Network Working Group                                  B. Haberman, Ed. Request for Comments: 5075                                      JHU APL Category: Standards Track                                      R. Hinden                                                                    Nokia                                                            November 2007                IPv6 Router Advertisement Flags Option
Status of This Memo
This document specifies an Internet standards track protocol for the    Internet community, and requests discussion and suggestions for
improvements.  Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol.  Distribution of this memo is unlimited.
Abstract
The IPv6 Neighbor Discovery’s 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 to the Router Advertisement message that expands
the available number of flag bits available.
Table of Contents
1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3.  Current Router Advertisement Flags  . . . . . . . . . . . . . . 2
4.  Flags Expansion Option  . . . . . . . . . . . . . . . . . . . . 3
5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5    7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5    7.2.  Informative References  . . . . . . . . . . . . . . . . . . 6 Haberman & Hinden          Standards Track                    [Page 1]
1.  Introduction
The IPv6 Neighbor Discovery Protocol’s (NDP) [RFC4861] 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.
2.  Terminology
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 [RFC2119].
3.  Current Router Advertisement Flags
Currently, the NDP Router Advertisement message contains the
following one-bit flags defined in published RFCs:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|M|O|H|Prf|P|R|R|
+-+-+-+-+-+-+-+-+
Figure 1: Router Advertisement Flags
o  M - Managed Address Configuration Flag [RFC4861]
o  O - Other Configuration Flag [RFC4861]
o  H - Mobile IPv6 Home Agent Flag [RFC3775]
o  Prf - Router Selection Preferences [RFC4191]
o  P - Neighbor Discovery Proxy Flag [RFC4389]
o  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. Haberman & Hinden          Standards Track                    [Page 2]
4.  Flags Expansion Option
The Neighbor Discovery specification [RFC4861] contains the
capability to define NDP options.  The following (Figure 2) is the
definition of the Expanded Flags Option (EFO) for NDP Router
Advertisement messages.
0                  1                  2                  3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Type      |    Length    |        Bit fields available ..
react routerv6+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... for assignment                                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Router Advertisement Expanded Flags Option
o  Type - TBD (to be assigned by IANA)
o  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.
o  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:
o  MUST only occur in Router Advertisement messages.
o  MUST occur prior to any additional options associated with any
flags set in this option.
o  MUST only occur once in the Router Advertisement message.
o  MUST NOT be added to a Router Advertisement message if no flags in      the option are set.
o  MUST set all unused flags to zero.
Haberman & Hinden          Standards Track                    [Page 3]
Upon reception, a receiver processing NDP messages containing this
option:
o  MUST ignore the option if it occurs in a message other than a
Router Advertisement.
o  MUST ignore all instances of the option except the first one
encountered in the Router Advertisement message.
o  MUST ignore the option if the Length is less than 1.
o  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 Figure 1.
Flag bits 0 to 7 are found in the Router Advertisement message header    defined in [RFC4861].
5.  IANA Considerations
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:
/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:
Haberman & Hinden          Standards Track                    [Page 4]
+---------------+---------------------------------------+-----------+  | RA Option Bit | Description                          | Reference |  +---------------+---------------------------------------+-----------+  | 0            | M - Managed Address Configuration    | [RFC4861] |  |              | Flag                                  |          |  | 1            | O - Other Configuration Flag          | [RFC4861] |  | 2            | H - Mobile IPv6 Home Agent Flag      | [RFC3775] |  | 3            | Prf - Router Selection Preferences    | [RFC4191] |  | 4            | Prf - Router Selection Preferences    | [RFC4191] |  | 5            | P - Neighbor Discovery Proxy Flag    | [RFC4389] |  | 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 [RFC2434].
6.  Security Considerations
This protocol shares the security issues of NDP that are documented
in the "Security Considerations" section of [RFC4861].
The inclusion of additional optional bit fields provides a potential    covert channel that is useful for passing information.
7.  References
7.1.  Normative References
[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
Haberman & Hinden          Standards Track                    [Page 5]

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