Updates to the IPv6 Multicast Addressing Architecture

This document updates the IPv6 addressing architecture by redefining reserved bits as generic flag bits (). The document also provides some clarifications related to the use of these flag bits (). This document updates , , and . These updates are logical consequences of the new processing rules in . Textual representation of IPv6 addresses included in the RFC updates follows the recommendation in .

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.

Bits 17-20 of a multicast address, where bit 1 is the most significant bit, are defined in and 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 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 , which proposed associating a meaning with one of the reserved bits. Moreover, 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. 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: The reading of may lead one to conclude that ff3x::/32 is the only allowed Source-Specific Multicast (SSM) IPv6 prefix block. 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.

This document changes Section 4 of as follows: OLD:

Note: [ADDRARCH] is a reference listed in . [ADDRARCH] has been since obsoleted by . NEW:

This document changes Section 6 of 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.

This document changes Section 2 of as follows: OLD:

NEW:

This document changes Section 3 of as follows: OLD:

NEW:

This document changes Section 4 of 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: It MUST be a multicast address with the R-bit set to 1. 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.

This document changes Section 7.1 of 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.

The same security considerations as those discussed in , , and are to be taken into account.

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.