IPv6 Router Advertisement Flags Option
obsoletes
- rfc-5075 — IPv6 Router Advertisement Flags Option
Extracted elements (11)
The 8-bit flags field in IPv6 Router Advertisement messages was being exhausted by multiple protocols reserving single-bit flags (M, O, H, Prf, P, R). Rather than redesigning the RA message header, an NDP option mechanism is used to expand the flag space by an additional 48 bits.
Flag bits 0–7 remain in the Router Advertisement message header as defined in RFC4861; bits 8–55 are carried in the EFO. Implementations that do not support the EFO will simply ignore the option, maintaining backward compatibility with RFC4861 NDP.
An implementation MUST set the EFO Length field to 1, MUST ignore any unrecognized data, and MUST be able to recognize the specific length in order to skip over unrecognized bits.
During transmission, the EFO MUST only occur in Router Advertisement messages, MUST occur prior to any additional options associated with any flags set in this option, MUST only occur once in the RA message, MUST NOT be added if no flags in the option are set, and MUST set all unused flags to zero.
Upon reception, a receiver MUST ignore the EFO if it occurs in a message other than a Router Advertisement, MUST ignore all instances except the first encountered in an RA message, MUST ignore the option if Length is less than 1, and MUST ignore any unknown flag bits.
The existing 8-bit RA flags field (bits 0–7) includes: M (Managed Address Configuration, RFC4861), O (Other Configuration, RFC4861), H (Mobile IPv6 Home Agent, RFC3775), Prf (Router Selection Preferences, 2 bits, RFC4191), P (ND Proxy, RFC4389), and 2 reserved bits.
The Expanded Flags Option (EFO) is a new NDP option (Type=26) for Router Advertisement messages that provides 48 additional flag bits (bits 8–55) beyond the 8 bits in the RA header. Bits are numbered left to right starting at bit offset 16 in the option.
IANA created a new registry for IPv6 ND Router Advertisement flags under icmpv6-parameters, consolidating existing RA header flags (bits 0–7 from RFC4861, RFC3775, RFC4191, RFC4389) and EFO bits (8–53 available for assignment, 54–55 reserved for private experimentation). New assignments require standards action or IESG approval.
IANA registered NDP option Type 26 ('RA Flags Extension Option') in the icmpv6-parameters registry for the Expanded Flags Option defined in this document.
The EFO shares all security issues of NDP documented in RFC4861. Additionally, the inclusion of additional optional bit fields provides a potential covert channel useful for passing information out-of-band.
The EFO is encoded as: Type (1 octet, value 26), Length (1 octet, MUST be set to 1), followed by 48 bits (6 octets) of flag fields available for assignment by IANA. Total option size is 8 octets (one 8-byte NDP option unit).