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rfc:rfc6513

Internet Engineering Task Force (IETF) E. Rosen, Ed. Request for Comments: 6513 Cisco Systems, Inc. Category: Standards Track R. Aggarwal, Ed. ISSN: 2070-1721 Juniper Networks

                                                         February 2012
                   Multicast in MPLS/BGP IP VPNs

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.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6513.

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Rosen & Aggarwal Standards Track [Page 1] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1. Introduction ....................................................5
 2.  Overview .......................................................5
    2.1. Optimality vs. Scalability .................................5
         2.1.1. Multicast Distribution Trees ........................7
         2.1.2. Ingress Replication through Unicast Tunnels .........8
    2.2. Multicast Routing Adjacencies ..............................8
    2.3. MVPN Definition ............................................9
    2.4. Auto-Discovery ............................................10
    2.5. PE-PE Multicast Routing Information .......................11
    2.6. PE-PE Multicast Data Transmission .........................11
    2.7. Inter-AS MVPNs ............................................12
    2.8. Optionally Eliminating Shared Tree State ..................13
 3. Concepts and Framework .........................................13
    3.1. PE-CE Multicast Routing ...................................13
    3.2. P-Multicast Service Interfaces (PMSIs) ....................14
         3.2.1. Inclusive and Selective PMSIs ......................15
         3.2.2. P-Tunnels Instantiating PMSIs ......................16
    3.3. Use of PMSIs for Carrying Multicast Data ..................18
    3.4. PE-PE Transmission of C-Multicast Routing .................20
         3.4.1. PIM Peering ........................................20
                3.4.1.1. Full per-MVPN PIM Peering across
                         an MI-PMSI ................................20
                3.4.1.2. Lightweight PIM Peering across an MI-PMSI .20
                3.4.1.3. Unicasting of PIM C-Join/Prune Messages ...21
         3.4.2. Using BGP to Carry C-Multicast Routing .............22
 4. BGP-Based Auto-Discovery of MVPN Membership ....................22
 5. PE-PE Transmission of C-Multicast Routing ......................25
    5.1. Selecting the Upstream Multicast Hop (UMH) ................25
         5.1.1. Eligible Routes for UMH Selection ..................26
         5.1.2. Information Carried by Eligible UMH Routes .........26
         5.1.3. Selecting the Upstream PE ..........................27
         5.1.4. Selecting the Upstream Multicast Hop ...............29
    5.2. Details of Per-MVPN Full PIM Peering over MI-PMSI .........29
         5.2.1. PIM C-Instance Control Packets .....................29

Rosen & Aggarwal Standards Track [Page 2] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

         5.2.2. PIM C-Instance Reverse Path Forwarding
                (RPF) Determination ................................30
    5.3. Use of BGP for Carrying C-Multicast Routing ...............31
         5.3.1. Sending BGP Updates ................................31
         5.3.2. Explicit Tracking ..................................32
         5.3.3. Withdrawing BGP Updates ............................32
         5.3.4. BSR ................................................33
 6. PMSI Instantiation .............................................33
    6.1. Use of the Intra-AS I-PMSI A-D Route ......................34
         6.1.1. Sending Intra-AS I-PMSI A-D Routes .................34
         6.1.2. Receiving Intra-AS I-PMSI A-D Routes ...............35
    6.2. When C-flows Are Specifically Bound to P-Tunnels ..........35
    6.3. Aggregating Multiple MVPNs on a Single P-Tunnel ...........35
         6.3.1. Aggregate Tree Leaf Discovery ......................36
         6.3.2. Aggregation Methodology ............................36
         6.3.3. Demultiplexing C-Multicast Traffic .................37
    6.4. Considerations for Specific Tunnel Technologies ...........38
         6.4.1. RSVP-TE P2MP LSPs ..................................39
         6.4.2. PIM Trees ..........................................41
         6.4.3. mLDP P2MP LSPs .....................................42
         6.4.4. mLDP MP2MP LSPs ....................................42
         6.4.5. Ingress Replication ................................42
 7. Binding Specific C-Flows to Specific P-Tunnels .................44
    7.1. General Considerations ....................................45
         7.1.1. At the PE Transmitting the C-Flow on the P-Tunnel ..45
         7.1.2. At the PE Receiving the C-flow from the P-Tunnel ...46
    7.2. Optimizing Multicast Distribution via S-PMSIs .............48
    7.3. Announcing the Presence of Unsolicited Flooded Data .......49
    7.4. Protocols for Binding C-Flows to P-Tunnels ................50
         7.4.1. Using BGP S-PMSI A-D Routes ........................50
                7.4.1.1. Advertising C-Flow Binding to P-Tunnel ....50
                7.4.1.2. Explicit Tracking .........................51
         7.4.2. UDP-Based Protocol .................................52
                7.4.2.1. Advertising C-Flow Binding to P-Tunnel ....52
                7.4.2.2. Packet Formats and Constants ..............53
         7.4.3. Aggregation ........................................55
 8. Inter-AS Procedures ............................................55
    8.1. Non-Segmented Inter-AS P-Tunnels ..........................56
         8.1.1. Inter-AS MVPN Auto-Discovery .......................56
         8.1.2. Inter-AS MVPN Routing Information Exchange .........56
         8.1.3. Inter-AS P-Tunnels .................................57
                8.1.3.1. PIM-Based Inter-AS P-Multicast Trees ......57
                8.1.3.2. The PIM MVPN Join Attribute ...............58
                         8.1.3.2.1. Definition .....................58
                         8.1.3.2.2. Usage ..........................59
    8.2. Segmented Inter-AS P-Tunnels ..............................60
 9. Preventing Duplication of Multicast Data Packets ...............60
    9.1. Methods for Ensuring Non-Duplication ......................61

Rosen & Aggarwal Standards Track [Page 3] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

         9.1.1. Discarding Packets from Wrong PE ...................62
         9.1.2. Single Forwarder Selection .........................63
         9.1.3. Native PIM Methods .................................63
    9.2. Multihomed C-S or C-RP ....................................63
    9.3. Switching from the C-RP Tree to the C-S Tree ..............63
         9.3.1. How Duplicates Can Occur ...........................63
         9.3.2. Solution Using Source Active A-D Routes ............65
 10. Eliminating PE-PE Distribution of (C-*,C-G) State .............67
    10.1. Co-Locating C-RPs on a PE ................................68
         10.1.1. Initial Configuration .............................68
         10.1.2. Anycast RP Based on Propagating Active Sources ....68
                10.1.2.1. Receiver(s) within a Site ................69
                10.1.2.2. Source within a Site .....................69
                10.1.2.3. Receiver Switching from Shared to
                          Source Tree ..............................69
    10.2. Using MSDP between a PE and a Local C-RP .................69
 11. Support for PIM-BIDIR C-Groups ................................71
    11.1. The VPN Backbone Becomes the RPL .........................72
         11.1.1. Control Plane .....................................72
         11.1.2. Data Plane ........................................73
    11.2. Partitioned Sets of PEs ..................................73
         11.2.1. Partitions ........................................73
         11.2.2. Using PE Distinguisher Labels .....................74
         11.2.3. Partial Mesh of MP2MP P-Tunnels ...................75
 12. Encapsulations ................................................75
    12.1. Encapsulations for Single PMSI per P-Tunnel ..............75
         12.1.1. Encapsulation in GRE ..............................75
         12.1.2. Encapsulation in IP ...............................76
         12.1.3. Encapsulation in MPLS .............................77
    12.2. Encapsulations for Multiple PMSIs per P-Tunnel ...........78
         12.2.1. Encapsulation in GRE ..............................78
         12.2.2. Encapsulation in IP ...............................78
    12.3. Encapsulations Identifying a Distinguished PE ............78
         12.3.1. For MP2MP LSP P-Tunnels ...........................78
         12.3.2. For Support of PIM-BIDIR C-Groups .................79
    12.4. General Considerations for IP and GRE Encapsulations .....79
         12.4.1. MTU (Maximum Transmission Unit) ...................79
         12.4.2. TTL (Time to Live) ................................80
         12.4.3. Avoiding Conflict with Internet Multicast .........80
    12.5. Differentiated Services ..................................81
 13. Security Considerations .......................................81
 14. IANA Considerations ...........................................83
 15. Acknowledgments ...............................................83
 16. References ....................................................84
    16.1. Normative References .....................................84
    16.2. Informative References ...................................85

Rosen & Aggarwal Standards Track [Page 4] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

1. Introduction

 [RFC4364] specifies the set of procedures that a Service Provider
 (SP) must implement in order to provide a particular kind of VPN
 service ("BGP/MPLS IP VPN") for its customers.  The service described
 therein allows IP unicast packets to travel from one customer site to
 another, but it does not provide a way for IP multicast traffic to
 travel from one customer site to another.
 This document extends the service defined in [RFC4364] so that it
 also includes the capability of handling IP multicast traffic.  This
 requires a number of different protocols to work together.  The
 document provides a framework describing how the various protocols
 fit together, and it also provides a detailed specification of some
 of the protocols.  The detailed specification of some of the other
 protocols is found in preexisting documents or in companion
 documents.
 A BGP/MPLS IP VPN service that supports multicast is known as a
 "Multicast VPN" or "MVPN".
 Both this document and its companion document [MVPN-BGP] discuss the
 use of various BGP messages and procedures to provide MVPN support.
 While every effort has been made to ensure that the two documents are
 consistent with each other, it is possible that discrepancies have
 crept in.  In the event of any conflict or other discrepancy with
 respect to the use of BGP in support of MVPN service, [MVPN-BGP] is
 to be considered to be the authoritative document.
 Throughout this document, we will use the term "VPN-IP route" to mean
 a route that is either in the VPN-IPv4 address family [RFC4364] or in
 the VPN-IPv6 address family [RFC4659].
 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. Overview

2.1. Optimality vs. Scalability

 In a "BGP/MPLS IP VPN" [RFC4364], 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").  Routing information from a
 particular VPN is maintained only by the Provider Edge routers (the
 "PE routers", or "PEs") that attach directly to sites of that VPN.
 Customer data travels through the P routers in tunnels from one PE to
 another (usually MPLS Label Switched Paths, LSPs), so to support the

Rosen & Aggarwal Standards Track [Page 5] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 VPN service the P routers only need to have routes to the PE routers.
 The PE-to-PE routing is optimal, but the amount of associated state
 in the P routers depends only on the number of PEs, not on the number
 of VPNs.
 However, in order to provide optimal multicast routing for a
 particular multicast flow, the P routers through which that flow
 travels have to hold state that is specific to that flow.  A
 multicast flow is identified by the (source, group) tuple where the
 source is the IP address of the sender and the group is the IP
 multicast group address of the destination.  Scalability would be
 poor if the amount of state in the P routers were proportional to the
 number of multicast flows in the VPNs.  Therefore, when supporting
 multicast service for a BGP/MPLS IP VPN, the optimality of the
 multicast routing must be traded off against the scalability of the P
 routers.  We explain this below in more detail.
 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 Customer Edge router (a "CE router"
 or "CE") sends to a PE, such that the IP destination address of the
 packets is a multicast group address, the packets are multicast
 control packets addressed to the PE router itself, or the packets are
 IP multicast data packets encapsulated in MPLS.
 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:
  1. 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 that is on the path
     to a receiver of that group;
  1. The packet is not received by any other PEs;
  1. While in the backbone, no more than one copy of the packet ever

traverses any link.

  1. While in the backbone, if bandwidth usage is to be optimized, the

packet traverses minimum cost trees rather than shortest path

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

Rosen & Aggarwal Standards Track [Page 6] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 Potentially, this would 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.  The SP also doesn't have any
 control over the number of transmitters in each group, nor over the
 distribution of the receivers.
 The procedures defined in this document allow an SP to provide
 multicast VPN service, without requiring the amount of state
 maintained by the P routers to be proportional to the number of
 multicast data flows in the VPNs.  The amount of state is traded off
 against the optimality of the multicast routing.  Enough flexibility
 is provided so that a given SP can make his own trade-offs between
 scalability and optimality.  An SP can even allow some multicast
 groups in some VPNs to receive optimal routing, while others do not.
 Of course, the cost of this flexibility is an increase in the number
 of options provided by the protocols.
 The basic technique for providing scalability is to aggregate a
 number of customer multicast flows onto a single multicast
 distribution tree through the P routers.  A number of aggregation
 methods are supported.
 The procedures defined in this document also accommodate the SP that
 does not want to build multicast distribution trees in his backbone
 at all; the ingress PE can replicate each multicast data packet and
 then unicast each replica through a tunnel to each egress PE that
 needs to receive the data.

2.1.1. Multicast Distribution Trees

 This document supports the use of 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".  An Inclusive Tree that carries the traffic of more
 than one MVPN is an "Aggregate Inclusive Tree".  An Inclusive Tree
 contains, as its members, all the PEs that attach to any of the MVPNs
 using the tree.
 With this option, even if each tree supports only one MVPN, the upper
 bound on the amount of state maintained by the P routers is
 proportional to the number of VPNs supported rather than to the
 number of multicast flows in those VPNs.  If the trees are
 unidirectional, it would be more accurate to say that the state is
 proportional to the product of the number of VPNs and the average
 number of PEs per VPN.  The amount of state maintained by the P
 routers can be further reduced by aggregating more MVPNs onto a
 single tree.  If each such tree supports a set of MVPNs, (call it an
 "MVPN aggregation set"), the state maintained by the P routers is

Rosen & Aggarwal Standards Track [Page 7] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 proportional to the product of the number of MVPN aggregation sets
 and the average number of PEs per MVPN.  Thus, the state does not
 grow linearly with the number of MVPNs.
 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 that enable a single multicast
 distribution tree in the backbone to 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.  By default, 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 that 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.

2.1.2. Ingress Replication through Unicast Tunnels

 This document also provides procedures for carrying MVPN data traffic
 through 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 it puts the entire replication load on the ingress PE
 router and makes no attempt to optimize the multicast routing.

2.2. Multicast Routing Adjacencies

 In BGP/MPLS IP VPNs [RFC4364], each CE (Customer Edge) 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 multicast routing adjacency of a PE router, but CE
 routers at different sites do not become multicast routing
 adjacencies of each other.
 We will use the term "C-tree" to refer to a multicast distribution
 tree whose nodes include CE routers.  (See Section 3.1 for further
 explication of this terminology.)

Rosen & Aggarwal Standards Track [Page 8] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 The multicast routing protocol on the PE-CE link is presumed to be
 PIM (Protocol Independent Multicast) [PIM-SM].  Both the ASM (Any-
 Source Multicast) and the SSM (Source-Specific Multicast) service
 models are supported.  Thus, both shared C-trees and source-specific
 C-trees are supported.  Shared C-trees may be unidirectional or
 bidirectional; in the latter case, the multicast routing protocol is
 presumed to be the BIDIR-PIM [BIDIR-PIM] "variant" of PIM-SM.  A CE
 router exchanges "ordinary" PIM control messages with the PE router
 to which it is attached.
 Support for PIM-DM (Dense Mode) is outside the scope of this
 document.
 The PEs attaching to a particular MVPN then have to exchange the
 multicast routing information with each other.  Two basic methods for
 doing this are defined: (1) PE-PE PIM and (2) BGP.  In the former
 case, the PEs need to be multicast routing adjacencies of each other.
 In the latter case, they do not.  For example, each PE may be a BGP
 adjacency of a route reflector (RR) and not of any other PEs.
 In order to support the "Carrier's Carrier" model of [RFC4364], mLDP
 (Label Distribution Protocol Extensions for Multipoint Label Switched
 Paths) [MLDP] may also be supported on the PE-CE interface.  The use
 of mLDP on the PE-CE interface is described in [MVPN-BGP].  The use
 of BGP on the PE-CE interface is not within the scope of this
 document.

2.3. MVPN Definition

 An MVPN is defined by two sets of sites: the Sender Sites set and the
 Receiver Sites set, with the following properties:
  1. Hosts within the Sender Sites set could originate multicast

traffic for receivers in the Receiver Sites set.

  1. Receivers not in the Receiver Sites set should not be able to

receive this traffic.

  1. Hosts within the Receiver Sites set could receive multicast

traffic originated by any host in the Sender Sites set.

  1. Hosts within the Receiver Sites set should not be able to receive

multicast traffic originated by any host that is not in the

     Sender Sites set.

Rosen & Aggarwal Standards Track [Page 9] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 A site could be both in the Sender Sites set and Receiver Sites set,
 which implies that hosts within such a site could both originate and
 receive multicast traffic.  An extreme case is when the Sender Sites
 set is the same as the Receiver Sites set, in which case all sites
 could originate and receive multicast traffic from each other.
 Sites within a given MVPN may be either within the same organization
 or in different organizations, which implies that an MVPN can be
 either an Intranet or an Extranet.
 A given site may be in more than one MVPN, which implies that MVPNs
 may overlap.
 Not all sites of a given MVPN have to be connected to the same
 service provider, which implies that an MVPN can span multiple
 service providers.
 Another way to look at MVPN is to say that an MVPN is defined by a
 set of administrative policies.  Such policies determine both the
 Sender Sites set and Receiver Sites set.  Such policies are
 established by MVPN customers, but implemented/realized by MVPN
 Service Providers using the existing BGP/MPLS VPN mechanisms, such as
 Route Targets (RTs), with extensions, as necessary.

2.4. Auto-Discovery

 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.  (Strictly speaking,
 a PE in the Receiver Sites set need only discover the other PEs in
 the Sender Sites set, and a PE in the Sender Sites set need only
 discover the other PEs in the Receiver Sites set.) This is referred
 to as "MVPN Auto-Discovery".
 This document discusses two ways of providing MVPN auto-discovery:
  1. BGP can be used for discovering and maintaining MVPN membership.

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 [RFC4364]) that is configured to contain
     the multicast routing information of that MVPN.  This auto-
     discovery option does not make any assumptions about the methods
     used for transmitting MVPN multicast data packets through the
     backbone.

Rosen & Aggarwal Standards Track [Page 10] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

  1. If it is known that the PE-PE multicast control packets (i.e.,

PIM packets) of a particular MVPN are to be transmitted through a

     non-aggregated Inclusive Tree supporting the ASM service model
     (e.g., through a tree that is created by non-SSM PIM-SM or by
     BIDIR-PIM), and if the PEs attaching to that MVPN are configured
     with the group address corresponding to that tree, then the PEs
     can auto-discover each other simply by joining the tree and then
     multicasting PIM Hellos over the tree.

2.5. PE-PE Multicast Routing Information

 The BGP/MPLS IP VPN [RFC4364] 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 BGP routing
 adjacency with other PE routers to exchange MVPN control information.
 BGP also provides reliable transport and uses incremental updates.
 Another option is the use of the currently existing "soft state" PIM
 standard [PIM-SM] that uses periodic complete updates.

2.6. PE-PE Multicast Data Transmission

 Like [RFC4364], this document decouples the procedures for exchanging
 routing information from the procedures for transmitting data
 traffic.  Hence, a variety of transport technologies may be used in
 the backbone.  For Inclusive Trees, these transport technologies
 include unicast PE-PE tunnels, using encapsulation in MPLS, IP, or
 GRE (Generic Routing Encapsulation), multicast distribution trees
 created by PIM (either unidirectional in the SSM or ASM service
 models or bidirectional) using IP/GRE encapsulation, point-to-
 multipoint LSPs created by RSVP - Traffic Engineering (RSVP-TE) or
 mLDP, and multipoint-to-multipoint LSPs created by mLDP.
 In order to aggregate traffic from multiple MVPNs onto a single
 multicast distribution tree, it is necessary to have a mechanism to
 enable the egresses of the tree to demultiplex the multicast traffic
 received over the tree and to associate each received packet with a
 particular MVPN.  This document specifies a mechanism whereby
 upstream label assignment [MPLS-UPSTREAM-LABEL] is used by the root
 of the tree to assign a label to each flow.  This label is used by

Rosen & Aggarwal Standards Track [Page 11] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 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 encapsulations for
 supporting the various SP multicast transport options.
 The specification for aggregating traffic of multiple MVPNs onto a
 single multipoint-to-multipoint LSP or onto a single bidirectional
 multicast distribution tree is outside the scope of this document.
 The specifications for using, as Selective Trees, multicast
 distribution trees that support the ASM service model are outside the
 scope of this document.  The specification for using multipoint-to-
 multipoint LSPs as Selective Trees is outside the scope of this
 document.
 This document assumes that when SP multicast trees are used, traffic
 for a particular multicast group is transmitted by a particular PE on
 only one SP multicast tree.  The use of multiple SP multicast trees
 for transmitting traffic belonging to a particular multicast group is
 outside the scope of this document.

