+---------------------+--------------+----------------------------+
 |      MPREFIX64      | IPv4 address | IPv4-Embedded IPv6 Address |
 +---------------------+--------------+----------------------------+
 |   ff3x:0:8000::/96  | 233.252.0.5  |   ff3x:0:8000::233.252.0.5 |
 +---------------------+--------------+----------------------------+

Figure 4: Example of SSM IPv4-embedded IPv6 address

5. IANA Considerations

This document requests IANA to reserve:

• ff3x:0:8000::/96 SSM range to embed an IPv4 multicast address in the last 32 bits.

6. Security Considerations

This document defines an algorithmic translation of an IPv6 multicast address into an IPv4 multicast address, and vice versa. The security considerations discussed in [RFC6052] are to be taken into consideration.

7. Acknowledgements

Many thanks to R. Bonica, B. Sarikaya, P. Savola, T. Tsou, C. Bormann, T. Chown, P. Koch, B. Haberman, and B. Hinden for their comments and review.

8. References

8.1. Normative References

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3956]	Savola, P. and B. Haberman, "Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address", RFC 3956, November 2004.
[I-D.ietf-6man-multicast-addr-arch-update]	Boucadair, M. and S. Venaas, "Updates to the IPv6 Multicast Addressing Architecture", Internet-Draft draft-ietf-6man-multicast-addr-arch-update-00, April 2013.
[RFC3306]	Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6 Multicast Addresses", RFC 3306, August 2002.
[RFC3307]	Haberman, B., "Allocation Guidelines for IPv6 Multicast Addresses", RFC 3307, August 2002.
[RFC6052]	Bao, C., Huitema, C., Bagnulo, M., Boucadair, M. and X. Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052, October 2010.
[RFC4607]	Holbrook, H. and B. Cain, "Source-Specific Multicast for IP", RFC 4607, August 2006.

8.2. Informative References

[I-D.ietf-softwire-multicast-prefix-option]	Boucadair, M., Qin, J., Tsou, T. and X. Deng, "DHCPv6 Option for IPv4-Embedded Multicast and Unicast IPv6 Prefixes", Internet-Draft draft-ietf-softwire-multicast-prefix-option-03, October 2012.
[RFC6676]	Venaas, S., Parekh, R., Van de Velde, G., Chown, T. and M. Eubanks, "Multicast Addresses for Documentation", RFC 6676, August 2012.
[I-D.ietf-mboned-v4v6-mcast-ps]	Jacquenet, C., Boucadair, M., Lee, Y., Qin, J., Tsou, T. and Q. Sun, "IPv4-IPv6 Multicast: Problem Statement and Use Cases", Internet-Draft draft-ietf-mboned-v4v6-mcast-ps-01, November 2012.
[I-D.ietf-softwire-dslite-multicast]	Qin, J., Boucadair, M., Jacquenet, C., Lee, Y. and Q. Wang, "Delivery of IPv4 Multicast Services to IPv4 Clients over an IPv6 Multicast Network", Internet-Draft draft-ietf-softwire-dslite-multicast-04, October 2012.
[I-D.ietf-softwire-mesh-multicast]	Xu, M., Cui, Y., Wu, J., Yang, S., Metz, C. and G. Shepherd, "Softwire Mesh Multicast", Internet-Draft draft-ietf-softwire-mesh-multicast-04, January 2013.
[I-D.ietf-behave-nat64-learn-analysis]	Korhonen, J. and T. Savolainen, "Analysis of solution proposals for hosts to learn NAT64 prefix", Internet-Draft draft-ietf-behave-nat64-learn-analysis-03, March 2012.
[RFC4566]	Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.

Appendix A. Motivations

A.1. Why an Address Format is Needed for Multicast IPv4-IPv6 Interconnection?

