery
         procedures.

      3. The C-Source address. This can be a prefix.

      4. The C-Group address. This can be a prefix.






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      5. The upstream PE for which the message is intended. This address
         is either taken from the RPF attribute of the route matching
         the C-Source, or, if the RPF attribute is not present, is the
         BGP next-hop on the path to the C-Source. If the C-Source is a
         wildcard (e.g. (*, G) information), the upstream PE is the BGP
         next-hop on the path to C-RP. When a PE receives a C-Join (S,G)
         from a CE, the PE checks its VRF to find the address of the
         (upstream) PE that is used to reach the C-Source (we call this
         Ingress-PE address). A procedure must be specified to ensure
         that if there is more than one PE through which S could be
         reached, all PEs in the MVPN will send the C-Join/Prunes for
         the given C-source to the same PE. This is achieved using the
         following procedure:

         If the next-hop interface on a PE's route to C-S, that is
         installed in the VRF, is a VRF interface than the PE should use
         that route to reach C-S. Else: from all the VPN-IPv4 routes
         that could be imported into the VRF and have exactly the same
         IPv4 prefix as the route in the VRF the PE uses the one with
         the highest next-hop address to determine the upstream PE to
         send C-Joins/C-Prunes for C-S.


   When a PE distributes this information via BGP, it must include a
   Route Target Extended Communities attribute. This RT must be an
   "Import RT" [RFC2547bis] of each VRF in the MVPN. The BGP
   distribution procedures used by [RFC2547bis] will then ensure that
   this information gets associated with the right VRFs. A PE will
   process this information only if the upstream PE address carried in
   the advertisement is the PE's own address, or if the PE is configured
   to be an anycast C-RP (section 9).

   The use of BGP to propagate MVPN routing information allows the use
   of Route Reflectors, and has the same characteristics, with respect
   to the routing adjacencies maintained, as the use of BGP for
   distributing unicast VPN routing information as in [RFC2547bis].


6. I-PMSI Instantiation

   This section describes how tunnels in the SP network can be used to
   instantiate an I-PMSI for an MVPN on a PE.   When C-multicast data is
   delivered on an I-PMSI, the data will go to all PEs that are on the
   path to receivers for that C-group, but may also go to PEs that are
   not on the path to receivers for that C-group.

   The tunnels which instantiate I-PMSIs can be either PE-PE unicast
   tunnels or P-multicast trees. When PE-PE unicast tunnels are used the



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   PMSI is said to be instantiated using ingress replication.

   [Editor's Note: MD trees described in [ROSEN-8, MVPN-BASE] are an
   example of P-multicast trees. Also Aggregate Trees described in
   [RAGGARWA-MCAST] are an example of P-multicast trees.]


6.1. MVPN Membership and Egress PE Auto-Discovery

   As described in section 4 a PE discovers the MVPN membership
   information of other PEs using BGP auto-discovery mechanisms or using
   a mechanism that instantiates a MI-PMSI interface. When a PE supports
   only a UI-PMSI service for an MVPN, it MUST rely on the BGP auto-
   discovery mechanisms for discovering this information. This
   information also results in a PE discovering the leaves of the P-
   multicast tree, which are the egress PEs that have members in one or
   more MVPNs mapped onto the tree.


6.1.1. Auto-Discovery for Ingress Replication

   In order for a PE to use Unicast Tunnels to send a C-multicast data
   packet for a particular MVPN to a set of remote PEs, the remote PEs
   must be able to correctly decapsulate such packets and to assign each
   one to the proper MVPN. This requires that the encapsulation used for
   sending packets through the tunnel have demultiplexing information
   which the receiver can associate with a particular MVPN.

   Thus an ingress PE must not transmit C-multicast data through a
   particular kind of unicast tunnel to a particular remote PE unless it
   knows that the remote PE has supports that particular tunnel
   encapsulation, and unless there is some agreement between the two PEs
   as to what values of the demultiplexing information represent which
   MVPNs.  If a PE is capable of ingress replication for an MVPN, it
   announces this as part of the BGP based MVPN membership auto-
   discovery process, described in section 4. The following information
   elements need to be advertised by a PE:

      1. The ability to support ingress replication for a particular
         MVPN, using a particular encapsulation format (e.g., MPLS,
         MPLS-in-GRE).

      2. A mapping of demultiplexing values to MVPNs.

         If the encapsulation format is MPLS or MPLS-in-something, the
         demultiplexing values may be downstream-assigned MPLS labels.
         The encapsulation procedures are described further in section
         11.



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         Other demultiplexing procedures for unicast are under
         consideration.


6.1.2. Auto-Discovery for P-Multicast Trees

   A PE announces the P-multicast technology it supports for a specified
   MVPN, as part of the BGP MVPN membership discovery. This allows other
   PEs to determine the P-multicast technology they can use for building
   P-multicast trees to instantiate an I-PMSI. If a PE has a default
   tree instantiation of an I-PMSI, it also announces the tree
   identifier as part of the auto-discovery, as well as announcing its
   aggregation capability.

   The announcement of a tree identifier at discovery time is only
   possible if the tree already exists (e.g., a preconfigured "traffic
   engineered" tunnel which is know to include all the necessary PEs),
   or if the tree can be constructed dynamically without any PE having
   to know in advance all the other PEs on the tree (e.g., PIM-created
   trees).


6.2. MVPN Routing Information Exchange

   When a PE doesn't support the use of a MI-PMSI for a given MVPN, it
   MUST either unicast MVPN routing information using PIM or else use
   BGP for exchanging the MVPN routing information. This is because
   there may be no MI-PMSI available for it to exchange MVPN routing
   information.


6.3. Aggregation

   A P-multicast tree can be used to instantiate a PMSI service for only
   one MVPN or for more than one MVPN. When a P-multicast tree is shared
   across multiple MVPNs it is termed an Aggregate Tree [RAGGARWA-
   MCAST]. The procedures described in this document allow a single SP
   multicast tree to be shared across multiple MVPNs. The procedures
   that are specific to aggregation are optional and are explicitly
   pointed out. Unless otherwise specified a P-multicast tree technology
   supports aggregation.

   Aggregate Trees allow a single P-multicast tree to be used across
   multiple MVPNs and hence state in the SP core grows per-set-of-MVPNs
   and not per MVPN.  Depending on the congruency of the aggregated
   MVPNs, this may result in trading off optimality of multicast
   routing.




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   An Aggregate Tree can be used by a PE to provide an UI-PMSI or MI-
   PMSI service for more than one MVPN. When this is the case the
   Aggregate Tree is said to have an inclusive mapping.


6.3.1. Aggregate Tree Leaf Discovery

   BGP MVPN membership discovery allows a PE to determine the different
   Aggregate Trees that it should create and the MVPNs that should be
   mapped onto each such tree. The leaves of an Aggregate Tree are
   determined by the PEs, supporting aggregation, that belong to all the
   MVPNs that are mapped onto the tree.


6.3.2. Aggregation Methodology

   This document does not specify any rules for determining whether or
   not the PMSIs of two particular MVPNs are to be instantiated by the
   same Aggregate Tree.  This determination can be made by
   implementation-specific heuristics, by configuration, or even perhaps
   by the use of offline tools.

   It is the intention of this document that the control procedures will
   always result in all the PEs of an MVPN to agree on the PMSIs which
   are to be used and on the tunnels used to instantiate those PMSIs.

   This section discusses potential methodologies with respect to
   aggregation.

   The "congruency" of aggregation is defined by the amount of overlap
   in the leaves of the customer trees that are aggregated on a SP tree.
   For Aggregate Trees with an inclusive mapping the congruency depends
   on the overlap in the membership of the MVPNs that are aggregated on
   the tree. If there is complete overlap i.e. all MVPNs have exactly
   the same sites, aggregation is perfectly congruent. As the overlap
   between the MVPNs that are aggregated reduces, i.e. the number of
   sites that are common across all the MVPNs reduces, the congruency
   reduces.

   If aggregation is done such that it is not perfectly congruent a PE
   may receive traffic for MVPNs to which it doesn't belong. As the
   amount of multicast traffic in these unwanted MVPNs increases
   aggregation becomes less optimal with respect to delivered traffic.
   Hence there is a tradeoff between reducing state and delivering
   unwanted traffic.

   An implementation should provide knobs to control the congruency of
   aggregation. These knobs are implementation dependent. Configuring



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   the percentage of sites that MVPNs must have in common to be
   aggregated, is an example of such a knob. This will allow a SP to
   deploy aggregation depending on the MVPN membership and traffic
   profiles in its network.  If different PEs or servers are setting up
   Aggregate Trees this will also allow a service provider to engineer
   the maximum amount of unwanted MVPNs hat a particular PE may receive
   traffic for.


