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rfc:rfc8114

Internet Engineering Task Force (IETF) M. Boucadair Request for Comments: 8114 Orange Category: Standards Track C. Qin ISSN: 2070-1721 Cisco

                                                          C. Jacquenet
                                                                Orange
                                                                Y. Lee
                                                               Comcast
                                                               Q. Wang
                                                         China Telecom
                                                            March 2017
      Delivery of IPv4 Multicast Services to IPv4 Clients over
                     an IPv6 Multicast Network

Abstract

 This document specifies a solution for the delivery of IPv4 multicast
 services to IPv4 clients over an IPv6 multicast network.  The
 solution relies upon a stateless IPv4-in-IPv6 encapsulation scheme
 and uses an IPv6 multicast distribution tree to deliver IPv4
 multicast traffic.  The solution is particularly useful for the
 delivery of multicast service offerings to customers serviced by
 Dual-Stack Lite (DS-Lite).

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
 http://www.rfc-editor.org/info/rfc8114.

Boucadair, et al. Standards Track [Page 1] RFC 8114 IPv4 over IPv6 Multicast March 2017

Copyright Notice

 Copyright (c) 2017 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.

Boucadair, et al. Standards Track [Page 2] RFC 8114 IPv4 over IPv6 Multicast March 2017

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
 2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
 3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
 4.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   6
   4.1.  IPv4-Embedded IPv6 Prefixes . . . . . . . . . . . . . . .   7
   4.2.  Multicast Distribution Tree Computation . . . . . . . . .   8
   4.3.  Multicast Data Forwarding . . . . . . . . . . . . . . . .   9
 5.  IPv4/IPv6 Address Mapping . . . . . . . . . . . . . . . . . .   9
   5.1.  Prefix Assignment . . . . . . . . . . . . . . . . . . . .   9
   5.2.  Multicast Address Translation Algorithm . . . . . . . . .  10
   5.3.  Textual Representation  . . . . . . . . . . . . . . . . .  10
   5.4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  10
 6.  Multicast B4 (mB4)  . . . . . . . . . . . . . . . . . . . . .  11
   6.1.  IGMP-MLD Interworking Function  . . . . . . . . . . . . .  11
   6.2.  Multicast Data Forwarding . . . . . . . . . . . . . . . .  12
   6.3.  Fragmentation . . . . . . . . . . . . . . . . . . . . . .  12
   6.4.  Host Built-In mB4 Function  . . . . . . . . . . . . . . .  12
   6.5.  Preserve the Scope  . . . . . . . . . . . . . . . . . . .  13
 7.  Multicast AFTR (mAFTR)  . . . . . . . . . . . . . . . . . . .  13
   7.1.  Routing Considerations  . . . . . . . . . . . . . . . . .  13
   7.2.  Processing PIM Messages . . . . . . . . . . . . . . . . .  14
   7.3.  Switching from Shared Tree to Shortest Path Tree  . . . .  15
   7.4.  Multicast Data Forwarding . . . . . . . . . . . . . . . .  15
   7.5.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .  15
 8.  Deployment Considerations . . . . . . . . . . . . . . . . . .  16
   8.1.  Other Operational Modes . . . . . . . . . . . . . . . . .  16
     8.1.1.  The IPv6 DR is Co-located with the mAFTR  . . . . . .  16
     8.1.2.  The IPv4 DR is Co-located with the mAFTR  . . . . . .  16
   8.2.  Load Balancing  . . . . . . . . . . . . . . . . . . . . .  16
   8.3.  mAFTR Policy Configuration  . . . . . . . . . . . . . . .  16
   8.4.  Static vs. Dynamic PIM Triggering . . . . . . . . . . . .  17
 9.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   9.1.  Firewall Configuration  . . . . . . . . . . . . . . . . .  17
 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
 11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
   11.1.  Normative References . . . . . . . . . . . . . . . . . .  18
   11.2.  Informative References . . . . . . . . . . . . . . . . .  19
 Appendix A.  Use Case: IPTV . . . . . . . . . . . . . . . . . . .  21
 Appendix B.  Older Versions of Group Membership Management
              Protocols  . . . . . . . . . . . . . . . . . . . . .  22
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  22
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

Boucadair, et al. Standards Track [Page 3] RFC 8114 IPv4 over IPv6 Multicast March 2017

1. Introduction

 DS-Lite [RFC6333] is an IPv4 address-sharing technique that enables
 operators to multiplex public IPv4 addresses while provisioning only
 IPv6 to users.  A typical DS-Lite scenario is the delivery of an IPv4
 service to an IPv4 user over an IPv6 network (denoted as a 4-6-4
 scenario).  [RFC6333] covers unicast services exclusively.
 This document specifies a generic solution for the delivery of IPv4
 multicast services to IPv4 clients over an IPv6 multicast network.
 The solution was developed with DS-Lite in mind (see more discussion
 below).  However, the solution is not limited to DS-Lite; it can also
 be applied in other deployment contexts, such as the ones described
 in [RFC7596] and [RFC7597].
 If customers have to access IPv4 multicast-based services through a
 DS-Lite environment, Address Family Transition Router (AFTR) devices
 will have to process all the Internet Group Management Protocol
 (IGMP) Report messages [RFC2236] [RFC3376] that have been forwarded
 by the Customer Premises Equipment (CPE) into the IPv4-in-IPv6
 tunnels.  From that standpoint, AFTR devices are likely to behave as
 a replication point for downstream multicast traffic, and the
 multicast packets will be replicated for each tunnel endpoint that
 IPv4 receivers are connected to.
 This kind of DS-Lite environment raises two major issues:
 1.  The IPv6 network loses the benefits of efficient multicast
     traffic forwarding because it is unable to deterministically
     replicate the data as close to the receivers as possible.  As a
     consequence, the downstream bandwidth in the IPv6 network will be
     vastly consumed by sending multicast data over a unicast
     infrastructure.
 2.  The AFTR is responsible for replicating multicast traffic and
     forwarding it into each tunnel endpoint connecting IPv4 receivers
     that have explicitly asked for the corresponding content.  This
     process may significantly consume the AFTR's resources and
     overload the AFTR.
 This document specifies an extension to the DS-Lite model to deliver
 IPv4 multicast services to IPv4 clients over an IPv6 multicast-
 enabled network.

