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IETF RFC 7371
Last modified on Friday, September 19th, 2014
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Internet Engineering Task Force (IETF) M. Boucadair
Request for Comments: 7371 France Telecom
Updates: 3306, 3956, 4291 S. Venaas
Category: Standards Track Cisco
ISSN: 2070-1721 September 2014
Updates to the IPv6 Multicast Addressing Architecture
 Abstract
This document updates the IPv6 multicast addressing architecture by
redefining the reserved bits as generic flag bits. The document also
provides some clarifications related to the use of these flag bits.
This document updates RFCs 3956, 3306, and 4291.
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 5741.
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/RFC 7371.
Boucadair & Venaas Standards Track PAGE 1 
RFC 7371 Multicast Flag Bits September 2014
Copyright Notice
Copyright (c) 2014 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.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................3
1.1. Requirements Language ......................................3
2. Addressing Architecture Update ..................................3
3. Flag Bits: New Processing Rules .................................4
4. RFC Updates .....................................................4
4.1. Updates to RFC 3306 ........................................4
4.1.1. Update #1 ...........................................4
4.1.2. Update #2 ...........................................6
4.2. Updates to RFC 3956 ........................................6
4.2.1. Update #1 ...........................................6
4.2.2. Update #2 ...........................................7
4.2.3. Update #3 ...........................................8
4.2.4. Update #4 ...........................................9
5. Security Considerations .........................................9
6. Acknowledgements ................................................9
7. References ......................................................9
7.1. Normative References .......................................9
7.2. Informative References ....................................10
Boucadair & Venaas Standards Track PAGE 2
RFC 7371 Multicast Flag Bits September 2014
1. Introduction
This document updates the IPv6 addressing architecture [RFC 4291] by
redefining reserved bits as generic flag bits (Section 2). The
document also provides some clarifications related to the use of
these flag bits (Section 3).
This document updates [RFC 3956], [RFC 3306], and [RFC 4291]. These
updates are logical consequences of the new processing rules in
Section 3.
Textual representation of IPv6 addresses included in the RFC updates
follows the recommendation in [RFC 5952].
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 [RFC 2119].
2. Addressing Architecture Update
Bits 17-20 of a multicast address, where bit 1 is the most
significant bit, are defined in [RFC 3956] and [RFC 3306] as reserved
bits. This document defines these bits as generic flag bits so that
they apply to any multicast address. These bits are referred to as
"ff2" (flag field 2), while the "flgs" bits in [RFC 4291] [RFC 3956]
are renamed to "ff1" (flag field 1).
Within this document, flag bits denote both ff1 and ff2.
Defining the bits 17-20 as flags for all IPv6 multicast addresses
allows addresses to be treated in a more uniform and generic way, and
allows for these bits to be defined in the future for different
purposes, irrespective of the specific type of multicast address.
For the record, this design choice was initially triggered by the
specification in [ADDR-FORMAT], which proposed associating a meaning
with one of the reserved bits. Moreover, [ADDR-FORMAT] also
considered the use of the last remaining flag in ff1, but that
approach was abandoned because it is not clear at this stage whether
there are other usage scenarios of the flag.
Section 4 specifies the updated structure of the addressing
architecture.
Further specification documents may define a meaning for these
flag bits.
Boucadair & Venaas Standards Track PAGE 3
RFC 7371 Multicast Flag Bits September 2014
3. Flag Bits: New Processing Rules
Some implementations and specification documents do not treat the
flag bits as separate bits but tend to use their combined value as a
4-bit integer. This practice is a hurdle for assigning a meaning to
the remaining flag bits. Below are listed some examples for
illustration purposes:
o The reading of [RFC 3306] may lead one to conclude that ff3x::/32
is the only allowed Source-Specific Multicast (SSM) IPv6 prefix
block.
o [RFC 3956] states that only ff70::/12 applies to Embedded-RP.
Particularly, implementations should not treat the fff0::/12 range
as Embedded-RP.
To avoid such confusion and to unambiguously associate a meaning with
the remaining flags, the following requirement is made:
Implementations MUST treat flag bits as separate bits.
