IPv4-Embedded IPv6 Multicast Address Format
draft-ietf-mboned-64-multicast-address-format-01

Abstract

This document specifies an extension to the IPv6 multicast addressing architecture to be used in the context of IPv4-IPv6 interconnection. In particular, this document defines an address format for IPv4-embedded IPv6 multicast addresses. This address format can be used for IPv4-IPv6 translation or encapsulation schemes.

This document updates RFC4291.

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].

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on August 30, 2012.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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

Table of Contents

• 1. Introduction
• 2. Terminology
• 3. IPv4-Embedded IPv6 Multicast Address Format: ASM Mode
• 4. IPv4-Embedded IPv6 Multicast Address Format: SSM Mode
• 5. Textual Representation
• 6. Multicast PREFIX64
• 7. Source IPv4 Address in the IPv6 Realm
• 8. Examples
• 9. IANA Considerations
• 10. Security Considerations
• 11. Acknowledgements
• 12. References
• Appendix A. Design Choices
• Appendix A.1. Location of the IPv4 Address
• Appendix A.2. Location of the M-bit
• Appendix A.3. Encapsulation vs. Translation
• Authors' Addresses

1. Introduction

This document specifies an extension to the IPv6 multicast addressing architecture to be used in the context of IPv4-IPv6 interconnection. In particular, this document defines an address format for IPv4-embedded IPv6 multicast addresses. This address format can be used for IPv4-IPv6 translation or encapsulation schemes.

This document updates [RFC4291].

This specification can be used in conjunction with other extensions such as building unicast prefix-based multicast IPv6 address [RFC3306] or embedding the rendezvous point [RFC3956].

This document is a companion document to [RFC6052] which focuses exclusively on IPv4-embedded IPv6 unicast addresses.

Details about design choices are documented in Appendix Appendix A.

2. Terminology

This document makes use of the following terms:

• IPv4-embedded IPv6 multicast address: denotes a multicast IPv6 address which includes in 32 bits an IPv4 address. Two types of IPv6 addresses are defined that carry an IPv4 address in the low-order 32 bits of the address. The format to build such addresses is defined in Section 3 for ASM mode and Section 4 for SSM mode.
• Multicast Prefix64 (or MPREFIX64 for short) refers to an IPv6 multicast prefix to be used to construct IPv4-embedded IPv6 multicast addresses.
• ASM_MPREFIX64: denotes a multicast Prefix64 used in ASM mode. It follows the format described in Section 3.
• SSM_MPREFIX64: denotes a multicast Prefix64 used in SSM mode. It follows the format described in Section 4.

3. IPv4-Embedded IPv6 Multicast Address Format: ASM Mode

To meet the requirements listed in Appendix Appendix A.2, the following address format is defined to enclose an IPv4 multicast address when ASM mode is used:

|   8    |  4 |  4 |  4 |             76               |    32    |
+--------+----+----+----+------------------------------+----------+
|11111111|flgs|scop|64IX|         sub-group-id         |v4 address|
+--------+----+----+----+-----------------------------------------+
                                              +-+-+-+-+
IPv4-IPv6 Interconnection bits (64IX):        |M|r|r|r|
                                              +-+-+-+-+

The description of the fields is as follows:

• "flgs" and "scop" fields are defined in [RFC4291].
• 64IX field (IPv4-IPv6 interconnection bits): The first bit is the M-bit. When "M-bit" is set to 1, it indicates that a multicast IPv4 address is embedded in the low-order 32 bits of the multicast IPv6 address. All the remaining bits are reserved and MUST be set to 0.
• sub-group-id: This field is configurable according to local policies of the entity managing the IPv4-IPv6 Interconnection Function. This field must follow the recommendations specified in [RFC3306] if unicast-based prefix is used or the recommendations specified in [RFC3956] if embedded-RP is used. The default value is all zeros.
• The low-order 32 bits MUST include an IPv4 multicast address when the M-bit is set to 1. The enclosed IPv4 multicast address SHOULD NOT be in 232/8 range.

4. IPv4-Embedded IPv6 Multicast Address Format: SSM Mode

As mentioned above, any IPv4-embedded IPv6 address used in SSM mode MUST be part of ff3x::/32 [RFC4607]. Figure 2 describes the format of the IPv6 multicast address to be used to enclose an IPv4 multicast address.

|   8    |  4 |  4 |    16     |  4 |       60         |    32    |
+--------+----+----+-----------+----+------------------+----------+
|11111111|0011|scop|00.......00|64IX|   sub-group-id   |v4 address|
+--------+----+----+-----------+----+------------------+----------+
                                              +-+-+-+-+
IPv4-IPv6 Interconnection bits (64IX):        |M|r|r|r|
                                              +-+-+-+-+

• Flags must be set to 0011.
• "scop" is defined in [RFC4291].
• 64IX field (IPv4-IPv6 interconnection bits): Same meaning as Section 3.
• sub-group-id: The default value is all zeros.
• The low-order 32 bits MUST include an IPv4 multicast address when the M-bit is set to 1. The embedded IPv4 address SHOULD be in the 232/8 range [RFC4607]. 232.0.0.1-232.0.0.255 range is being reserved to IANA.

5. 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.

6. Multicast PREFIX64

For the delivery of the IPv4-IPv6 multicast interconnection services, a dedicated multicast prefix denoted as MPREFIX64 should be provisioned to any function requiring to build an IPv4-embedded IPv6 multicast address based on an IPv4 multicast address. MPREFIX64 can be of ASM or SSM type. When both modes are used, two prefixes are required to be provisioned.