Boucadair, et al. Standards Track [Page 4] RFC 8114 IPv4 over IPv6 Multicast March 2017

 This document describes a stateless translation mechanism that
 supports either Source-Specific Multicast (SSM) or Any-Source
 Multicast (ASM) operation.  The recommendation in Section 1 of
 [RFC4607] is that multicast services use SSM where possible; the
 operation of the translation mechanism is also simplified when SSM is
 used, e.g., considerations for placement of the IPv6 Rendezvous Point
 (RP) are no longer relevant.

1.1. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

2. Terminology

 This document makes use of the following terms:
 IPv4-embedded IPv6 address:  an IPv6 address that embeds a 32-bit-
    encoded IPv4 address.  An IPv4-embedded IPv6 address can be
    unicast or multicast.
 mPrefix64:  a dedicated multicast IPv6 prefix for constructing
    IPv4-embedded IPv6 multicast addresses. mPrefix64 can be of two
    types: ASM_mPrefix64 used in Any-Source Multicast (ASM) mode or
    SSM_mPrefix64 used in Source-Specific Multicast (SSM) mode
    [RFC4607].  The size of this prefix is /96.
       Note: "64" is used as an abbreviation for IPv6-IPv4
       interconnection.
 uPrefix64:  a dedicated IPv6 unicast prefix for constructing
    IPv4-embedded IPv6 unicast addresses [RFC6052].  This prefix may
    be either the Well-Known Prefix (i.e., 64:ff9b::/96) or a Network-
    Specific Prefix (NSP).
 Multicast AFTR (mAFTR):  a functional entity that supports an
    IPv4-IPv6 multicast interworking function (refer to Figure 3).  It
    receives and encapsulates the IPv4 multicast packets into IPv4-in-
    IPv6 packets.  Also, it behaves as the corresponding IPv6
    multicast source for the encapsulated IPv4-in-IPv6 packets.
 Multicast Basic Bridging BroadBand (mB4):  a functional entity that
    supports an IGMP-MLD Interworking function (refer to Section 6.1)
    that translates the IGMP messages into the corresponding Multicast
    Listener Discovery (MLD) messages and sends the MLD messages to
    the IPv6 network.  In addition, the mB4 decapsulates IPv4-in-IPv6
    multicast packets.

Boucadair, et al. Standards Track [Page 5] RFC 8114 IPv4 over IPv6 Multicast March 2017

 PIMv4:  refers to Protocol Independent Multicast (PIM) when deployed
    in an IPv4 infrastructure (i.e., IPv4 transport capabilities are
    used to exchange PIM messages).
 PIMv6:  refers to PIM when deployed in an IPv6 infrastructure (i.e.,
    IPv6 transport capabilities are used to exchange PIM messages).
 Host portion of the MLD protocol:  refers to the part of MLD that
    applies to all multicast address listeners (Section 6 of
    [RFC3810]).  As a reminder, MLD specifies separate behaviors for
    multicast address listeners (i.e., hosts or routers that listen to
    multicast packets) and multicast routers.
 Router portion of IGMP:  refers to the part of IGMP that is performed
    by multicast routers (Section 6 of [RFC3376]).
 DR:  refers to the Designated Router as defined in [RFC7761].

3. Scope

 This document focuses only on the subscription to IPv4 multicast
 groups and the delivery of IPv4-formatted content to IPv4 receivers
 over an IPv6-only network.  In particular, only the following case is
 covered:
    IPv4 receivers access IPv4 multicast content over IPv6-only
    multicast-enabled networks.
 This document does not cover the source/receiver heuristics, where
 IPv4 receivers can also behave as IPv4 multicast sources.  This
 document assumes that hosts behind the mB4 are IPv4 multicast
 receivers only.  Also, the document covers the host built-in mB4
 function.

4. Solution Overview

 In the DS-Lite specification [RFC6333], an IPv4-in-IPv6 tunnel is
 used to carry bidirectional IPv4 unicast traffic between a B4 and an
 AFTR.  The solution specified in this document provides an IPv4-in-
 IPv6 encapsulation scheme to deliver unidirectional IPv4 multicast
 traffic from an mAFTR to an mB4.
 An overview of the solution is provided in this section; it is
 intended as an introduction to how it works but is not normative.
 For the normative specifications of the two new functional elements,
 mB4 and mAFTR (Figure 1), refer to Sections 6 and 7.