2.7. Inter-AS MVPNs

 [RFC4364] describes different options for supporting BGP/MPLS IP
 unicast VPNs whose provider backbones contain more than one
 Autonomous System (AS).  These are known as "inter-AS VPNs".  In an
 inter-AS VPN, the ASes may belong to the same provider or to
 different providers.  This document describes how inter-AS MVPNs can
 be supported for each of the unicast BGP/MPLS VPN inter-AS options.
 This document also specifies a model where inter-AS MVPN service can
 be offered without requiring a single SP multicast tree to span
 multiple ASes.  In this model, an inter-AS multicast tree consists of
 a number of "segments", one per AS, that are stitched together at AS
 boundary points.  These are known as "segmented inter-AS trees".
 Each segment of a segmented inter-AS tree may use a different
 multicast transport technology.
 It is also possible to support inter-AS MVPNs with non-segmented
 source trees that extend across AS boundaries.

Rosen & Aggarwal Standards Track [Page 12] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

2.8. Optionally Eliminating Shared Tree State

 This document also discusses some options and protocol extensions
 that can be used to eliminate the need for the PE routers to
 distribute to each other the (*,G) and (*,G,rpt) states that occur
 when the VPNs are creating unidirectional C-trees to support the ASM
 service model.

3. Concepts and Framework

3.1. PE-CE Multicast Routing

 Support of multicast in BGP/MPLS IP VPNs is modeled closely after the
 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" (VPN
 Routing and Forwarding Table), defined in [RFC4364], 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 do not attempt to provide support for every
 possible multicast routing protocol that could possibly run on the
 PE-CE link.  Rather, we consider multicast C-instances only for the
 following multicast routing protocols:
  1. PIM Sparse Mode (PIM-SM), supporting the ASM service model
  1. PIM Sparse Mode, supporting the SSM service model
  1. PIM Bidirectional Mode (BIDIR-PIM), which uses bidirectional

C-trees to support the ASM service model.

 In order to support the "Carrier's Carrier" model of [RFC4364], mLDP
 may also be supported on the PE-CE interface.  The use of mLDP on the
 PE-CE interface is described in [MVPN-BGP].

Rosen & Aggarwal Standards Track [Page 13] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 The use of BGP on the PE-CE interface is not within the scope of this
 document.
 As the only multicast C-instances discussed by this document are 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 the 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.
 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 that is processed by the PIM
 P-instance, and a C-Join would be a PIM Join that 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.  A C-tree is a multicast distribution tree
 constructed and maintained by the PIM C-instances.  A C-flow is a
 stream of multicast packets with a common C-source address and a
 common C-group address.  We will use the notation "(C-S,C-G)" to
 identify specific C-flows.  If a particular C-tree is a shared tree
 (whether unidirectional or bidirectional) rather than a source-
 specific tree, we will sometimes speak of the entire set of flows
 traveling that tree, identifying the set as "(C-*,C-G)".

3.2. P-Multicast Service Interfaces (PMSIs)

 A PE must have the ability to forward multicast data packets received
 from a CE 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 one or more PMSIs connecting those PE routers
 and/or subsets thereof.  A PMSI is a conceptual "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 the packet
 will be able to determine the MVPN to which the packet belongs.

Rosen & Aggarwal Standards Track [Page 14] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 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 "P-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 that is used to instantiate a
 PMSI may allow a single P-tunnel to carry the data of multiple PMSIs.
 In this document, we make 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 "P-tunnel" is used to refer
 to the transport mechanism that instantiates a service.
 PMSIs are used to carry C-multicast data traffic.  The C-multicast
 data traffic travels along a C-tree, but in the SP backbone all
 C-trees are tunneled through P-tunnels.  Thus, we will sometimes talk
 of a P-tunnel carrying one or more C-trees.
 Some of the options for passing multicast control traffic among the
 PEs do so by sending the control traffic through a PMSI; other
 options do not send control traffic through a PMSI.

3.2.1. Inclusive and Selective PMSIs

 We will distinguish between three different kinds of PMSIs:
  1. "Multidirectional Inclusive" PMSI (MI-PMSI)
     A Multidirectional Inclusive PMSI is one that 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 P-tunnel or
     P-tunnels that instantiate 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.
  1. "Unidirectional Inclusive" PMSI (UI-PMSI)
     A Unidirectional Inclusive PMSI is one that 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

Rosen & Aggarwal Standards Track [Page 15] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

     MVPN.  There is, at most, one UI-PMSI per PE per MVPN, though the
     P-tunnel that instantiates a UI-PMSI may, in fact, carry the data
     of more than one PMSI.
  1. "Selective" PMSI (S-PMSI).
     A Selective PMSI is one that 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.  The P-tunnel
     that instantiates a given S-PMSI may carry data from multiple
     S-PMSIs.
 In later sections, we describe the role played by these different
 kinds of PMSIs.  We will use the term "I-PMSI" when we are not
 distinguishing between "MI-PMSIs" and "UI-PMSIs".

3.2.2. P-Tunnels Instantiating PMSIs

 The P-tunnels that are used to instantiate PMSIs will be referred to
 as "P-tunnels".  A number of different tunnel setup techniques can be
 used to create the P-tunnels that instantiate the PMSIs.  Among these
 are the following:
  1. PIM
     A PMSI can be instantiated as (a set of) Multicast Distribution
     trees created by the PIM P-instance ("P-trees").
     The multicast distribution trees that instantiate I-PMSIs may be
     either shared trees or source-specific trees.
     This document (along with [MVPN-BGP]) specifies procedures for
     identifying a particular (C-S,C-G) flow and assigning it to a
     particular S-PMSI.  Such an S-PMSI is most naturally instantiated
     as a source-specific tree.
     The use of shared trees (including bidirectional trees) to
     instantiate S-PMSIs is outside the scope of this document.
     The use of PIM-DM to create P-tunnels is not supported.
     P-tunnels may be shared by multiple MVPNs (i.e., a given P-tunnel
     may be the instantiation of multiple PMSIs), as long as the
     tunnel encapsulation provides some means of demultiplexing the
     data traffic by MVPN.

Rosen & Aggarwal Standards Track [Page 16] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

  1. mLDP
     mLDP Point-to-Multipoint (P2MP) LSPs or Multipoint-to-Multipoint
     (MP2MP) LSPs can be used to instantiate I-PMSIs.
     An S-PMSI or a UI-PMSI could be instantiated as a single mLDP
     P2MP LSP, whereas an MI-PMSI would have to be instantiated as a
     set of such LSPs (each PE in the MVPN being the root of one such
     LSP) or as a single MP2MP LSP.
     Procedures for sharing MP2MP LSPs across multiple MVPNs are
     outside the scope of this document.
     The use of MP2MP LSPs to instantiate S-PMSIs is outside the scope
     of this document.
     Section 11.2.3 discusses a way of using a partial mesh of MP2MP
     LSPs to instantiate a PMSI.  However, a full specification of the
     necessary procedures is outside the scope of this document.
  1. RSVP-TE
     A PMSI may be instantiated as one or more RSVP-TE Point-to-
     Multipoint (P2MP) LSPs.  An S-PMSI or a UI-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.
  1. A Mesh of Unicast P-Tunnels.
     If a PMSI is implemented as a mesh of unicast P-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 P-tunnels among that MVPN's PEs.  A UI-PMSI or an S-PMSI
     can be instantiated as a partial mesh.
 It can be seen that each method of implementing PMSIs has its own
 area of applicability.  Therefore, this specification allows for the
 use of any of these methods.  At first glance, this may seem like an
 overabundance of options.  However, the history of multicast
 development and deployment should make it clear that there is no one
 option that is always acceptable.  The use of segmented inter-AS
 trees does allow each SP to select the option that it finds most
 applicable in its own environment, without causing any other SP to
 choose that same option.

Rosen & Aggarwal Standards Track [Page 17] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 SPECIFYING THE CONDITIONS UNDER WHICH A PARTICULAR TREE-BUILDING
 METHOD IS APPLICABLE IS OUTSIDE THE SCOPE OF THIS DOCUMENT.
 The choice of the tunnel technique belongs to the sender router and
 is a local policy decision of that router.  The procedures defined
 throughout this document do not mandate that the same tunnel
 technique be used for all P-tunnels going through a given provider
 backbone.  However, it is expected that any tunnel technique that can
 be used by a PE for a particular MVPN is also supported by all the
 other PEs having VRFs for the MVPN.  Moreover, the use of ingress
 replication by any PE for an MVPN implies that all other PEs MUST use
 ingress replication for this MVPN.

3.3. Use of PMSIs for Carrying Multicast Data

 Each PE supporting a particular MVPN must have a way of discovering
 the following information:
  1. The set of other PEs in its AS that are attached to sites of that

MVPN, and the set of other ASes that have PEs attached to sites

     of that MVPN.  However, if non-segmented inter-AS trees are used
     (see Section 8.1), then each PE needs to know the entire set of
     PEs attached to sites of that MVPN.
  1. If segmented inter-AS trees are to be used, the set of border

routers in its AS that support inter-AS connectivity for that

     MVPN.
  1. If the MVPN is configured to use an MI-PMSI, the information

needed to set up and to use the P-tunnels instantiating the

     MI-PMSI.
  1. For each other PE, whether the PE supports Aggregate Trees for

the MVPN, and if so, the demultiplexing information that must be

     provided so that the other PE can determine whether a packet that
     it received on an Aggregate Tree belongs to this MVPN.
 In some cases, the information above is provided by means of the BGP-
 based auto-discovery procedures discussed in Section 4 of this
 document and in Section 9 of [MVPN-BGP].  In other cases, this
 information is provided after discovery is complete, by means of
 procedures discussed in Section 7.4.  In either case, the information
 that is provided must be sufficient to enable the PMSI to be bound to
 the identified P-tunnel, to enable the P-tunnel to be created if it
 does not already exist, and to enable the different PMSIs that may
 travel on the same P-tunnel to be properly demultiplexed.

Rosen & Aggarwal Standards Track [Page 18] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 If an MVPN uses an MI-PMSI, then the information needed to identify
 the P-tunnels that instantiate the MI-PMSI has to be known to the PEs
 attached to the MVPN before any data can be transmitted on the
 MI-PMSI.  This information is either statically configured or auto-
 discovered (see Section 4).  The actual process of constructing the
 P-tunnels (e.g., via PIM, RSVP-TE, or mLDP) SHOULD occur as soon as
 this information is known.
 When MI-PMSIs are used, they may serve as the default method of
 carrying C-multicast data traffic.  When we say that an MI-PMSI is
 the "default" method of carrying C-multicast data traffic for a
 particular MVPN, we mean that it is not necessary to use any special
 control procedures to bind a particular C-flow to the MI-PMSI; any
 C-flows that have not been bound to other PMSIs will be assumed to
 travel through the MI-PMSI.
 There is no requirement to use MI-PMSIs as the default method of
 carrying C-flows.  It is possible to adopt a policy in which all
 C-flows are carried on UI-PMSIs or S-PMSIs.  In this case, if an
 MI-PMSI is not used for carrying routing information, it is not
 needed at all.
 Even when an MI-PMSI is used as the default method of carrying an
 MVPN's C-flows, if a particular C-flow has certain characteristics,
 it may be desirable to migrate it from the MI-PMSI to an S-PMSI.
 These characteristics, as well as the procedures for migrating a
 C-flow from an MI-PMSI to an S-PMSI, are discussed in Section 7.
 Sometimes a set of C-flows are traveling the same, shared, C-tree
 (e.g., either unidirectional or bidirectional), and it may be
 desirable to move the whole set of C-flows as a unit to an S-PMSI.
 Procedures for doing this are outside the scope of this
 specification.
 Some of the procedures for transmitting C-multicast routing
 information among the PEs require that the routing information be
 sent over an MI-PMSI.  Other procedures do not use an MI-PMSI to
 transmit the C-multicast routing information.
 For a given MVPN, whether an MI-PMSI is used to carry C-multicast
 routing information is independent from whether an MI-PMSI is used as
 the default method of carrying the C-multicast data traffic.
 As previously stated, it is possible to send all C-flows on a set of
 S-PMSIs, omitting any usage of I-PMSIs.  This prevents PEs from
 receiving data that they don't need, at the cost of requiring
 additional P-tunnels, and additional signaling to bind the C-flows to
 P-tunnels.  Cost-effective instantiation of S-PMSIs is likely to

Rosen & Aggarwal Standards Track [Page 19] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 require Aggregate P-trees, which, in turn, makes it necessary for the
 transmitting PE to know which PEs need to receive which multicast
 streams.  This is known as "explicit tracking", and the procedures to
 enable explicit tracking may themselves impose a cost.  This is
 further discussed in Section 7.4.1.2.

3.4. PE-PE Transmission of C-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 be either configured or discovered as part of the auto-discovery
 process.

3.4.1. PIM Peering

3.4.1.1. Full per-MVPN PIM Peering across an MI-PMSI

 If the set of PEs attached to a given MVPN are connected via an
 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 that 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 procedure is fully specified in Section 5.2.

3.4.1.2. Lightweight PIM Peering across an MI-PMSI

 The procedure of the previous Section has the following
 disadvantages:
  1. 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.

Rosen & Aggarwal Standards Track [Page 20] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

  1. 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 the BGP-based auto-discovery
 procedure of Section 4.  In that case, the periodic Hellos would
 serve no function and could 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/Prune messages is not as simple to
 eliminate.  If and when "refresh reduction" procedures are specified
 for PIM, it may be useful to incorporate them, so as to make the
 lightweight PIM peering procedures even more lightweight.
 Lightweight PIM peering is not specified in this document.

3.4.1.3. Unicasting of PIM C-Join/Prune Messages

 PIM does not require that the C-Join/Prune messages that 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 that 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
 Reduction may be considered to be a prerequisite for the procedure of
 unicasting the C-Join/Prune messages.
 When the C-Join/Prune messages are unicast, they are not transmitted
 on a PMSI at all.  Note that the procedure of unicasting the
 C-Join/Prune messages is different than the procedure of transmitting
 the C-Join/Prune messages on an MI-PMSI that is instantiated as a
 mesh of unicast P-tunnels.
 If there are multiple PEs that can be used to reach a given C-source,
 procedures described in Sections 5.1 and 9 MUST be used to ensure
 that duplicate packets do not get delivered.

Rosen & Aggarwal Standards Track [Page 21] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 Procedures for unicasting the PIM control messages are not further
 specified in this document.

3.4.2. Using BGP to Carry C-Multicast Routing

 It is possible to use BGP to carry C-multicast routing information
 from PE to PE, dispensing entirely with the transmission of
 C-Join/Prune messages from PE to PE.  This is discussed in Section
 5.3 and fully specified in [MVPN-BGP].

4. BGP-Based Auto-Discovery of MVPN Membership

 BGP-based auto-discovery is done by means of a new address family,
 the MCAST-VPN address family.  (This address family also has other
 uses, as will be seen later.)  Any PE that attaches to an MVPN must
 issue a BGP Update message containing an NLRI ("Network Layer
 Reachability Information" element) in this address family, along with
 a specific set of attributes.  In this document, we specify the
 information that must be contained in these BGP Updates in order to
 provide auto-discovery.  The encoding details, along with the
 complete set of detailed procedures, are specified in a separate
 document [MVPN-BGP].
 This section specifies the intra-AS BGP-based auto-discovery
 procedures.  When segmented inter-AS trees are used, additional
 procedures are needed, as specified in [MVPN-BGP].  (When segmented
 inter-AS trees are not used, the inter-AS procedures are almost
 identical to the intra-AS procedures.)
 BGP-based auto-discovery uses a particular kind of MCAST-VPN route
 known as an "auto-discovery route", or "A-D route".  In particular,
 it uses two kinds of "A-D routes": the "Intra-AS I-PMSI A-D route"
 and the "Inter-AS I-PMSI A-D route".  (There are also additional
 kinds of A-D routes, such as the Source Active A-D routes, which are
 used for purposes that go beyond auto-discovery.  These are discussed
 in subsequent sections.)
 The Inter-AS I-PMSI A-D route is used only when segmented inter-AS
 P-tunnels are used, as specified in [MVPN-BGP].
 The "Intra-AS I-PMSI A-D route" is originated by the PEs that are
 (directly) connected to the site(s) of an MVPN.  It is distributed to
 other PEs that attach to sites of the MVPN.  If segmented inter-AS
 P-tunnels are used, then the Intra-AS I-PMSI A-D routes are not
 distributed outside the AS where they originate; if segmented inter-
 AS P-tunnels are not used, then the Intra-AS I-PMSI A-D routes are,
 despite their name, distributed to all PEs attached to the VPN, no
 matter what AS the PEs are in.

Rosen & Aggarwal Standards Track [Page 22] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 The NLRI of an Intra-AS I-PMSI A-D route must contain the following
 information:
  1. The route type (i.e., Intra-AS I-PMSI A-D route).
  1. The IP address of the originating PE.
  1. An RD ("Route Distinguisher", [RFC4364]) configured locally for

the MVPN. This is an RD that can be prepended to that IP address

     to form a globally unique VPN-IP address of the PE.
 Intra-AS I-PMSI A-D routes carry the following attributes:
  1. Route Target Extended Communities attribute.
     One or more of these MUST be carried by each Intra-AS I-PMSI A-D
     route.  If any other PE has one of these Route Targets configured
     for import into 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.  More specifically,
     it allows a PE in the Receiver Sites set to discover the PEs in
     the Sender Sites set of the MVPN, and the PEs in the Sender Sites
     set of the MVPN to discover the PEs in the Receiver Sites set of
     the MVPN.  The PEs in the Receiver Sites set would be configured
     to import the Route Targets advertised in the BGP A-D routes by
     PEs in the Sender Sites set.  The PEs in the Sender Sites set
     would be configured to import the Route Targets advertised in the
     BGP A-D routes by PEs in the Receiver Sites set.
  1. PMSI Tunnel attribute.
     This attribute is present whenever the MVPN uses an MI-PMSI or
     when it uses a UI-PMSI rooted at the originating router.  It
     contains the following information:
  • tunnel technology, which may be one of the following:
           + Bidirectional multicast tree created by BIDIR-PIM,
           + Source-specific multicast tree created by PIM-SM,
             supporting the SSM service model,
           + Set of trees (one shared tree and a set of source trees)
             created by PIM-SM using the ASM service model,
           + Point-to-multipoint LSP created by RSVP-TE,
           + Point-to-multipoint LSP created by mLDP,

Rosen & Aggarwal Standards Track [Page 23] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

           + multipoint-to-multipoint LSP created by mLDP
           + unicast tunnel
  • P-tunnel identifier
         Before a P-tunnel can be constructed to instantiate the
         I-PMSI, the PE must be able to create a unique identifier for
         the tunnel.  The syntax of this identifier depends on the
         tunnel technology used.
         Each PE attaching to a given MVPN must be configured with
         information specifying the allowable encapsulations to use
         for that MVPN, as well as the particular one of those
         encapsulations that the PE is to identify in the PMSI Tunnel
         attribute of the Intra-AS I-PMSI A-D routes that it
         originates.
  • Multi-VPN aggregation capability and demultiplexor value.
         This specifies whether the P-tunnel is capable of 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.
  1. PE Distinguisher Labels Attribute
     Sometimes it is necessary for one PE to advertise an upstream-
     assigned MPLS label that identifies another PE.  Under certain
     circumstances to be discussed later, a PE that is the root of a
     multicast P-tunnel will bind an MPLS label value to one or more
     of the PEs that belong to the P-tunnel, and it will distribute
     these label bindings using Intra-AS I-PMSI A-D routes.
     Specification of when this must be done is provided in Sections
     6.4.4 and 11.2.2.  We refer to these as "PE Distinguisher
     Labels".
     Note that, as specified in [MPLS-UPSTREAM-LABEL], PE
     Distinguisher Label values are unique only in the context of the
     IP address identifying the root of the P-tunnel; they are not
     necessarily unique per tunnel.

