Multicast VPN Using Bit Index Explicit Replication (BIER)
RFC 8556

Internet Engineering Task Force (IETF)                     E. Rosen, Ed.
Request for Comments: 8556                                  M. Sivakumar
Category: Standards Track                                  T. Przygienda
ISSN: 2070-1721                                   Juniper Networks, Inc.
                                                               S. Aldrin
                                                            Google, Inc.
                                                             A. Dolganow
                                                                   Nokia
                                                              April 2018

       Multicast VPN Using Bit Index Explicit Replication (BIER)

Abstract

   The Multicast Virtual Private Network (MVPN) specifications require
   the use of multicast tunnels ("P-tunnels") that traverse a service
   provider's backbone network.  The P-tunnels are used for carrying
   multicast traffic across the backbone.  A variety of P-tunnel types
   are supported.  Bit Index Explicit Replication (BIER) is a new
   architecture that provides optimal multicast forwarding through a
   "multicast domain", without requiring intermediate routers to
   maintain any per-flow state or to engage in an explicit tree-building
   protocol.  This document specifies the protocol and procedures that
   allow MVPN to use BIER as the method of carrying multicast traffic
   over a service provider's backbone network.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8556.

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Copyright Notice

   Copyright (c) 2018 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

   (https://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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Use of the PMSI Tunnel Attribute in x-PMSI A-D Routes . . . .   5
     2.1.  MPLS Label  . . . . . . . . . . . . . . . . . . . . . . .   7
     2.2.  Explicit Tracking . . . . . . . . . . . . . . . . . . . .   9
       2.2.1.  Using the LIR Flag  . . . . . . . . . . . . . . . . .  10
       2.2.2.  Using the LIR-pF Flag . . . . . . . . . . . . . . . .  10
   3.  Use of the PMSI Tunnel Attribute in Leaf A-D Routes . . . . .  11
   4.  Data Plane  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.1.  Encapsulation and Transmission  . . . . . . . . . . . . .  12
     4.2.  Disposition . . . . . . . . . . . . . . . . . . . . . . .  14
       4.2.1.  At a BFER That Is an Egress PE  . . . . . . . . . . .  14
       4.2.2.  At a BFER That Is a P-tunnel Segmentation Boundary  .  14
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  16
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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1.  Introduction

   [RFC6513] and [RFC6514] specify the protocols and procedures that a
   Service Provider (SP) can use to provide Multicast Virtual Private
   Network (MVPN) service to its customers.  Multicast tunnels are
   created through an SP's backbone network; these are known as
   "P-tunnels".  The P-tunnels are used for carrying multicast traffic
   across the backbone.  The MVPN specifications allow the use of
   several different kinds of P-tunnel technology.

   Bit Index Explicit Replication (BIER) ([RFC8279]) is an architecture
   that provides optimal multicast forwarding through a "multicast
   domain", without requiring intermediate routers to maintain any per-
   flow state or to engage in an explicit tree-building protocol.  The
   purpose of the current document is to specify the protocols and
   procedures needed in order to provide MVPN service using BIER to
   transport the multicast traffic over the backbone.

   Although BIER does not explicitly build and maintain multicast
   tunnels, one can think of BIER as using a number of implicitly
   created tunnels through a "BIER domain".  In particular, one can
   think of there as being one Point-to-Multipoint (P2MP) tunnel from
   each Bit Forwarding Ingress Router (BFIR) to all the Bit Forwarding
   Egress Routers (BFERs) in the BIER domain, where a BIER domain is
   generally co-extensive with an IGP network.  These "tunnels" are not
   specific to any particular VPN.  However, the MVPN architecture
   provides protocols and procedures that allow the traffic of multiple
   MVPNs to be aggregated on a single P-tunnel.  In this document, we
   specify how to use these multi-VPN aggregation procedures to enable
   BIER to transport traffic from multiple MVPNs.

   MVPN traffic must sometimes traverse more than one IGP domain,
   whereas BIER only carries multicast traffic within a single IGP
   domain.  However, the MVPN specifications allow P-tunnels to be
   segmented (the concept of MVPN segmentation is defined in [RFC6513]
   and [RFC6514]), where the segmentation points may either be
   Autonomous System Border Routers (ASBRs) as described in [RFC6514],
   or Area Border Routers (ABRs) as described in [RFC7524].  As long as
   the segmentation points are capable of acting as BFIRs and BFERs,
   BIER can be used to provide some or all of the segments of a
   P-tunnel.

