    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

                                ------------
                              /              \
                             |  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]