Rosen & Aggarwal Standards Track [Page 24] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

5. PE-PE Transmission of C-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.  This
 is known as the "PE-PE transmission of C-multicast routing
 information".
 This section specifies the procedures used for PE-PE transmission of
 C-multicast routing information.  Not every procedure mentioned in
 Section 3.4 is specified here.  Rather, this section focuses on two
 particular procedures:
  1. Full PIM Peering.
     This procedure is fully specified herein.
  1. Use of BGP to distribute C-multicast routing
     This procedure is described herein, but the full specification
     appears in [MVPN-BGP].
 Those aspects of the procedures that apply to both of the above are
 also specified fully herein.
 Specification of other procedures is outside the scope of this
 document.

5.1. Selecting the Upstream Multicast Hop (UMH)

 When a PE receives a C-Join/Prune message from a CE, the message
 identifies a particular multicast flow as belonging either to a
 source-specific tree (S,G) or to a shared tree (*,G).  Throughout
 this section, we use the term "C-root" to refer to S, in the case of
 a source-specific tree, or to the Rendezvous Point (RP) for G, in the
 case of (*,G).  If the route to the C-root is across the VPN
 backbone, then the PE needs to find the "Upstream Multicast Hop"
 (UMH) for the (S,G) or (*,G) flow.  The UMH is either the PE at which
 (S,G) or (*,G) data packets enter the VPN backbone or the Autonomous
 System Border Router (ASBR) at which those data packets enter the
 local AS when traveling through the VPN backbone.  The process of
 finding the upstream multicast hop for a given C-root is known as
 "upstream multicast hop selection".

Rosen & Aggarwal Standards Track [Page 25] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

5.1.1. Eligible Routes for UMH Selection

 In the simplest case, the PE does the upstream hop selection by
 looking up the C-root in the unicast VRF associated with the PE-CE
 interface over which the C-Join/Prune message was received.  The
 route that matches the C-root will contain the information needed to
 select the UMH.
 However, in some cases, the CEs may be distributing to the PEs a
 special set of routes that are to be used exclusively for the purpose
 of upstream multicast hop selection, and not used for unicast routing
 at all.  For example, when BGP is the CE-PE unicast routing protocol,
 the CEs may be using Subsequent Address Family Identifier 2 (SAFI 2)
 to distribute a special set of routes that are to be used for, and
 only for, upstream multicast hop selection.  When OSPF [OSPF] is the
 CE-PE routing protocol, the CE may use an MT-ID (Multi-Topology
 Identifier) [OSPF-MT] of 1 to distribute a special set of routes that
 are to be used for, and only for, upstream multicast hop selection.
 When a CE uses one of these mechanisms to distribute to a PE a
 special set of routes to be used exclusively for upstream multicast
 hop selection, these routes are distributed among the PEs using SAFI
 129, as described in [MVPN-BGP].  Whether the routes used for
 upstream multicast hop selection are (a) the "ordinary" unicast
 routes or (b) a special set of routes that are used exclusively for
 upstream multicast hop selection is a matter of policy.  How that
 policy is chosen, deployed, or implemented is outside the scope of
 this document.  In the following, we will simply refer to the set of
 routes that are used for upstream multicast hop selection, the
 "Eligible UMH routes", with no presumptions about the policy by which
 this set of routes was chosen.

5.1.2. Information Carried by Eligible UMH Routes

 Every route that is eligible for UMH selection SHOULD carry a VRF
 Route Import Extended Community [MVPN-BGP].  However, if BGP is used
 to distribute C-multicast routing information, or if the route is
 from a VRF that belongs to a multi-AS VPN as described in option b of
 Section 10 of [RFC4364], then the route MUST carry a VRF Route Import
 Extended Community.  This attribute identifies the PE that originated
 the route.
 If BGP is used for carrying C-multicast routes, OR if "Segmented
 inter-AS Tunnels" are used, then every UMH route MUST also carry a
 Source AS Extended Community [MVPN-BGP].
 These two attributes are used in the upstream multicast hop selection
 procedures described below.

Rosen & Aggarwal Standards Track [Page 26] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

5.1.3. Selecting the Upstream PE

 The first step in selecting the upstream multicast hop for a given
 C-root is to select the Upstream PE router for that C-root.
 The PE that received the C-Join message from a CE looks in the VRF
 corresponding to the interfaces over which the C-Join was received.
 It finds the Eligible UMH route that is the best match for the C-root
 specified in that C-Join.  Call this the "Installed UMH Route".
 Note that the outgoing interface of the Installed UMH Route may be
 one of the interfaces associated with the VRF, in which case the
 upstream multicast hop is a CE and the route to the C-root is not
 across the VPN backbone.
 Consider the set of all VPN-IP routes that (a) are eligible to be
 imported into the VRF (as determined by their Route Targets), (b) are
 eligible to be used for upstream multicast hop selection, and (c)
 have exactly the same IP prefix (not necessarily the same RD) as the
 installed UMH route.
 For each route in this set, determine the corresponding Upstream PE
 and Upstream RD.  If a route has a VRF Route Import Extended
 Community, the route's Upstream PE is determined from it.  If a route
 does not have a VRF Route Import Extended Community, the route's
 Upstream PE is determined from the route's BGP Next Hop.  In either
 case, the Upstream RD is taken from the route's NLRI.
 This results in a set of triples of <route, Upstream PE, Upstream
 RD>.
 Call this the "UMH Route Candidate Set".  Then, the PE MUST select a
 single route from the set to be the "Selected UMH Route".  The
 corresponding Upstream PE is known as the "Selected Upstream PE", and
 the corresponding Upstream RD is known as the "Selected Upstream RD".
 There are several possible procedures that can be used by a PE to
 select a single route from the candidate set.
 The default procedure, which MUST be implemented, is to select the
 route whose corresponding Upstream PE address is numerically highest,
 where a 32-bit IP address is treated as a 32-bit unsigned integer.
 Call this the "default Upstream PE selection".  For a given C-root,
 provided that the routing information used to create the candidate
 set is stable, all PEs will have the same default Upstream PE
 selection.  (Though different default Upstream PE selections may be
 chosen during a routing transient.)

Rosen & Aggarwal Standards Track [Page 27] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 An alternative procedure that MUST be implemented, but which is
 disabled by default, is the following.  This procedure ensures that,
 except during a routing transient, each PE chooses the same Upstream
 PE for a given combination of C-root and C-G.
    1. The PEs in the candidate set are numbered from lowest to
       highest IP address, starting from 0.
    2. The following hash is performed:
  1. A bytewise exclusive-or of all the bytes in the C-root

address and the C-G address is performed.

  1. The result is taken modulo n, where n is the number of PEs

in the candidate set. Call this result N.

 The Selected Upstream PE is then the one that appears in position N
 in the list of step 1.
 Other hashing algorithms are allowed as well, but not required.
 The alternative procedure allows a form of "equal cost load
 balancing".  Suppose, for example, that from egress PEs PE3 and PE4,
 source C-S can be reached, at equal cost, via ingress PE PE1 or
 ingress PE PE2.  The load balancing procedure makes it possible for
 PE1 to be the ingress PE for (C-S,C-G1) data traffic while PE2 is the
 ingress PE for (C-S,C-G2) data traffic.
 Another procedure, which SHOULD be implemented, is to use the
 Installed UMH Route as the Selected UMH Route.  If this procedure is
 used, the result is likely to be that a given PE will choose the
 Upstream PE that is closest to it, according to the routing in the SP
 backbone.  As a result, for a given C-root, different PEs may choose
 different Upstream PEs.  This is useful if the C-root is an anycast
 address, and can also be useful if the C-root is in a multihomed site
 (i.e., a site that is attached to multiple PEs).  However, this
 procedure is more likely to lead to steady state duplication of
 traffic unless (a) PEs discard data traffic that arrives from the
 "wrong" Upstream PE or (b) data traffic is carried only in non-
 aggregated S-PMSIs.  This issue is discussed at length in Section 9.
 General policy-based procedures for selecting the UMH route are
 allowed but not required, and they are not further discussed in this
 specification.

Rosen & Aggarwal Standards Track [Page 28] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

5.1.4. Selecting the Upstream Multicast Hop

 In certain cases, the Selected Upstream Multicast Hop is the same as
 the Selected Upstream PE.  In other cases, the Selected Upstream
 Multicast Hop is the ASBR that is the BGP Next Hop of the Selected
 UMH Route.
 If the Selected Upstream PE is in the local AS, then the Selected
 Upstream PE is also the Selected Upstream Multicast Hop.  This is the
 case if any of the following conditions holds:
  1. The Selected UMH Route has a Source AS Extended Community, and

the Source AS is the same as the local AS,

  1. The Selected UMH Route does not have a Source AS Extended

Community, but the route's BGP Next Hop is the same as the

     Upstream PE.
 Otherwise, the Selected Upstream Multicast Hop is an ASBR.  The
 method of determining just which ASBR it is depends on the particular
 inter-AS signaling method being used (PIM or BGP) and on whether
 segmented or non-segmented inter-AS tunnels are used.  These details
 are presented in later sections.

5.2. Details of Per-MVPN Full 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 full PIM adjacencies with
 each other over that LAN interface.
 The use of PIM when an MI-PMSI is not in use is outside the scope of
 this document.
 To form full PIM adjacencies, the PEs execute the standard PIM
 procedures on the LAN interface, including the generation and
 processing of PIM Hello, Join/Prune, Assert, DF (Designated
 Forwarder) election, and other PIM control messages.  These are
 executed independently for each C-instance.  PIM "Join suppression"
 SHOULD be enabled.

5.2.1. PIM C-Instance Control Packets

 All IPv4 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 P-tunnel that is being used to instantiate the

Rosen & Aggarwal Standards Track [Page 29] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 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.
 The handling of IPv6 PIM C-instance control packets will be specified
 in a follow-on document.
 As specified in Section 5.1.2, when PIM is being used to distribute
 C-multicast routing information, any PE distributing VPN-IP routes
 that are eligible for use as UMH routes SHOULD include a VRF Route
 Import Extended Community with each route.  For a given VRF, the
 Global Administrator field of the VRF Route Import Extended Community
 MUST be set to the same IP address that the PE places in the IP
 source address field of the PE-PE PIM control messages it originates
 from that VRF.
 Note that BSR (Bootstrap Router Mechanism for PIM) [BSR] messages are
 treated the same as PIM C-instance control packets, and BSR
 processing is regarded as an integral part of the PIM C-instance
 processing.

5.2.2. PIM C-Instance Reverse Path Forwarding (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.  Therefore, it is necessary to specify special
 procedures for determining when the MI-PMSI is to be regarded as the
 "RPF Interface" for a particular C-address.
 The PE follows the procedures of Section 5.1 to determine the
 Selected UMH Route.  If that route is NOT a VPN-IP route learned from
 BGP as described in [RFC4364], 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 Selected UMH Route is a VPN-IP route whose outgoing
 interface is not one of the interfaces associated with the VRF, then
 PIM will consider the RPF 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-root is the MI-PMSI, it is necessary for PIM to determine the "RPF
 neighbor" for that C-root.  This will be one of the other PEs that is
 a PIM adjacency over the MI-PMSI.  In particular, it will be the
 "Selected Upstream PE", as defined in Section 5.1.

Rosen & Aggarwal Standards Track [Page 30] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

5.3. Use of BGP for Carrying C-Multicast Routing

 It is possible to use BGP to carry C-multicast routing information
 from PE to PE, dispensing entirely with the transmission of
 C-Join/Prune messages from PE to PE.  This section describes the
 procedures for carrying intra-AS multicast routing information.
 Inter-AS procedures are described in Section 8.  The complete
 specification of both sets of procedures and of the encodings can be
 found in [MVPN-BGP].

5.3.1. Sending BGP Updates

 The MCAST-VPN address family is used for this purpose.  MCAST-VPN
 routes used for the purpose of carrying C-multicast routing
 information are distinguished from those used for the purpose of
 carrying auto-discovery information by means of a "route type" field
 that is encoded into the NLRI.  The following information is required
 in BGP to advertise the MVPN routing information.  The NLRI contains
 the following:
  1. The type of C-multicast route
     There are two types:
  • source tree join
  • shared tree join
  1. The C-group address
  1. The C-source address (In the case of a shared tree join, this is

the address of the C-RP.)

  1. The Selected Upstream RD corresponding to the C-root address

(determined by the procedures of Section 5.1).

 Whenever a C-multicast route is sent, it must also carry the Selected
 Upstream Multicast Hop corresponding to the C-root address
 (determined by the procedures of Section 5.1).  The Selected Upstream
 Multicast Hop must be encoded as part of a Route Target Extended
 Community to facilitate the optional use of filters that can prevent
 the distribution of the update to BGP speakers other than the
 Upstream Multicast Hop.  See Section 10.1.3 of [MVPN-BGP] for the
 details.
 There is no C-multicast route corresponding to the PIM function of
 pruning a source off the shared tree when a PE switches from a
 (C-*,C-G) tree to a (C-S,C-G) tree.  Section 9 of this document

Rosen & Aggarwal Standards Track [Page 31] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 specifies a mandatory procedure that ensures that if any PE joins a
 (C-S,C-G) source tree, all other PEs that have joined or will join
 the (C-*,C-G) shared tree will also join the (C-S,C-G) source tree.
 This eliminates the need for a C-multicast route that prunes C-S off
 the (C-*,C-G) shared tree when switching from (C-*,C-G) to (C-S,C-G)
 tree.

5.3.2. Explicit Tracking

 Note that the upstream multicast hop is NOT part of the NLRI in the
 C-multicast BGP routes.  This means that if several PEs join the same
 C-tree, the BGP routes they distribute to do so are regarded by BGP
 as comparable routes, and only one will be installed.  If a route
 reflector is being used, this further means that the PE that is used
 to reach the C-source will know only that one or more of the other
 PEs have joined the tree, but it won't know which one.  That is, this
 BGP update mechanism does not provide "explicit tracking".  Explicit
 tracking is not provided by default because it increases the amount
 of state needed and thus decreases scalability.  Also, as
 constructing the C-PIM messages to send "upstream" for a given tree
 does not depend on knowing all the PEs that are downstream on that
 tree, there is no reason for the C-multicast route type updates to
 provide explicit tracking.
 There are some cases in which explicit tracking is necessary in order
 for the PEs to set up certain kinds of P-trees.  There are other
 cases in which explicit tracking is desirable in order to determine
 how to optimally aggregate multicast flows onto a given aggregate
 tree.  As these functions have to do with the setting up of
 infrastructure in the P-network, rather than with the dissemination
 of C-multicast routing information, any explicit tracking that is
 necessary is handled by sending a particular type of A-D route known
 as "Leaf A-D routes".
 Whenever a PE sends an A-D route with a PMSI Tunnel attribute, it can
 set a bit in the PMSI Tunnel attribute indicating "Leaf Information
 Required".  A PE that installs such an A-D route MUST respond by
 generating a Leaf A-D route, indicating that it needs to join (or be
 joined to) the specified PMSI Tunnel.  Details can be found in
 [MVPN-BGP].

5.3.3. Withdrawing BGP Updates

 A PE removes itself from a C-multicast tree (shared or source) by
 withdrawing the corresponding BGP Update.

Rosen & Aggarwal Standards Track [Page 32] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 If a PE has pruned a C-source from a shared C-multicast tree, and it
 needs to "unprune" that source from that tree, it does so by
 withdrawing the route that pruned the source from the tree.

5.3.4. BSR

 BGP does not provide a method for carrying the control information of
 BSR packets received by a PE from a CE.  BSR is supported by
 transmitting the BSR control messages from one PE in an MVPN to all
 the other PEs in that MVPN.
 When a PE needs to transmit a BSR message for a particular MVPN to
 other PEs, it must put its own IP address into the BSR message as the
 IP source address.  As specified in Section 5.1.2, when a PE
 distributes VPN-IP routes that are eligible for use as UMH routes,
 the PE MUST include a VRF Route Import Extended Community with each
 route.  For a given MVPN, a single such IP address MUST be used, and
 that same IP address MUST be used as the source address in all BSR
 packets that the PE transmits to other PEs.
 The BSR message may be transmitted over any PMSI that will deliver
 the message to all the other PEs in the MVPN.  If no such PMSI has
 been instantiated yet, then an appropriate P-tunnel must be
 advertised, and the C-flow whose C-source address is the address of
 the PE itself, and whose multicast group is ALL-PIM-ROUTERS
 (224.0.0.13), must be bound to it.  This can be done using the
 procedures described in Sections 7.3 and 7.4.  Note that this is NOT
 meant to imply that the other PIM control packets from the PIM
 C-instance are to be transmitted to the other PEs.
 When a PE receives a BSR message for a particular MVPN from some
 other PE, the PE accepts the message only if the IP source address in
 that message is the Selected Upstream PE (see Section 5.1.3) for the
 IP address of the Bootstrap router.  Otherwise, the PE simply
 discards the packet.  If the PE accepts the packet, it does normal
 BSR processing on it, and it may forward a BSR message to one or more
 CEs as a result.

6. PMSI Instantiation

 This section provides the procedures for using P-tunnels to
 instantiate a PMSI.  It describes the procedures for setting up and
 maintaining the P-tunnels as well as for sending and receiving C-data
 and/or C-control messages on the P-tunnels.  However, procedures for
 binding particular C-flows to particular P-tunnels are discussed in
 Section 7.

Rosen & Aggarwal Standards Track [Page 33] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 PMSIs can be instantiated either by P-multicast trees or by PE-PE
 unicast tunnels.  In the latter case, the PMSI is said to be
 instantiated by "ingress replication".
 This specification supports a number of different methods for setting
 up P-multicast trees: these are detailed below.  A P-tunnel may
 support a single VPN (a non-aggregated P-multicast tree) or multiple
 VPNs (an aggregated P-multicast tree).

6.1. Use of the Intra-AS I-PMSI A-D Route

6.1.1. Sending Intra-AS I-PMSI A-D Routes

 When a PE is provisioned to have one or more VRFs that provide MVPN
 support, the PE announces its MVPN membership information using
 Intra-AS I-PMSI A-D routes, as discussed in Section 4 and detailed in
 Section 9.1.1 of [MVPN-BGP].  (Under certain conditions, detailed in
 [MVPN-BGP], the Intra-AS I-PMSI A-D route may be omitted.)
 Generally, the Intra-AS I-PMSI A-D route will have a PMSI Tunnel
 attribute that identifies a P-tunnel that is being used to
 instantiate the I-PMSI.  Section 9.1.1 of [MVPN-BGP] details certain
 conditions under which the PMSI Tunnel attribute may be omitted (or
 in which a PMSI Tunnel attribute with the "no tunnel information
 present" bit may be sent).
 As a special case, when (a) C-PIM control messages are to be sent
 through an MI-PMSI and (b) the MI-PMSI is instantiated by a P-tunnel
 technique for which each PE needs to know only a single P-tunnel
 identifier per VPN, then the use of the Intra-AS I-PMSI A-D routes
 MAY be omitted, and static configuration of the tunnel identifier
 used instead.  However, this is not recommended for long-term use,
 and in all other cases, the Intra-AS I-PMSI A-D routes MUST be used.
 The PMSI Tunnel attribute MAY contain an upstream-assigned MPLS
 label, assigned by the PE originating the Intra-AS I-PMSI A-D route.
 If this label is present, the P-tunnel can be carrying data from
 several MVPNs.  The label is used on the data packets traveling
 through the tunnel to identify the MVPN to which those data packets
 belong.  (The specified label identifies the packet as belonging to
 the MVPN that is identified by the RTs of the Intra-AS I-PMSI A-D
 route.)
 See Section 12.2 for details on how to place the label in the
 packet's label stack.