   Procedures to support MVPN customers who are using Bidirectional PIM
   (BIDIR-PIM) are outside the scope of this document.

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   This document uses the following terminology from [RFC8279]:

   o  BFR: Bit-Forwarding Router.

   o  BFIR: Bit-Forwarding Ingress Router.

   o  BFER: Bit-Forwarding Egress Router.

   This document uses the following terminology from [RFC6513]:

   o  MVPN: Multicast Virtual Private Network -- a VPN [RFC4364] in
      which multicast service is offered.

   o  P-tunnel: A multicast tunnel through the network of one or more
      SPs.  P-tunnels are used to transport MVPN multicast data

   o  PMSI: Provider Multicast Service Interface.  PMSI is an
      abstraction that represents a multicast service for carrying
      packets.  A PMSI is instantiated via one or more P-tunnels.

   o  C-S: A multicast source address, identifying a multicast source
      located at a VPN customer site.

   o  C-G: A multicast group address used by a VPN customer.

   o  C-flow: A customer multicast flow.  Each C-flow is identified by
      the ordered pair (source address, group address), where each
      address is in the customer's address space.  The identifier of a
      particular C-flow is usually written as (C-S,C-G).

      Sets of C-flows can be identified by the use of the "C-*" wildcard
      (see [RFC6625]), e.g., (C-*,C-G).

   o  I-PMSI A-D Route: Inclusive PMSI Auto-Discovery route.  Carried in
      BGP Update messages, these routes are used to advertise the
      default P-tunnel for a particular MVPN.

   o  S-PMSI A-D route: Selective PMSI Auto-Discovery route.  Carried in
      BGP Update messages, these routes are used to advertise the fact
      that particular C-flows are bound to (i.e., are traveling through)
      particular P-tunnels.

   o  x-PMSI A-D route: A route that is either an I-PMSI A-D route or an
      S-PMSI A-D route.

   o  Leaf A-D route: A route that a multicast egress node sends in
      order to join a particular P-tunnel.

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   o  PMSI Tunnel attribute (PTA): In an x-PMSI A-D route, the Network
      Layer Reachability Information (NLRI) of the route identifies a
      PMSI.  The BGP attribute known as the PMSI Tunnel attribute is
      attached to such a route in order to identify a particular
      P-tunnel that is associated with the PMSI.  When C-flows of
      multiple VPNs are carried in a single P-tunnel, this attribute
      also carries the information needed to multiplex and demultiplex
      the C-flows.  A PTA can also be carried by a Leaf A-D root.  In
      this case, it contains information that is needed in order for the
      originator of the route to join the specified P-tunnel.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Use of the PMSI Tunnel Attribute in x-PMSI A-D Routes

   As defined in [RFC6514], the PMSI Tunnel attribute (PTA) carried by
   an x-PMSI A-D route identifies the P-tunnel that is used to
   instantiate a particular PMSI.  If a PMSI is to be instantiated by
   BIER, the PTA is constructed by a BFIR.

   If segmented P-tunnels are not being used, the PTA attached to a
   given x-PMSI A-D route is constructed by the router that originated
   the route (typically by the ingress Provider Edge (PE) router), and
   the PTA is not changed as the route is propagated.

   If segmented P-tunnels are being used, the PTA attached to a given
   x-PMSI A-D route by the route's originator may be replaced at a
   segmentation point (a BFER) by a PTA identifying the next segment of
   the P-tunnel.  If the next segment of the P-tunnel is instantiated by
   BIER, the segmentation point serves as the BFIR for that next
   segment.

   In either case, a PTA is constructed by a BFIR as follows (see
   Figure 1):

   The PTA contains the following fields:

   o  Tunnel Type: IANA has assigned 0x0B as the tunnel type codepoint
      for "BIER" in the "P-Multicast Service Interface Tunnel (PMSI
      Tunnel) Tunnel Types" registry.  This codepoint is used to
      indicate that the PMSI is instantiated by BIER.

      Although BIER does not actually create tunnels, the MVPN
      procedures treat BIER as if it were a type of tunnel.

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   o  Tunnel Identifier: When the tunnel type is BIER, this field
      contains three subfields:

      1.  The first subfield is a single octet, containing a BIER
          sub-domain-id (see [RFC8279]).  This indicates that packets
          sent on the PMSI will be sent on the specified BIER
          sub-domain.  How that sub-domain is chosen is outside the
          scope of this document.