6.3.3. Encapsulation of the Aggregate Tree

   An Aggregate Tree may use an IP/GRE encapsulation or a MPLS
   encapsulation. The protocol type in the IP/GRE header in the former
   case and the protocol type in the data link header in the latter need
   further explanation. This will be specified in a separate document.


6.3.4. Demultiplexing C-multicast traffic

   When multiple MVPNs are aggregated onto one P-Multicast tree,
   determining the tree over which the packet is received is not
   sufficient to determine the MVPN to which the packet belongs.  The
   packet must also carry some demultiplexing information to allow the
   egress PEs to determine the MVPN to which the packet belongs.  Since
   the packet has been multicast through the P network, any given
   demultiplexing value must have the same meaning to all the egress
   PEs.  The demultiplexing value is a MPLS label that corresponds to
   the multicast VRF to which the packet belongs. This label is placed
   by the ingress PE immediately beneath the P-Multicast tree header.
   Each of the egress PEs must be able to associate this MPLS label with
   the same MVPN.  If downstream label assignment were used this would
   require all the egress PEs in the MVPN to agree on a common label for
   the MVPN. Instead the MPLS label is upstream assigned [MPLS-
   UPSTREAM-LABEL]. The label bindings are advertised via BGP updates
   originated the ingress PEs.

   This procedure requires each egress PE to support a separate label
   space for every other PE. The egress PEs create a forwarding entry
   for the upstream assigned MPLS label, allocated by the ingress PE, in
   this label space. Hence when the egress PE receives a packet over an
   Aggregate Tree, it first determines the tree that the packet was
   received over. The tree identifier determines the label space in
   which the upstream assigned MPLS label lookup has to be performed.
   The same label space may be used for all P-multicast trees rooted at
   the same ingress PE, or an implementation may decide to use a
   separate label space for every P-multicast tree.

   The encapsulation format is either MPLS or MPLS-in-something (e.g.



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   MPLS-in-GRE). When MPLS is used, this label will appear immediately
   below the label that identifies the P-multicast tree.  When MPLS-in-
   GRE is used, this label will be the top MPLS label that appears when
   the GRE header is stripped off.

   When IP encapsulation is used for the P-multicast Tree, whatever
   information that particular encapsulation format uses for identifying
   a particular tunnel is used to determine the label space in which the
   MPLS label is looked up.

   If the P-multicast tree uses MPLS encapsulation, the P-multicast tree
   is itself identified by an MPLS label.  The egress PE MUST NOT
   advertise IMPLICIT NULL or EXPLICIT NULL for that tree.  Once the
   label representing the tree is popped off the MPLS label stack, the
   next label is the demultiplexing information that allows the proper
   MVPN to be determined.

   This specification requires that, to support this sort of
   aggregation, there be at least one upstream-assigned label per MVPN.
   It does not require that there be only one.  For example, an ingress
   PE could assign a unique label to each C-(S,G).  (This could be done
   using the same technique this is used to assign a particular C-(S,G)
   to an S-PMSI.)


6.4. Mapping Received Packets to MVPNs

   When an egress PE receives a C-multicast data packet over a P-
   multicast tree, it needs to forward the packet to the CEs that have
   receivers in the packet's C-multicast group. It also needs to
   determine the RPF interface for the C-multicast data packet. In order
   to do this the egress PE needs to determine the tunnel that the
   packet was received on. The PE can then determine the MVPN that the
   packet belongs to and if needed do any further lookups that are
   needed to forward the packet.


6.4.1. Unicast Tunnels

   When ingress replication is used, the MVPN to which the received C-
   multicast data packet belongs can be determined by the MPLS label
   that was allocated by the egress. This label is distributed by the
   egress.  This also determines the RPF interface for the C-multicast
   data packet.







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6.4.2. Non-Aggregated P-Multicast Trees

   If a P-multicast tree is associated with only one MVPN, determining
   the P-multicast tree on which a packet was received is sufficient to
   determine the packet's MVPN. All that the egress PE needs to know is
   the MVPN the P-multicast tree is associated with.

   There are different ways in which the egress PE can learn this
   association:

      a) Configuration. The P-multicast tree that a particular MVPN
         belongs to is configured on each PE.

         [Editor's Note: PIM-SM Default MD trees in [ROSEN-8] and
         [MVPN-BASE] are examples of configuring the P-multicast tree
         and MVPN association]

      b) BGP based advertisement of the P-multicast tree - MPVN mapping
         after the root of the tree discovers the leaves of the tree.
         The root of the tree sets up the tree after discovering each of
         the PEs that belong to the MVPN.  It then advertises the P-
         multicast tree - MVPN mapping to each of the leaves.  This
         mechanism can be used with both source initiated trees [e.g.
         RSVP-TE P2MP LSPs] and receiver initiated trees [e.g. PIM
         trees].

         [Editor's Note: Aggregate tree advertisements in [RAGGARWA-
         MCAST] are examples of this.]

      c) BGP based advertisment of the P-multicast tree - MVPN mapping
         as part of the MVPN membership discovery. The root of the tree
         advertises, to each of the other PEs that belong to the MVPN,
         the P-multicast tree that the MVPN is associated with. This
         implies that the root doesn't need to know the leaves of the
         tree beforehand. This is possible only for receiver initiated
         trees e.g. PIM based trees.

         [Editor's Note: PIM-SSM discovery in [ROSEN-8] is an example of
         the above]

   Both of the above require the BGP based advertisement to contain the
   P-multicast tree identifier. This identifier is encoded as a BGP
   attribute and contains the following elements:

     - Tunnel Type.






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     - Tunnel identifier. The semantics of the identifier is determined
       by the tunnel type.



6.4.3. Aggregate P-Multicast Trees

   Once a PE sets up an Aggregate Tree it needs to announce the C-
   multicast groups being mapped to this tree to other PEs in the
   network. This procedure is referred to as Aggregate Tree discovery.
   For an Aggregate Tree with an inclusive mapping this discovery
   implies announcing:

     - The mapping of all MVPNs mapped to the Tree.

     - For each MVPN mapped onto the tree the inner label allocated for
       it by the ingress PE. The use of this label is explained in the
       demultiplexing procedures of section 6.3.4.

     - The P-multicast tree Identifier

   The egress PE creates a logical interface corresponding to the tree
   identifier. This interface is the RPF interface for all the <C-
   Source, C-Group> entries mapped to that tree.

   When PIM is used to setup P-multicast trees, the egress PE also Joins
   the P-Group Address corresponding to the tree. This results in setup
   of the PIM P-multicast tree.


6.5. I-PMSI Instantiation Using Ingress Replication

   As described in section 3 a PMSI can be instantiated using Unicast
   Tunnels between the PEs that are participating in the MVPN. In this
   mechanism the ingress PE replicates a C-multicast data packet
   belonging to a particular MVPN and sends a copy to all or a subset of
   the PEs that belong to the MVPN. A copy of the packet is tunnelled to
   a remote PE over an Unicast Tunnel to the remote PE. IP/GRE Tunnels
   or MPLS LSPs are examples of unicast tunnels that may be used. Note
   that the same Unicast Tunnel can be used to transport packets
   belonging to different MVPNs.

   Ingress replication can be used to instantiate a UI-PMSI.. The PE
   sets up unicast tunnels to each of the remote PEs that support
   ingress replication. For a given MVPN all C-multicast data packets
   are sent to each of the remote PEs in the MVPN that support ingress
   replication. Hence a remote PE may receive C-multicast data packets
   for a group even if it doesn't have any receivers in that group.



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   Ingress replication can also be used to instantiate a MI-PMSI. In
   this case each PE has a mesh of unicast tunnels to every other PE in
   that MVPN.

   However when ingress replication is used it is recommended that only
   S-PMSIs be used. Instantiation of S-PMSIs with ingress replication is
   described in section 7.2.


6.6. Establishing P-Multicast Trees

   The architecture outlined in this document places no limitations on
   the protocols used to instantiate P-multicast trees. However any
   protocol specific procedures, are described only for PIM-SM, PIM-SSM,
   PIM-Bidir and RSVP-TE P2MP LSPs.