Rosen & Aggarwal Standards Track [Page 34] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 The Intra-AS I-PMSI A-D route may contain a "PE Distinguisher Labels"
 attribute.  This contains a set of bindings between upstream-assigned
 labels and PE addresses.  The PE that originated the route may use
 this to bind an upstream-assigned label to one or more of the other
 PEs that belong to the same MVPN.  The way in which PE Distinguisher
 Labels are used is discussed in Sections 6.4.1, 6.4.3, 11.2.2, and
 12.3.  Other uses of the PE Distinguisher Labels attribute are
 outside the scope of this document.

6.1.2. Receiving Intra-AS I-PMSI A-D Routes

 The action to be taken when a PE receives an Intra-AS I-PMSI A-D
 route for a particular MVPN depends on the particular P-tunnel
 technology that is being used by that MVPN.  If the P-tunnel
 technology requires tunnels to be built by means of receiver-
 initiated joins, the PE SHOULD join the tunnel immediately.

6.2. When C-flows Are Specifically Bound to P-Tunnels

 This situation is discussed in Section 7.

6.3. Aggregating Multiple MVPNs on a Single P-Tunnel

 When a P-multicast tree is shared across multiple MVPNs, it is termed
 an "Aggregate Tree".  The procedures described in this document allow
 a single SP multicast tree to be shared across multiple MVPNs.
 Unless otherwise specified, P-multicast tree technology supports
 aggregation.
 All procedures that are specific to multi-MVPN aggregation are
 OPTIONAL and are explicitly pointed out.
 Aggregate Trees allow a single P-multicast tree to be used across
 multiple MVPNs so that state in the SP core grows per set of MVPNs
 and not per MVPN.  Depending on the congruence of the aggregated
 MVPNs, this may result in trading off optimality of multicast
 routing.
 An Aggregate Tree can be used by a PE to provide a 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.

Rosen & Aggarwal Standards Track [Page 35] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

6.3.1. Aggregate Tree Leaf Discovery

 BGP MVPN membership discovery (Section 4) 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.
 If an Aggregate Tree is used to instantiate one or more S-PMSIs, then
 it may be desirable for the PE at the root of the tree to know which
 PEs (in its MVPN) are receivers on that tree.  This enables the PE to
 decide when to aggregate two S-PMSIs, based on congruence (as
 discussed in the next section).  Thus, explicit tracking may be
 required.  Since the procedures for disseminating C-multicast routes
 do not provide explicit tracking, a type of A-D route known as a
 "Leaf A-D route" is used.  The PE that wants to assign a particular
 C-multicast flow to a particular Aggregate Tree can send an A-D
 route, which elicits Leaf A-D routes from the PEs that need to
 receive that C-multicast flow.  This provides the explicit tracking
 information needed to support the aggregation methodology discussed
 in the next section.  For more details on Leaf A-D routes, please
 refer to [MVPN-BGP].

6.3.2. Aggregation Methodology

 This document does not specify the mandatory implementation of any
 particular set of 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 agreeing on the PMSIs that
 are to be used and on the tunnels used to instantiate those PMSIs.
 This section discusses potential methodologies with respect to
 aggregation.
 The "congruence" of aggregation is defined by the amount of overlap
 in the leaves of the customer trees that are aggregated on an SP
 tree.  For Aggregate Trees with an inclusive mapping, the congruence
 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 congruence reduces.

Rosen & Aggarwal Standards Track [Page 36] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 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 trade-off between reducing state and delivering
 unwanted traffic.
 An implementation should provide knobs to control the congruence of
 aggregation.  These knobs are implementation dependent.  Configuring
 the percentage of sites that MVPNs must have in common to be
 aggregated is an example of such a knob.  This will allow an 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 for which a particular PE may
 receive traffic.

6.3.3. Demultiplexing C-Multicast Traffic

 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 with which the P-multicast tree is associated.
 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-assigned labels 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 by 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 over which the packet
 was received.  The tree identifier determines the label space in
 which the upstream-assigned MPLS label lookup has to be performed.

Rosen & Aggarwal Standards Track [Page 37] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 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.
 A full specification of the procedures to support aggregation on
 shared trees or on MP2MP LSPs is outside the scope of this document.
 The encapsulation format is either MPLS or MPLS-in-something (e.g.,
 MPLS-in-GRE [MPLS-IP]).  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,C-G).  (This could be
 done using the same technique that is used to assign a particular
 (C-S,C-G) to an S-PMSI, see Section 7.4.)
 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.  In order to do this,
 the egress PE needs to determine the P-tunnel on which the packet was
 received.  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. Considerations for Specific Tunnel Technologies

 While it is believed that the architecture specified in this document
 places no limitations on the protocols used for setting up and
 maintaining P-tunnels, the only protocols that have been explicitly
 considered are PIM-SM (both the SSM and ASM service models are

Rosen & Aggarwal Standards Track [Page 38] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 considered, as are bidirectional trees), RSVP-TE, mLDP, and BGP.
 (BGP's role in the setup and maintenance of P-tunnels is to "stitch"
 together the intra-AS segments of a segmented inter-AS P-tunnel.)

6.4.1. RSVP-TE P2MP LSPs

 If an I-PMSI is to be instantiated as one or more non-segmented
 P-tunnels, where the P-tunnels are RSVP-TE P2MP LSPs, then only the
 PEs that are at the head ends of those LSPs will ever include the
 PMSI Tunnel attribute in their Intra-AS I-PMSI A-D routes.  (These
 will be the PEs in the "Sender Sites set".)
 If an I-PMSI is to be instantiated as one or more segmented
 P-tunnels, where some of the intra-AS segments of these tunnels are
 RSVP-TE P2MP LSPs, then only a PE or ASBR that is at the head end of
 one of these LSPs will ever include the PMSI Tunnel attribute in its
 Inter-AS I-PMSI A-D route.
 Other PEs send Intra-AS I-PMSI A-D routes without PMSI Tunnel
 attributes.  (These will be the PEs that are in the "Receiver Sites
 set" but not in the "Sender Sites set".)  As each "Sender Site" PE
 receives an Intra-AS I-PMSI A-D route from a PE in the Receiver Sites
 set, it adds the PE originating that Intra-AS I-PMSI A-D route to the
 set of receiving PEs for the P2MP LSP.  The PE at the head end MUST
 then use RSVP-TE [RSVP-P2MP] signaling to add the receiver PEs to the
 P-tunnel.
 When RSVP-TE P2MP LSPs are used to instantiate S-PMSIs, and a
 particular C-flow is to be bound to the LSP, it is necessary to use
 explicit tracking so that the head end of the LSP knows which PEs
 need to receive data from the specified C-flow.  If the binding is
 done using S-PMSI A-D routes (see Section 7.4.1), the "Leaf
 Information Required" bit MUST be set in the PMSI Tunnel attribute.
 RSVP-TE P2MP LSPs can optionally support aggregation of multiple
 MVPNs.
 If an RSVP-TE P2MP LSP Tunnel is used for only a single MVPN, the
 mapping between the LSP and the MVPN can either be configured or be
 deduced from the procedures used to announce the LSP (e.g., from the
 RTs in the A-D route that announced the LSP).  If the LSP is used for
 multiple MVPNs, the set of MVPNs using it (and the corresponding MPLS
 labels) is inferred from the PMSI Tunnel attributes that specify the
 LSP.
 If an RSVP-TE P2MP LSP is being used to carry a set of C-flows
 traveling along a bidirectional C-tree, using the procedures of
 Section 11.2, the head end MUST include the PE Distinguisher Labels

Rosen & Aggarwal Standards Track [Page 39] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 attribute in its Intra-AS I-PMSI A-D route or S-PMSI A-D route, and
 it MUST provide an upstream-assigned label for each PE that it has
 selected as the Upstream PE for the C-tree's RPA (Rendezvous Point
 Address).  See Section 11.2 for details.
 A PMSI Tunnel attribute specifying an RSVP-TE P2MP LSP contains the
 following information:
  1. The type of the tunnel is set to RSVP-TE P2MP Tunnel
  1. The RSVP-TE P2MP Tunnel's SESSION Object.
  1. Optionally, the RSVP-TE P2MP LSP's SENDER_TEMPLATE Object. This

object is included when it is desired to identify a particular

     P2MP TE LSP.
 Demultiplexing the C-multicast data packets at the egress PE follows
 procedures described in Section 6.3.3.  As specified in Section
 6.3.3, an egress PE MUST NOT advertise IMPLICIT NULL or EXPLICIT NULL
 for an RSVP-TE P2MP LSP that is carrying traffic for one or more
 MVPNs.
 If (and only if) a particular RSVP-TE P2MP LSP is possibly carrying
 data from multiple MVPNs, the following special procedures apply:
  1. A packet in a particular MVPN, when transmitted into the LSP,

must carry the MPLS label specified in the PMSI Tunnel attribute

     that announced that LSP as a P-tunnel for that for that MVPN.
  1. Demultiplexing the C-multicast data packets at the egress PE is

done by means of the MPLS label that rises to the top of the

     stack after the label corresponding to the P2MP LSP is popped
     off.
 It is possible that at the time a PE learns, via an A-D route with a
 PMSI Tunnel attribute, that it needs to receive traffic on a
 particular RSVP-TE P2MP LSP, the signaling to set up the LSP will not
 have been completed.  In this case, the PE needs to wait for the
 RSVP-TE signaling to take place before it can modify its forwarding
 tables as directed by the A-D route.
 It is also possible that the signaling to set up an RSVP-TE P2MP LSP
 will be completed before a given PE learns, via a PMSI Tunnel
 attribute, of the use to which that LSP will be put.  The PE MUST
 discard any traffic received on that LSP until that time.

Rosen & Aggarwal Standards Track [Page 40] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 In order for the egress PE to be able to discard such traffic, it
 needs to know that the LSP is associated with an MVPN and that the
 A-D route that binds the LSP to an MVPN or to a particular a C-flow
 has not yet been received.  This is provided by extending [RSVP-P2MP]
 with [RSVP-OOB].

6.4.2. PIM Trees

 When the P-tunnels are PIM trees, the PMSI Tunnel attribute contains
 enough information to allow each other PE in the same MVPN to use
 P-PIM signaling to join the P-tunnel.
 If an I-PMSI is to be instantiated as one or more PIM trees, then the
 PE that is at the root of a given PIM tree sends an Intra-AS I-PMSI
 A-D route containing a PMSI Tunnel attribute that contains all the
 information needed for other PEs to join the tree.
 If PIM trees are to be used to instantiate an MI-PMSI, each PE in the
 MVPN must send an Intra-AS I-PMSI A-D route containing such a PMSI
 Tunnel attribute.
 If a PMSI is to be instantiated via a shared tree, the PMSI Tunnel
 attribute identifies the P-group address.  The RP or RPA
 corresponding to the P-group address is not specified.  It must, of
 course, be known to all the PEs.  It is presupposed that the PEs use
 one of the methods for automatically learning the RP-to-group
 correspondences (e.g., Bootstrap Router Protocol [BSR]), or else that
 the correspondence is configured.
 If a PMSI is to be instantiated via a source-specific tree, the PMSI
 Tunnel attribute identifies the PE router that is the root of the
 tree, as well as a P-group address.  The PMSI Tunnel attribute always
 specifies whether the PIM tree is to be a unidirectional shared tree,
 a bidirectional shared tree, or a source-specific tree.
 If PIM trees are being used to instantiate S-PMSIs, the above
 procedures assume that each PE router has a set of group P-addresses
 that it can use for setting up the PIM-trees.  Each PE must be
 configured with this set of P-addresses.  If the P-tunnels are
 source-specific trees, then the PEs may be configured with
 overlapping sets of group P-addresses.  If the trees are not source-
 specific, 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 source-specific trees are used, so the use of
 source-specific trees is strongly recommended whenever unidirectional
 trees are desired.

Rosen & Aggarwal Standards Track [Page 41] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 Specification of the full set of procedures for using bidirectional
 PIM trees to instantiate S-PMSIs is outside the scope of this
 document.
 Details for constructing the PMSI Tunnel attribute identifying a PIM
 tree can be found in [MVPN-BGP].

6.4.3. mLDP P2MP LSPs

 When the P-tunnels are mLDP P2MP trees, each Intra-AS I-PMSI A-D
 route has a PMSI Tunnel attribute containing enough information to
 allow each other PE in the same MVPN to use mLDP signaling to join
 the P-tunnel.  The tunnel identifier consists of a P2MP Forwarding
 Equivalence Class (FEC) Element [mLDP].
 An mLDP P2MP LSP may be used to carry the traffic of multiple VPNs,
 if the PMSI Tunnel attribute specifying it contains a non-zero MPLS
 label.
 If an mLDP P2MP LSP is being used to carry the set of flows traveling
 along a particular bidirectional C-tree, using the procedures of
 Section 11.2, the root of the LSP MUST include the PE Distinguisher
 Labels attribute in its Intra-AS I-PMSI A-D route or S-PMSI A-D
 route, and it MUST provide an upstream-assigned label for the PE that
 it has selected to be the Upstream PE for the C-tree's RPA.  See
 Section 11.2 for details.

6.4.4. mLDP MP2MP LSPs

 The specification of the procedures for assigning C-flows to mLDP
 MP2MP LSPs that serve as P-tunnels is outside the scope of this
 document.

6.4.5. 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 tunneled to
 a remote PE over a Unicast Tunnel to the remote PE.  IP/GRE Tunnels
 or MPLS LSPs are examples of unicast tunnels that may be used.  The
 same Unicast Tunnel can be used to transport packets belonging to
 different MVPNs
 In order for a PE to use Unicast P-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

Rosen & Aggarwal Standards Track [Page 42] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 one to the proper MVPN.  This requires that the encapsulation used
 for sending packets through the P-tunnel have demultiplexing
 information that the receiver can associate with a particular MVPN.
 If ingress replication is being used to instantiate the PMSIs for an
 MVPN, the PEs announce this as part of the BGP-based MVPN membership
 auto-discovery process, described in Section 4.  The PMSI Tunnel
 attribute specifies ingress replication; it also specifies a
 downstream-assigned MPLS label.  This label will be used to identify
 that a particular packet belongs to the MVPN that the Intra-AS I-PMSI
 A-D route belongs to (as inferred from its RTs).  If PE1 specifies a
 particular label value for a particular MVPN, then any other PE
 sending PE1 a packet for that MVPN through a unicast P-tunnel must
 put that label on the packet's label stack.  PE1 then treats that
 label as the demultiplexor value identifying the MVPN in question.
 Ingress replication may be used to instantiate any kind of PMSI.
 When ingress replication is done, it is RECOMMENDED, except in the
 one particular case mentioned in the next paragraph, that explicit
 tracking be done and that the data packets of a particular C-flow
 only get sent to those PEs that need to see the packets of that
 C-flow.  There is never any need to use the procedures of Section 7.4
 for binding particular C-flows to particular P-tunnels.
 The particular case in which there is no need for explicit tracking
 is the case where ingress replication is being used to create a
 one-hop ASBR-ASBR inter-AS segment of an segmented inter-AS P-tunnel.
 Section 9.1 specifies three different methods that can be used to
 prevent duplication of multicast data packets.  Any given deployment
 must use at least one of those methods.  Note that the method
 described in Section 9.1.1 ("Discarding Packets from Wrong PE")
 presupposes that the egress PE of a P-tunnel can, upon receiving a
 packet from the P-tunnel, determine the identity of the PE that
 transmitted the packet into the P-tunnel.  SPs that use ingress
 replication to instantiate their PMSIs are cautioned against this use
 for this purpose of unicast P-tunnel technologies that do not allow
 the egress PE to identify the ingress PE (e.g., MP2P LSPs for which
 penultimate-hop-popping is done).  Deployment of ingress replication
 with such P-tunnel technology MUST NOT be done unless it is known
 that the deployment relies entirely on the procedures of Sections
 9.1.2 or 9.1.3 for duplicate prevention.

Rosen & Aggarwal Standards Track [Page 43] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

7. Binding Specific C-Flows to Specific P-Tunnels

 As discussed previously, Intra-AS I-PMSI A-D routes may (or may not)
 have PMSI Tunnel attributes, identifying P-tunnels that can be used
 as the default P-tunnels for carrying C-multicast traffic, i.e., for
 carrying C-multicast traffic that has not been specifically bound to
 another P-tunnel.
 If none of the Intra-AS I-PMSI A-D routes originated by a particular
 PE for a particular MVPN carry PMSI Tunnel attributes at all (or if
 the only PMSI Tunnel attributes they carry have type "No tunnel
 information present"), then there are no default P-tunnels for that
 PE to use when transmitting C-multicast traffic in that MVPN to other
 PEs.  In that case, all such C-flows must be assigned to specific
 P-tunnels using one of the mechanisms specified in Section 7.4.  That
 is, all such C-flows are carried on P-tunnels that instantiate
 S-PMSIs.
 There are other cases where it may be either necessary or desirable
 to use the mechanisms of Section 7.4 to identify specific C-flows and
 bind them to or unbind them from specific P-tunnels.  Some possible
 cases are as follows:
  1. The policy for a particular MVPN is to send all C-data on

S-PMSIs, even if the Intra-AS I-PMSI A-D routes carry PMSI Tunnel

     attributes.  (This is another case where all C-data is carried on
     S-PMSIs; presumably, the I-PMSIs are used for control
     information.)
  1. It is desired to optimize the routing of the particular C-flow,

which may already be traveling on an I-PMSI, by sending it

     instead on an S-PMSI.
  1. If a particular C-flow is traveling on an S-PMSI, it may be

considered desirable to move it to an I-PMSI (i.e., optimization

     of the routing for that flow may no longer be considered
     desirable).
  1. It is desired to change the encapsulation used to carry the

C-flow, e.g., because one now wants to aggregate it on a P-tunnel

     with flows from other MVPNs.
 Note that if Full PIM Peering over an MI-PMSI (Section 5.2) is being
 used, then from the perspective of the PIM state machine, the
 "interface" connecting the PEs to each other is the MI-PMSI, even if
 some or all of the C-flows are being sent on S-PMSIs.  That is, from

Rosen & Aggarwal Standards Track [Page 44] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 the perspective of the C-PIM state machine, when a C-flow is being
 sent or received on an S-PMSI, the output or input interface
 (respectively) is considered to be the MI-PMSI.
 Section 7.1 discusses certain general considerations that apply
 whenever a specified C-flow is bound to a specified P-tunnel using
 the mechanisms of Section 7.4.  This includes the case where the
 C-flow is moved from one P-tunnel to another as well as the case
 where the C-flow is initially bound to an S-PMSI P-tunnel.
 Section 7.2 discusses the specific case of using the mechanisms of
 Section 7.4 as a way of optimizing multicast routing by switching
 specific flows from one P-tunnel to another.
 Section 7.3 discusses the case where the mechanisms of Section 7.4
 are used to announce the presence of "unsolicited flooded data" and
 to assign such data to a particular P-tunnel.
 Section 7.4 specifies the protocols for assigning specific C-flows to
 specific P-tunnels.  These protocols may be used to assign a C-flow
 to a P-tunnel initially or to switch a flow from one P-tunnel to
 another.
 Procedures for binding to a specified P-tunnel the set of C-flows
 traveling along a specified C-tree (or for so binding a set of
 C-flows that share some relevant characteristic), without identifying
 each flow individually, are outside the scope of this document.