Boucadair, et al. Standards Track [Page 6] RFC 8114 IPv4 over IPv6 Multicast March 2017

  1. ———–

/ \

                       |  IPv4 network  |
                        \              /
                          ------------
            IPv4 multicast  :   |   ^  PIMv4 Join
                            v   |   :
                         +-------------+
                         |    mAFTR    |
                         +-------------+
           IPv6 multicast  |:|  |   ^  PIMv6 Join (PIMv6
           (IPv4 embedded) |:|  |   :   routers in between)
                          ------------
                        /              \
                       |  IPv6 network  |
                        \              /
                          ------------
                           |:|  |   ^  MLD Report
                           |v|  |   :
                          +-----------+
                          |    mB4    |
                          +-----------+
            IPv4 multicast  :   |   ^  IGMP Report
                            v   |   :
                          +-----------+
                          |   IPv4    |
                          | receiver  |
                          +-----------+
                   Figure 1: Functional Architecture

4.1. IPv4-Embedded IPv6 Prefixes

 In order to map the addresses of IPv4 multicast traffic with IPv6
 multicast addresses, an IPv6 multicast prefix (mPrefix64) and an IPv6
 unicast prefix (uPrefix64) are provided to the mAFTR and the mB4
 elements, both of which contribute to the computation and the
 maintenance of the IPv6 multicast distribution tree that extends the
 IPv4 multicast distribution tree into the IPv6 multicast network.
 The IPv4/IPv6 address mapping is stateless.
 The mAFTR and the mB4 use mPrefix64 to convert an IPv4 multicast
 address (G4) into an IPv4-embedded IPv6 multicast address (G6).  The
 mAFTR and the mB4 use uPrefix64 to convert an IPv4 source address
 (S4) into an IPv4-embedded IPv6 address (S6).  The mAFTR and the mB4
 must use the same mPrefix64 and uPrefix64; they also run the same
 algorithm for building IPv4-embedded IPv6 addresses.  Refer to
 Section 5 for more details about the address mapping.

Boucadair, et al. Standards Track [Page 7] RFC 8114 IPv4 over IPv6 Multicast March 2017

4.2. Multicast Distribution Tree Computation

 When an IPv4 receiver connected to the device that embeds the mB4
 capability wants to subscribe to an IPv4 multicast group, it sends an
 IGMP Report message towards the mB4.  The mB4 creates the IPv6
 multicast group (G6) address using mPrefix64 and the original IPv4
 multicast group address.  If the receiver sends a source-specific
 IGMPv3 Report message, the mB4 will create the IPv6 source address
 (S6) using uPrefix64 and the original IPv4 source address.
 The mB4 uses the G6 (and both S6 and G6 in SSM) to create the
 corresponding MLD Report message.  The mB4 sends the Report message
 towards the IPv6 network.  The PIMv6 DR receives the MLD Report
 message and sends the PIMv6 Join message to join the IPv6 multicast
 distribution tree.  It can send either PIMv6 Join (*,G6) in ASM or
 PIMv6 Join (S6,G6) in SSM to the mAFTR.
 The mAFTR acts as the IPv6 DR to which the uPrefix64-derived S6 is
 connected.  The mAFTR will receive the source-specific PIMv6 Join
 message (S6,G6) from the IPv6 multicast network.  If the mAFTR is the
 Rendezvous Point (RP) of G6, it will receive the any-source PIMv6
 Join message (*,G6) from the IPv6 multicast network.  If the mAFTR is
 not the RP of G6, it will send the PIM Register message to the RP of
 G6 located in the IPv6 multicast network.  For the sake of
 simplicity, it is recommended to configure the mAFTR as the RP for
 the IPv4-embedded IPv6 multicast groups it manages; no registration
 procedure is required under this configuration.
 When the mAFTR receives the PIMv6 Join message (*,G6), it will
 extract the IPv4 multicast group address (G4).  If the mAFTR is the
 RP of G4 in the IPv4 multicast network, it will create a (*,G4) entry
 (if such entry does not already exist) in its own IPv4 multicast
 routing table.  If the mAFTR is not the RP of G4, it will send the
 corresponding PIMv4 Join message (*,G4) towards the RP of G4 in the
 IPv4 multicast network.
 When the mAFTR receives the PIMv6 Join message (S6,G6), it will
 extract the IPv4 multicast group address (G4) and IPv4 source address
 (S4) and send the corresponding (S4,G4) PIMv4 Join message directly
 to the IPv4 source.
 A branch of the multicast distribution tree is thus constructed,
 comprising both an IPv4 part (from the mAFTR upstream) and an IPv6
 part (from mAFTR downstream towards the mB4).
 The mAFTR advertises the route of uPrefix64 with an IPv6 Interior
 Gateway Protocol (IGP), so as to represent the IPv4-embedded IPv6
 source in the IPv6 multicast network and to allow IPv6 routers to run

Boucadair, et al. Standards Track [Page 8] RFC 8114 IPv4 over IPv6 Multicast March 2017

 the Reverse Path Forwarding (RPF) check procedure on incoming
 multicast traffic.  Injecting internal /96 routes is not problematic
 given the recommendation in [RFC7608] that requires that forwarding
 processes must be designed to process prefixes of any length up to
 /128.