Arguments why an IPv6 address format is needed to embed multicast IPv4 address are quite similar to those of [RFC6052]. Concretely, the definition of a multicast address format embedding a multicast IPv4 address allows:

• Stateless IPv4-IPv6 header translation of multicast flows;
• Stateless IPv4-IPv6 PIM interworking function;
• Stateless IGMP-MLD interworking function (e.g., required for an IPv4 receiver to access to IPv4 multicast content via an IPv6 network);
• Stateless (local) synthesis of IPv6 address when IPv4 multicast address are embedded in application payload (e.g., SDP [RFC4566]);
• Except the provisioning of the same MPREFIX64, no coordination is required between IPv4-IPv6 PIM interworking function, IGMP-MLD interworking function, IPv4-IPv6 Interconnection Function and any ALG (Application Level Gateway) in the path;
• Minimal operational constraints on the multicast address management: IPv6 multicast addresses can be constructed using what has been deployed for IPv4 delivery mode.

A.2. Why Identifying an IPv4-Embedded IPv6 Multicast Address is Required?

Reserving a dedicated multicast prefix for IPv4-IPv6 interconnection purposes is a means to guide the address selection process at the receiver side; in particular it assists the receiver to select the appropriate IP multicast address while avoiding to involve an IPv4-IPv6 interconnection function in the path.

Two use cases to illustrate this behavior are provided below:[I-D.ietf-behave-nat64-learn-analysis] in a multicast context.

1. An ALG is required to help an IPv6 receiver to select the appropriate IP address when only the IPv4 address is advertised (e.g., using SDP); otherwise the access to the IPv4 multicast content can not be offered to the IPv6 receiver. The ALG may be located downstream the receiver. As such, the ALG does not know in advance whether the receiver is dual-stack or IPv6-only. The ALG may be tuned to insert both the original IPv4 address and corresponding IPv6 multicast address. If a dedicated prefix is not used, a dual-stack receiver may prefer to use the IPv6 address to receive the multicast content. This address selection would require multicast flows to cross an IPv4-IPv6 interconnection function.
2. In order to avoid involving an ALG in the path, an IPv4-only source can advertise both its IPv4 address and IPv4-Embedded IPv6 Multicast Address. If a dedicated prefix is not reserved, a dual-stack receiver may prefer to use the IPv6 address to receive the multicast content. This address selection would require multicast flows to cross an IPv4-IPv6 interconnection function.

Reserving dedicated IPv6 multicast prefixes for IPv4-IPv6 interconnection purposes mitigates the issues discussed in

A.3. Location of the IPv4 Address

There is no strong argument to allow for flexible options to encode the IPv4 address inside the multicast IPv6 address. The option retained by the authors is to encode the multicast IPv4 address in the low-order 32 bits of the IPv6 address.

This choice is also motivated by the need to be compliant with [RFC3306] and [RFC3956].

Appendix B. Design Considerations

The following constraints should be met when reserving dedicated prefix(es) to be used for IPv4/IPv6 multicast interconnection:

1:

Belong to SSM prefix range (preferably ff3x::/32) and be compatible with unicast-based prefix [RFC3306] for SSM. Note that [RFC3306] suggests to set "plen" to 0 and "network-prefix" to 0. As such, any prefix in the 33-96 range can be convenient. Given [RFC4607] indicates future specifications may allow a non-zero network prefix field, a /33 would allow for future extensions but it has the drawback of reserving a large block. A /96 would be adequate for the use cases already identified in [I-D.ietf-mboned-v4v6-mcast-ps]. In the event of any concrete extension, reserving additional prefixes may be considered.

2:

Be compatible with embedded-RP [RFC3956] and unicast-based prefix [RFC3306] for ASM. This results in associating a meaning with one of the reserved bits in [I-D.ietf-6man-multicast-addr-arch-update]. Defining the 17-20 bits range to have a meaning and be used for IPv4/IPv6 transition has the advantage of allowing for future extensions but it may be seen as a waste of the multicast address space. Consequently, using one of the reserved bits (in the range 17-20) from the unicast-based IPv6 multicast address format [RFC3306] is preferred.

Meeting (1) and (2) with the same reserved bit is not feasible without modifying embedded-RP and unicast-based prefix specifications; this option is avoided.

As a consequence, this document proposes to reserve a multicast prefix for SSM and define one bit of the unicast prefix-based multicast IPv6 address for ASM when embedding IPv4 multicast address in an IPv6 multicast address.

Authors' Addresses