7.1. General Considerations

7.1.1. At the PE Transmitting the C-Flow on the P-Tunnel

 The decision to bind a particular C-flow (designated as (C-S,C-G)) to
 a particular P-tunnel, or to switch a particular C-flow to a
 particular P-tunnel, is always made by the PE that is to transmit the
 C-flow onto the P-tunnel.
 Whenever a PE moves a particular C-flow from one P-tunnel, say P1, to
 another, say P2, care must be taken to ensure that there is no steady
 state duplication of traffic.  At any given time, the PE transmits
 the C-flow either on P1 or on P2, but not on both.
 When a particular PE, say PE1, decides to bind a particular C-flow to
 a particular P-tunnel, say P2, the following procedures MUST be
 applied:

Rosen & Aggarwal Standards Track [Page 45] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

  1. PE1 must issue the required control plane information to signal

that the specified C-flow is now bound to P-tunnel P2 (see

     Section 7.4).
  1. If P-tunnel P2 needs to be constructed from the root downwards,

PE1 must initiate the signaling to construct P2. This is only

     required if P2 is an RSVP-TE P2MP LSP.
  1. If the specified C-flow is currently bound to a different

P-tunnel, say P1, then:

  • PE1 MUST wait for a "switch-over" delay before sending

traffic of the C-flow on P-tunnel P2. It is RECOMMENDED to

         allow this delay to be configurable.
  • Once the "switch-over" delay has elapsed, PE1 MUST send

traffic for the C-flow on P2 and MUST NOT send it on P1. In

         no case is any C-flow packet sent on both P-tunnels.
 When a C-flow is switched from one P-tunnel to another, the purpose
 of running a switch-over timer is to minimize packet loss without
 introducing packet duplication.  However, jitter may be introduced
 due to the difference in transit delays between the old and new
 P-tunnels.
 For best effect, the switch-over timer should be configured to a
 value that is "just long enough" (a) to allow all the PEs to learn
 about the new binding of C-flow to P-tunnel and (b) to allow the PEs
 to construct the P-tunnel, if it doesn't already exist.
 If, after such a switch, the "old" P-tunnel P1 is no longer needed,
 it SHOULD be torn down and the resources supporting it freed.  The
 procedures for "tearing down" a P-tunnel are specific to the P-tunnel
 technology.
 Procedures for binding sets of C-flows traveling along specified
 C-trees (or sets of C-flows sharing any other characteristic) to a
 specified P-tunnel (or for moving them from one P-tunnel to another)
 are outside the scope of this document.

7.1.2. At the PE Receiving the C-flow from the P-Tunnel

 Suppose that a particular PE, say PE1, learns, via the procedures of
 Section 7.4, that some other PE, say PE2, has bound a particular
 C-flow, designated as (C-S,C-G), to a particular P-tunnel, say P2.
 Then, PE1 must determine whether it needs to receive (C-S,C-G)
 traffic from PE2.

Rosen & Aggarwal Standards Track [Page 46] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 If BGP is being used to distribute C-multicast routing information
 from PE to PE, the conditions under which PE1 needs to receive
 (C-S,C-G) traffic from PE2 are specified in Section 12.3 of
 [MVPN-BGP].
 If PIM over an MI-PMSI is being used to distribute C-multicast
 routing from PE to PE, PE1 needs to receive (C-S,C-G) traffic from
 PE2 if one or more of the following conditions holds:
  1. PE1 has (C-S,C-G) state such that PE2 is PE1's Upstream PE for

(C-S,C-G), and PE1 has downstream neighbors ("non-null olist")

     for the (C-S,C-G) state.
  1. PE1 has (C-*,C-G) state with an Upstream PE (not necessarily PE2)

and with downstream neighbors ( "non-null olist"), but PE1 does

     not have (C-S,C-G) state.
  1. Native PIM methods are being used to prevent steady-state packet

duplication, and PE1 has either (C-*,C-G) or (C-S,C-G) state such

     that the MI-PMSI is one of the downstream interfaces.  Note that
     this includes the case where PE1 is itself sending (C-S,C-G)
     traffic on an S-PMSI.  (In this case, PE1 needs to receive the
     (C-S,C-G) traffic from PE2 in order to allow the PIM Assert
     mechanism to function properly.)
 Irrespective of whether BGP or PIM is being used to distribute
 C-multicast routing information, once PE1 determines that it needs to
 receive (C-S,C-G) traffic from PE2, the following procedures MUST be
 applied:
  1. PE1 MUST take all necessary steps to be able to receive the

(C-S,C-G) traffic on P2.

  • If P2 is a PIM tunnel or an mLDP LSP, PE1 will need to use

PIM or mLDP (respectively) to join P2 (unless it is already

         joined to P2).
  • PE1 may need to modify the forwarding state for (C-S,C-G) to

indicate that (C-S,C-G) traffic is to be accepted on P2. If

         P2 is an Aggregate Tree, this also implies setting up the
         demultiplexing forwarding entries based on the inner label as
         described in Section 6.3.3
  1. If PE1 was previously receiving the (C-S,C-G) C-flow on another

P-tunnel, say P1, then:

  • PE1 MAY run a switch-over timer, and until it expires, SHOULD

accept traffic for the given C-flow on both P1 and P2;

Rosen & Aggarwal Standards Track [Page 47] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

  • If, after such a switch, the "old" P-tunnel P1 is no longer

needed, it SHOULD be torn down and the resources supporting

         it freed.  The procedures for "tearing down" a P-tunnel are
         specific to the P-tunnel technology.
  1. If PE1 later determines that it no longer needs to receive any of

the C-multicast data that is being sent on a particular P-tunnel,

     it may initiate signaling (specific to the P-tunnel technology)
     to remove itself from that tunnel.

7.2. 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 that includes
 only those PEs that are transmitters and/or receivers (or at least
 includes fewer PEs that are neither).
 If explicit tracking is being done, S-PMSI creation can also be
 triggered on other criteria.  For instance, there could be a "pseudo-
 wasted bandwidth" criterion: switching to an 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 (a) the total bandwidth wasted by
 many sparsely subscribed low-bandwidth groups may be large and (b)
 there's no point to moving a high-bandwidth group to an S-PMSI if all
 the PEs have receivers for it.
 Switching a (C-S,C-G) stream to an S-PMSI may require the root of the
 S-PMSI to determine the egress PEs that need to receive the (C-S,C-G)
 traffic.  This is true in the following cases:
  1. If the P-tunnel is a source-initiated tree, such as an RSVP-TE

P2MP Tunnel, the PE needs to know the leaves of the tree before

     it can instantiate the S-PMSI.
  1. 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.

Rosen & Aggarwal Standards Track [Page 48] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 The above two cases require that explicit tracking be done for the
 (C-S,C-G) stream.  The root of the S-PMSI MAY decide to do explicit
 tracking of this stream only after it has determined to move the
 stream to an S-PMSI, or it MAY have been doing explicit tracking all
 along.
 If the S-PMSI is instantiated by a P-multicast tree, the PE at the
 root of the tree must signal the leaves of the tree that the
 (C-S,C-G) stream is now bound to the S-PMSI.  Note that the PE could
 create the identity of the P-multicast tree prior to the actual
 instantiation of the P-tunnel.
 If the S-PMSI is instantiated by a source-initiated P-multicast tree
 (e.g., an RSVP-TE P2MP tunnel), the PE at the root of the tree must
 establish the source-initiated P-multicast tree to the leaves.  This
 tree MAY have been established before the leaves receive the S-PMSI
 binding, or it MAY be established after the leaves receive the
 binding.  The leaves MUST NOT switch to the S-PMSI until they receive
 both the binding and the tree signaling message.

7.3. Announcing the Presence of Unsolicited Flooded Data

 A PE may receive "unsolicited" data from a CE, where the data is
 intended to be flooded to the other PEs of the same MVPN and then on
 to other CEs.  By "unsolicited", we mean that the data is to be
 delivered to all the other PEs of the MVPN, even though those PEs may
 not have sent any control information indicating that they need to
 receive that data.
 For example, if the BSR [BSR] is being used within the MVPN, BSR
 control messages may be received by a PE from a CE.  These need to be
 forwarded to other PEs, even though no PE ever issues any kind of
 explicit signal saying that it wants to receive BSR messages.
 If a PE receives a BSR message from a CE, and if the CE's MVPN has an
 MI-PMSI, then the PE can just send BSR messages on the appropriate
 P-tunnel.  Otherwise, the PE MUST announce the binding of a
 particular C-flow to a particular P-tunnel, using the procedures of
 Section 7.4.  The particular C-flow in this case would be
 (C-IPaddress_of_PE, ALL-PIM-ROUTERS).  The P-tunnel identified by the
 procedures of Section 7.4 may or may not be one that was previously
 identified in the PMSI Tunnel attribute of an I-PMSI A-D route.
 Further procedures for handling BSR may be found in Sections 5.2.1
 and 5.3.4.

Rosen & Aggarwal Standards Track [Page 49] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 Analogous procedures may be used for announcing the presence of other
 sorts of unsolicited flooded data, e.g., dense mode data or data from
 proprietary protocols that presume messages can be flooded.  However,
 a full specification of the procedures for traffic other than BSR
 traffic is outside the scope of this document.

7.4. Protocols for Binding C-Flows to P-Tunnels

 We describe two protocols for binding C-flows to P-tunnels.
 These protocols can be used for moving C-flows from I-PMSIs to
 S-PMSIs, as long as the S-PMSI is instantiated by a P-multicast tree.
 (If the S-PMSI is instantiated by means of ingress replication, the
 procedures of Section 6.4.5 suffice.)
 These protocols can also be used for other cases in which it is
 necessary to bind specific C-flows to specific P-tunnels.

7.4.1. Using BGP S-PMSI A-D Routes

 Not withstanding the name of the mechanism "S-PMSI A-D routes", the
 mechanism to be specified in this section may be used any time it is
 necessary to advertise a binding of a C-flow to a particular
 P-tunnel.

7.4.1.1. Advertising C-Flow Binding to P-Tunnel

 The ingress PE informs all the PEs that are on the path to receivers
 of the (C-S,C-G) of the binding of the P-tunnel to the (C-S,C-G).
 The BGP announcement is done by sending an update for the MCAST-VPN
 address family.  An S-PMSI A-D route is used, containing the
 following information:
    1. The IP address of the originating PE.
    2. The RD configured locally for the MVPN.  This is required to
       uniquely identify the (C-S,C-G) as the addresses could overlap
       between different MVPNs.  This is the same RD value used in the
       auto-discovery process.
    3. The C-S address.
    4. The C-G address.
    5. A PE MAY use a single P-tunnel to aggregate two or more
       S-PMSIs.  If the PE already advertised unaggregated S-PMSI A-D
       routes for these S-PMSIs, then a decision to aggregate them
       requires the PE to re-advertise these routes.  The re-

Rosen & Aggarwal Standards Track [Page 50] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

       advertised routes MUST be the same as the original ones, except
       for the PMSI Tunnel attribute.  If the PE has not previously
       advertised S-PMSI A-D routes for these S-PMSIs, then the
       aggregation requires the PE to advertise (new) S-PMSI A-D
       routes for these S-PMSIs.  The PMSI Tunnel attribute in the
       newly advertised/re-advertised routes MUST carry the identity
       of the P-tunnel that aggregates the S-PMSIs.
       If all these aggregated S-PMSIs belong to the same MVPN, and
       this MVPN uses PIM as its C-multicast routing protocol, then
       the corresponding S-PMSI A-D routes MAY carry an MPLS upstream-
       assigned label [MPLS-UPSTREAM-LABEL].  Moreover, in this case,
       the labels MUST be distinct on a per-MVPN basis, and MAY be
       distinct on a per-route basis.
       If all these aggregated S-PMSIs belong to the MVPN(s) that use
       mLDP as its C-multicast routing protocol, then the
       corresponding S-PMSI A-D routes MUST carry an MPLS upstream-
       assigned label [MPLS-UPSTREAM-LABEL], and these labels MUST be
       distinct on a per-route (per-mLDP-FEC) basis, irrespective of
       whether the aggregated S-PMSIs belong to the same or different
       MVPNs.
 When a PE distributes this information via BGP, it must include the
 following:
    1. An identifier for the particular P-tunnel to which the stream
       is to be bound.  This identifier is a structured field that
       includes the following information:
  • The type of tunnel
  • 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.

7.4.1.2. Explicit Tracking

 If the PE wants to enable explicit tracking for the specified flow,
 it also indicates this in the A-D route it uses to bind the flow to a
 particular P-tunnel.  Then, any PE that receives the A-D route will

Rosen & Aggarwal Standards Track [Page 51] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 respond with a "Leaf A-D route" in which it identifies itself as a
 receiver of the specified flow.  The Leaf A-D route will be withdrawn
 when the PE is no longer a receiver for the flow.
 If the PE needs to enable explicit tracking for a flow without at the
 same time binding the flow to a specific P-tunnel, it can do so by
 sending an S-PMSI A-D route whose NLRI identifies the flow and whose
 PMSI Tunnel attribute has its tunnel type value set to "no tunnel
 information present" and its "leaf information required" bit set to
 1.  This will elicit the Leaf A-D routes.  This is useful when the PE
 needs to know the receivers before selecting a P-tunnel.

7.4.2. UDP-Based Protocol

 This procedure carries its control messages in UDP and requires that
 the MVPN have an MI-PMSI that can be used to carry the control
 messages.

7.4.2.1. Advertising C-Flow Binding to P-Tunnel

 In order for a given PE to move a particular C-flow to a particular
 P-tunnel, an "S-PMSI Join message" is sent periodically on the
 MI-PMSI.  (Notwithstanding the name of the mechanism, the mechanism
 may be used to bind a flow to any P-tunnel.)  The S-PMSI Join message
 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:
  1. An identifier for the particular multicast stream that is to be

bound to the P-tunnel. This can be represented as an (S,G) pair.

  1. An identifier for the particular P-tunnel to which the stream is

to be bound. This identifier is a structured field that 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.
  • If (and only if) the identified P-tunnel is aggregating

several S-PMSIs, any demultiplexing information needed by the

         tunnel encapsulation protocol to identify a particular
         S-PMSI.

Rosen & Aggarwal Standards Track [Page 52] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 If the policy for the MVPN is that traffic is sent/received by
 default over an MI-PMSI, then traffic for a particular C-flow can be
 switched back to the MI-PMSI simply by ceasing to send S-PMSI Joins
 for that C-flow.
 Note that an S-PMSI Join that is not received over a PMSI (e.g., one
 that is received directly from a CE) is an illegal packet that MUST
 be discarded.

7.4.2.2. Packet Formats and Constants

 The S-PMSI Join message is encapsulated within UDP and has the
 following type/length/value (TLV) encoding:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |            Length           |     Value       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               .                               |
     |                               .                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type (8 bits)
 Length (16 bits): the total number of octets in the Type, Length, and
 Value fields combined
 Value (variable length)
 In this specification, only one type of S-PMSI Join is defined.  A
 Type 1 S-PMSI Join is used when the S-PMSI tunnel is a PIM tunnel
 that is used to carry a single multicast stream, where the packets of
 that stream have IPv4 source and destination IP addresses.
 The S-PMSI Join format to use when the C-source and C-group are IPv6
 addresses will be defined in a follow-on document.

Rosen & Aggarwal Standards Track [Page 53] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |           Length            |    Reserved     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           C-source                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           C-group                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           P-group                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type (8 bits): 1
 Length (16 bits): 16
 Reserved (8 bits): This field SHOULD be zero when transmitted, and
 MUST be ignored when received.
 C-source (32 bits): the IPv4 address of the traffic source in the
 VPN.
 C-group (32 bits): the IPv4 address of the multicast traffic
 destination address in the VPN.
 P-group (32 bits): the IPv4 group address that the PE router is going
 to use to encapsulate the flow (C-source, C-group).
 The P-group identifies the S-PMSI P-tunnel, and the (C-S,C-G)
 identifies the multicast flow that is carried in the P-tunnel.
 The protocol uses the following constants.
 [S-PMSI_DELAY]:
     Once an S-PMSI Join message has been sent, the PE router that is
     to transmit onto the S-PMSI will delay this amount of time before
     it begins using the S-PMSI.  The default value is 3 seconds.
 [S-PMSI_TIMEOUT]:
     If a PE (other than the transmitter) does not receive any packets
     over the S-PMSI P-tunnel for this amount of time, the PE will
     prune itself from the S-PMSI P-tunnel, and will expect (C-S,C-G)
     packets to arrive on an I-PMSI.  The default value is 3 minutes.
     This value must be consistent among PE routers.

Rosen & Aggarwal Standards Track [Page 54] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 [S-PMSI_HOLDOWN]:
     If the PE that transmits onto the S-PMSI does not see any
     (C-S,C-G) packets for this amount of time, it will resume sending
     (C-S,C-G) packets on an I-PMSI.
     This is used to avoid oscillation when traffic is bursty.  The
     default value is 1 minute.
 [S-PMSI_INTERVAL]:
     The interval the transmitting PE router uses to periodically send
     the S-PMSI Join message.  The default value is 60 seconds.

7.4.3. Aggregation

 S-PMSIs can be aggregated on a P-multicast tree.  The S-PMSI to
 (C-S,C-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.

8. Inter-AS Procedures

 If an MVPN has sites in more than one AS, it requires one or more
 PMSIs to be instantiated by inter-AS P-tunnels.  This document
 describes two different types of inter-AS P-tunnel:
    1. "Segmented inter-AS P-tunnels"
       A segmented inter-AS P-tunnel consists of a number of
       independent segments that are stitched together at the ASBRs.
       There are two types of segment: inter-AS segments and intra-AS
       segments.  The segmented inter-AS P-tunnel consists of
       alternating intra-AS and inter-AS segments.
       Inter-AS segments connect adjacent ASBRs of different ASes;
       these "one-hop" segments are instantiated as unicast P-tunnels.
       Intra-AS segments connect ASBRs and PEs that are in the same
       AS.  An intra-AS segment may be of whatever technology is
       desired by the SP that administers the that AS.  Different
       intra-AS segments may be of different technologies.
       Note that the intra-AS segments of inter-AS P-tunnels form a
       category of P-tunnels that is distinct from simple intra-AS
       P-tunnels; we will rely on this distinction later (see Section
       9).

Rosen & Aggarwal Standards Track [Page 55] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

       A segmented inter-AS P-tunnel can be thought of as a tree that
       is rooted at a particular AS, and that has, as its leaves, the
       other ASes that need to receive multicast data from the root
       AS.
    2. "Non-segmented Inter-AS P-tunnels"
       A non-segmented inter-AS P-tunnel is a single P-tunnel that
       spans AS boundaries.  The tunnel technology cannot change from
       one point in the tunnel to the next, so all ASes through which
       the P-tunnel passes must support that technology.  In essence,
       AS boundaries are of no significance to a non-segmented inter-
       AS P-tunnel.
 Section 10 of [RFC4364] describes three different options for
 supporting unicast inter-AS BGP/MPLS IP VPNs, known as options A, B,
 and C.  We describe below how both segmented and non-segmented inter-
 AS trees can be supported when options B or C are used.  (Option A
 does not pass any routing information through an ASBR at all, so no
 special inter-AS procedures are needed.)

8.1. Non-Segmented Inter-AS P-Tunnels

 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.

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.  However, please note that, if non-segmented inter-AS
 P-tunnels are to be used, then the Intra-AS I-PMSI A-D routes MUST be
 distributed across AS boundaries!