4.3. Multicast Data Forwarding

 When the mAFTR receives an IPv4 multicast packet, it will encapsulate
 the packet into an IPv6 multicast packet using the IPv4-embedded IPv6
 multicast address as the destination address and an IPv4-embedded
 IPv6 unicast address as the source address.  The encapsulated IPv6
 multicast packet will be forwarded down the IPv6 multicast
 distribution tree, and the mB4 will eventually receive the packet.
 The IPv6 multicast network treats the IPv4-in-IPv6 encapsulated
 multicast packets as native IPv6 multicast packets.  The IPv6
 multicast routers use the outer IPv6 header to make their forwarding
 decisions.
 When the mB4 receives the IPv6 multicast packet (to G6) derived by
 mPrefix64, it decapsulates it and forwards the original IPv4
 multicast packet towards the receivers subscribing to G4.
 Note: At this point, only IPv4-in-IPv6 encapsulation is defined;
 however, other types of encapsulation could be defined in the future.

5. IPv4/IPv6 Address Mapping

5.1. Prefix Assignment

 A dedicated IPv6 multicast prefix (mPrefix64) is provisioned to the
 mAFTR and the mB4.  The mAFTR and the mB4 use the mPrefix64 to form
 an IPv6 multicast group address from an IPv4 multicast group address.
 The mPrefix64 can be of two types: ASM_mPrefix64 (an mPrefix64 used
 in ASM mode) or SSM_mPrefix64 (an mPrefix64 used in SSM mode).  The
 mPrefix64 MUST be derived from the corresponding IPv6 multicast
 address space (e.g., the SSM_mPrefix64 must be in the range of the
 multicast address space specified in [RFC4607]).
 The IPv6 part of the multicast distribution tree can be seen as an
 extension of the IPv4 part of the multicast distribution tree.  The
 IPv4 source address MUST be mapped to an IPv6 source address.  An
 IPv6 unicast prefix (uPrefix64) is provisioned to the mAFTR and the
 mB4.  The mAFTR and the mB4 use the uPrefix64 to form an IPv6 source
 address from an IPv4 source address as specified in [RFC6052].  The

Boucadair, et al. Standards Track [Page 9] RFC 8114 IPv4 over IPv6 Multicast March 2017

 uPrefix-formed IPv6 source address will represent the original IPv4
 source in the IPv6 multicast network.  The uPrefix64 MUST be derived
 from the IPv6 unicast address space.
 The multicast address translation MUST follow the algorithm defined
 in Section 5.2.
 The mPrefix64 and uPrefix64 can be configured in the mB4 using a
 variety of methods, including an out-of-band mechanism, manual
 configuration, or a dedicated provisioning protocol (e.g., using
 DHCPv6 [RFC8115]).
 The stateless translation mechanism described in Section 5 does not
 preclude use of Embedded-RP [RFC3956] [RFC7371].

5.2. Multicast Address Translation Algorithm

 IPv4-embedded IPv6 multicast addresses are composed according to the
 following algorithm:
 o  Concatenate the 96 bits of the mPrefix64 and the 32 bits of the
    IPv4 address to obtain a 128-bit address.
 The IPv4 multicast addresses are extracted from the IPv4-embedded
 IPv6 multicast addresses according to the following algorithm:
 o  If the multicast address has a pre-configured mPrefix64, extract
    the last 32 bits of the IPv6 multicast address.
 An IPv4 source is represented in the IPv6 realm with its
 IPv4-converted IPv6 address [RFC6052].

5.3. Textual Representation

 The embedded IPv4 address in an IPv6 multicast address is included in
 the last 32 bits; therefore, dotted decimal notation can be used.

5.4. Examples

  Group address mapping example:
  +---------------------+--------------+----------------------------+
  |      mPrefix64      | IPv4 address | IPv4-Embedded IPv6 address |
  +---------------------+--------------+----------------------------+
  |  ff0x::db8:0:0/96   |  233.252.0.1 |   ff0x::db8:233.252.0.1    |
  +---------------------+--------------+----------------------------+

Boucadair, et al. Standards Track [Page 10] RFC 8114 IPv4 over IPv6 Multicast March 2017

  Source address mapping example when a /96 is used:
  +---------------------+--------------+----------------------------+
  |      uPrefix64      | IPv4 address | IPv4-Embedded IPv6 address |
  +---------------------+--------------+----------------------------+
  |    2001:db8::/96    |  192.0.2.33  |     2001:db8::192.0.2.33   |
  +---------------------+--------------+----------------------------+
 IPv4 and IPv6 addresses used in this example are derived from the
 IPv4 and IPv6 blocks reserved for documentation, as per [RFC6676].
 The unicast IPv4 address of the above example is derived from the
 documentation address block defined in [RFC6890].

6. Multicast B4 (mB4)

6.1. IGMP-MLD Interworking Function

 The IGMP-MLD Interworking function combines the IGMP/MLD Proxying
 function and the address-synthesizing operations.  The IGMP/MLD
 Proxying function is specified in [RFC4605].  The address translation
 is stateless and MUST follow the address mapping specified in
 Section 5.
 The mB4 performs the host portion of the MLD protocol on the upstream
 interface.  The composition of IPv6 membership in this context is
 constructed through address-synthesizing operations and MUST
 synchronize with the membership database maintained in the IGMP
 domain.  MLD messages are sent natively to the direct-connected IPv6
 multicast routers (they will be processed by the PIM DR).  The mB4
 also performs the router portion of IGMP on the downstream
 interface(s).  Refer to [RFC4605] for more details.
          +----------+   IGMP  +-------+   MLD   +---------+
          |   IPv4   |---------|  mB4  |---------|   PIM   |
          | Receiver |         |       |         |    DR   |
          +----------+         +-------+         +---------+
                    Figure 2: IGMP-MLD Interworking
 If SSM is deployed, the mB4 MUST construct the IPv6 source address
 (or retrieve the IPv4 source address) using the uPrefix64.  The mB4
 MAY create a membership database that associates the IPv4-IPv6
 multicast groups with the interfaces (e.g., WLAN and Wired Ethernet)
 facing IPv4 multicast receivers.