   A P-multicast tree can be either a source tree or a shared tree. A
   source tree is used to carry traffic only for the multicast VRFs that
   exist locally on the root of the tree i.e. for which the root has
   local CEs. The root is a PE router. Source P-multicast trees can be
   instantiated using PIM-SM, PIM-SSM and RSVP-TE P2MP LSPs.

   A shared tree on the other hand can be used to carry traffic
   belonging to VRFs that exist on other PEs as well. The root of a
   shared tree is not necessarily one of the PEs in the MVPN. All PEs
   that use the shared tree will send MVPN data or control packets to
   the root of the shared tree. This may require an unicast tunnel
   between each of these PEs and the root. The root will then send them
   on the shared tree and all the PEs that are leaves of the shared tree
   will receive the packets. For example a RP based PIM-SM tree would be
   a shared tree. Shared trees can be instantiated using PIM-SM, PIM-
   SSM, PIM-Bidir and RSVP-TE P2MP LSPs. Aggregation support for PIM-
   Bidir based P-multicast trees is for further study. Shared trees
   require all the PEs to discover the root of the shared tree for a
   MVPN. To achieve this the root of a shared tree advertises as part of
   the BGP based MVPN membership discovery:

     - The capability to setup a shared tree for a specified MVPN.

     - A downstream assigned label that is to be used by each PE to
       encapsulate a MVPN data packet, when they send this packet to the
       root of the shared tree.

     - A downstream assigned label that is to be used by each PE to
       encapsulate a MVPN control packet, when they send this packet to
       the root of the shared tree.





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   Both a source tree and a shared tree can be used to instantiate an
   I-PMSI.  If a source tree is used to instantiate an UI-PMSI for a
   MVPN, all the other PEs that belong to the MVPN, must be leaves of
   the source tree. If a shared tree is used to instantiate a UI-PMSI
   for a MVPN, all the PEs that are members of the MVPN must be leaves
   of the shared tree.


6.7. RSVP-TE P2MP LSPs

   This section describes procedures that are specific to the usage of
   RSVP-TE P2MP LSPs for instantiating a UI-PMSI. The RSVP-TE P2MP LSP
   can be either a source tree or a shared tree. Procedures in [RSVP-
   TE-P2MP] are used to signal the LSP. The LSP is signalled after the
   root of the LSP discovers the leaves. The egress PEs are discovered
   using the MVPN membership procedures described in section 4. RSVP-TE
   P2MP LSPs can optionally support aggregation.


6.7.1. P2MP TE LSP Tunnel - MVPN Mapping

   P2MP TE LSP Tunnel to MVPN mapping can be learned at the egress PEs
   using either option (a) or option (b) described in section 6.4.2.
   Option (b) i.e. BGP based advertisements of the P2MP TE LSP Tunnel -
   MPVN mapping require that the root of the tree include the P2MP TE
   LSP Tunnel identifier as the tunnel identifier in the BGP
   advertisements. This identifier contains the following information
   elements:
     - The type of the tunnel is set to RSVP-TE P2MP Tunnel

     - RSVP-TE P2MP Tunnel's SESSION Object

     - Optionally RSVP-TE P2MP LSP's SENDER_TEMPLATE Object. This object
       is included when it is desired to identify a particular P2MP TE
       LSP.


6.7.2. Demultiplexing C-Multicast Data Packets

   Demultiplexing the C-multicast data packets at the egress PE follow
   procedures described in section 6.3.4. The RSVP-TE P2MP LSP Tunnel
   must be signaled with penultimate-hop-popping (PHP) off. Signalling
   the P2MP TE LSP Tunnel with PHP off requires an extension to RSVP-TE
   which will be described later.







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7. Optimizing Multicast Distribution via S-PMSIs

   Whenever a particular multicast stream is being sent on an I-PMSI, it
   is likely that the data of that stream is being sent to PEs that do
   not require it.  If a particular stream has a significant amount of
   traffic,  it may be beneficial to move it to an S-PMSI which includes
   only those PEs that are transmitters and/or receivers (or at least
   includes fewer PEs that are neither).

   S-PMSI creation can be triggered on other criteria than mere
   bandwidth once Join suppression is disabled. For instance there could
   be a "pseudo wasted bandwidth" criteria : switching to a S-PMSI would
   be done if the bandwidth multiplied by the number of uninterested PEs
   (PE that are receiving the stream but have no receivers) is above a
   specified threshold. The motivation is that many sparsely subscribed
   low-bandwidth groups may waste much bandwidth, and using an S-PMSI
   for a high bandwidth multicast stream for which all PEs have
   receivers is of no use either

   Switching to a S-PMSI requires the root of the S-PMSI to a) Discover
   the egress PEs that will receive traffic using the S-PMSI b) Setup
   the S-PMSI and c) If required, signal the binding of the multicast
   stream to the S-PMSI to the leaves of the tunnel used to instantiate
   the S-PMSI.

   Step (c) is required only when the tunnel is a P-multicast tree and
   we specify two methods for achieving this.


7.1. Egress PE Discovery

   S-PMSI instantiation using a tunnel may imply using an existing
   tunnel or creating a new tunnel. Depending on the type of tunnel used
   the PE may or may not need to know the PEs that have receivers in the
   C-(S, G) bound to the S-PMSI. If the PE needs to know the egress PEs
   before instantiating the S-PMSI, it MUST receive C-Joins from each of
   the egress PEs. To achieve this either unicast PIM or BGP or MI-PMSI
   with Join suppression disabled MUST be used to is used to transmit
   C-Joins. There are two cases in which the PE needs to know the egress
   PEs:

     - If the tunnel is a source initiated tree, such as a RSVP-TE P2MP
       Tunnel, the PE needs to know the leaves of the tree before it can
       instantiate the S-PMSI.







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     - If a PE instantiates multiple S-PMSIs, belonging to different
       MVPNs, using one P-multicast tree, such a tree is termed an
       Aggregate Tree with a selective mapping. The setting up of such
       an Aggregate Tree requires the ingress PE to know all the other
       PEs that have receivers for multicast groups that are mapped onto
       the tree. This is learned from the C-Joins received by the
       ingress PE. Hence the leaves of the Aggregate Tree are discovered
       using C-Joins.


7.2. S-PMSI Instantiation Using Ingress Replication

   As described in section 6.1.1, ingress replication can be used to
   instantiate a UI-PMSI. However this can result in a PE receiving
   packets for a multicast group for which it doesn't have any
   receivers. This can be avoided if the ingress PE tracks the remote
   PEs which have receivers in a particular C-multicast group.  In order
   to do this it needs to receive C-Joins from each of the remote PEs.
   It then replicates the C-multicast data packet and sends it to only
   those egress PEs which are on the path to a receiver of that C-group.
   It is possible that each PE that is using ingress replication
   instantiates only S-PMSIs. It is also possible that some PEs
   instantiate UI-PMSIs while others instantiate only S-PMSIs. In both
   these cases the PE MUST either unicast MVPN routing information using
   PIM or it should use BGP for exchanging the MVPN routing information.
   This is because there may be no MI-PMSI available for it to exchange
   MVPN routing information.

   Note that the use of ingress replication doesn't require any extra
   procedures for signaling the binding of the S-PMSI from the ingress
   PE to the egress PEs.  The procedures described for I-PMSIs are
   sufficient.


7.3. Protocol for Switching to S-PMSIs

   We describe two protocols for switching to S-PMSIs.  These protocols
   can be used when the tunnel that instantiates the S-PMSI is a P-
   multicast tree.


7.3.1. A UDP-based Protocol for Switching to S-PMSIs

   This procedure can be used for any MVPN which has an MI-PMSI.
   Traffic from all multicast streams in a given MPVN is sent, by
   default, on the MI-PMSI.  Consider a single multicast stream within a
   given MVPN, and consider a PE which is attached to a source of
   multicast traffic for that stream.  The PE can be configured to move



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   the stream from the MI-PMSI to an S-PMSI if certain configurable
   conditions are met.  To do this, it needs to inform all the PEs which
   attach to receivers for stream.  These PEs need to start listening
   for traffic on the S-PMSI, and the transmitting PE may start sending
   traffic on the S-PMSI when it is reasonably certain that all
   receiving PEs are listening on the S-PMSI.


7.3.1.1. Binding a Stream to an S-PMSI

   When a PE which attaches to a transmitter for a particular multicast
   stream notices that the conditions for moving the stream to an S-PMSI
   are met, it begins to periodically send an "S-PMSI Join Message" on
   the MI-PMSI.  The S-PMSI Join is a UDP-encapsulated message whose
   destination address is ALL-PIM-ROUTERS (224.0.0.13), and whose
   destination port is 3232.