8.1.2. Inter-AS MVPN Routing Information Exchange

 When non-segmented inter-AS P-tunnels are used, MVPN C-multicast
 routing information may be exchanged by means of PIM peering across
 an MI-PMSI or by means of BGP carrying C-multicast routes.
 When PIM peering is used to distribute the C-multicast routing
 information, a PE that sends C-PIM Join/Prune messages for a
 particular (C-S,C-G) must be able to identify the PE that is its PIM
 adjacency on the path to S.  This is the "Selected Upstream PE"
 described in Section 5.1.3.

Rosen & Aggarwal Standards Track [Page 56] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 If BGP (rather than PIM) is used to distribute the C-multicast
 routing information, and if option b of Section 10 of [RFC4364] is in
 use, then the C-multicast 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 C-multicast
 routes are not installed in the ASBRs.  The handling of the
 C-multicast routes in either case is thus exactly analogous to the
 handling of unicast VPN-IP routes in the corresponding case.

8.1.3. Inter-AS P-Tunnels

 The procedures described earlier in this document can be used to
 instantiate either an I-PMSI or an S-PMSI with inter-AS P-tunnels.
 Specific tunneling techniques require some explanation.
 If ingress replication is used, the inter-AS PE-PE P-tunnels will use
 the inter-AS tunneling procedures for the tunneling technology used.
 Procedures in [RSVP-P2MP] are used for inter-AS RSVP-TE P2MP
 P-tunnels.
 Procedures for using PIM to set up the P-tunnels are discussed in the
 next section.

8.1.3.1. PIM-Based Inter-AS P-Multicast Trees

 When PIM is used to set up a non-segmented inter-AS P-multicast tree,
 the PIM Join/Prune messages used to join the tree contain the IP
 address of the Upstream PE.  However, there are two special
 considerations that must be taken into account:
  1. It is possible that the P routers within one or more of the ASes

will not have routes to the Upstream PE. For example, if an AS

     has a "BGP-free core", the P routers in an AS will not have
     routes to addresses outside the AS.
  1. If the PIM Join/Prune message must travel through several ASes,

it is possible that the ASBRs will not have routes to he PE

     routers.  For example, in an inter-AS VPN constructed according
     to "option b" of Section 10 of [RFC4364], the ASBRs do not
     necessarily have routes to the PE routers.
 In either case, "ordinary" PIM Join/Prune messages cannot be routed
 to the Upstream PE.  Therefore, in that case, the PIM Join/Prune
 messages MUST contain the "PIM MVPN Join attribute".  This allows the
 multicast distribution tree to be properly constructed, even if
 routes to PEs in other ASes do not exist in the given AS's IGP and

Rosen & Aggarwal Standards Track [Page 57] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 even if the routes to those PEs do not exist in BGP.  The use of a
 PIM MVPN Join attribute in the PIM messages allows the inter-AS trees
 to be built.
 The PIM MVPN Join attribute adds the following information to the PIM
 Join/Prune messages: a "proxy address", which contains the address of
 the next ASBR on the path to the Upstream PE.  When the PIM
 Join/Prune arrives at the ASBR that is identified by the "proxy
 address", that ASBR must change the proxy address to identify the
 next hop ASBR.
 This information allows the PIM Join/Prune to be routed through an
 AS, even if the P routers of that AS do not have routes to the
 Upstream PE.  However, this information is not sufficient to enable
 the ASBRs to route the Join/Prune if the ASBRs themselves do not have
 routes to the Upstream PE.
 However, even if the ASBRs do not have routes to the Upstream PE, the
 procedures of this document ensure that they will have Intra-AS
 I-PMSI A-D routes that lead to the Upstream PE.  (Recall that if non-
 segmented inter-AS P-tunnels are being used, the ASBRs and PEs will
 have Intra-AS I-PMSI A-D routes that have been distributed inter-AS.)
 So, rather than having the PIM Join/Prune messages routed by the
 ASBRs along a route to the Upstream PE, the PIM Join/Prune messages
 MUST be routed along the path determined by the Intra-AS I-PMSI A-D
 routes.
 The basic format of a PIM Join attribute is specified in
 [PIM-ATTRIB].  The details of the PIM MVPN Join attribute are
 specified in the next section.

8.1.3.2. The PIM MVPN Join Attribute

8.1.3.2.1. Definition

 In [PIM-ATTRIB], the notion of a "join attribute" is defined, and a
 format for included join attributes in PIM Join/Prune messages is
 specified.  We now define a new join attribute, which we call the
 "MVPN Join attribute".
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |F|E| Attr_Type | Length        |     Proxy IP address
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  |      RD
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-.......

Rosen & Aggarwal Standards Track [Page 58] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 The Attr_Type field of the MVPN Join attribute is set to 1.
 The F bit is set to 0.
 Two information fields are carried in the MVPN Join attribute:
  1. Proxy IP address: The IP address of the node towards which the

PIM Join/Prune message is to be forwarded. This will be either

      an IPv4 or an IPv6 address, depending on whether the PIM
      Join/Prune message itself is IPv4 or IPv6.
  1. RD: An eight-byte RD. This immediately follows the proxy IP

address.

 The PIM message also carries the address of the Upstream PE.
 In the case of an intra-AS MVPN, the proxy and the Upstream PE are
 the same.  In the case of an inter-AS MVPN, the proxy will be the
 ASBR that is the exit point from the local AS on the path to the
 Upstream PE.

8.1.3.2.2. Usage

 When a PE router originates a PIM Join/Prune message in order to set
 up an inter-AS PMSI, it does so as a result of having received a
 particular Intra-AS I-PMSI A-D route or S-PMSI A-D route.  It
 includes an MVPN Join attribute whose fields are set as follows:
  1. If the Upstream PE is in the same AS as the local PE, then the

proxy field contains the address of the Upstream PE. Otherwise,

     it contains the address of the BGP Next Hop of the route to the
     Upstream PE.
  1. The RD field contains the RD from the NLRI of the Intra-AS A-D

route.

  1. The Upstream PE field contains the address of the PE that

originated the Intra-AS I-PMSI A-D route or S-PMSI A-D route

     (obtained from the NLRI of that route).
 When a PIM router processes a PIM Join/Prune message with an MVPN
 Join attribute, it first checks to see if the proxy field contains
 one of its own addresses.
 If not, the router uses the proxy IP address in order to determine
 the RPF interface and neighbor.  The MVPN Join attribute must be
 passed upstream unchanged.

Rosen & Aggarwal Standards Track [Page 59] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 If the proxy address is one of the router's own IP addresses, then
 the router looks in its BGP routing table for an Intra-AS A-D route
 whose NLRI consists of the Upstream PE address prepended with the RD
 from the Join attribute.  If there is no match, the PIM message is
 discarded.  If there is a match, the IP address from the BGP next hop
 field of the matching route is used in order to determine the RPF
 interface and neighbor.  When the PIM Join/Prune is forwarded
 upstream, the proxy field is replaced with the address of the BGP
 next hop, and the RD and Upstream PE fields are left unchanged.
 The use of non-segmented inter-AS trees constructed via BIDIR-PIM is
 outside the scope of this document.

8.2. Segmented Inter-AS P-Tunnels

 The procedures for setting up and maintaining segmented inter-AS
 Inclusive and Selective P-tunnels may be found in [MVPN-BGP].

9. Preventing Duplication of Multicast Data Packets

 Consider the case of an egress PE that receives packets of a
 particular C-flow, (C-S,C-G), over a non-aggregated S-PMSI.  The
 procedures described so far will never cause the PE to receive
 duplicate copies of any packet in that stream.  It is possible that
 the (C-S,C-G) stream is carried in more than one S-PMSI; this may
 happen when the site that contains C-S is multihomed to more than one
 PE.  However, a PE that needs to receive (C-S,C-G) packets only joins
 one of these S-PMSIs, and so it only receives one copy of each
 packet.  However, if the data packets of stream (C-S,C-G) are carried
 in either an I-PMSI or an aggregated S-PMSI, then the procedures
 specified so far make it possible for an egress PE to receive more
 than one copy of each data packet.  Additional procedures are needed
 to either make this impossible or ensure that the egress PE does not
 forward duplicates to the CE routers.
 This section covers only the situation where the C-trees are
 unidirectional, in either the ASM or SSM service models.  The case
 where the C-trees are bidirectional is considered separately in
 Section 11.
 There are two cases where the procedures specified so far make it
 possible for an egress PE to receive duplicate copies of a multicast
 data packet.  These are as follows:
    1. The first case occurs when both of the following conditions
       hold:

Rosen & Aggarwal Standards Track [Page 60] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

          a. an MVPN site that contains C-S or C-RP is multihomed to
             more than one PE, and
          b. either an I-PMSI or an aggregated S-PMSI is used for
             carrying the packets originated by C-S.
       In this case, an egress PE may receive one copy of the packet
       from each PE to which the site is homed.  This case is
       discussed further in Section 9.2.
    2. The second case occurs when all of the following conditions
       hold:
          a. the IP destination address of the customer packet, C-G,
             identifies a multicast group that is operating in ASM
             mode and whose C-multicast tree is set up using PIM-SM,
          b. an MI-PMSI is used for carrying the data packets, and
          c. a router or a CE in a site connected to the egress PE
             switches from the C-RP tree to the C-S tree.
       In this case, it is possible to get one copy of a given packet
       from the ingress PE attached to the C-RP's site and one from
       the ingress PE attached to the C-S's site.  This case is
       discussed further in Section 9.3.
 Additional procedures are therefore needed to ensure that no MVPN
 customer sees steady state multicast data packet duplication.  There
 are three procedures that may be used:
    1. Discarding data packets received from the "wrong" PE
    2. Single Forwarder Selection
    3. Native PIM methods
 These methods are described in Section 9.1.  Their applicability to
 the two scenarios where duplication is possible is discussed in
 Sections 9.2 and 9.3.

9.1. Methods for Ensuring Non-Duplication

 Every MVPN MUST use at least one of the three methods for ensuring
 non-duplication.

Rosen & Aggarwal Standards Track [Page 61] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

9.1.1. Discarding Packets from Wrong PE

 Per Section 5.1.3, an egress PE, say PE1, chooses a specific Upstream
 PE, for given (C-S,C-G).  When PE1 receives a (C-S,C-G) packet from a
 PMSI, it may be able to identify the PE that transmitted the packet
 onto the PMSI.  If that transmitter is other than the PE selected by
 PE1 as the Upstream PE, then PE1 can drop the packet.  This means
 that the PE will see a duplicate, but the duplicate will not get
 forwarded.
 The method used by an egress PE to determine the ingress PE for a
 particular packet, received over a particular PMSI, depends on the
 P-tunnel technology that is used to instantiate the PMSI.  If the
 P-tunnel is a P2MP LSP, a PIM-SM or PIM-SSM tree, or a unicast
 P-tunnel that uses IP encapsulation, then the tunnel encapsulation
 contains information that can be used (possibly along with other
 state information in the PE) to determine the ingress PE, as long as
 the P-tunnel is instantiating an intra-AS PMSI or an inter-AS PMSI
 which is supported by a non-segmented inter-AS tunnel.
 Even when inter-AS segmented P-tunnels are used, if an aggregated
 S-PMSI is used for carrying the packets, the tunnel encapsulation
 must have some information that can be used to identify the PMSI; in
 turn, that implicitly identifies the ingress PE.
 Consider the case of an I-PMSI that spans multiple ASes and that is
 instantiated by segmented inter-AS P-tunnels.  Suppose it is carrying
 data that is traveling along a particular C-tree.  Suppose also that
 the C-root of that C-tree is multihomed to two or more PEs, and that
 each such PE is in a different AS than the others.  Then, if there is
 any duplicate traffic, the duplicates will arrive on a different
 P-tunnel.  Specifically, if the PE was expecting the traffic on a
 particular inter-AS P-tunnel, duplicate traffic will arrive either on
 an intra-AS P-tunnel (not an intra-AS segment of an inter-AS
 P-tunnel) or on some other inter-AS P-tunnel.  To detect duplicates,
 the PE has to keep track of which inter-AS A-D route the PE uses for
 sending MVPN multicast routing information towards the C-S/C-RP.  The
 PE MUST process received (multicast) traffic originated by C-S/C-RP
 only from the inter-AS P-tunnel that was carried in the best Inter-AS
 A-D route for the MVPN and that was originated by the AS that
 contains C-S/C-RP (where "the best" is determined by the PE).  The PE
 MUST discard, as duplicates, all other multicast traffic originated
 by the C-S/C-RP, but received on any other P-tunnel.
 If, for a given MVPN, (a) an MI-PMSI is used for carrying multicast
 data packets, (b) the MI-PMSI is instantiated by a segmented inter-AS
 P-tunnel, (c) the C-S or C-RP is multihomed to different PEs, and (d)
 at least two such PEs are in the same AS, then, depending on the

Rosen & Aggarwal Standards Track [Page 62] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 tunneling technology used to instantiate the MI-PMSI, it may not
 always be possible for the egress PE to determine the Upstream PE.
 In that case, the procedure of Sections 9.1.2 or 9.1.3 must be used.
 NB: Section 10 describes an exception case where PE1 has to accept a
 packet even if it is not from the Selected Upstream PE.

9.1.2. Single Forwarder Selection

 Section 5.1 specifies a procedure for choosing a "default Upstream PE
 selection", such that (except during routing transients) all PEs will
 choose the same default Upstream PE.  To ensure that duplicate
 packets are not sent through the backbone (except during routing
 transients), an ingress PE does not forward to the backbone any
 (C-S,C-G) multicast data packet it receives from a CE, unless the PE
 is the default Upstream PE selection.
 One difference in effect between this procedure and the procedure of
 Section 9.1.1 is that this procedure sends only one copy of each
 packet to each egress PE, rather than sending multiple copies and
 forcing the egress PE to discard all but one.

9.1.3. Native PIM Methods

 If PE-PE multicast routing information for a given MVPN is being
 disseminated by running PIM over an MI-PMSI, then native PIM methods
 will prevent steady state data packet duplication.  The PIM Assert
 mechanism prevents steady state duplication in the scenario of
 Section 9.2, even if Single Forwarder Selection is not done.  The PIM
 Prune(S,G,rpt) mechanism addresses the scenario of Section 9.3.

9.2. Multihomed C-S or C-RP

 Any of the three methods of Section 9.1 will prevent steady state
 duplicates in the case of a multihomed C-S or C-RP.

9.3. Switching from the C-RP Tree to the C-S Tree

9.3.1. How Duplicates Can Occur

 If some PEs are on the C-S tree and some are on the C-RP tree, then a
 PE may also receive duplicate data traffic after a (C-*,C-G) to
 (C-S,C-G) switch.
 If PIM is being used on an MI-PMSI to disseminate multicast routing
 information, native PIM methods (in particular, the use of the
 Prune(S,G,rpt) message) prevent steady state data duplication in this
 case.

Rosen & Aggarwal Standards Track [Page 63] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 If BGP C-multicast routing is being used, then the procedure of
 Section 9.1.1, if applicable, can be used to prevent duplication.
 However, if that procedure is not applicable, then the procedure of
 Section 9.1.2 is not sufficient to prevent steady state data
 duplication in all scenarios.
 In the scenario in which (a) BGP C-multicast routing is being used,
 (b) there are inter-site shared C-trees, and (c) there are inter-site
 source C-trees, additional procedures are needed.  To see this,
 consider the following topology:
                      CE1---C-RP
                       |
                       |
                CE2---PE1-- ... --PE2---CE5---C-S
                            ...
         C-R1---CE3---PE3-- ... --PE4---CE4---C-R2
 Suppose that C-R1 and C-R2 use PIM to join the (C-*,C-G) tree, where
 C-RP is the RP corresponding to C-G.  As a result, CE3 and CE4 will
 send PIM Join(*,G) messages to PE3 and PE4, respectively.  This will
 cause PE3 and PE4 to originate C-multicast Shared Tree Join Routes,
 specifying (C-*,C-G).  These routes will identify PE1 as the Upstream
 PE.
 Now suppose that C-S is a transmitter for multicast group C-G, and
 that C-S sends its multicast data packets to C-RP in PIM Register
 messages.  Then, PE1 will receive (C-S,C-G) data packets from CE1,
 and will forward them over an I-PMSI to PE3 and PE4, who will forward
 them, in turn, to CE3 and CE4, respectively.
 When C-R1 receives (C-S,C-G) data packets, it may decide to join the
 (C-S,C-G) source tree, by sending a PIM Join(S,G) to CE3.  This will,
 in turn, cause CE3 to send a PIM Join(S,G) to PE3, which will, in
 turn, cause PE3 to originate a C-multicast Source Tree Join Route,
 specifying (C-S,C-G) and identifying PE2 as the Upstream PE.  As a
 result, when PE2 receives (C-S,C-G) data packets from CE5, it will
 forward them on a PMSI to PE3.

Rosen & Aggarwal Standards Track [Page 64] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 At this point, the following situation exists:
  1. If PE1 receives (C-S,C-G) packets from CE1, PE1 must forward them

on the I-PMSI, because PE4 is still expecting to receive the

     (C-S,C-G) packets from PE1.
  1. PE3 must continue to receive packets from the I-PMSI, since there

may be other sources transmitting C-G traffic and PE3 currently

     has no other way to receive that traffic.
  1. PE3 must also receive (C-S,C-G) traffic from PE2.
 As a result, PE3 may receive two copies of each (C-S,C-G) packet.
 The procedure of Section 9.1.2 (Single Forwarder Selection) does not
 prevent PE3 from receiving two copies, because it does not prevent
 one PE from forwarding (C-S,C-G) traffic along the shared C-tree
 while another forwards (C-S,C-G) traffic along a source-specific
 C-tree.
 So if PE3 cannot apply the method of Section 9.1.1 (Discarding
 Packets from Wrong PE), perhaps because the tunneling technology does
 not allow the egress PE to identify the ingress PE, then additional
 procedures are needed.

9.3.2. Solution Using Source Active A-D Routes

 The issue described in Section 9.3.1 is resolved through the use of
 Source Active A-D routes.  In the remainder this section, we provide
 an example of how this works, along with an informal description of
 the procedures.
 A full and precise specification of the relevant procedures can be
 found in Section 13 of [MVPN-BGP].  In the event of any conflicts or
 other discrepancies between the description below and the description
 in [MVPN-BGP], [MVPN-BGP] is to be considered to be the authoritative
 document.
 Please note that the material in this section only applies when
 inter-site shared trees are being used.
 Whenever a PE creates an (C-S,C-G) state as a result of receiving a
 C-multicast route for (C-S,C-G) from some other PE, and the C-G group
 is an ASM group, the PE that creates the state MUST originate a
 Source Active A-D route (see [MVPN-BGP], Section 4.5).  The NLRI of
 the route includes C-S and C-G.  By default, the route carries the
 same set of Route Targets as the Intra-AS I-PMSI A-D route of the
 MVPN originated by the PE.  Using the normal BGP procedures, the

Rosen & Aggarwal Standards Track [Page 65] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 route is propagated to all the PEs of the MVPN.  For more details,
 see Section 13.1 ("Source within a Site - Source Active
 Advertisement") of [MVPN-BGP].
 When, as a result of receiving a new Source Active A-D route, a PE
 updates its VRF with the route, the PE MUST check if the newly
 received route matches any (C-*,C-G) entries.  If (a) there is a
 matching entry, (b) the PE does not have (C-S,C-G) state in its MVPN
 Tree Information Base (MVPN-TIB) for (C-S,C-G) carried in the route,
 and (c) the received route is selected as the best (using the BGP
 route selection procedures), then the PE takes the following action:
  1. If the PE's (C-*,C-G) state has a PMSI as a downstream interface,

the PE acts as if all the other PEs had pruned C-S off the

     (C-*,C-G) tree.  That is:
  • If the PE receives (C-S,C-G) traffic from a CE, it does not

transmit it to other PEs.