Boucadair, et al. Standards Track [Page 11] RFC 8114 IPv4 over IPv6 Multicast March 2017

6.2. Multicast Data Forwarding

 When the mB4 receives an IPv6 multicast packet, it MUST check the
 group address and the source address.  If the IPv6 multicast group
 prefix is mPrefix64 and the IPv6 source prefix is uPrefix64, the mB4
 MUST decapsulate the IPv6 header [RFC2473]; the decapsulated IPv4
 multicast packet will be forwarded through each relevant interface
 following standard IPv4 multicast forwarding procedures.  Otherwise,
 the mB4 MUST silently drop the packet.
 As an illustration, if a packet is received from source
 2001:db8::192.0.2.33 and needs to be forwarded to group
 ff3x:20:2001:db8::233.252.0.1, the mB4 decapsulates it into an IPv4
 multicast packet using 192.0.2.33 as the IPv4 source address and
 using 233.252.0.1 as the IPv4 destination multicast group.  This
 example assumes that the mB4 is provisioned with uPrefix64
 (2001:db8::/96) and mPrefix64 (ff3x:20:2001:db8::/96).

6.3. Fragmentation

 Encapsulating IPv4 multicast packets into IPv6 multicast packets that
 will be forwarded by the mAFTR towards the mB4 along the IPv6
 multicast distribution tree reduces the effective MTU size by the
 size of an IPv6 header.  In this specification, the data flow is
 unidirectional from the mAFTR to the mB4.  The mAFTR MUST fragment
 the oversized IPv6 packet after the encapsulation into two IPv6
 packets.  The mB4 MUST reassemble the IPv6 packets, decapsulate the
 IPv6 header, and forward the IPv4 packet to the hosts that have
 subscribed to the corresponding multicast group.  Further
 considerations about fragmentation issues are documented in Sections
 5.3 and 6.3 of [RFC6333].

6.4. Host Built-In mB4 Function

 If the mB4 function is implemented in the host that is directly
 connected to an IPv6-only network, the host MUST implement the
 behaviors specified in Sections 6.1, 6.2, and 6.3.  The host MAY
 optimize the implementation to provide an Application Programming
 Interface (API) or kernel module to skip the IGMP-MLD Interworking
 function.  Optimization considerations are out of scope of this
 specification.

Boucadair, et al. Standards Track [Page 12] RFC 8114 IPv4 over IPv6 Multicast March 2017

6.5. Preserve the Scope

 When several mPrefix64s are available, if each enclosed IPv4-embedded
 IPv6 multicast prefix has a distinct scope, the mB4 MUST select the
 appropriate IPv4-embedded IPv6 multicast prefix whose scope matches
 the IPv4 multicast address used to synthesize an IPv4-embedded IPv6
 multicast address (specific mappings are listed in Section 8 of
 [RFC2365]).  Mapping is achieved such that the scope of the selected
 IPv6 multicast prefix does not exceed the original IPv4 multicast
 scope.  If the mB4 is instructed to preserve the scope but no IPv6
 multicast prefix that matches the IPv4 multicast scope is found, IPv6
 multicast address mapping SHOULD fail.
 The mB4 MAY be configured to not preserve the scope when enforcing
 the address translation algorithm.
 Consider that an mB4 is configured with two mPrefix64s,
 ff0e::db8:0:0/96 (global scope) and ff08::db8:0:0/96 (organization
 scope).  If the mB4 receives an IGMP Report message from an IPv4
 receiver to subscribe to 233.252.0.1, it checks which mPrefix64 to
 use in order to preserve the scope of the requested IPv4 multicast
 group.  In this example, given that 233.252.0.1 is intended for
 global use, the mB4 creates the IPv6 multicast group (G6) address
 using ff0e::db8:0:0/96 and the original IPv4 multicast group address
 (233.252.0.1): ff0e::db8:233.252.0.1.

7. Multicast AFTR (mAFTR)

7.1. Routing Considerations

 The mAFTR is responsible for interconnecting the IPv4 multicast
 distribution tree with the corresponding IPv6 multicast distribution
 tree.  The mAFTR MUST use the uPrefix64 to build the IPv6 source
 addresses of the multicast group address derived from mPrefix64.  In
 other words, the mAFTR MUST be the multicast source whose address is
 derived from uPrefix64.
 The mAFTR MUST advertise the route towards uPrefix64 with the IPv6
 IGP.  This is needed by the IPv6 multicast routers so that they
 acquire the routing information to discover the source.