   The S-PMSI Join Message contains the following information:

     - An identifier for the particular multicast stream which is to be
       bound to the S-PMSI.   This can be represented as an (S,G) pair.

     - An identifier for the particular S-PMSI to which the stream is to
       be bound.  This identifier is a structured field which includes
       the following information:

         * The type of tunnel used to instantiate the S-PMSI

         * An identifier for the tunnel.  The form of the identifier
           will depend upon the tunnel type.  The combination of tunnel
           identifier and tunnel type should contain enough information
           to enable all the PEs to "join" the tunnel and receive
           messages from it.

         * Any demultiplexing information needed by the tunnel
           encapsulation protocol to identify the particular S-PMSI.
           This allows a single tunnel to aggregate multiple S-PMSIs.
           If a particular tunnel is not aggregating multiple S-PMSIs,
           then no demultiplexing information is needed.

   A PE router which is not connected to a receiver will still receive
   the S-PMSI Joins, and MAY cache the information contained therein.
   Then if the PE later finds that it is attached to a receiver, it can
   immediately start listening to the S-PMSI.

   Upon receiving the S-PMSI Join, PE routers connected to receivers for
   the specified stream will take whatever action is necessary to start
   receiving multicast data packets on the S-PMSI.  The precise action



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   taken will depend upon the tunnel type.

   After a configurable delay, the PE router which is sending the S-PMSI
   Joins will start transmitting the stream's data packets on the S-
   PMSI.

   When the pre-configured conditions are no longer met for a particular
   stream, e.g. the traffic stops, the PE router connected to the source
   stops announcing S-PMSI Joins for that stream.  Any PE that does not
   receive, over a configurable interval, an S-PMSI Join for a
   particular stream will stop listening to the S-PMSI.


7.3.1.2. Packet Formats and Constants

   To be included.


7.3.2. A BGP-based Protocol for Switching to S-PMSIs

   This procedure can be used for a MVPN that is using either a UI-PMSI
   or a MI-PMSI. Consider a single multicast stream for a C-(S, G)
   within a given MVPN, and consider a PE which is attached to a source
   of multicast traffic for that stream. The PE can be configured to
   move the stream from the MI-PMSI or UI-PMSI to an S-PMSI if certain
   configurable conditions are met. Once a PE decides to move the C-(S,
   G) for a given MVPN to a S-PMSI, it needs to instantiate the S-PMSI
   using a tunnel and announce to all the egress PEs, that are on the
   path to receivers of the C-(S, G), of the binding of the S-PMSI to
   the C-(S, G). The announcement is done using BGP.  Depending on the
   tunneling technology used, this announcement may be done before or
   after setting up the tunnel. The source and egress PEs have to switch
   to using the S-PMSI for the C-(S, G).


7.3.2.1. Advertising C-(S, G) Binding to a S-PMSI using BGP

   The ingress PE informs all the PEs that are on the path to receivers
   of the C-(S, G) of the binding of the S-PMSI to the C-(S, G). The BGP
   announcement is done using an update with a new <AFI, SAFI>. The
   update contains the following:

      a) The originating PE's address.

      b) The RD configured locally for the MVPN. This is required to
         uniquely identify the <C-Source, C-Group> as the addresses
         could overlap between different MVPNs




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      c) If the S-PMSI is instantiated using an Aggregate Tree with a
         selective mapping, an inner label allocated by the Aggregate
         Tree root for the <C-Source, C-Group>. This allows a single
         tunnel to aggregate multiple S-PMSIs. This is the upstream
         label the usage of which is described in section 6.3.4.

      d) The C-Source address. This address can be a prefix in order to
         allow a range of C-Source addresses to be mapped to an
         Aggregate Tree.

      e) The C-Group address. This address can be a range in order to
         allow a range of C-Group addresses to be mapped to an Aggregate
         Tree.

   When a PE distributes this information via BGP, it must include the
   following:

      1. An identifier for the particular S-PMSI to which the stream is
         to be bound.  This identifier is a structured field which
         includes the following information:

           * The type of tunnel used to instantiate the S-PMSI

           * An identifier for the tunnel.  The form of the identifier
             will depend upon the tunnel type.  The combination of
             tunnel identifier and tunnel type should contain enough
             information to enable all the PEs to "join" the tunnel and
             receive messages from it.


      2. Route Target Extended Communities attribute. This is used as
         described in section 4.


7.3.2.2. Switching to S-PMSI

   After the egress PEs receive the announcement they setup their
   forwarding path to receive traffic on the S-PMSI if they have one or
   more receivers interested in the <C-S, C-G> bound to the S-PMSI. This
   involves changing the RPF interface for the relevant <C-S, C-G>
   entries to the interface that is used to instantiate the S-PMSI. If
   an Aggregate Tree is used to instantiate a S-PMSI this also implies
   setting up the demultiplexing forwarding entries based on the inner
   label as described in section 6.3.4.  The egress PEs may perform the
   switch to the S-PMSI once the advertisement from the ingress PE is
   received or wait for a preconfigured timer to do so.

   A source PE may use one of two approaches to decide when to start



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   transmitting data on the S-PMSI. In the first approach once the
   source PE instantiates the S-PMSI, it starts sending multicast
   packets for <C-S, C-G> entries mapped to the S-PMSI on both that as
   well as on the I-PMSI, which is currently used to send traffic for
   the <C-S, C-G>. After some preconfigured timer the PE stops sending
   multicast packets for <C-S, C-G> on the I-PMSI. In the second
   approach after a certain pre-configured delay after advertising the
   <C-S, C-G> entry bound to a S-PMSI,  the source PE begins to send
   traffic on the S-PMSI. At this point it stops to send traffic for the
   <C-S, C-G> on the I-PMSI. This traffic is instead transmitted on the
   S-PMSI.


7.4. Aggregation

   S-PMSIs can be aggregated on a P-multicast tree. The S-PMSI to C-(S,
   G) binding advertisement supports aggregation. Furthermore the
   aggregation procedures of section 6.3 apply. It is also possible to
   aggregate both S-PMSIs and I-PMSIs on the same P-multicast tree.


7.5. Instantiating the S-PMSI with a PIM Tree

   The procedures of section 7.3 tell a PE when it must start listening
   and stop listening to a particular S-PMSI.  Those procedures also
   specify the method for instantiating the S-PMSI.  In this section, we
   provide the procedures to be used when the S-PMSI is instantiated as
   a PIM tree.  The PIM tree is created by the PIM P-instance.

   If a single PIM tree is being used to aggregate multiple S-PMSIs,
   then the PIM tree to which a given stream is bound may have already
   been joined by a given receiving PE.  If the tree does not already
   exist, then the appropriate PIM procedures to create it must be
   executed in the P-instance.

   If the S-PMSI for a particular multicast stream is instantiated as a
   PIM-SM or PIM-Bidir tree, the S-PMSI identifier will specify the RP
   and the group P-address, and the PE routers which have receivers for
   that stream must build a shared tree toward the RP.

   If the S-PMSI is instantiated as a PIM-SSM tree, the PE routers build
   a source tree toward the PE router that is advertising the S-PMSI
   Join.  The IP address root of the tree is the same as the source IP
   address which appears in the S-PMSI Join.  In this case, the tunnel
   identifier in the S-PMSI Join will only need to specify a group P-
   address.

   The above procedures assume that each PE router has a set of group



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   P-addresses that it can use for setting up the PIM-trees.  Each PE
   must be configured with this set of P-addresses.  If PIM-SSM is used
   to set up the tunnels, then the PEs may be with overlapping sets of
   group P-addresses.  If PIM-SSM is not used, then each PE must be
   configured with a unique set of group P-addresses (i.e., having no
   overlap with the set configured at any other PE router).  The
   management of this set of addresses is thus greatly simplified when
   PIM-SSM is used, so the use of PIM-SSM is strongly recommended
   whenever PIM trees are used to instantiate S-PMSIs.

   If it is known that all the PEs which need to receive data traffic on
   a given S-PMSI can support aggregation of multiple  S-PMSIs on a
   single PIM tree, then the transmitting PE, may, at its discretion,
   decide to bind the S-PMSI to a PIM  tree which is already bound to
   one or more other S-PMSIs, from the same or from different MVPNs.  In
   this case, appropriate demultiplexing information must be signaled.


7.6. Instantiating S-PMSIs using RSVP-TE P2MP Tunnels

   RSVP-TE P2MP Tunnels can be used for instantiating S-PMSIs.
   Procedures described in the context of I-PMSIs in section 6.7 apply.