  • Depending on the PIM state of the PE's PE-CE interfaces, the

PE may or may not need to invoke PIM procedures to prune C-S

         off the (C-*,C-G) tree by sending a PIM Prune(S,G,rpt) to one
         or more of the CEs.  This is determined by ordinary PIM
         procedures.  If this does need to be done, the PE SHOULD
         delay sending the Prune until it first runs a timer; this
         helps ensure that the source is not pruned from the shared
         tree until all PEs have had time to receive the Source Active
         A-D route.
  1. If the PE's (C-*,C-G) state does not have a PMSI as a downstream

interface, the PE sets up its forwarding path to receive

     (C-S,C-G) traffic from the originator of the selected Source
     Active A-D route.
 Whenever a PE deletes the (C-S,C-G) state that was previously created
 as a result of receiving a C-multicast route for (C-S,C-G) from some
 other PE, the PE that deletes the state also withdraws the Source
 Active A-D route (if there is one) that was advertised when the state
 was created.
 In the example topology of Section 9.3.1, this procedure will cause
 PE2 to generate a Source Active A-D route for (C-S,C-G).  When this
 route is received, PE4 will set up its forwarding state to expect
 (C-S,C-G) packets from PE2.  PE1 will change its forwarding state so
 that (C-S,C-G) packets that it receives from CE1 are not forwarded to
 any other PEs.  (Note that PE1 may still forward (C-S,C-G) packets
 received from CE1 to CE2, if CE2 has receivers for C-G and those

Rosen & Aggarwal Standards Track [Page 66] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 receivers did not switch from the (C-*,C-G) tree to the (C-S,C-G)
 tree.)  As a result, PE3 and PE4 do not receive duplicate packets of
 the (C-S,C-G) C-flow.
 With this procedure in place, there is no need to have any kind of
 C-multicast route that has the semantics of a PIM Prune(S,G,rpt)
 message.
 It is worth noting that if, as a result of this procedure, a PE sets
 up its forwarding state to receive (C-S,C-G) traffic from the source
 tree, the UMH is not necessarily the same as it would be if the PE
 had joined the source tree as a result of receiving a PIM Join for
 the same source tree from a directly attached CE.
 Note that the mechanism described in Section 7.4.1 can be leveraged
 to advertise an S-PMSI binding along with the source active messages.
 This is accomplished by using the same BGP Update message to carry
 both the NLRI of the S-PMSI A-D route and the NLRI of the Source
 Active A-D route.  (Though an implementation processing the received
 routes cannot assume that this will always be the case.)

10. Eliminating PE-PE Distribution of (C-*,C-G) State

 In the ASM service model, a node that wants to become a receiver for
 a particular multicast group G first joins a shared tree, rooted at a
 rendezvous point.  When the receiver detects traffic from a
 particular source, it has the option of joining a source tree, rooted
 at that source.  If it does so, it has to prune that source from the
 shared tree, to ensure that it receives packets from that source on
 only one tree.
 Maintaining the shared tree can require considerable state, as it is
 necessary not only to know who the upstream and downstream nodes are,
 but to know which sources have been pruned off which branches of the
 share tree.
 The BGP-based signaling procedures defined in this document and in
 [MVPN-BGP] eliminate the need for PEs to distribute to each other any
 state having to do with which sources have been pruned off a shared
 C-tree.  Those procedures do still allow multicast data traffic to
 travel on a shared C-tree, but they do not allow a situation in which
 some CEs receive (S,G) traffic on a shared tree and some on a source
 tree.  This results in a considerable simplification of the PE-PE
 procedures with minimal change to the multicast service seen within
 the VPN.  However, shared C-trees are still supported across the VPN
 backbone.  That is, (C-*,C-G) state is distributed PE-PE, but
 (C-*,C-G,rpt) state is not.

Rosen & Aggarwal Standards Track [Page 67] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 In this section, we specify a number of optional procedures that go
 further and that completely eliminate the support for shared C-trees
 across the VPN backbone.  In these procedures, the PEs keep track of
 the active sources for each C-G.  As soon as a CE tries to join the
 (*,G) tree, the PEs instead join the (S,G) trees for all the active
 sources.  Thus, all distribution of (C-*,C-G) state is eliminated.
 These procedures are optional because they require some additional
 support on the part of the VPN customer and because they are not
 always appropriate.  (For example, a VPN customer may have his own
 policy of always using shared trees for certain multicast groups.)
 There are several different options, described in the following sub-
 sections.

10.1. 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 Any-Source Multicast (ASM) mode [RFC4607] on
 the VPN customer site.  To quote from [MVPN-REQ]:
    In the case of PIM-SM, when a source starts to emit traffic toward
    a group (in ASM mode), if sources and receivers are located in VPN
    sites that are different than that of the RP, then traffic may
    transiently 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 traffic peak, even short, may not be convenient depending on
    the traffic and link bandwidth.
    Thus, a VPN solution MAY provide features that solve or help
    mitigate this potential 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
 section describes how "anycast-RP" can be used to achieve this.  This
 is described below.

10.1.1. Initial Configuration

 For a particular MVPN, at least one or more PEs that have sites in
 that MVPN, act as an RP for the sites of that MVPN connected to these
 PEs.  Within each MVPN, all of these RPs use the same (anycast)
 address.  All of these RPs use the Anycast RP technique.

10.1.2. Anycast RP Based on Propagating Active Sources

 This mechanism is based on propagating active sources between RPs.

Rosen & Aggarwal Standards Track [Page 68] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

10.1.2.1. Receiver(s) within a Site

 The PE that receives a C-Join message for (*,G) does not send the
 information that it has receiver(s) for G until it receives
 information about active sources for G from an Upstream PE.
 On receiving this (described in the next section), the downstream PE
 will respond with a Join message for (C-S,C-G).  Sending this
 information could be done using any of the procedures described in
 Section 5.  Only the Upstream PE will process this information.

10.1.2.2. Source within a Site

 When a PE receives a PIM Register message from a site that belongs to
 a given VPN, PE follows the normal PIM anycast RP procedures.  It
 then advertises the source and group of the multicast data packet
 carried in the PIM Register message to other PEs in BGP using the
 following information elements:
  1. Active source address
  1. Active group address
  1. Route target of the MVPN.
 This advertisement goes to all the PEs that belong to that MVPN.
 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 there are, then it generates
 an advertisement for (C-S,C-G) as specified in the previous section.

10.1.2.3. 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.

10.2. Using MSDP between a PE and a Local C-RP

 Section 10.1 describes the case where each PE is a C-RP.  This
 enables the PEs to know the active multicast sources for each MVPN,
 and they can then use BGP to distribute this information to each
 other.  As a result, the PEs do not have to join any shared C-trees,
 and this results in a simplification of the PE operation.
 In another deployment scenario, the PEs are not themselves C-RPs, but
 use Multicast Source Discovery Protocol (MSDP) [RFC3618] to talk to
 the C-RPs.  In particular, a PE that attaches to a site that contains
 a C-RP becomes an MSDP peer of that C-RP.  That PE then uses BGP to

Rosen & Aggarwal Standards Track [Page 69] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 distribute the information about the active sources to the other PEs.
 When the PE determines, by MSDP, that a particular source is no
 longer active, then it withdraws the corresponding BGP Update.  Then,
 the PEs do not have to join any shared C-trees, and they do not have
 to be C-RPs either.
 MSDP provides the capability for a Source Active (SA) message to
 carry an encapsulated data packet.  This capability can be used to
 allow an MSDP speaker to receive the first (or first several)
 packet(s) of an (S,G) flow, even though the MSDP speaker hasn't yet
 joined the (S,G) tree.  (Presumably, it will join that tree as a
 result of receiving the SA message that carries the encapsulated data
 packet.)  If this capability is not used, the first several data
 packets of an (S,G) stream may be lost.
 A PE that is talking MSDP to an RP may receive such an encapsulated
 data packet from the RP.  The data packet should be decapsulated and
 transmitted to the other PEs in the MVPN.  If the packet belongs to a
 particular (S,G) flow, and if the PE is a transmitter for some S-PMSI
 to which (S,G) has already been bound, the decapsulated data packet
 should be transmitted on that S-PMSI.  Otherwise, if an I-PMSI exists
 for that MVPN, the decapsulated data packet should be transmitted on
 it.  (If a MI-PMSI exists, this would typically be used.)  If neither
 of these conditions hold, the decapsulated data packet is not
 transmitted to the other PEs in the MVPN.  The decision as to whether
 and how to transmit the decapsulated data packet does not affect the
 processing of the SA control message itself.
 Suppose that PE1 transmits a multicast data packet on a PMSI, where
 that data packet is part of an (S,G) flow, and PE2 receives that
 packet from that PMSI.  According to Section 9, if PE1 is not the PE
 that PE2 expects to be transmitting (S,G) packets, then PE2 must
 discard the packet.  If an MSDP-encapsulated data packet is
 transmitted on a PMSI, as specified above, this rule from Section 9
 would likely result in the packet being discarded.  Therefore, if
 MSDP-encapsulated data packets being decapsulated and transmitted on
 a PMSI, we need to modify the rules of Section 9 as follows:
    1. If the receiving PE, PE2, has already joined the (S,G) tree,
       and has chosen PE1 as the Upstream PE for the (S,G) tree, but
       this packet does not come from PE1, PE2 must discard the
       packet.
    2. If the receiving PE, PE2, has not already joined the (S,G)
       tree, but is a PIM adjacency to a CE that is downstream on the
       (*,G) tree, the packet should be forwarded to the CE.

Rosen & Aggarwal Standards Track [Page 70] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

11. Support for PIM-BIDIR C-Groups

 In BIDIR-PIM, each multicast group is associated with a Rendezvous
 Point Address (RPA).  The Rendezvous Point Link (RPL) is the link
 that attaches to the RPA.  Usually, it's a LAN where the RPA is in
 the IP subnet assigned to the LAN.  The root node of a BIDIR-PIM tree
 is a node that has an interface on the RPL.
 On any LAN (other than the RPL) that is a link in a BIDIR-PIM tree,
 there must be a single node that has been chosen to be the DF.  (More
 precisely, for each RPA there is a single node that is the DF for
 that RPA.)  A node that receives traffic from an upstream interface
 may forward it on a particular downstream interface only if the node
 is the DF for that downstream interface.  A node that receives
 traffic from a downstream interface may forward it on an upstream
 interface only if that node is the DF for the downstream interface.
 If, for any period of time, there is a link on which each of two
 different nodes believes itself to be the DF, data forwarding loops
 can form.  Loops in a bidirectional multicast tree can be very
 harmful.  However, any election procedure will have a convergence
 period.  The BIDIR-PIM DF election procedure is very complicated,
 because it goes to great pains to ensure that if convergence is not
 extremely fast, then there is no forwarding at all until convergence
 has taken place.
 Other variants of PIM also have a DF election procedure for LANs.
 However, as long as the multicast tree is unidirectional,
 disagreement about who the DF is can result only in duplication of
 packets, not in loops.  Therefore, the time taken to converge on a
 single DF is of much less concern for unidirectional trees and it is
 for bidirectional trees.
 In the MVPN environment, if PIM signaling is used among the PEs, then
 the standard LAN-based DF election procedure can be used.  However,
 election procedures that are optimized for a LAN may not work as well
 in the MVPN environment.  So, an alternative to DF election would be
 desirable.
 If BGP signaling is used among the PEs, an alternative to DF election
 is necessary.  One might think that the "Single Forwarder Selection"
 procedures described in Sections 5 and 9 could be used to choose a
 single PE "DF" for the backbone (for a given RPA in a given MVPN).
 However, that is still likely to leave a convergence period of at
 least several seconds during which loops could form, and there could
 be a much longer convergence period if there is anything disrupting
 the smooth flow of BGP Updates.  So, a simple procedure like that is
 not sufficient.

Rosen & Aggarwal Standards Track [Page 71] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 The remainder of this section describes two different methods that
 can be used to support BIDIR-PIM while eliminating the DF election.

11.1. The VPN Backbone Becomes the RPL

 On a per-MVPN basis, this method treats the whole service provider(s)
 infrastructure as a single RPL.  We refer to such an RPL as an "MVPN-
 RPL".  This eliminates the need for the PEs to engage in any "DF
 election" procedure because BIDIR-PIM does not have a DF on the RPL.
 However, this method can only be used if the customer is
 "outsourcing" the RPL/RPA functionality to the SP.
 An MVPN-RPL could be realized either via an I-PMSI (this I-PMSI is on
 a per-MVPN basis and spans all the PEs that have sites of a given
 MVPN), via a collection of S-PMSIs, or even via a combination of an
 I-PMSI and one or more S-PMSIs.

11.1.1. Control Plane

 Associated with each MVPN-RPL is an address prefix that is
 unambiguous within the context of the MVPN associated with the MVPN-
 RPL.
 For a given MVPN, each VRF connected to an MVPN-RPL of that MVPN is
 configured to advertise to all of its connected CEs the address
 prefix of the MVPN-RPL.
 Since, in BIDIR-PIM, there is no Designated Forwarder on an RPL, in
 the context of MVPN-RPL, there is no need to perform the Designated
 Forwarder election among the PEs (note it is still necessary to
 perform the Designated Forwarder election between a PE and its
 directly attached CEs, but that is done using plain BIDIR-PIM
 procedures).
 For a given MVPN, a PE connected to an MVPN-RPL of that MVPN should
 send multicast data (C-S,C-G) on the MVPN-RPL only if at least one
 other PE connected to the MVPN-RPL has a downstream multicast state
 for C-G.  In the context of MVPN, this is accomplished by requiring a
 PE that has a downstream state for a particular C-G of a particular
 VRF present on the PE to originate a C-multicast route for (C-*,C-G).
 The RD of this route should be the same as the RD associated with the
 VRF.  The RTs carried by the route should be such as to ensure that
 the route gets distributed to all the PEs of the MVPN.

Rosen & Aggarwal Standards Track [Page 72] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

11.1.2. Data Plane

 A PE that receives (C-S,C-G) multicast data from a CE should forward
 this data on the MVPN-RPL of the MVPN the CE belongs to only if the
 PE receives at least one C-multicast route for (C-*, C-G).
 Otherwise, the PE should not forward the data on the RPL/I-PMSI.
 When a PE receives a multicast packet with (C-S,C-G) on an MVPN-RPL
 associated with a given MVPN, the PE forwards this packet to every
 directly connected CE of that MVPN, provided that the CE sends Join
 (C-*,C-G) to the PE (provided that the PE has the downstream
 (C-*,C-G) state).  The PE does not forward this packet back on the
 MVPN-RPL.  If a PE has no downstream (C-*,C-G) state, the PE does not
 forward the packet.

11.2. Partitioned Sets of PEs

 This method does not require the use of the MVPN-RPL, and it does not
 require the customer to outsource the RPA/RPL functionality to the
 SP.

11.2.1. Partitions

 Consider a particular C-RPA, call it C-R, in a particular MVPN.
 Consider the set of PEs that attach to sites that have senders or
 receivers for a BIDIR-PIM group C-G, where C-R is the RPA for C-G.
 (As always, we use the "C-" prefix to indicate that we are referring
 to an address in the VPN's address space rather than in the
 provider's address space.)
 Following the procedures of Section 5.1, each PE in the set
 independently chooses some other PE in the set to be its "Upstream
 PE" for those BIDIR-PIM groups with RPA C-R.  Optionally, they can
 all choose the "default selection" (described in Section 5.1) to
 ensure that each PE to choose the same Upstream PE.  Note that if a
 PE has a route to C-R via a VRF interface, then the PE may choose
 itself as the Upstream PE.
 The set of PEs can now be partitioned into a number of subsets.
 We'll say that PE1 and PE2 are in the same partition if and only if
 there is some PE3 such that PE1 and PE2 have each chosen PE3 as the
 Upstream PE for C-R.  Note that each partition has exactly one
 Upstream PE.  So it is possible to identify the partition by
 identifying its Upstream PE.
 Consider packet P, and let PE1 be its ingress PE.  PE1 will send the
 packet on a PMSI so that it reaches the other PEs that need to
 receive it.  This is done by encapsulating the packet and sending it

Rosen & Aggarwal Standards Track [Page 73] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 on a P-tunnel.  If the original packet is part of a PIM-BIDIR group
 (its ingress PE determines this from the packet's destination address
 C-G), and if the VPN backbone is not the RPL, then the encapsulation
 MUST carry information that can be used to identify the partition to
 which the ingress PE belongs.
 When PE2 receives a packet from the PMSI, PE2 must determine, by
 examining the encapsulation, whether the packet's ingress PE belongs
 to the same partition (relative to the C-RPA of the packet's C-G) to
 which the PE2 itself belongs.  If not, PE2 discards the packet.
 Otherwise, PE2 performs the normal BIDIR-PIM data packet processing.
 With this rule in place, harmful loops cannot be introduced by the
 PEs into the customer's bidirectional tree.
 Note that if there is more than one partition, the VPN backbone will
 not carry a packet from one partition to another.  The only way for a
 packet to get from one partition to another is for it to go up
 towards the RPA and then down another path to the backbone.  If this
 is not considered desirable, then all PEs should choose the same
 Upstream PE for a given C-RPA.  Then, multiple partitions will only
 exist during routing transients.

11.2.2. Using PE Distinguisher Labels

 If a given P-tunnel is to be used to carry packets traveling along a
 bidirectional C-tree, then, EXCEPT for the case described in Sections
 11.1 and 11.2.3, the packets that travel on that P-tunnel MUST carry
 a PE Distinguisher Label (defined in Section 4), using the
 encapsulation discussed in Section 12.3.
 When a given PE transmits a given packet of a bidirectional C-group
 to the P-tunnel, the packet will carry the PE Distinguisher Label
 corresponding to the partition, for the C-group's C-RPA, that
 contains the transmitting PE.  This is the PE Distinguisher Label
 that has been bound to the Upstream PE of that partition; it is not
 necessarily the label that has been bound to the transmitting PE.
 Recall that the PE Distinguisher Labels are upstream-assigned labels
 that are assigned and advertised by the node that is at the root of
 the P-tunnel.  The information about PE Distinguisher Labels is
 distributed with Intra-AS I-PMSI A-D routes and/or S-PMSI A-D routes
 by encoding it into the PE Distinguisher Labels attribute carried by
 these routes.
 When a PE receives a packet with a PE label that does not identify
 the partition of the receiving PE, then the receiving PE discards the
 packet.

Rosen & Aggarwal Standards Track [Page 74] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 Note that this procedure does not necessarily require the root of a
 P-tunnel to assign a PE Distinguisher Label for every PE that belongs
 to the tunnel.  If the root of the P-tunnel is the only PE that can
 transmit packets to the P-tunnel, then the root needs to assign PE
 Distinguisher Labels only for those PEs that the root has selected to
 be the UMHs for the particular C-RPAs known to the root.

11.2.3. Partial Mesh of MP2MP P-Tunnels

 There is one case in which support for BIDIR-PIM C-groups does not
 require the use of a PE Distinguisher Label.  For each C-RPA, suppose
 a distinct MP2MP LSP is used as the P-tunnel serving that C-RPA's
 partition.  Then, for a given packet, a PE receiving the packet from
 a P-tunnel can infer the partition from the tunnel.  So, PE
 Distinguisher Labels are not needed in this case.

12. 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.