      2.  The second subfield is a two-octet field containing the BFR-id
          in the sub-domain identified in the first subfield of the
          router that is constructing the PTA.

      3.  The third subfield is the BFR-prefix (see [RFC8279]) of the
          router (a BFIR) that is constructing the PTA.  The BFR-prefix
          will either be a /32 IPv4 address or a /128 IPv6 address.
          Whether the address is IPv4 or IPv6 can be inferred from the
          total length of the PTA.

          The BFR-prefix need not be the same IP address that is carried
          in any other field of the x-PMSI A-D route, even if the BFIR
          is the originating router of the x-PMSI A-D route.

      Failure to properly set the Tunnel Identifier field cannot be
      detected by the protocol and will result in improper delivery of
      the data packets sent on the PMSI.

   o  MPLS Label: This field MUST contain an upstream-assigned non-zero
      MPLS label.  It is assigned by the router (a BFIR) that constructs
      the PTA.  Constraints on the way in which a BFIR selects this
      label are discussed in Section 2.1.

      Failure to follow the constraints on label assignment cannot be
      detected by the protocol and may result in improper handling of
      data packets by the egress PE routers.

   o  Flags: When the tunnel type is BIER, two of the flags in the PTA
      Flags field are meaningful.  Details about the use of these flags
      can be found in Section 2.2.

      *  Leaf Information Required per Flow (LIR-pF): This flag is
         introduced in [RFC8534].  A BFIR SHOULD NOT set this flag
         UNLESS it knows that all the BFERs in the BIER domain (or at
         least all the BFERs to which it needs to transmit) support this
         flag.  (How this is known is outside the scope of this
         document.)  Procedures for the use of this flag are given in
         Section 2.2.2.  Support for this flag is OPTIONAL.

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      *  Leaf Information Required (LIR): see Section 2.2.1.

          +---------------------------------+
          |  Flags (1 octet)                |
          +---------------------------------+
          |  Tunnel Type = 0x0B (1 octet)   |
          +---------------------------------+
          |  MPLS Label (3 octets)          |
          +---------------------------------+
          |  Sub-domain-id (1 octet)        |  <---
          +---------------------------------+     |
          |  BFR-id (2 octets)              |     |-- Tunnel
          +---------------------------------+     |   Identifier
          |  BFR-prefix (4 or 16 octets)    |  <---
          +---------------------------------+

                 Figure 1: PMSI Tunnel Attribute for BIER

   If a PTA specifying tunnel type BIER is attached to an x-PMSI A-D
   route, the route MUST NOT be distributed beyond the boundaries of a
   BIER domain.  That is, any routers that receive the route must be in
   the same BIER domain as the originator of the route.  If the
   originator is in more than one BIER domain, the route must be
   distributed only within the BIER domain in which the BFR-prefix in
   the PTA uniquely identifies the originator.  As with all MVPN routes,
   distribution of these routes is controlled by the provisioning of
   Route Targets (RTs).  Thus, the requirement expressed in this
   paragraph is really a requirement on the way the Route Targets are
   provisioned.

2.1.  MPLS Label

   The MPLS Label carried in the PTA is an upstream-assigned label.

   If two PTAs contain the same BFR-prefix in their respective Tunnel
   Identifier fields, then the labels carried in those PTAs MUST come
   from the same label space (see Section 7 of [RFC5331]).  An
   implementation may choose to use this fact when setting up the tables
   it uses to interpret the upstream-assigned labels.

   Suppose that a BFIR attaches a PTA to each of two x-PMSI A-D routes,
   and both PTAs specify a tunnel type of BIER.

   o  If the two routes do not carry the same set of RTs, then their
      respective PTAs MUST contain different MPLS label values.

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   o  If the two routes do not have the same Address Family Identifier
      (AFI) value, then their respective PTAs MUST contain different
      MPLS label values.  This ensures that when an egress PE receives a
      data packet with the given label, the egress PE can infer from the
      label whether the payload is an IPv4 packet or an IPv6 packet.

   o  If the BFIR is an ingress PE supporting MVPN extranet ([RFC7900])
      functionality, and if the two routes originate from different VPN
      Routing and Forwarding tables (VRFs) on this ingress PE, then the
      respective PTAs of the two routes MUST contain different MPLS
      label values.

   o  If the BFIR is an ingress PE supporting the "Extranet Separation"
      feature of MVPN extranet (see Section 7.3 of [RFC7900]), and if
      one of the routes carries the "Extranet Separation" extended
      community but the other does not, then the respective PTAs of the
      two routes MUST contain different MPLS label values.

   o  If segmented P-tunnels are being used, then the respective PTAs of
      the two routes MUST contain different MPLS label values whenever
      the respective NLRIs of the two routes are not identical.  The
      MPLS label can then be used at the next segmentation point to
      switch packets from one P-tunnel segment directly to the next,
      without requiring the segmentation points to contain any other
      multicast forwarding state.  This is explained further below; see
      also Section 4.