4. RFC Updates
4.1. Updates to RFC 3306
4.1.1. Update #1
This document changes Section 4 of [RFC 3306] as follows:
OLD:
| 8 | 4 | 4 | 8 | 8 | 64 | 32 |
+--------+----+----+--------+--------+----------------+----------+
|11111111|flgs|scop|reserved| plen | network prefix | group ID |
+--------+----+----+--------+--------+----------------+----------+
+-+-+-+-+
flgs is a set of 4 flags: |0|0|P|T|
+-+-+-+-+
o P = 0 indicates a multicast address that is not assigned
based on the network prefix. This indicates a multicast
address as defined in [ADDRARCH].
o P = 1 indicates a multicast address that is assigned based
on the network prefix.
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RFC 7371 Multicast Flag Bits September 2014
o If P = 1, T MUST be set to 1, otherwise the setting of the T
bit is defined in Section 2.7 of [ADDRARCH].
The reserved field MUST be zero.
Note: [ADDRARCH] is a reference listed in [RFC 3306]. [ADDRARCH]
has been since obsoleted by [RFC 4291].
NEW:
| 8 | 4 | 4 | 4 | 4 | 8 | 64 | 32 |
+--------+----+----+----+----+--------+----------------+----------+
|11111111|ff1 |scop|ff2 |rsvd| plen | network prefix | group ID |
+--------+----+----+----+----+--------+----------------+----------+
+-+-+-+-+
ff1 (flag field 1) is a set of 4 flags: |X|Y|P|T|
+-+-+-+-+
X and Y may each be set to 0 or 1. Note that X is for future
assignment, while a meaning is associated with Y in RFC 3956.
o P = 0 indicates a multicast address that is not assigned
based on the network prefix. This indicates a multicast
address as defined in [RFC 4291].
o P = 1 indicates a multicast address that is assigned based
on the network prefix.
o If P = 1, T MUST be set to 1; otherwise, the setting of the
T bit is defined in Section 2.7 of [RFC 4291].
+-+-+-+-+
ff2 (flag field 2) is a set of 4 flags: |r|r|r|r|
+-+-+-+-+
where "rrrr" are for future assignment as additional flag bits.
r bits MUST each be sent as zero and MUST be ignored on receipt.
Flag bits denote both ff1 and ff2.
Boucadair & Venaas Standards Track PAGE 5
RFC 7371 Multicast Flag Bits September 2014
4.1.2. Update #2
This document changes Section 6 of [RFC 3306] as follows:
OLD:
These settings create an SSM range of FF3x::/32 (where 'x' is any
valid scope value). The source address field in the IPv6 header
identifies the owner of the multicast address.
NEW:
If the flag bits in ff1 are set to 0011, these settings create an
SSM range of ff3x::/32 (where 'x' is any valid scope value). The
source address field in the IPv6 header identifies the owner of
the multicast address. ff3x::/32 is not the only allowed SSM
prefix range. For example, if the most significant flag bit in
ff1 is set, then we would get the SSM range ffbx::/32.
4.2. Updates to RFC 3956
4.2.1. Update #1
This document changes Section 2 of [RFC 3956] as follows:
OLD:
As described in [RFC 3306], the multicast address format is
as follows:
| 8 | 4 | 4 | 8 | 8 | 64 | 32 |
+--------+----+----+--------+----+----------------+----------+
|11111111|flgs|scop|reserved|plen| network prefix | group ID |
+--------+----+----+--------+----+----------------+----------+
Where flgs are "0011". (The first two bits are as yet undefined,
sent as zero and ignored on receipt.)
Boucadair & Venaas Standards Track PAGE 6
RFC 7371 Multicast Flag Bits September 2014
NEW:
The multicast address format is as follows:
| 8 | 4 | 4 | 4 | 4 | 8 | 64 | 32 |
+--------+----+----+----+----+----+----------------+----------+
|11111111|ff1 |scop|ff2 |rsvd|plen| network prefix | group ID |
+--------+----+----+----+----+----+----------------+----------+
+-+-+-+-+
ff1 (flag field 1) is a set of four flags: |X|R|P|T|
+-+-+-+-+
where X is for future assignment as an additional flag bit.
X may be set to 0 or 1.
+-+-+-+-+
ff2 (flag field 2) is a set of 4 flags: |r|r|r|r|
+-+-+-+-+
where "rrrr" are for future assignment as additional flag bits.
r bits MUST each be sent as zero and MUST be ignored
on receipt.
Flag bits denote both ff1 and ff2.
