RFC7371
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/rfc7371.
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Contents
Introduction
This document updates the IPv6 addressing architecture RFC4291 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 RFC3956, RFC3306, and RFC4291. 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 RFC5952.
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.
Addressing Architecture Update
Bits 17-20 of a multicast address, where bit 1 is the most significant bit, are defined in RFC3956 and RFC3306 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 RFC4291 RFC3956 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.
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 RFC3306 may lead one to conclude that ff3x::/32
is the only allowed Source-Specific Multicast (SSM) IPv6 prefix block.
o RFC3956 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.
RFC Updates
Updates to RFC 3306
Update #1
This document changes Section 4 of RFC3306 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.
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 RFC3306. [ADDRARCH] has been since obsoleted by RFC4291.
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 RFC4291.
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 RFC4291.
+-+-+-+-+
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.
Update #2
This document changes Section 6 of RFC3306 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.
Updates to RFC 3956
Update #1
This document changes Section 2 of RFC3956 as follows:
OLD:
As described in RFC3306, 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.)
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.
Update #2
This document changes Section 3 of RFC3956 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 RFC3306. 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.
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 RFC3306, 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 RFC3306 are interpreted as embedding the RP interface ID, as specified in this memo.
Update #3
This document changes Section 4 of RFC3956 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.
Update #4
This document changes Section 7.1 of RFC3956 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.
Security Considerations
The same security considerations as those discussed in RFC3956, RFC3306, and RFC4291 are to be taken into account.
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.
References
Normative References
RFC2119 Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
RFC3306 Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
Multicast Addresses", RFC 3306, August 2002.
RFC3956 Savola, P. and B. Haberman, "Embedding the Rendezvous
Point (RP) Address in an IPv6 Multicast Address",
RFC 3956, November 2004.
RFC4291 Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
RFC5952 Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 2010.
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: [email protected]
Stig Venaas Cisco USA
EMail: [email protected]
