Fixing the C-Flag in the Extended Address Registration Option (EARO)
updates
- rfc-8928 — Address-Protected Neighbor Discovery for Low-Power and Lossy Networks
Extracted elements (13)
RFC 8928 incorrectly referred to the EARO as the 'Enhanced Address Registration Option' rather than the 'Extended Address Registration Option.' RFC 9927 corrects this terminology throughout.
RFC 8928 placed the C-flag at bit position 3 of the EARO flags field but never registered it with IANA. RFC 9685 subsequently defined the P-Field at bits 2 and 3 with proper IANA registration, creating an overlap. To resolve this conflict, RFC 9927 repositions the C-flag to bit 1.
The updates introduced by RFC 9927 are not backward compatible with RFC 8928. However, because there are no known implementations or deployments of RFC 8928, no transition plan is required.
All reserved fields in the EARO MUST be set to zero by the sender and MUST be ignored by the receiver.
The Status field in NS(EARO) messages MUST be set to 0 unless the registration is for a prefix (i.e., the F-flag is set and prefix length is provided).
The C-flag is a 1-bit flag at bit 1 of the EARO flags field, moved from its original position in RFC 8928. When set, it indicates that the ROVR field contains a Crypto-ID and that the 6LN MAY be challenged for ownership of the registered address.
The P-field is a 2-bit Registered Address Type Indicator (RATInd) at bits 2–3 of the EARO flags field (defined by RFC 9685). It indicates whether the registered address is unicast, multicast, anycast, or derived from a registered unicast prefix.
The ROVR (Registration Ownership Verifier) is a variable-length field of 64, 128, 192, or 256 bits used to verify ownership of a registered IPv6 address. When the C-flag is set, this field contains a Crypto-ID as defined in RFC 8928.
The Status field is a 6-bit unsigned integer present only in NA(EARO) response messages, used to indicate the status of a registration. It is defined in RFC 8505 and resized by RFC 9010; values are in the IANA 'Address Registration Option Status Values' registry.
IANA has updated the 'Address Registration Option Flags' registry (within the ICMPv6 Parameters registry group) to assign bit number 1 to the C-Flag, referencing both RFC 9927 and RFC 8928.
This specification does not introduce any new security considerations beyond those already discussed in RFC 8928 and RFC 8505.
The EARO format for use in NA messages differs from the NS format in bits 16–23: the NS format carries F and Prefix Length in that field, while the NA format carries a 2-bit reserved 'r' field and a 6-bit Status field. The second 32-bit word and ROVR field are identical across both formats.
The EARO format for use in NS messages is a 32-bit header (Type, Length, F, Prefix Length, Opaque) followed by a second 32-bit word containing flags |r|C|P|I|R|T|, TID, and Registration Lifetime, followed by a variable-length ROVR field (64, 128, 192, or 256 bits). The C-flag now occupies bit 1 of the flags byte.