IPv6 Router Advertisement Flags Option
obsoleted by
- rfc-5175 — IPv6 Router Advertisement Flags Option
Extracted elements (12)
The NDP Router Advertisement message has only an 8-bit field for single-bit flags, and several protocols have already reserved flags, with others preparing to exhaust the remaining bits. This document defines an option that expands the available flag bits by adding 48 additional bits to NDP messages.
Flag bits 0–7 remain in the Router Advertisement message header as defined in RFC 4861; bits 8–55 are located in the EFO, numbered left to right starting at bit offset 16 in the option, continuing the same numbering scheme.
A receiver MUST ignore the EFO if the Length field is less than 1, and MUST ignore any unknown flag bits within the option.
An implementation MUST set the 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, allowing for future expansion.
During transmission, the EFO MUST only occur in Router Advertisement messages, MUST occur prior to any additional options associated with flags set in the option, and MUST only occur once in the RA message.
The EFO MUST NOT be added to a Router Advertisement message if no flags in the option are set, and MUST set all unused flag bits to zero.
Upon reception, a receiver MUST ignore the EFO if it occurs in a message other than a Router Advertisement, and MUST ignore all instances of the option except the first one encountered.
The Expanded Flags Option (EFO) is a new NDP option for Router Advertisement messages that provides 48 additional flag bits (bits 8–55), extending the 8 flag bits already present in the RA message header.
IANA assigned Type 26 to the RA Flags Extension Option in the ICMPv6 Parameters registry for IPv6 Neighbor Discovery options.
IANA created a new registry for IPv6 ND Router Advertisement flags covering bits 0–55: bits 0–5 assigned to existing flags (M, O, H, Prf, Prf, P from RFC 4861/3775/4191/4389), bits 6–53 reserved and available for assignment, bits 54–55 reserved for private experimentation. New assignments require standards action or IESG approval per RFC 2434.
This protocol shares the security issues of NDP documented in the Security Considerations of RFC 4861. Additionally, the inclusion of extra optional bit fields provides a potential covert channel useful for passing information.
The EFO is a 64-bit (8-octet, Length=1) NDP option: an 8-bit Type field (assigned value 26), an 8-bit Length field, and 48 bits available for flag assignment numbered 8–55 from left to right starting at bit offset 16 in the option.