12.1. Encapsulations for Single PMSI per P-Tunnel

12.1.1. Encapsulation in GRE

 GRE encapsulation can be used for any PMSI that is instantiated by a
 mesh of unicast P-tunnels, as well as for any PMSI that is
 instantiated by one or more PIM P-tunnels of any sort.
    Packets received        Packets in transit      Packets forwarded
    at the ingress PE       in the service          by the egress PEs
                            provider network
                         +---------------+
                         |  P-IP Header  |
                         +---------------+
                         |      GRE      |
 ++=============++       ++=============++       ++=============++
 || C-IP Header ||       || C-IP Header ||       || C-IP Header ||
 ++=============++ >>>>> ++=============++ >>>>> ++=============++
 || C-Payload   ||       || C-Payload   ||       || C-Payload   ||
 ++=============++       ++=============++       ++=============++

Rosen & Aggarwal Standards Track [Page 75] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 The IP Protocol Number field in the P-IP header MUST be set to 47.
 The Protocol Type field of the GRE header is set to either 0x800 or
 0x86dd, depending on whether the C-IP header is IPv4 or IPv6,
 respectively.
 When an encapsulated packet is transmitted by a particular PE, the
 source IP address in the P-IP header must be the same address that
 the PE uses to identify itself in the VRF Route Import Extended
 Communities that it attaches to any of VPN-IP routes eligible for UMH
 determination that it advertises via BGP (see Section 5.1).
 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 P-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.
 [RFC2784] specifies an optional GRE checksum and [RFC2890] 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 the GRE checksum field must follow [RFC2784].
 To facilitate high speed implementation, this document recommends
 that the ingress PE routers encapsulate VPN packets without setting
 the checksum or sequence fields.

12.1.2. Encapsulation in IP

 IP-in-IP [RFC2003] is also a viable option.  The following diagram
 shows the progression of the packet as it enters and leaves the
 service provider network.

Rosen & Aggarwal Standards Track [Page 76] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 Packets received        Packets in transit      Packets forwarded
 at the ingress PE       in the service          by the egress PEs
                         provider network
                         +---------------+
                         |  P-IP Header  |
 ++=============++       ++=============++       ++=============++
 || C-IP Header ||       || C-IP Header ||       || C-IP Header ||
 ++=============++ >>>>> ++=============++ >>>>> ++=============++
 || C-Payload   ||       || C-Payload   ||       || C-Payload   ||
 ++=============++       ++=============++       ++=============++
 When the P-IP header is an IPv4 header, its Protocol Number field is
 set to either 4 or 41, depending on whether the C-IP header is an
 IPv4 header or an IPv6 header, respectively.
 When the P-IP header is an IPv6 header, its Next Header field is set
 to either 4 or 41, depending on whether the C-IP header is an IPv4
 header or an IPv6 header, respectively.
 When an encapsulated packet is transmitted by a particular PE, the
 source IP address in the P-IP header must be the same address that
 the PE uses to identify itself in the VRF Route Import Extended
 Communities that it attaches to any of VPN-IP routes eligible for UMH
 determination that it advertises via BGP (see Section 5.1).

12.1.3. Encapsulation in MPLS

 If the PMSI is instantiated as a P2MP MPLS LSP or a MP2MP LSP, MPLS
 encapsulation is used.  Penultimate-hop-popping MUST be disabled for
 the LSP.
 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 appropriate additional MPLS
 encapsulation procedures may be used.
 Packets received        Packets in transit      Packets forwarded
 at the ingress PE       in the service          by the egress PEs
                         provider network
                         +---------------+
                         | P-MPLS Header |
 ++=============++       ++=============++       ++=============++
 || C-IP Header ||       || C-IP Header ||       || C-IP Header ||
 ++=============++ >>>>> ++=============++ >>>>> ++=============++
 || C-Payload   ||       || C-Payload   ||       || C-Payload   ||
 ++=============++       ++=============++       ++=============++

Rosen & Aggarwal Standards Track [Page 77] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

12.2. Encapsulations for Multiple PMSIs per P-Tunnel

 The encapsulations for transmitting multicast data messages when
 there are multiple PMSIs per P-tunnel are based on the encapsulation
 for a single PMSI per P-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.

12.2.1. Encapsulation in GRE

 Rather than the IP-in-GRE encapsulation discussed in Section 12.1.1,
 we use the MPLS-in-GRE encapsulation.  This is specified in
 [MPLS-IP].  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]).

12.2.2. Encapsulation in IP

 Rather than the IP-in-IP encapsulation discussed in Section 12.1.2,
 we use the MPLS-in-IP encapsulation.  This is specified in [MPLS-IP].
 The IP protocol number field MUST be set to the value identifying the
 payload as an MPLS unicast packet.  (There is no "MPLS multicast
 packet" protocol number.)

12.3. Encapsulations Identifying a Distinguished PE

12.3.1. For MP2MP LSP P-Tunnels

 As discussed in Section 9, if a multicast data packet is traveling on
 a unidirectional C-tree, it is highly desirable for the PE that
 receives the packet from a PMSI to be able to determine the identity
 of the PE that transmitted the data packet onto the PMSI.  The
 encapsulations of the previous sections all provide this information,
 except in one case.  If a PMSI is being instantiated by an MP2MP LSP,
 then the encapsulations discussed so far do not allow one to
 determine the identity of the PE that transmitted the packet onto the
 PMSI.
 Therefore, when a packet traveling on a unidirectional C-tree is
 traveling on a MP2MP LSP P-tunnel, it MUST carry, as its second
 label, a label that has been bound to the packet's ingress PE.  This
 label is an upstream-assigned label that the LSP's root node has
 bound to the ingress PE and has distributed via the PE Distinguisher

Rosen & Aggarwal Standards Track [Page 78] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 Labels attribute of a PMSI A-D route (see Section 4).  This label
 will appear immediately beneath the labels that are discussed in
 Sections 12.1.3 and 12.2.
 A full specification of the procedures for advertising and for using
 the PE Distinguisher Labels attribute in this case is outside the
 scope of this document.

12.3.2. For Support of PIM-BIDIR C-Groups

 As was discussed in Section 11, when a packet belongs to a PIM-BIDIR
 multicast group, the set of PEs of that packet's VPN can be
 partitioned into a number of subsets, where exactly one PE in each
 partition is the Upstream PE for that partition.  When such packets
 are transmitted on a PMSI, unless the procedures of Section 11.2.3
 are being used, it is necessary for the packet to carry information
 identifying a particular partition.  This is done by having the
 packet carry the PE Distinguisher Label corresponding to the Upstream
 PE of one partition.  For a particular P-tunnel, this label will have
 been advertised by the node that is the root of that P-tunnel.  (A
 full specification of the procedures for advertising PE Distinguisher
 Labels is out of the scope of this document.)
 This label needs to be used whenever a packet belongs to a PIM-BIDIR
 C-group, no matter what encapsulation is used by the P-tunnel.
 Hence, the encapsulations of Section 12.2 MUST be used.  If the
 P-tunnel contains only one PMSI, the PE label replaces the label
 discussed in Section 12.2.  If the P-tunnel contains multiple PMSIs,
 the PE label follows the label discussed in Section 12.2.
 In general, PE Distinguisher Labels can be carried if the
 encapsulation is MPLS, MPLS-in-IP, or MPLS-in-GRE.  However,
 procedures for advertising and using PE Distinguisher Labels when the
 encapsulation is LDP-based MP2P MPLS is outside the scope of this
 specification.

12.4. General Considerations for IP and GRE Encapsulations

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

12.4.1. MTU (Maximum Transmission Unit)

 It is the responsibility of the originator of a C-packet to ensure
 that the packet is small enough to reach all of its destinations,
 even when it is encapsulated within IP or GRE.

Rosen & Aggarwal Standards Track [Page 79] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 When a packet is encapsulated in IP or GRE, the router that does the
 encapsulation MUST set the DF bit in the outer header.  This ensures
 that the decapsulating router will not need to reassemble the
 encapsulating packets before performing decapsulation.
 In some cases, the encapsulating router may know that a particular
 C-packet is too large to reach its destinations.  Procedures by which
 it may know this are outside the scope of the current document.
 However, if this is known, then:
  1. If the DF bit is set in the IP header of a C-packet that is known

to be too large, the router will discard the C-packet as being

     "too large" and follow normal IP procedures (which may require
     the return of an ICMP message to the source).
  1. If the DF bit is not set in the IP header of a C-packet that is

known to be too large, the router MAY fragment the packet before

     encapsulating it and then encapsulate each fragment separately.
     Alternatively, the router MAY discard the packet.
 If the router discards a packet as too large, it should maintain
 Operations, Administration, and Maintenance (OAM) information related
 to this behavior, allowing the operator to properly troubleshoot the
 issue.
 Note that if the entire path of the P-tunnel does not support an MTU
 that is large enough to carry the a particular encapsulated C-packet,
 and if the encapsulating router does not do fragmentation, then the
 customer will not receive the expected connectivity.

12.4.2. TTL (Time to Live)

 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.

12.4.3. 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 that it used in support of MVPN
 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.

Rosen & Aggarwal Standards Track [Page 80] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

12.5. Differentiated Services

 The setting of the DS (Differentiated Services) field in the delivery
 IP header should follow the guidelines outlined in [RFC2983].
 Setting the Traffic Class field [RFC5462] in the delivery MPLS header
 should follow the guidelines in [RFC3270].  An SP may also choose to
 deploy any of additional Differentiated Services mechanisms that the
 PE routers support for the encapsulation in use.  Note that the type
 of encapsulation determines the set of Differentiated Services
 mechanisms that may be deployed.

13. Security Considerations

 This document describes an extension to the procedures of [RFC4364],
 and hence shares the security considerations described in [RFC4364]
 and [RFC4365].
 When GRE encapsulation is used, the security considerations of
 [MPLS-IP] are also relevant.  Additionally, the security
 considerations of [RFC4797] are relevant as it discusses implications
 on packet spoofing in the context of BGP/MPLS IP VPNs.
 The security considerations of [MPLS-HDR] apply when MPLS
 encapsulation is used.
 This document makes use of a number of control protocols: PIM
 [PIM-SM], BGP [MVPN-BGP], mLDP [MLDP], and RSVP-TE [RSVP-P2MP].
 Security considerations relevant to each protocol are discussed in
 the respective protocol specifications.
 If one uses the UDP-based protocol for switching to S-PMSI (as
 specified in Section 7.4.2), then an S-PMSI Join message (i.e., a UDP
 packet with destination port 3232 and destination address ALL-PIM-
 ROUTERS) that is not received over a PMSI (e.g., one received
 directly from a CE router) is an illegal packet and MUST be dropped.
 The various procedures for P-tunnel construction have security issues
 that are specific to the way that the P-tunnels are used in this
 document.  When P-tunnels are constructed via such techniques as PIM,
 mLDP, or RSVP-TE, each P or PE router receiving a control message
 MUST ensure that the control message comes from another P or PE
 router, not from a CE router.  (Interpreting an mLDP or PIM or RSVP-
 TE control message from a CE router as referring to a P-tunnel would
 be a bug.)
 A PE MUST NOT accept BGP routes of the MCAST-VPN address family from
 a CE.

Rosen & Aggarwal Standards Track [Page 81] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 If BGP is used as a CE-PE routing protocol, then when a PE receives
 an IP route from a CE, if this route carries the VRF Route Import
 Extended Community, the PE MUST remove this Community from the route
 before turning it into a VPN-IP route.  Routes that a PE advertises
 to a CE MUST NOT carry the VRF Route Import Extended Community.
 An ASBR may receive, from one SP's domain, an mLDP, PIM, or RSVP-TE
 control message that attempts to extend a P-tunnel from one SP's
 domain into another SP's domain.  This is perfectly valid if there is
 an agreement between the SPs to jointly provide an MVPN service.  In
 the absence of such an agreement, however, this could be an
 illegitimate attempt to intercept data packets.  By default, an ASBR
 MUST NOT allow P-tunnels to extend beyond AS boundaries.  However, it
 MUST be possible to configure an ASBR to allow this on a specified
 set of interfaces.
 Many of the procedures in this document cause the SP network to
 create and maintain an amount of state that is proportional to
 customer multicast activity.  If the amount of customer multicast
 activity exceeds expectations, this can potentially cause P and PE
 routers to maintain an unexpectedly large amount of state, which may
 cause control and/or data plane overload.  To protect against this
 situation, an implementation should provide ways for the SP to bound
 the amount of state it devotes to the handling of customer multicast
 activity.
 In particular, an implementation SHOULD provide mechanisms that allow
 an SP to place limitations on the following:
  1. total number of (C-*,C-G) and/or (C-S,C-G) states per VRF
  1. total number of P-tunnels per VRF used for S-PMSIs
  1. total number of P-tunnels traversing a given P router
 A PE implementation MAY also provide mechanisms that allow an SP to
 limit the rate of change of various MVPN-related states on PEs, as
 well as the rate at which MVPN-related control messages may be
 received by a PE from the CEs and/or sent from the PE to other PEs.
 An implementation that provides the procedures specified in Sections
 10.1 or 10.2 MUST provide the capability to impose an upper bound on
 the number of Source Active A-D routes generated and on how
 frequently they may be originated.  This MUST be provided on a per-
 PE, per-MVPN granularity.

Rosen & Aggarwal Standards Track [Page 82] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 Lack of the mechanisms that allow an SP to limit the rate of change
 of various MVPN-related states on PEs, as well as the rate at which
 MVPN-related control messages may be received by a PE from the CEs
 and/or sent from the PE to other PEs may result in the control plane
 overload on the PE, which in turn would adversely impact all the
 customers connected to that PE, as well as to other PEs.
 See also the Security Considerations section of [MVPN-BGP].

14. IANA Considerations

 Section 7.4.2 defines the "S-PMSI Join message", which is carried in
 a UDP datagram whose port number is 3232.  This port number had
 already been assigned by IANA to "MDT port".  The reference has been
 updated to this document.
 IANA has created a registry for the "S-PMSI Join message Type Field".
 Assignments are to be made according to the policy "IETF Review" as
 defined in [RFC5226].  The value 1 has been registered with a
 reference to this document.  The description reads "PIM IPv4 S-PMSI
 (unaggregated)".
 [PIM-ATTRIB] establishes a registry for "PIM Join attribute Types".
 IANA has assigned the value 1 to the "MVPN Join Attribute" with a
 reference to this document.
 IANA has assigned SAFI 129 to "Multicast for BGP/MPLS IP Virtual
 Private Networks (VPNs)" with a reference to this document and
 [MVPN-BGP].

15. Acknowledgments

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

Rosen & Aggarwal Standards Track [Page 83] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

16. References

16.1. Normative References

 [MLDP]        Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and
               B. Thomas, "Label Distribution Protocol Extensions for
               Point-to-Multipoint and Multipoint-to-Multipoint Label
               Switched Paths", RFC 6388, November 2011.
 [MPLS-HDR]    Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
               Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
               Encoding", RFC 3032, January 2001.
 [MPLS-IP]     Worster, T., Rekhter, Y., and E. Rosen, Ed.,
               "Encapsulating MPLS in IP or Generic Routing
               Encapsulation (GRE)", RFC 4023, March 2005.
 [MPLS-MCAST-ENCAPS]
               Eckert, T., Rosen, E., Ed., Aggarwal, R., and Y.
               Rekhter, "MPLS Multicast Encapsulations", RFC 5332,
               August 2008.
 [MPLS-UPSTREAM-LABEL]
               Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
               Label Assignment and Context-Specific Label Space", RFC
               5331, August 2008.
 [MVPN-BGP]    Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter,
               "BGP Encodings and Procedures for Multicast in MPLS/BGP
               IP VPNs", RFC 6514, February 2012.
 [OSPF]        Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
               1998.
 [OSPF-MT]     Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
               Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
               RFC 4915, June 2007.
 [PIM-ATTRIB]  Boers, A., Wijnands, I., and E. Rosen, "The Protocol
               Independent Multicast (PIM) Join Attribute Format", RFC
               5384, November 2008.
 [PIM-SM]      Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
               "Protocol Independent Multicast - Sparse Mode (PIM-SM):
               Protocol Specification (Revised)", RFC 4601, August
               2006.

Rosen & Aggarwal Standards Track [Page 84] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4364]     Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
               Networks (VPNs)", RFC 4364, February 2006.
 [RFC4659]     De Clercq, J., Ooms, D., Carugi, M., and F. Le
               Faucheur, "BGP-MPLS IP Virtual Private Network (VPN)
               Extension for IPv6 VPN", RFC 4659, September 2006.
 [RFC5462]     Andersson, L. and R. Asati, "Multiprotocol Label
               Switching (MPLS) Label Stack Entry: "EXP" Field Renamed
               to "Traffic Class" Field", RFC 5462, February 2009.
 [RSVP-OOB]    Ali, Z., Swallow, G., and R. Aggarwal, "Non-Penultimate
               Hop Popping Behavior and Out-of-Band Mapping for RSVP-
               TE Label Switched Paths", RFC 6511, February 2012.
 [RSVP-P2MP]   Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
               Yasukawa, Ed., "Extensions to Resource Reservation
               Protocol - Traffic Engineering (RSVP-TE) for Point-to-
               Multipoint TE Label Switched Paths (LSPs)", RFC 4875,
               May 2007.

16.2. Informative References

 [ADMIN-ADDR]  Meyer, D., "Administratively Scoped IP Multicast", BCP
               23, RFC 2365, July 1998.
 [BIDIR-PIM]   Handley, M., Kouvelas, I., Speakman, T., and L.
               Vicisano, "Bidirectional Protocol Independent Multicast
               (BIDIR-PIM)", RFC 5015, October 2007.
 [BSR]         Bhaskar, N., Gall, A., Lingard, J., and S. Venaas,
               "Bootstrap Router (BSR) Mechanism for Protocol
               Independent Multicast (PIM)", RFC 5059, January 2008.
 [MVPN-REQ]    Morin, T., Ed., "Requirements for Multicast in Layer 3
               Provider-Provisioned Virtual Private Networks
               (PPVPNs)", RFC 4834, April 2007.
 [RFC2003]     Perkins, C., "IP Encapsulation within IP", RFC 2003,
               October 1996.
 [RFC2784]     Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
               Traina, "Generic Routing Encapsulation (GRE)", RFC
               2784, March 2000.

Rosen & Aggarwal Standards Track [Page 85] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

 [RFC2890]     Dommety, G., "Key and Sequence Number Extensions to
               GRE", RFC 2890, September 2000.
 [RFC2983]     Black, D., "Differentiated Services and Tunnels", RFC
               2983, October 2000.
 [RFC3270]     Le Faucheur, F., Wu, L., Davie, B., Davari, S.,
               Vaananen, P., Krishnan, R., Cheval, P., and J.
               Heinanen, "Multi-Protocol Label Switching (MPLS)
               Support of Differentiated Services", RFC 3270, May
               2002.
 [RFC3618]     Fenner, B., Ed., and D. Meyer, Ed., "Multicast Source
               Discovery Protocol (MSDP)", RFC 3618, October 2003.
 [RFC4365]     Rosen, E., "Applicability Statement for BGP/MPLS IP
               Virtual Private Networks (VPNs)", RFC 4365, February
               2006.
 [RFC4607]     Holbrook, H. and B. Cain, "Source-Specific Multicast
               for IP", RFC 4607, August 2006.
 [RFC4797]     Rekhter, Y., Bonica, R., and E. Rosen, "Use of Provider
               Edge to Provider Edge (PE-PE) Generic Routing
               Encapsulation (GRE) or IP in BGP/MPLS IP Virtual
               Private Networks", RFC 4797, January 2007.
 [RFC5226]     Narten, T. and H. Alvestrand, "Guidelines for Writing
               an IANA Considerations Section in RFCs", BCP 26, RFC
               5226, May 2008.

Rosen & Aggarwal Standards Track [Page 86] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

Contributing Authors

 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
 EMail: 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
 IJsbrand Wijnands
 Cisco Systems, Inc.
 170 Tasman Drive
 San Jose, CA, 95134
 EMail: 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

Rosen & Aggarwal Standards Track [Page 87] RFC 6513 Multicast in MPLS/BGP IP VPNs February 2012

Editors' Addresses

 Eric C. Rosen
 Cisco Systems, Inc.
 1414 Massachusetts Avenue
 Boxborough, MA, 01719
 EMail: erosen@cisco.com
 Rahul Aggarwal
 Juniper Networks
 1194 North Mathilda Ave.
 Sunnyvale, CA 94089
 EMail: raggarwa_1@yahoo.com

Rosen & Aggarwal Standards Track [Page 88]

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