8. Inter-AS Procedures

   This document describes two PMSI instantiation models for supporting
   an Inter-AS MVPN service:

      1. A model where Inter-AS MVPN service is delivered over a PMSI
         that is instantiated using a tunnel that spans multiple ASs.

         [Editor's Note: This is the model in [ROSEN-8] and [MVPN-
         BASE].]

      2. A model where Inter-AS MVPN service is delivered over a PMSI
         that is instantiated without requiring a tunnel to span
         multiple ASs. Each AS can have a different tunnel. The PMSI is
         instantiated using an "overlay tunnel" which is overlayed on
         each of the intra-AS tunnels. This allows an Inter-AS MVPN
         service to be provided with each AS potentially supporting a
         different MVPN transport technology.

   [RFC2547bis] describes different options for supporting Inter-AS
   BGP/MPLS unicast VPNs. Inter-AS MVPNs can be supported for each of
   these unicast BGP/MPLS VPN Inter-AS options, for both the PMSI
   instantiation models mentioned above. No further clarifications are
   required for option A.  We describe these two instantiation models in



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   the sub-sections below.


8.1. Tunnel Spans Multiple ASs

   In this model, the previously described discovery and tunnel setup
   mechanisms are used, even though the PEs belonging to a given MVPN
   may be in different ASes.  The ASBRs play no special role, but
   function merely as P routers.


8.1.1. Inter-AS MVPN Auto-Discovery

   The previously described BGP-based auto-discovery mechanisms work "as
   is" when an MVPN contains PEs that are in different Autonomous
   Systems.


8.1.2. Inter-AS MVPN Routing Information Exchange

   MVPN routing information exchange can be done by PIM peering (either
   lightweight or full) across an MI-PMSI, or by unicasting PIM
   messages.  The method of using BGP to send MVPN routing information
   can also be used.

   If any form of PIM peering is used, a PE that sends C-PIM Join/Prune
   messages for a particular C-(S,G) must be able to identify the PE
   which is its PIM adjacency on the path to S.  The identity of the PIM
   adjacency is determined from the RPF attribute associated with the
   VPN-IPv4 route to S.

   If no RPF attribute is present, then the identity of the PIM
   adjacency is taken from the BGP Next Hop attribute of the VPN-IPv4
   route to S.  Note that this will not give the correct result if
   option b of section 10 of [rfc2547bis] is used.  To avoid this
   possibility of error, the RPF attribute SHOULD always be present if
   MVPN routing information is to be distributed by PIM.

   If BGP (rather than PIM) is used to distribute the MVPN routing
   information, and if option b of section 10 of [rfc2547bis] is in use,
   then the MVPN routes will be installed in the ASBRs along the path
   from each multicast source in the MVPN to each multicast receiver in
   the MVPN.  If option b is not in use, the MVPN routes are not
   installed in the ASBRs.  The handling of MVPN routes in either case
   is thus exactly analogous to the handling of unicast VPN-IPv4 routes
   in the corresponding case.





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8.1.3. Inter-AS I-PMSI

   The procedures described earlier in this document can be used to
   instantiate an I-PMSI with inter-AS tunnels. Specific tunneling
   techniques require some explanation:

      1. If ingress replication is used, the inter-AS PE-PE tunnels will
         use the inter-AS tunneling procedures for the tunneling
         technology used.

      2. Inter-AS PIM-SM or PIM-SSM based trees rely on a PE joining a
         (P-S, P-G) tuple where P-S is the address of a PE in another
         AS. This (P-S, P-G) tuple is learned using the MVPN membership
         and BGP MVPN-tunnel binding procedures described earlier.
         However, if the source of the tree is in a different AS than a
         particular P router, it is possible that the P router will not
         have a route to the source.  For example, the remote AS may be
         using BGP to distribute a route to the source, but a particular
         P router may be part of a "BGP-free core", in which the P
         routers are not aware of BGP-distributed routes.

         In such a case it is necessary for a PE to to tell PIM to
         construct the tree through a particular BGP speaker, the "BGP
         next hop" for the tree source.  This can be accomplished with a
         PIM extension, in which the P-PIM Join/Prune messages carry a
         new "proxy" field which contains the address of that BGP next
         hop.  As the P-multicast tree is constructed, it is built
         towards the proxy (the BGP next hop) rather than towards P-S,
         so the P routers will not need to have a route to P-S.

         Support for inter-AS trees using PIM-Bidir are for further
         study.

         When the BGP-based discovery procedures for MVPN are in place,
         one can distinguish two different inter-AS routes to a
         particular P-S:

           - BGP will install a unicast route to P-S along a particular
             path, using the IPv4 AFI/SAFI ;

           - A PE's MVPN auto-discovery information is advertised by
             sending a BGP update whose  NLRI  is in a special address
             family (AFI/SAFI) used for this purpose.  The  NLRI of the
             address family contains the  IPv4 address of the PE, as
             well as an RD.  If the NLRI contains the IPv4 address of
             P-S, this in effect creates a second route to P-S.  This
             route might follow a different path than the route in the
             unicast IPv4 family.



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         When building a PIM tree towards P-S, it may be desirable to
         build it along the route on which the MVPN auto-discovery
         AFI/SAFI is installed, rather than along the route on which the
         IPv4 AFI/SAFI is installed.  This enables the inter-AS portion
         of the tree to follow a path which is specifically chosen for
         multicast (i.e., it allows the inter-AS multicast topology to
         be "non-congruent" to the inter-AS unicast topology).

         In order for P routers to send P-Join/Prune messages along this
         path, they need to make use of the "proxy" field extension
         discussed above.  The PIM message must also contain the full
         NLRI in the MVPN auto-discovery family, so that the BGP
         speakers can look up that NLRI to find the BGP next hop.

      3. Procedures in [RSVP-TE-P2MP] are used for inter-AS RSVP-TE P2MP
         Tunnels.


8.1.4. Inter-AS S-PMSI

   The leaves of the tunnel are discovered using the MVPN routing
   information.  Procedures for setting up the tunnel are similar to the
   ones described in section 8.2.3 for an inter-AS I-PMSI.


8.2. Overlay Inter-AS Tunnel

8.2.1. Inter-AS MVPN Auto-Discovery

   The BGP based MVPN membership discovery procedures of section 4 are
   used to auto-discover the inter-AS MVPN membership.

   In this case, for a given MVPN in an AS, the objective is to form a
   spanning tree of MVPN membership, rooted at the AS. The nodes of this
   tree are ASs.  The leaves of this tree are only those ASs that have
   at least one PE with a member in the MVPN. The overlay tunnel used to
   instantiate an inter-AS PMSI must traverse this spanning tree. A
   given AS needs to announce to another AS only the fact that it has
   membership in a given MVPN. It doesn't need to announce the
   membership of each PE in the AS to other ASs.

   A PE in a given AS advertises its MVPN membership to all its IBGP
   peers.  This IBGP peer may be a route reflector which in turn
   advertises this information to only its IBGP peers. In this manner
   all the PEs and ASBRs in the AS learn this membership information.

   Each ASBR then advertises the "AS MVPN membership" to its neighbor
   ASBRs using EBGP. This advertisement must not be advertised to the



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   PEs/ASBRs in the the same AS as this ASBR. This advertisement in
   effect may take place using Route Reflectors. The advertisement
   carries the following information elements:

      a. A Route Distinguisher for the MVPN. Each ASBR in the AS must
         use the same RD when advertising this information to other
         ASBRs. To accomplish this either the MVPN Route Target can be
         used as the RD or each ASBR can pick the RD advertised by a PE
         with the lowest BGP next-hop address.

      b. Origin AS number. This AS number must be encoded in an IP
         address.  As defined in [RFC1940] an IP address from network
         128.0.0.0 is used to encode a next hop that is a domain. The
         least significant two octets contain the DI, which is an
         Internet Autonomous System number.

      c. The announcing ASBR's local address as the next-hop for the
         above information elements.

   An ASBR in an AS that receives the above information from its EBGP
   peers, changes the next-hop to self, and announces it to its IBGP
   peers. It also runs BGP path selection on the information elements
   (a, b) described above.

   If the ASBR is the next-hop for the best path it advertises:

      a) To its neighbor ASBR, from which it received the information
         element, an ASBR-ASBR tunnel binding. This binding as described
         in section 6 can be used by the neighbor ASBR to send traffic
         to this ASBR.

      b) To the PEs in its AS an I-PMSI tunnel binding, for the AS MVPN
         membership received from the other AS, as described in section
         6.