Boucadair, et al. Standards Track [Page 13] RFC 8114 IPv4 over IPv6 Multicast March 2017

7.2. Processing PIM Messages

 The mAFTR MUST interwork PIM Join/Prune messages for (*,G6) and
 (S6,G6) on their corresponding (*,G4) and (S4,G4).  The following
 text specifies the expected behavior of the mAFTR for PIM Join
 messages.
                              +---------+
                     ---------|  mAFTR  |---------
                       PIMv6  |uPrefix64|  PIMv4
                              |mPrefix64|
                              +---------+
              Figure 3: PIMv6-PIMv4 Interworking Function
 The mAFTR contains two separate Tree Information Bases (TIBs): the
 IPv4 Tree Information Base (TIB4) and the IPv6 Tree Information Base
 (TIB6), which are bridged by one IPv4-in-IPv6 virtual interface.  It
 should be noted that TIB implementations may vary (e.g., some may
 rely upon a single integrated TIB without any virtual interface), but
 they should follow this specification for the sake of global and
 functional consistency.
 When an mAFTR receives a PIMv6 Join message (*,G6) with an IPv6
 multicast group address (G6) that is derived from the mPrefix64, it
 MUST check its IPv6 Tree Information Base (TIB6).  If there is an
 entry for this G6 address, it MUST check whether the interface
 through which the PIMv6 Join message has been received is in the
 outgoing interface (oif) list.  If not, the mAFTR MUST add the
 interface to the oif list.  If there is no entry in the TIB6, the
 mAFTR MUST create a new entry (*,G6) for the multicast group.
 Whether or not the IPv4-in-IPv6 virtual interface is set as the
 incoming interface of the newly created entry is up to the
 implementation, but it should comply with the mAFTR's multicast data
 forwarding behavior (see Section 7.4).
 The mAFTR MUST extract the IPv4 multicast group address (G4) from the
 IPv4-embedded IPv6 multicast address (G6) contained in the PIMv6 Join
 message.  The mAFTR MUST check its IPv4 Tree Information Base (TIB4).
 If there is an entry for G4, it MUST check whether the IPv4-in-IPv6
 virtual interface is in the outgoing interface list.  If not, the
 mAFTR MUST add the interface to the oif list.  If there is no entry
 for G4, the mAFTR MUST create a new (*,G4) entry in its TIB4 and
 initiate the procedure for building the shared tree in the IPv4
 multicast network without any additional requirement.

Boucadair, et al. Standards Track [Page 14] RFC 8114 IPv4 over IPv6 Multicast March 2017

 If the mAFTR receives a source-specific Join message, the (S6,G6) is
 processed rather than (*,G6).  The procedures of processing (S6,G6)
 and (*,G6) are almost the same.  Differences have been detailed in
 [RFC7761].

7.3. Switching from Shared Tree to Shortest Path Tree

 When the mAFTR receives the first IPv4 multicast packet, it may
 extract the source address (S4) from the packet and send an Explicit
 PIMv4 (S4,G4) Join message directly to S4.  The mAFTR switches from
 the shared Rendezvous Point Tree (RPT) to the Shortest Path Tree
 (SPT) for G4.
 For IPv6 multicast routers to switch to the SPT, there is no new
 requirement.  IPv6 multicast routers may send an Explicit PIMv6 Join
 to the mAFTR once the first (S6,G6) multicast packet arrives from
 upstream multicast routers.

7.4. Multicast Data Forwarding

 When the mAFTR receives an IPv4 multicast packet, it checks its TIB4
 to find a matching entry and then forwards the packet to the
 interface(s) listed in the outgoing interface list.  If the IPv4-in-
 IPv6 virtual interface also belongs to this list, the packet is
 encapsulated with the mPrefix64-derived and uPrefix64-derived
 IPv4-embedded IPv6 addresses to form an IPv6 multicast packet
 [RFC2473].  Then another lookup is made by the mAFTR to find a
 matching entry in the TIB6.  Whether or not the RPF check for the
 second lookup is performed is up to the implementation and is out of
 the scope of this document.  The IPv6 multicast packet is then
 forwarded along the IPv6 multicast distribution tree, based upon the
 outgoing interface list of the matching entry in the TIB6.
 As an illustration, if a packet is received from source 192.0.2.33
 and needs to be forwarded to group 233.252.0.1, the mAFTR
 encapsulates it into an IPv6 multicast packet using
 ff3x:20:2001:db8::233.252.0.1 as the IPv6 destination multicast group
 and using 2001:db8::192.0.2.33 as the IPv6 source address.

7.5. Scope

 The Scope field of IPv4-in-IPv6 multicast addresses should be valued
 accordingly (e.g., to "E" for global scope) in the deployment
 environment.  This specification does not discuss the scope value
 that should be used.
 The considerations in Section 6.5 are to be followed by the mAFTR.

Boucadair, et al. Standards Track [Page 15] RFC 8114 IPv4 over IPv6 Multicast March 2017

8. Deployment Considerations

8.1. Other Operational Modes

8.1.1. The IPv6 DR is Co-located with the mAFTR

 The mAFTR can embed the MLD Querier function (as well as the PIMv6
 DR) for optimization purposes.  When the mB4 sends an MLD Report
 message to this mAFTR, the mAFTR should process the MLD Report
 message that contains the IPv4-embedded IPv6 multicast group address
 and then send the corresponding PIMv4 Join message (Figure 4).
                              +---------+
                     ---------|  mAFTR  |---------
                        MLD   |uPrefix64|  PIMv4
                              |mPrefix64|
                              +---------+
               Figure 4: MLD-PIMv4 Interworking Function
 Discussions about the location of the mAFTR capability and related
 ASM or SSM multicast design considerations are out of the scope of
 this document.

