IPv6 Router Advertisement Flags Option

The IPv6 Neighbor Discovery Protocol's (NDP) Router Advertisement message contains an 8-bit field reserved for single-bit flags. Several protocols have reserved flags in this field and others are preparing to reserve a sufficient number of flags to exhaust the field. This document defines an option for the Router Advertisement message that expands the available number of flag bits by adding an additional 48 flag bits to NDP messages.

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 .

Currently, the NDP Router Advertisement message contains the following one-bit flags defined in published RFCs:

M - Managed Address Configuration Flag O - Other Configuration Flag H - Mobile IPv6 Home Agent Flag Prf - Router Selection Preferences P - Neighbor Discovery Proxy Flag R - Reserved With other protocols in the works (e.g., Detecting Network Attachment) that want to use flags in the NDP messages, it is necessary to define an expansion capability to support new features.

The Neighbor Discovery specification contains the capability to define NDP options. The following () is the definition of the Expanded Flags Option (EFO) for NDP Router Advertisement messages.

Type - TBD (to be assigned by IANA) Length - The length MUST be checked when processing the option in order to allow for future expansion of this option. An implementation of this specification MUST set the Length to 1, MUST ignore any unrecognized data, and MUST be able to recognize the specific length in order to skip over unrecognized bits. Bits - allocated by IANA The definition and usage of these bits is to be found in the document requesting their allocation. During the construction/transmission, this option: 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 Router Advertisement message. MUST NOT be added to a Router Advertisement message if no flags in the option are set. MUST set all unused flags to zero. Upon reception, a receiver processing NDP messages containing this option: MUST ignore the option if it occurs in a message other than a Router Advertisement. MUST ignore all instances of the option except the first one encountered in the Router Advertisement message. MUST ignore the option if the Length is less than 1. MUST ignore any unknown flag bits. The bit fields within the option are numbered from left to right, from 8 to 55 (starting as bit offset 16 in the option) and follow the numbering of the flag bits in the RA option described in . Flag bits 0 to 7 are found in the Router Advertisement message header defined in .

IANA has defined a new IPv6 Neighbor Discovery option for the option defined in this document of the form: Type Description Reference 26 RA Flags Extension Option [RFC5075] The registry for these options can be found at: http://www.iana.org/assignments/icmpv6-parameters IANA has created a new registry for IPv6 ND Router Advertisement flags. This should include the current flags in the RA option and in the extension option defined in this document. The new registry has been added to the icmpv6-parameters as shown above. The format for the registry is: RA Option Bit Description Reference 0 M - Managed Address Configuration Flag 1 O - Other Configuration Flag 2 H - Mobile IPv6 Home Agent Flag 3 Prf - Router Selection Preferences 4 Prf - Router Selection Preferences 5 P - Neighbor Discovery Proxy Flag 6-53 R - Reserved; Available for assignment 54-55 Private Experimentation The assignment of new RA flags in the RA option header and the bits defined in the RA extension option defined in this document require standards action or IESG approval .

This protocol shares the security issues of NDP that are documented in the "Security Considerations" section of . The inclusion of additional optional bit fields provides a potential covert channel that is useful for passing information.