   When segmented P-tunnels are being used, a segmentation point, call
   it "B1", may receive an x-PMSI A-D route whose PTA specifies BIER
   from within a given BIER domain.  This means that BIER is being used
   for the previous segment of a segmented P-tunnel.  If the next
   segment is also of type BIER, B1 will be the BFIR for the next
   segment.  That is, B1 is a BFER of one BIER domain (corresponding to
   the previous segment) and a BFIR of another BIER domain
   (corresponding to the next segment).  B1 needs to replace the PTA of
   the x-PMSI A-D route with a new PTA, specifying its own BFR-prefix
   and specifying an upstream-assigned label assigned by B1 itself.

   Suppose that B1 has received two x-PMSI A-D routes, R1 and R2, where:

   o  R1 and R2 each have a PTA specifying BIER.

   o  R1's PTA specifies BFR-prefix B2 and Label L2.

   o  R2's PTA specifies BFR-prefix B3 and Label L3.

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   Suppose B1 decides to propagate both R1 and R2, replacing each PTA
   with a new PTA specifying BIER.  Suppose these new PTAs specify
   labels L4 and L5,respectively.  Then L4 and L5 MUST be different
   (upstream-assigned) label values, UNLESS both of the following
   conditions hold:

   o  R1 and R2 have the same value in the Originating Router field of
      their respective NLRIs, and

   o  B2 is equal to B3, and

   o  L2 is equal to L3.

   The segmentation point (B1, in this example) MUST also program its
   data plane appropriately.  For example, when:

   o  B1 receives a BIER packet for which it is a BFER, and

   o  the BIER header specifies the BFIR-id that corresponds to B2, and

   o  the BIER payload is an MPLS packet with upstream-assigned label,
      and

   o  the top label value is L2,

   then the data plane must be programmed to replace L2 with L4 and to
   re-encapsulate the packet in a BIER header with B1's BFR-id in the
   BFIR-id field.  The BitString of the new BIER header is determined by
   applying the procedures for MVPN explicit tracking (see Section 2.2)
   in the BIER domain of the next segment, i.e., in the BIER domain for
   which B1 is the BFIR).

2.2.  Explicit Tracking

   When using BIER to transport an MVPN data packet through a BIER
   domain, an ingress PE functions as a BFIR (see [RFC8279]).  The BFIR
   must determine the set of BFERs to which the packet needs to be
   delivered.  This can be done in either of two ways:

   1.  Using the explicit tracking mechanism based on the "Leaf
       Information Required" flag specified in [RFC6513] and [RFC6514].
       This method is further described in Section 2.2.1.

   2.  Using the OPTIONAL explicit tracking mechanism based on the
       LIR-pF flag specified in [RFC8534].  This method, further
       described in Section 2.2.2, may be used if (and only if)
       segmented P-tunnels are not being used.

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2.2.1.  Using the LIR Flag

   To determine the set of BFERs to which the packets of a given C-flow
   must be sent, a BFIR MUST originate a (C-S,C-G) S-PMSI A-D route for
   the given C-flow.  It MUST attach a PTA to that route and MUST set
   the Leaf Information Required (LIR) flag in the PTA.  Per [RFC6514],
   the BFERs that need to receive that C-flow will respond with
   (C-S,C-G) Leaf A-D routes.  By matching the received Leaf A-D routes
   to the originated S-PMSI A-D routes, the originator of the S-PMSI A-D
   route determines the set of BFERs that need to receive the multicast
   data flow that is identified in the NLRI of S-PMSI A-D route.