The structure of the MPREFIX64 follows the guidelines specified in Section 3 for the ASM mode and Section 4 when SSM mode is used.

The RECOMMENDED MPREFIX64 length is /96 (as shown in Figure 3).

The format of the MPREFIX64 should follow what is specified in [RFC3306] and [RFC3956] if corresponding mechanisms are used.

• The format specified in Section 3 uses some reserved bits defined in [RFC3306] and [RFC3956]: the first of the reserved bits now has a meaning, while the remaining bits MUST be set to 0.

ASM Mode:

|   8    |  4 |  4 |  4 |             76               |    32    |
+--------+----+----+----+------------------------------+----------+
|11111111|flgs|scop|64IX|         sub-group-id         |v4 address|
+--------+----+----+----+------------------------------+----------+
|                                                      |          | 
v                                                      v          v
+------------------------------------------------------+----------+
|                ASM_MPREFIX64                         |v4 address|
+------------------------------------------------------+----------+

SSM Mode:

|   8    |  4 |  4 |    16     |  4 |       60         |    32    |
+--------+----+----+-----------+----+------------------+----------+
|11111111|0011|scop|00.......00|64IX|   sub-group-id   |v4 address|
+--------+----+----+-----------+----+------------------+----------+
|                                                      |          | 
v                                                      v          v
+------------------------------------------------------+----------+
|                SSM_MPREFIX64                         |v4 address|
+------------------------------------------------------+----------+

7. Source IPv4 Address in the IPv6 Realm

An IPv4 source is represented in the IPv6 realm with its IPv4-converted IPv6 address [RFC6052].

8. Examples

Figure 4 provides an example of ASM IPv4-Embedded IPv6 Address while Figure 5 provides an example of SSM IPv4-Embedded IPv6 Address.

 +---------------------+--------------+----------------------------+
 |      MPREFIX64      | IPv4 address | IPv4-embedded IPv6 address |
 +---------------------+--------------+----------------------------+
 | ffxx:8000:abc::/96  |  224.1.2.3   |  ffxx:8000:abc::224.1.2.3  |
 +---------------------+--------------+----------------------------+

 +---------------------+--------------+----------------------------+
 |      MPREFIX64      | IPv4 address | IPv4-embedded IPv6 address |
 +---------------------+--------------+----------------------------+
 |   ff3x:0:8000::/96  |  232.1.2.3   |   ff3x:0:8000::232.1.2.3   |
 +---------------------+--------------+----------------------------+

9. IANA Considerations

Authors of this document request to reserve:

• ff3x:0:8000/33 SSM block to embed an IPv4 multicast address in the last 32 bits.
• ffxx:8000/17 ASM block to embed an IPv4 multicast address in the last 32 bits.

10. Security Considerations

This document defined an address format to embed an IPv4 multicast address in an IPv6 multicast address. The same security considerations as those discussed in [RFC6052] are to be taken into consideration.

11. Acknowledgements

Many thanks to R. Bonica, B. Sarikaya, P. Savola and T. Tsou for their comments and review.

12. References

Appendix A. Design Choices

Appendix A.1. 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 A.2. Location of the M-bit

Figure 6 is a reminder of the IPv6 multicast address format as defined in [RFC4291]:

   |   8    |  4 |  4 |                  112 bits                   |
   +------ -+----+----+---------------------------------------------+
   |11111111|flgs|scop|                  group ID                   |
   +--------+----+----+---------------------------------------------+
                                    +-+-+-+-+
      flgs is a set of 4 flags:     |0|R|P|T|
                                    +-+-+-+-+
   *  "T-bit" is defined in [RFC4291]; 
   *  "P-bit" is defined in [RFC3306]
   *  "R-bit" is defined in [RFC3956]

It was tempting to use the remaining flag to indicate whether an IPv6 address embeds an IPv4 address or not. This choice has been abandoned by the authors for various reasons:

• ff3x::/32 is defined as SSM. Defining a new flag would require standards and implementations to also treat ffbx::/32 as SSM.
• Prefixes starting with ff7x are defined as embedded-RP, but not prefixes starting with fffx. Below is provided an excerpt from [RFC3956]:
  • " ...the encoding and the protocol mode used when the two high-order bits in "flgs" are set to 11 ("fff0::/12") is intentionally unspecified until such time that the highest-order bit is defined. Without further IETF specification, implementations SHOULD NOT treat the fff0::/12 range as Embedded-RP."
  • as such defining a new flag would require implementations to also treat ff7x::/12 as embedded-RP prefix.
• This is the last remaining flag and at this stage we are not sure whether there is other usage scenarios of the flag.

As a conclusion, the remaining flag is not used to indicate an IPv6 multicast address embeds an IPv4 multicast address. However the following constraints should be met:

• (1) Belong to 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.
• (2) Be compatible with embedded-RP [RFC3956] and unicast-based prefix [RFC3306] for ASM;
• (3) Avoid ff3x::4000:0001-ff3x::7fff:ffff which is reserved for IANA.

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

As a consequence, two multicast blocks are proposed to be used when embedding IPv4 address: one block for ASM (Section 3 ) and another one for the SSM (Section 4).

Appendix A.3. Encapsulation vs. Translation

IPv4-IPv6 encapsulator and translator may be embedded in the same device or even implemented with the same software module. In order to help the function whether an encapsulated IPv6 multicast packets or translated IPv6 ones are to be transferred. It was tempting to define an S-bit for that purpose but this choice has been abandoned in favor of the recommendation to use distinct MPREFIX64 for each scheme.

As such, there is no need to reserve a bit in the IPv6 multicast address to separate between the translation and the encapsulation schemes.

Authors' Addresses