   The ASBR announces the best path to its EBGP peers, with next-hop as
   self.  Thus the AS MVPN membership information propagates across
   multiple ASs along a spanning tree. BGP AS-Path based loop prevention
   mechanism prevents loops from forming as this information propagates.


8.2.2. Inter-AS MVPN Routing Information Exchange

   All of the MVPN routing information exchange methods specified in
   section 5 can be supported across ASs.

   The objective in this case is to propagate the MVPN routing
   information to the remote PE, in the reverse direction of the AS MVPN



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   routing information announced by the remote PE's origin AS. This is
   to build S-PMSIs that utilize overlay inter-AS tunnels as described
   in section 8.2.4. This information is processed by each ASBR along
   this reverse path. To achieve this the PE that is generating the MVPN
   routing advertisement, first determines the source AS of the remote
   PE. It then determines from the received AS MVPN membership
   information, the ASBR that is the next-hop for the best path of the
   source AS MVPN membership. The MVPN membership information is sent to
   this ASBR, if unicast PIM is used. If BGP is used the upstream
   address is set to this ASBR and the ASBR then further propagates the
   BGP advertisement.


8.2.3. Inter-AS I-PMSI

   One option for instantiating an inter-AS I-PMSI is to use different
   tunnels in each AS and stitch these tunnels together using MPLS label
   switching. This results in an overlay inter-AS tunnel. This allows
   each AS to use a different tunneling technology. If P-multicast trees
   are used, it allows each AS to use a different multicast tree
   protocol.

   The overlay inter-AS tunnel, for an I-PMSI is rooted at the router
   that instantiates the I-PMSI. It traverses the AS MVPN membership
   spanning tree.  Each ASBR on the spanning tree advertises a tunnel
   binding for the MVPN to its upstream ASBR. For a given AS only one
   ASBR advertises this binding for a given AS MVPN membership to its
   upstream ASBR. This ensures that there is only one exit point in an
   AS to reach another AS for the inter-AS PMSI.  It also ensures that
   there is only one entry point in an AS to receive traffic from
   another AS for the inter-AS PMSI. This tunnel binding carries the
   following information elements:

      a. The AS MVPN membership for which the tunnel binding is being
         advertised.

      b. A downstream label that the ASBR allocates to receive traffic
         from its upstream ASBR on the inter-AS overlay tunnel. This is
         the tunnel binding.

   Each downstream ASBR that is on the spanning tree (as determined by
   the AS MVPN information best path selection) also instantiates an I-
   PMSI using an intra-AS tunnel, for the MVPN.

   A C-multicast data packet is sent using an intra-AS tunnel by the PE
   that first receives this packet. An ASBR forwards this packet to any
   locally connected MVPN receivers for the multicast stream. If this
   ASBR has received a tunnel binding for the AS MVPN membership that it



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   advertised to a neighboring ASBR, it also forwards this packet to the
   neighboring ASBR. In this case the packet is encapsulated in the
   downstream MPLS label received from the neighboring ASBR. The
   neighboring ASBR delivers this packet to any locally connected MVPN
   receivers for that multicast stream. It also transports this packet
   on an intra-AS tunnel for the MVPN and the other PEs and ASBRs in the
   AS then receive this packet. The other ASBRs then repeat the
   procedure followed by the ASBR in the origin AS and the packet
   traverses the overlay inter-AS tunnel along a spanning tree.


8.2.3.1. Support for 2547 Unicast VPN Inter-AS Methods

   The above procedures for setting up an inter-AS I-PMSI can be
   supported for each of the 2547 unicast VPN inter-AS models. These
   procedures do not depend on the method used to exchange unicast VPN
   routes. For Option B and Option C they do require MPLS encapsulation
   between the ASBRs.


8.2.4. Inter-AS S-PMSI

   One option for instantiating an inter-AS S-PMSI is to use different
   tunnels in each AS and stitch these tunnels together using MPLS label
   switching. This results in an inter-AS overlay tunnel for the S-PMSI.
   The procedures are conceptually similar to the ones described for an
   I-PMSI inter-AS overlay tunnel.

   The PE that decides to set up a S-PMSI, advertises the S-PMSI tunnel
   binding using procedures in section 7.3.2 to the routers in its own
   AS. The <C-S, C-G> membership for which the S-PMSI is instantiated,
   is propagated along an inter-AS spanning tree. This spanning tree
   traverses the same ASBRs as the AS MVPN membership spanning tree. In
   addition to the information elements described in section 7.3.2
   (Origin AS, RD, next-hop) the C-S and C-G is also advertised. A
   downstream ASBR on the spanning tree sends back a tunnel binding for
   AS <C-S, C-G> information. If the downstream ASBR instantiates a S-
   PMSI for the AS <C-S, C-G> it sends back a downstream label that is
   used to forward the packet along its intra-AS S-PMSI for the <C-S,
   C-G>. However the downstream ASBR may decide to use an I-PMSI
   instead, in which case it sends back the same label that it
   advertised for the I-PMSI. If the downstream ASBR instantiates a S-
   PMSI, it further propagates the <C-S, C-G> membership to its
   downstream ASs, else it does not.

   An AS can instantiate an intra-AS S-PMSI for the inter-AS S-PMSI
   overlay tunnel only if the upstream AS instantiates a S-PMSI. The
   procedures allow each AS to determine whether it wishes to setup a



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   S-PMSI or not and the AS is not forced to setup a S-PMSI just because
   the upstream AS decides to do so.

   The leaves of an intra-AS S-PMSI tunnel will be the PEs that have
   local receivers that are interested in <C-S, C-G> and the ASBRs that
   have received MVPN routing information for <C-S, C-G>. Note that an
   AS can determine these ASBRs leaves <C-S, C-G> as the MVPN routing
   information is propagated and processed by each ASBR on the AS MVPN
   membership spanning tree.

   The C-multicast data traffic is sent on the S-PMSI by the originating
   PE.  When it reaches an ASBR that is on the spanning tree, it is
   delivered to local receivers, if any, and is also forwarded to the
   neighbor ASBR after being encapsulated in the label advertised by the
   neighbor. The neighbor ASBR either transports this packet on the S-
   PMSI for the multicast stream or an I-PMSI, delivering it to the
   ASBRs in its own AS. These ASBRs in turn repeat the procedures of the
   origin AS ASBRs and the multicast packet traverses the spanning tree.


9. Deployment Models

   This section describes some optional deployment models and specific
   procedures for those deployment models.


9.1. Co-locating C-RPs on a PE

   [MVPN-REQ] describes C-RP engineering as an issue when PIM-SM (or
   bidir-PIM) is used in ASM mode on the VPN customer site. To quote
   from [MVPN-REQ]:

   "In some cases this engineering problem is not trivial: for instance,
   if sources and receivers are located in VPN sites that are different
   than that of the RP, then traffic may flow twice through the SP
   network and the CE-PE link of the RP (from source to RP, and then
   from RP to receivers) ; this is obviously not ideal.  A multicast VPN
   solution SHOULD propose a way to help on solving this RP engineering
   issue."

   One of the C-RP deployment models is for the customer to outsource
   the RP to the provider. In this case the provider may co-locate the
   RP on the PE that is connected to the customer site [MVPN-REQ]. This
   model is introduced in [RP-MVPN]. This section describes how
   anycast-RP can be used for achieving this. It can either be done
   without advertising the active sources or by also advertising the
   active sources. This is described below.




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9.1.1. Initial Configuration

   For each VPN site connected to a PE the PE acts as an RP. Within each
   VPN all these RPs use the same (anycast) address. All these RPs use
   the Anycast RP technique.


9.1.2. Anycast RP Based on C-(*, G) Advertisements

9.1.2.1. Receiver(s) Within a Site

   When a host within a VPN site connected to a given PE joins a
   particular multicast group G, the designated router connected to the
   host would send C-Join for (*,G) to the PE (as the PE acts as an RP
   within the site).  If the site is multi-homed (means the site is
   connected to more than one PE), then all the PEs connected to the
   site act as (anycast) RP. So, the Join would arrive at one of these
   PEs (the one that is closest from the routing point of view to the
   designated router that originated Join (*,G)).