   Suppose that an ingress PE has originated an I-PMSI A-D route or a
   wildcard S-PMSI A-D route [RFC6625] with a PTA specifying a tunnel
   type of BIER.  Now suppose that the ingress PE originates an S-PMSI
   A-D route specifying (C-S,C-G), where (C-S,C-G) "matches" (according
   to the rules of [RFC6625]) the wildcard S-PMSI A-D route or the
   I-PMSI A-D route.  Instead of attaching a PTA specifying BIER to the
   (C-S,C-G) route, the ingress PE MAY attach a PTA specifying a tunnel
   type of "no tunnel information".  This is equivalent to attaching the
   same PTA attached to the matching "less specific" route.

2.2.2.  Using the LIR-pF Flag

   If segmented P-tunnels are not being used, the BFIR can determine the
   set of BFERs that need to receive the packets of a given (C-S,C-G)
   C-flow as follows.  The BFIR MUST originate a wildcard S-PMSI A-D
   route (either (C-*,C-*), (C-*,C-G), or (C-S,C-G)), and the PTA of
   that route MUST use the following settings:

   o  The LIR-pF flag MUST be set.

   o  The tunnel type MUST be set to BIER.

   o  A non-zero MPLS label MUST be specified.

   Per [RFC8534], a BFER that needs to receive (C-S,C-G) traffic from
   the BFIR will respond with a Leaf A-D route.

   A BFIR MUST NOT use this method of finding the set of BFERs needing
   to receive a given C-flow unless it knows that all those BFERs
   support the LIR-pF flag.  How this is known is outside the scope of
   this document.

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   This method greatly reduces the number of S-PMSI A-D routes that a
   BFIR needs to originate; it can now originate as few as one such
   route (a (C-*,C-*) S-PMSI A-D route), rather than one for each
   C-flow.  However, the method does not provide a way for the BFIR to
   assign a distinct label to each C-flow.  Therefore, it cannot be used
   when segmented P-tunnels are in use (see Section 4 for an
   explanation).

   Note: If a BFIR originates a (C-*,C-*) S-PMSI A-D route with the
   LIR-pF flag set but also originates a more specific wildcard route
   that matches a particular (C-S,C-G), the BFERs will not originate
   Leaf A-D routes for that (C-S,C-G) unless the LIR-pF flag is also set
   in the more specific wildcard route.  If the BFIR also originates a
   (C-S,C-G) S-PMSI A-D route without the LIR flag set, the BFERs will
   not originate Leaf A-D routes for that (C-S,C-G) unless the LIR flag
   is also set in that route.

3.  Use of the PMSI Tunnel Attribute in Leaf A-D Routes

   Before an egress PE can receive a (C-S,C-G) flow from a given ingress
   PE via BIER, the egress PE must have received one of the following
   x-PMSI A-D routes from the ingress PE:

   o  A (C-S,C-G) S-PMSI A-D route (i.e., an S-PMSI A-D route whose NLRI
      encodes (C-S,C-G)) and whose PTA specifies a tunnel type of BIER.
      If such a route is found, we refer to it as the "matching x-PMSI
      A-D route."

   o  A "less specific" x-PMSI A-D route (one specifying (C-*,C-*),
      (C-*,C-G), or (C-S,C-G)) whose PTA specifies a tunnel type of
      BIER, and that is the egress PE's "match for reception" of
      (C-S,C-G).

      The rules for determining which x-PMSI A-D route is the match for
      reception are given in [RFC6625].  However, these rules are
      modified here to exclude any x-PMSI A-D route that does not have a
      PTA or whose PTA specifies "no tunnel type".

      If such a route is found, we refer to it as the "matching x-PMSI
      A-D route."

   If no matching x-PMSI A-D route for (C-S,C-G) is found, the egress PE
   cannot receive the (C-S,C-G) flow from the ingress PE via BIER until
   such time as a matching route is received.

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   When an egress PE determines that it needs to receive a (C-S,C-G)
   flow from a particular ingress PE via BIER, it originates a Leaf A-D
   route.  Construction of the Leaf A-D route generally follows the
   procedures specified in [RFC6514] or, optionally, the procedures
   specified in [RFC8534].  However, when BIER is being used, the Leaf
   A-D route MUST carry a PTA that is constructed as follows:

   1.  The tunnel type MUST be set to BIER.

   2.  The MPLS Label field SHOULD be set to zero.

   3.  The sub-domain-id subfield of the Tunnel Identifier field (as
       defined in Section 2) MUST be set to the corresponding value from
       the PTA of the matching x-PMSI A-D route.

   4.  The BFR-id subfield of the Tunnel Identifier field MUST be set to
       the BFR-id in the sub-domain identified by the sub-domain-id
       subfield of the egress PE (BFER).