8.1.2. The IPv4 DR is Co-located with the mAFTR

 If the mAFTR is co-located with the IPv4 DR connected to the original
 IPv4 source, it may simply use the uPrefix64 and mPrefix64 prefixes
 to build the IPv4-embedded IPv6 multicast packets, and the sending of
 PIMv4 Join messages becomes unnecessary.

8.2. Load Balancing

 For robustness and load distribution purposes, several nodes in the
 network can embed the mAFTR function.  In such case, the same IPv6
 prefixes (i.e., mPrefix64 and uPrefix64) and algorithm to build
 IPv4-embedded IPv6 addresses must be configured on those nodes.

8.3. mAFTR Policy Configuration

 The mAFTR may be configured with a list of IPv4 multicast groups and
 sources.  Only multicast flows bound to the configured addresses
 should be handled by the mAFTR.  Otherwise, packets are silently
 dropped.

Boucadair, et al. Standards Track [Page 16] RFC 8114 IPv4 over IPv6 Multicast March 2017

8.4. Static vs. Dynamic PIM Triggering

 To optimize the usage of network resources in current deployments,
 all multicast streams are conveyed in the core network while only the
 most popular ones are forwarded in the aggregation/access networks
 (static mode).  Less popular streams are forwarded in the access
 network upon request (dynamic mode).  Depending on the location of
 the mAFTR in the network, two modes can be envisaged: static and
 dynamic.
 Static Mode:  The mAFTR is configured to instantiate permanent
    (S6,G6) and (*,G6) entries in its TIB6 using a pre-configured
    (S4,G4) list.
 Dynamic Mode:  The instantiation or withdrawal of (S6,G6) or (*,G6)
    entries is triggered by the receipt of PIMv6 messages.

9. Security Considerations

 Besides multicast scoping considerations (see Sections 6.5 and 7.5),
 this document does not introduce any new security concerns in
 addition to those discussed in Section 5 of [RFC6052], Section 10 of
 [RFC3810], and Section 6 of [RFC7761].
 Unlike solutions that map IPv4 multicast flows to IPv6 unicast flows,
 this document does not exacerbate Denial-of-Service (DoS) attacks.
 An mB4 SHOULD be provided with appropriate configuration information
 to preserve the scope of a multicast message when mapping an IPv4
 multicast address into an IPv4-embedded IPv6 multicast address and
 vice versa.

9.1. Firewall Configuration

 The CPE that embeds the mB4 function SHOULD be configured to accept
 incoming MLD messages and traffic forwarded to multicast groups
 subscribed to by receivers located in the customer premises.

10. IANA Considerations

 This document does not require any IANA actions.

Boucadair, et al. Standards Track [Page 17] RFC 8114 IPv4 over IPv6 Multicast March 2017

11. References

11.1. 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,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2365]  Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
            RFC 2365, DOI 10.17487/RFC2365, July 1998,
            <http://www.rfc-editor.org/info/rfc2365>.
 [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
            IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
            December 1998, <http://www.rfc-editor.org/info/rfc2473>.
 [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
            Thyagarajan, "Internet Group Management Protocol, Version
            3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
            <http://www.rfc-editor.org/info/rfc3376>.
 [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
            Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
            DOI 10.17487/RFC3810, June 2004,
            <http://www.rfc-editor.org/info/rfc3810>.
 [RFC4605]  Fenner, B., He, H., Haberman, B., and H. Sandick,
            "Internet Group Management Protocol (IGMP) / Multicast
            Listener Discovery (MLD)-Based Multicast Forwarding
            ("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605,
            August 2006, <http://www.rfc-editor.org/info/rfc4605>.
 [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
            IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
            <http://www.rfc-editor.org/info/rfc4607>.
 [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
            Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
            DOI 10.17487/RFC6052, October 2010,
            <http://www.rfc-editor.org/info/rfc6052>.
 [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
            Stack Lite Broadband Deployments Following IPv4
            Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,
            <http://www.rfc-editor.org/info/rfc6333>.

Boucadair, et al. Standards Track [Page 18] RFC 8114 IPv4 over IPv6 Multicast March 2017

 [RFC7608]  Boucadair, M., Petrescu, A., and F. Baker, "IPv6 Prefix
            Length Recommendation for Forwarding", BCP 198, RFC 7608,
            DOI 10.17487/RFC7608, July 2015,
            <http://www.rfc-editor.org/info/rfc7608>.
 [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
            Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
            Multicast - Sparse Mode (PIM-SM): Protocol Specification
            (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
            2016, <http://www.rfc-editor.org/info/rfc7761>.