   When a PE receives a Join (*,G) from a CE, the PE sends to all other
   PEs that are in the same VPN as the CE, the information that it has
   receiver(s) for G. Sending this information could be done using any
   of the procedures described in section 5. If BGP is used the upstream
   PE address is set to 0. A PE will process the message after it
   imports it into the VRF only if it is configured to be an anycast C-
   RP.If unicast PIM is used then a unicast PIM message will have to be
   sent to each of the other PEs. The upstream neighbor address for a
   unicast PIM message sent to a PE is the destination PE's address. If
   a MI-PMSI is used than further clarification is needed on the
   upstream neighbor address of the PIM message and will be provided in
   a future revision.


9.1.2.2. Source within a Site

   When a host S within a VPN site starts sending (multicast) packets to
   a particular group G, the designated router connected to the host
   would encapsulate these messages into PIM-Register and will send it
   to the PE connected to the site (as the PE acts as an (anycast) RP
   for that site).

   When a PE receives PIM-Register from a site that belongs to a given
   VPN, PE follows the normal PIM procedures. In addition, if the PE
   receives information from other PEs that other sites of that VPN have
   receivers for the group G, carried in the PIM-Register, the PE sends
   the multicast data on an I-PMSI or a S-PMSI.




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9.1.2.3. Receiver Switching from Shared to Source Tree

   When a receivers within a VPN site switches from shared to source
   tree the receiver sends Join C-(S,G) towards C-S, and also Prune
   (S,G,RPT) towards its (proxy) RP.


9.1.2.3.1. Handling Join (S,G)

   If PE already has an existing Join(*,G) state, then when the PE
   receives Join(S,G) the PE does not need to advertise this information
   to the other PEs.

   If Join(*,G) is expired, but Join(S,G) is still present, the PE has
   to advertise replacement of C-(*,G) with C-(S,G).

   Likewise, if PE does not have an existing Join(*,G) state, when PE
   receives Join(S,G) the PE has to advertise C-(S,G).

   The PE determines the upstream PE address as specified in section
   5.2. Once this is determined, the PE sends <S,G, Ingress-PE>
   information to all other PEs that are in the same VPN as the CE.
   Sending this information could be done with either of the methods
   described in section 5.


9.1.2.3.2. Handling Prune (S,G, RPT)

   When PE receives Prune(S,G,RPT) from CE, and the PE is receiving
   traffic for the (S,G) on an I-PMSI the PE does nothing; and if the PE
   receives (S,G) on an S-PMSI associated with (S,G) the PE informs the
   root of that S-PMSI that the PE is no longer interested in (S,G).
   This can be accomplished using any of the mechanisms of section 5.


9.1.2.3.3. Receiving information from other PEs

   When a PE receives <S, G, Ingress-PE> information from some other PE,
   the local PE checks whether it is the Ingress-PE, and if yes, then
   the PE sends Join (S,G) towards S. Otherwise, if PE has been sending
   Join (S,G) to one of its CEs, the PE now sends Prune (S,G) to that
   CE.  Otherwise, the PE does nothing.









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9.1.3. Anycast RP Based on Propagating Active Sources

   The second mechanism is based on propagating active sources between
   RPs.

   [Editor's Note: This is the model in [RP-MVPN].]

   The essential difference between this mechanism and the mechanism
   specified in the previous section is that when a PE discovers that
   there is a source in a site attached to that PE, the PE advertises
   this source in BGP.


9.1.3.1. Receiver(s) Within a Site

   The PE which receives C-Join for (*,G) or (S,G) does not send the
   information that it has receiver(s) for G until it receives source
   active information from an upstream PE.

   On receiving source active information (described in the next
   section), the downstream PE will respond with Join for C-(S,G).
   Sending this information could be done using any of the procedures
   described in section 5. If BGP is used, the ingress address is set to
   the upstream PE's address which has triggered the source active
   information. Only the upstream PE will process this information. If
   unicast PIM is used then a unicast PIM message will have to be sent
   to the PE upstream PE that has triggered the source active
   information. If a MI-PMSI is used than further clarification is
   needed on the upstream neighbor address of the PIM message and will
   be provided in a future revision.


9.1.3.2. Source Within a Site

   When a PE receives PIM-Register from a site that belongs to a given
   VPN, PE follows the normal PIM procedures. It then advertises this
   source and group to other PEs in BGP using the following information
   elements:

     - Active source address

     - Active group address

     - Ingress PE address







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     - Route target of the MVPN.

   This advertisement goes to all the PEs. When a PE receives this
   advertisement, it checks whether there are any receivers in the sites
   attached to the PE for the group carried in the source active
   advertisement. If yes, then it generates an advertisement for C-(S,G)
   as specified in the previous section.

   Note that the mechanism described in section 7.3.2. can be leveraged
   to advertise a S-PMSI binding along with the source active messages.


9.1.3.3. Receiver Switching from Shared to Source Tree

   No additional procedures are required when multicast receivers in
   customer's site shift from shared tree to source tree.


10. BGP Advertisements

10.1. Functions and Information Elements

   The information elements signalled in BGP and the functions of these
   information elements has been described in detail earlier.


10.2. Encoding

   The encoding will be described later.


11. Encapsulations

   The BGP-based auto-discovery procedures will ensure that the PEs in a
   single MVPN only use tunnels that they can all support, and for a
   given kind of tunnel, that they only use encapsulations that they can
   all support.


11.1. Encapsulations for Single PMSI per Tunnel

11.1.1. Encapsulation in GRE

   GRE encapsulation can be used for any PMSI that is instantiated by a
   mesh of unicast tunnels, as well as for any PMSI that is instantiated
   by one or more PIM tunnels of any sort.





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   Packets received        Packets in transit      Packets forwarded
   at ingress PE           in the service          by egress PEs
                           provider network

                           +---------------+
                           |  P-IP Header  |
                           +---------------+
                           |      GRE      |
   ++=============++       ++=============++       ++=============++
   || C-IP Header ||       || C-IP Header ||       || C-IP Header ||
   ++=============++ >>>>> ++=============++ >>>>> ++=============++
   || C-Payload   ||       || C-Payload   ||       || C-Payload   ||
   ++=============++       ++=============++       ++=============++


   The IPv4 Protocol Number field in the P-IP Header must be set to 47.
   The Protocol Type field of the GRE Header must be set to 0x800.

   When an encapsulated packet is transmitted by a particular PE, the
   source IP address in the P-IP header must be the same address as is
   advertised by that PE in the RPF attribute [ref].

   If the PMSI is instantiated by a PIM tree, the destination IP address
   in the P-IP header is the group P-address associated with that tree.
   The GRE key field value is omitted.

   If the PMSI is instantiated by unicast tunnels, the destination IP
   address is the address of the destination PE, and the optional GRE
   Key field is used to identify a particular MVPN.  In this case, each
   PE would have to advertise a key field value for each MVPN; each PE
   would assign the key field value that it expects to receive.

   [GRE2784] specifies an optional GRE checksum, and [GRE2890] specifies
   an optional GRE sequence number fields.

   The GRE sequence number field is not needed because the transport
   layer services for the original application will be provided by the
   C-IP Header.

   The use of GRE checksum field must follow [GRE2784].

   To facilitate high speed implementation, this document recommends
   that the ingress PE routers encapsulate VPN packets without setting
   the checksum, or sequence fields.






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11.1.2. Encapsulation in IP

   IP-in-IP [IPIP1853] is also a viable option.  When it is used, the
   IPv4 Protocol Number field is set to 4. The following diagram shows
   the progression of the packet as it enters and leaves the service
   provider network.


   Packets received        Packets in transit      Packets forwarded
   at ingress PE           in the service          by egress PEs
                           provider network

                           +---------------+
                           |  P-IP Header  |
   ++=============++       ++=============++       ++=============++
   || C-IP Header ||       || C-IP Header ||       || C-IP Header ||
   ++=============++ >>>>> ++=============++ >>>>> ++=============++
   || C-Payload   ||       || C-Payload   ||       || C-Payload   ||
   ++=============++       ++=============++       ++=============++



11.1.3. Encapsulation in MPLS

   If the PMSI is instantiated as a P2MP MPLS LSP, MPLS encapsulation is
   used. Penultimate-hop-popping must be disabled for the P2MP MPLS LSP.
   If the PMSI is instantiated as an RSVP-TE P2MP LSP, additional MPLS
   encapsulation procedures are used, as specified in [RSVP-TE-P2MP].

   If other methods of assigning MPLS labels to multicast distribution
   trees are in use, these multicast distribution trees may be used as
   appropriate to instantiate PMSIs, and any additional MPLS
   encapsulation procedures may be used.


