   5.  The BFR-prefix field of the Tunnel Identifier field (as defined
       in Section 2) MUST be set to the egress PE's (BFER's) BFR-prefix.

       The BFR-prefix need not be the same IP address that is carried in
       any other field of the Leaf A-D route.

   When an ingress PE receives such a Leaf A-D route, it learns the
   BFR-prefix of the egress PE from the PTA.  The ingress PE does not
   make any use the value of the PTA's MPLS label field.

   Failure to properly construct the PTA cannot always be detected by
   the protocol and will cause improper delivery of the data packets.

4.  Data Plane

   The MVPN application plays the role of the "multicast flow overlay"
   as described in [RFC8279].

4.1.  Encapsulation and Transmission

   To transmit an MVPN data packet, an ingress PE follows the rules of
   [RFC6625] to find the x-PMSI A-D route that is a "match for
   transmission" for that packet.  (In applying the rules of [RFC6625],
   any S-PMSI A-D route with a PTA specifying "no tunnel information" is
   ignored.)  If the matching route has a PTA specifying BIER, the
   (upstream-assigned) MPLS label from that PTA is pushed on the
   packet's label stack.  Then the packet is encapsulated in a BIER

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   header.  That is, the ingress PE functions as a BFIR.  The BIER
   sub-domain used for transmitting the packet is specified in the PTA
   of the above-mentioned x-PMSI A-D route.

   In order to create the proper BIER header for a given packet, the
   BFIR must know all the BFERs that need to receive that packet.  It
   determines this by finding all the Leaf A-D routes that correspond to
   the S-PMSI A-D route that is the packet's match for transmission.
   There are two different cases to consider:

   1.  The S-PMSI A-D route that is the match for transmission carries a
       PTA that has the LIR flag set but does not have the LIR-pF flag
       set.

       In this case, the corresponding Leaf A-D routes are those whose
       "route key" field is identical to the NLRI of the S-PMSI A-D
       route.

   2.  The S-PMSI A-D route that is the match for transmission carries a
       PTA that has the LIR-pF flag.

       In this case, the corresponding Leaf A-D routes are those whose
       "route key" field is derived from the NLRI of the S-PMSI A-D
       route according to the procedures described in Section 5.2 of
       [RFC8534].

   The Leaf A-D route from a given BFER will contain a PTA that
   specifies the BFER's BFR-prefix.  With this information, the BFIR can
   construct the BIER BitString.

   However, if the PTA of the Leaf A-D route from a given BFER specifies
   a sub-domain other than the one being used for transmitting the
   packet, the bit for that BFER cannot be determined and that BFER will
   not receive the packet.

   The BIER-encapsulated packet is then forwarded, according to the
   procedures described in [RFC8279] and [RFC8296].  (See especially
   Section 3, "Imposing and Processing the BIER Encapsulation" in
   [RFC8296].)

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4.2.  Disposition

   When a BFER receives an MVPN multicast data packet that has been
   BIER-encapsulated, the BIER layer passes the following information to
   the multicast flow overlay:

   o  The sub-domain-id and the BFIR-id from the BIER header.  (As the
      sub-domain-id is inferred from the BIFT-id field of the BIER
      header, an implementation might choose to pass the BIFT-id rather
      than the sub-domain-id; this is an implementation matter.)

   o  The "payload", which is an MPLS packet whose top label is an
      upstream-assigned label.  In the data plane, the BFIR-id and the
      sub-domain-id provide the context in which the upstream-assigned
      label is interpreted.

   By looking up the upstream-assigned label in the appropriate context,
   the multicast flow overlay determines whether the BFER is an egress
   PE for the packet.

   Note that if segmented P-tunnels are in use, a BFER might be a
   P-tunnel segmentation border router rather than an egress PE, or a
   BFER might be both an egress PE and a P-tunnel segmentation border
   router.  Depending upon the role of the BFER for the given packet, it
   may need to follow the procedures of Section 4.2.1, the procedures of
   Section 4.2.2, or both.

4.2.1.  At a BFER That Is an Egress PE

   From looking up the packet's upstream-assigned label in the context
   of the packet's BFIR-prefix, the egress PE determines the egress VRF
   for the packet.  From the IP header of the payload, the multicast
   states of the VRF, the upstream-assigned label, and the BFR-prefix,
   the egress PE can determine whether the packet needs to be forwarded
   out one or more VRF interfaces.