4.2.2. Update #2
This document changes Section 3 of [RFC 3956] as follows:
OLD:
| 8 | 4 | 4 | 4 | 4 | 8 | 64 | 32 |
+--------+----+----+----+----+----+----------------+----------+
|11111111|flgs|scop|rsvd|RIID|plen| network prefix | group ID |
+--------+----+----+----+----+----+----------------+----------+
+-+-+-+-+
flgs is a set of four flags: |0|R|P|T|
+-+-+-+-+
When the highest-order bit is 0, R = 1 indicates a multicast address
that embeds the address on the RP. Then P MUST be set to 1, and
consequently T MUST be set to 1, as specified in [RFC 3306]. In
effect, this implies the prefix FF70::/12. In this case, the last 4
bits of the previously reserved field are interpreted as embedding
the RP interface ID, as specified in this memo.
Boucadair & Venaas Standards Track PAGE 7
RFC 7371 Multicast Flag Bits September 2014
The behavior is unspecified if P or T is not set to 1, as then the
prefix would not be FF70::/12. Likewise, 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.
NEW:
| 8 | 4 | 4 | 4 | 4 | 8 | 64 | 32 |
+--------+----+----+----+----+----+----------------+----------+
|11111111|ff1 |scop|ff2 |RIID|plen| network prefix | group ID |
+--------+----+----+----+----+----+----------------+----------+
+-+-+-+-+
ff1 is a set of four flags: |X|R|P|T|
+-+-+-+-+
where X is for future assignment as an additional flag bit.
X may be set to 0 or 1.
R = 1 indicates a multicast address that embeds the address of the
RP. Then, P MUST be set to 1, and consequently T MUST be set
to 1, according to [RFC 3306], as this is a special case of
unicast-prefix-based addresses. This implies that, for instance,
prefixes ff70::/12 and fff0::/12 are embedded RP prefixes. When
the R-bit is set, the last 4 bits of the field that were reserved
in [RFC 3306] are interpreted as embedding the RP interface ID, as
specified in this memo.
4.2.3. Update #3
This document changes Section 4 of [RFC 3956] as follows:
OLD:
o It MUST be a multicast address with "flgs" set to 0111, that is, to
be of the prefix FF70::/12,
NEW:
o It MUST be a multicast address with the R-bit set to 1.
o It MUST have the P-bit and T-bit both set to 1 when using the
embedding in this document as it is a prefix-based address.
Boucadair & Venaas Standards Track PAGE 8
RFC 7371 Multicast Flag Bits September 2014
4.2.4. Update #4
This document changes Section 7.1 of [RFC 3956] as follows:
OLD:
To avoid loops and inconsistencies, for addresses in the range
FF70::/12, the Embedded-RP mapping MUST be considered the longest
possible match and higher priority than any other mechanism.
NEW:
To avoid loops and inconsistencies, for addresses with the R-bit
set to 1, the Embedded-RP mapping MUST be considered the longest
possible match and higher priority than any other mechanism.
5. Security Considerations
The same security considerations as those discussed in [RFC 3956],
[RFC 3306], and [RFC 4291] are to be taken into account.
6. Acknowledgements
Special thanks to Brian Haberman for the discussions prior to the
publication of this document.
Many thanks to Jouni Korhonen, Tatuya Jinmei, Charlie Kaufman, and
Ben Campbell for their review.
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3306] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
Multicast Addresses", RFC 3306, August 2002.
[RFC 3956] Savola, P. and B. Haberman, "Embedding the Rendezvous
Point (RP) Address in an IPv6 Multicast Address",
RFC 3956, November 2004.
[RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC 5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 2010.
Boucadair & Venaas Standards Track PAGE 9
RFC 7371 Multicast Flag Bits September 2014
7.2. Informative References
[ADDR-FORMAT]
Boucadair, M., Qin, J., Lee, Y., Venaas, S., Li, X., and
M. Xu, "IPv6 Multicast Address With Embedded IPv4
Multicast Address", Work in Progress, April 2013.
Authors' Addresses
Mohamed Boucadair
France Telecom
Rennes 35000
France
EMail: mohamed.boucadair@orange.com
Stig Venaas
Cisco
USA
EMail: stig@cisco.com
Boucadair & Venaas Standards Track PAGE 10
RFC TOTAL SIZE: 18327 bytes
PUBLICATION DATE: Friday, September 19th, 2014
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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