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   Packets received        Packets in transit      Packets forwarded
   at ingress PE           in the service          by egress PEs
                           provider network

                           +---------------+
                           | P-MPLS Header |
   ++=============++       ++=============++       ++=============++
   || C-IP Header ||       || C-IP Header ||       || C-IP Header ||
   ++=============++ >>>>> ++=============++ >>>>> ++=============++
   || C-Payload   ||       || C-Payload   ||       || C-Payload   ||
   ++=============++       ++=============++       ++=============++



11.2. Encapsulations for Multiple PMSIs per Tunnel

   The encapsulations for transmitting multicast data messages when
   there are multiple PMSIs per tunnel are based on the encapsulation
   for a single PMSI per tunnel, but with an MPLS label used for
   demultiplexing.

   The label is upstream-assigned and distributed via BGP as specified
   in section 4.  The label must enable the receiver to select the
   proper VRF, and may enable the receiver to select a particular
   multicast routing entry within that VRF.


11.2.1. Encapsulation in GRE

   Rather than the IP-in-GRE encapsulation discussed in section 11.1.1,
   we use the MPLS-in-GRE encapsulation.  This is specified in [draft-
   mpls-in-ip-or-gre].  The GRE protocol type MUST be set to 0x8847.
   [The reason for using the unicast rather than the multicast value is
   specified in [MPLS-MCAST-ENCAPS].


11.2.2. Encapsulation in IP

   Rather than the IP-in-IP encapsulation discussed in section 11.1.2,
   we use the MPLS-in-IP encapsulation.  This is specified in [draft-
   mpls-in-ip-or-gre].  The IP protocol number MUST be set to the value
   identifying the payload as an MPLS unicast packet. [There is no "MPLS
   multicast packet" protocol number.]







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11.3. Encapsulations for Unicasting PIM Control Messages

   When PIM control messages are unicast, rather than being sent on an
   MI-PMSI, the the receiving PE needs to determine the particular MVPN
   whose multicast routing information is being carried in the PIM
   message.  One method is to use a downstream-assigned MPLS label which
   the receiving PE has allocated for this specific purpose.  The label
   would be distributed via BGP.  This can be used with an MPLS, MPLS-
   in-GRE, or MPLS-in-IP encapsulation.

   A possible alternative to modify the PIM messages themselves so that
   they carry information which can be used to identify a particular
   MVPN, such as an RT.

   This area is still under consideration.



11.4. General Considerations for IP and GRE Encaps

   These apply also to the MPLS-in-IP and MPLS-in-GRE encapsulations.


11.4.1. MTU

   Path MTU discovery cannot be relied upon to ensure that the
   transmitter sends packets which are small enough to reach all the
   destinations.  This requires that:

      a. The ingress PE router (one that does the encapsulation) must
         not set the DF bit in the outer header, and

      b. If the "DF" bit is cleared in the IP header of the C-Packet,
         fragment the C-Packet before encapsulation if appropriate.
         This is very important in practice due to the fact that the
         performance of reassembly function is significantly lower than
         that of decapsulating and forwarding packets on today's router
         implementations.


11.4.2. TTL

   The ingress PE should not copy the TTL field from the payload IP
   header received from a CE router to the delivery IP or MPLS header.
   The setting of the TTL of the delivery header is determined by the
   local policy of the ingress PE router.





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11.4.3. Differentiated Services

   By default, the setting of the DS field in the delivery IP header
   should follow the guidelines outlined in [DIFF2983].  Setting the EXP
   field in the delivery MPLS header should follow the guidelines in
   [REF]. An SP may also choose to deploy any of the additional
   mechanisms the PE routers support.


11.4.4. Avoiding Conflict with Internet Multicast

   If the SP is providing Internet multicast, distinct from its VPN
   multicast services, and using PIM based P-multicast trees, it must
   ensure that the group P-addresses which it used in support of MPVN
   services are distinct from any of the group addresses of the Internet
   multicasts it supports.  This is best done by using administratively
   scoped addresses [ADMIN-ADDR].

   The group C-addresses need not be distinct from either the group P-
   addresses or the Internet multicast addresses.


12. Security Considerations

   To be supplied.


13. IANA Considerations

   To be supplied.


14. Other Authors

   Sarveshwar Bandi, Yiqun Cai, Thomas Morin, Yakov Rekhter, IJsbrands
   Wijnands, Seisho Yasukawa


15. Other Contributors

   Significant contributions were made Arjen Boers, Toerless Eckert,
   Adrian Farrel, Luyuan Fang, Dino Farinacci, Lenny Guiliano, Shankar
   Karuna, Anil Lohiya, Tom Pusateri, Ted Qian, Robert Raszuk, Tony
   Speakman, Dan Tappan.







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16. Authors' Addresses


      Rahul Aggarwal (Editor)
      Juniper Networks
      1194 North Mathilda Ave.
      Sunnyvale, CA 94089
      Email: rahul@juniper.net



      Sarveshwar Bandi
      Motorola
      Vanenburg IT park, Madhapur,
      Hyderabad, India
      Email: sarvesh@motorola.com



      Yiqun Cai
      Cisco Systems, Inc.
      170 Tasman Drive
      San Jose, CA, 95134
      E-mail: ycai@cisco.com



      Thomas Morin
      France Telecom R & D
      2, avenue Pierre-Marzin
      22307 Lannion Cedex
      France
      Email: thomas.morin@francetelecom.com



      Yakov Rekhter
      Juniper Networks
      1194 North Mathilda Ave.
      Sunnyvale, CA 94089
      Email: yakov@juniper.net










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      Eric C. Rosen (Editor)
      Cisco Systems, Inc.
      1414 Massachusetts Avenue
      Boxborough, MA, 01719
      E-mail: erosen@cisco.com



      IJsbrand Wijnands
      Cisco Systems, Inc.
      170 Tasman Drive
      San Jose, CA, 95134
      E-mail: ice@cisco.com



      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



17. Normative References

   [MVPN-REQ] T. Morin, Ed., "Requirements for Multicast in L3
   Provider-Provisioned VPNs", draft-ietf-l3vpn-ppvpn-mcast-reqts-00.txt

   [2547bis] "BGP/MPLS VPNs", Rosen, Rekhter, et. al., September 2003,
   draft-ietf-l3vpn-rfc2547bis-01.txt

   [RFC2119] "Key words for use in RFCs to Indicate Requirement
   Levels.", Bradner, March 1997

   [PIM-SM]  "Protocol Independent Multicast - Sparse Mode (PIM-SM)",
   Fenner, Handley, Holbrook, Kouvelas, October 2003, draft-ietf-pim-
   sm-v2-new-08.txt

   [RSVP-P2MP] R. Aggarwal, et. al., "Extensions to RSVP-TE for Point to
   Multipoint TE LSPs", draft-ietf-mpls-rsvp-te-p2mp-01.txt

   [RFC3107] Y. Rekhter, E. Rosen, "Carrying Label Information in BGP-
   4", RFC3107.




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   [MPLS-IP] T. Worster, Y. Rekhter, E. Rosen, "Encapsulating MPLS in IP
   or Generic Routing Encapsulation (GRE)", draft-ietf-mpls-in-ip-or-
   gre-08.txt

   [MPLS-MCAST-ENCAPS] T. Eckert, E. Rosen, R. Aggarwal, Y. Rekhter,
   "MPLS Multicast Encapsulations", draft-rosen-mpls-multicast-encaps-
   00.txt, April 2005

   [MPLS-UPSTREAM-LABEL] R. Aggarwal, Y. Rekhter, E. Rosen, draft-
   raggarwa-mpls-upstream-label-00.txt, "MPLS Upstream Label Assignment
   and Context Specific Label Space", draft-raggarwa-mpls-upstream-
   label-00.txt, January 2005


18. Informative References

   [ROSEN-8] E. Rosen, Y. Cai, I. Wijnands, "Multicast in MPLS/BGP IP
   VPNs", draft-rosen-vpn-mcast-08.txt

   [MVPN-PIM] R. Aggarwal, A. Lohiya, T. Pusateri, Y. Rekhter, "Base
   Specification for Multicast in MPLS/BGP VPNs", draft-raggarwa-l3vpn-
   2547-mvpn-00.txt

   [RAGGARWA-MCAST] R. Aggarwal, et. al., "Multicast in BGP/MPLS VPNs
   and VPLS", draft-raggarwa-l3vpn-mvpn-vpls-mcast--01.txt".

   [RP-MVPN] S. Yasukawa, et. al., "BGP/MPLS IP Multicast VPNs", draft-
   yasukawa-l3vpn-p2mp-mcast-00.txt


19. Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.






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20. Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.




























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