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
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.
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.
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).
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].
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.
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.
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.
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.
| 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.
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.
| 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.