11.2. Informative References

 [RFC2236]  Fenner, W., "Internet Group Management Protocol, Version
            2", RFC 2236, DOI 10.17487/RFC2236, November 1997,
            <http://www.rfc-editor.org/info/rfc2236>.
 [RFC3956]  Savola, P. and B. Haberman, "Embedding the Rendezvous
            Point (RP) Address in an IPv6 Multicast Address",
            RFC 3956, DOI 10.17487/RFC3956, November 2004,
            <http://www.rfc-editor.org/info/rfc3956>.
 [RFC6676]  Venaas, S., Parekh, R., Van de Velde, G., Chown, T., and
            M. Eubanks, "Multicast Addresses for Documentation",
            RFC 6676, DOI 10.17487/RFC6676, August 2012,
            <http://www.rfc-editor.org/info/rfc6676>.
 [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
            "Special-Purpose IP Address Registries", BCP 153,
            RFC 6890, DOI 10.17487/RFC6890, April 2013,
            <http://www.rfc-editor.org/info/rfc6890>.
 [RFC7371]  Boucadair, M. and S. Venaas, "Updates to the IPv6
            Multicast Addressing Architecture", RFC 7371,
            DOI 10.17487/RFC7371, September 2014,
            <http://www.rfc-editor.org/info/rfc7371>.
 [RFC7596]  Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
            Farrer, "Lightweight 4over6: An Extension to the Dual-
            Stack Lite Architecture", RFC 7596, DOI 10.17487/RFC7596,
            July 2015, <http://www.rfc-editor.org/info/rfc7596>.
 [RFC7597]  Troan, O., Ed., Dec, W., Li, X., Bao, C., Matsushima, S.,
            Murakami, T., and T. Taylor, Ed., "Mapping of Address and
            Port with Encapsulation (MAP-E)", RFC 7597,
            DOI 10.17487/RFC7597, July 2015,
            <http://www.rfc-editor.org/info/rfc7597>.

Boucadair, et al. Standards Track [Page 19] RFC 8114 IPv4 over IPv6 Multicast March 2017

 [RFC8115]  Boucadair, M., Qin, J., Tsou, T., and X. Deng, "DHCPv6
            Option for IPv4-Embedded Multicast and Unicast IPv6
            Prefixes", RFC 8115, DOI 10.17487/RFC8115, March 2017,
            <http://www.rfc-editor.org/info/rfc8115>.

Boucadair, et al. Standards Track [Page 20] RFC 8114 IPv4 over IPv6 Multicast March 2017

Appendix A. Use Case: IPTV

 IPTV generally includes two categories of service offerings:
 o  Video on Demand (VoD) that streams unicast video content to
    receivers.
 o  Multicast live TV broadcast services.
 Two types of provider are involved in the delivery of this service:
 o  Content Providers, who usually own the content that is multicast
    to receivers.  Content providers may contractually define an
    agreement with network providers to deliver content to receivers.
 o  Network Providers, who provide network connectivity services
    (e.g., network providers are responsible for carrying multicast
    flows from head-ends to receivers).
 Note that some contract agreements prevent a network provider from
 altering the content as sent by the content provider for various
 reasons.  Depending on these contract agreements, multicast streams
 should be delivered unaltered to the requesting users.
 Most current IPTV content is likely to remain IPv4-formatted and out
 of the control of network providers.  Additionally, there are
 numerous legacy receivers (e.g., IPv4-only Set-Top Boxes (STBs)) that
 can't be upgraded or easily replaced to support IPv6.  As a
 consequence, IPv4 service continuity must be guaranteed during the
 transition period, including the delivery of multicast services such
 as Live TV Broadcasting to users.

Boucadair, et al. Standards Track [Page 21] RFC 8114 IPv4 over IPv6 Multicast March 2017

Appendix B. Older Versions of Group Membership Management Protocols

 Given the multiple versions of group membership management protocols,
 mismatch issues may arise at the mB4 (refer to Section 6.1).
 If IGMPv2 operates on the IPv4 receivers while MLDv2 operates on the
 MLD Querier, or if IGMPv3 operates on the IPv4 receivers while MLDv1
 operates on the MLD Querier, a version mismatch issue will be
 encountered.  To solve this problem, the mB4 should perform the
 router portion of IGMP, which is similar to the corresponding MLD
 version (IGMPv2 for MLDv1 or IGMPv3 for MLDv2) operating in the IPv6
 domain.  Then, the protocol interaction approach specified in
 Section 7 of [RFC3376] can be applied to exchange signaling messages
 with the IPv4 receivers on which the different version of IGMP is
 operating.
 Note that the support of IPv4 SSM requires MLDv2 to be enabled in the
 IPv6 network.

Acknowledgements

 The authors would like to thank Dan Wing for his guidance in the
 early discussions that initiated this work.  We also thank Peng Sun,
 Jie Hu, Qiong Sun, Lizhong Jin, Alain Durand, Dean Cheng, Behcet
 Sarikaya, Tina Tsou, Rajiv Asati, Xiaohong Deng, and Stig Venaas for
 their valuable comments.
 Many thanks to Ian Farrer for the review.
 Thanks to Zhen Cao, Tim Chown, Francis Dupont, Jouni Korhonen, and
 Stig Venaas for the directorates review.

Boucadair, et al. Standards Track [Page 22] RFC 8114 IPv4 over IPv6 Multicast March 2017

Authors' Addresses

 Mohamed Boucadair
 Orange
 Rennes  35000
 France
 Email: mohamed.boucadair@orange.com
 Chao Qin
 Cisco
 Shanghai
 China
 Email: jacni@jacni.com
 Christian Jacquenet
 Orange
 Rennes  35000
 France
 Email: christian.jacquenet@orange.com
 Yiu L. Lee
 Comcast
 United States of America
 Email: yiu_lee@cable.comcast.com
 URI:   http://www.comcast.com
 Qian Wang
 China Telecom
 China
 Phone: +86 10 58502462
 Email: 13301168516@189.cn

Boucadair, et al. Standards Track [Page 23]

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