4.2.2.  At a BFER That Is a P-tunnel Segmentation Boundary

   When segmented P-tunnels are being used, a BFER that receives a BIER-
   encapsulated MVPN multicast data packet may need to be forwarded on
   its next P-tunnel segment.  The choice of the next P-tunnel segment
   for the packet depends upon the C-flow to which the packet belongs.
   As long as the BFIR has assigned the MPLS label according to the
   constraints specified in Section 2.1, the BFIR will have assigned
   distinct upstream-assigned MPLS labels to distinct C-flows.  The BFER
   can thus select the proper "next P-tunnel segment" for a given packet
   simply by looking up the upstream-assigned label that immediately
   follows the BIER header.

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5.  IANA Considerations

   IANA has assigned the codepoint 0x0B to BIER in the "P-Multicast
   Service Interface Tunnel (PMSI Tunnel) Tunnel Types" registry.

6.  Security Considerations

   The procedures of this document do not, in themselves, provide
   privacy, integrity, or authentication for the control plane or the
   data plane.  For a discussion of the security considerations
   regarding the use of BIER, please see [RFC8279] and [RFC8296].
   Security considerations regarding VPN technology based on [RFC4364],
   [RFC6513], and [RFC6514] can be found in those RFCs.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              RFC 5331, DOI 10.17487/RFC5331, August 2008,
              <https://www.rfc-editor.org/info/rfc5331>.

   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <https://www.rfc-editor.org/info/rfc6513>.

   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
              <https://www.rfc-editor.org/info/rfc6514>.

   [RFC6625]  Rosen, E., Ed., Rekhter, Y., Ed., Hendrickx, W., and
              R. Qiu, "Wildcards in Multicast VPN Auto-Discovery
              Routes", RFC 6625, DOI 10.17487/RFC6625, May 2012,
              <https://www.rfc-editor.org/info/rfc6625>.

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RFC 8556                     MVPN with BIER                   April 2018

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,
              <https://www.rfc-editor.org/info/rfc8279>.

   [RFC8296]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
              for Bit Index Explicit Replication (BIER) in MPLS and Non-
              MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
              2018, <https://www.rfc-editor.org/info/rfc8296>.

   [RFC8534]  Dolganow, A., Kotalwar, J., Rosen, E., Ed., and Z. Zhang,
              "Explicit Tracking with Wildcard Routes in Multicast VPN",
              RFC 8534, DOI 10.17487/RFC8534, February 2019,
              <https://www.rfc-editor.org/info/rfc8534>.

7.2.  Informative References

   [RFC7524]  Rekhter, Y., Rosen, E., Aggarwal, R., Morin, T.,
              Grosclaude, I., Leymann, N., and S. Saad, "Inter-Area
              Point-to-Multipoint (P2MP) Segmented Label Switched Paths
              (LSPs)", RFC 7524, DOI 10.17487/RFC7524, May 2015,
              <https://www.rfc-editor.org/info/rfc7524>.

   [RFC7900]  Rekhter, Y., Ed., Rosen, E., Ed., Aggarwal, R., Cai, Y.,
              and T. Morin, "Extranet Multicast in BGP/IP MPLS VPNs",
              RFC 7900, DOI 10.17487/RFC7900, June 2016,
              <https://www.rfc-editor.org/info/rfc7900>.

Acknowledgments

   The authors wish to thank Jeffrey Zhang for his ideas and
   contributions to this work.  We also thank Stig Venaas for his review
   and comments.

Contributors

   IJsbrand Wijnands
   Cisco Systems, Inc.
   De Kleetlaan 6a
   Diegem  1831
   Belgium
   Email: ice@cisco.com

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Authors' Addresses

   Eric C. Rosen (editor)
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, Massachusetts  01886
   United States of America

   Email: erosen52@gmail.com

   Mahesh Sivakumar
   Juniper Networks, Inc.
   1137 Innovation Way
   Sunnyvale, California  94089
   United States of America

   Email: sivakumar.mahesh@gmail.com

   Tony Przygienda
   Juniper Networks, Inc.
   1137 Innovation Way
   Sunnyvale, California  94089
   United States of America

   Email: prz@juniper.net

   Sam K Aldrin
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, California
   United States of America

   Email: aldrin.ietf@gmail.com

   Andrew Dolganow
   Nokia
   438B Alexandra Rd #08-07/10
   Alexandra Technopark
   Singapore  119968

   Email: andrew.dolganow@nokia.com

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