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IETF RFC 5175
IPv6 Router Advertisement Flags Option
Last modified on Wednesday, March 5th, 2008
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Network Working Group B. Haberman, Ed.
Request for Comments: 5175 JHU APL
Obsoletes: 5075 R. Hinden
Category: Standards Track Nokia
March 2008
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 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 
RFC 5175 IPv6 RA Flags Options March 2008
1. Introduction
The IPv6 Neighbor Discovery Protocol's (NDP) [RFC 4861] 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 [RFC 2119].
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 [RFC 4861]
o O - Other Configuration Flag [RFC 4861]
o H - Mobile IPv6 Home Agent Flag [RFC 3775]
o Prf - Router Selection Preferences [RFC 4191]
o P - Neighbor Discovery Proxy Flag [RFC 4389]
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
RFC 5175 IPv6 RA Flags Options March 2008
4. Flags Expansion Option
The Neighbor Discovery specification [RFC 4861] 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 ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... for assignment |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Router Advertisement Expanded Flags Option
o Type - 26
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
RFC 5175 IPv6 RA Flags Options March 2008
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 [RFC 4861].
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 | [RFC 5175] |
+------+---------------------------+-----------+
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:
Haberman & Hinden Standards Track PAGE 4
RFC 5175 IPv6 RA Flags Options March 2008
+---------------+---------------------------------------+-----------+
| RA Option Bit | Description | Reference |
+---------------+---------------------------------------+-----------+
| 0 | M - Managed Address Configuration | [RFC 4861] |
| | Flag | |
| 1 | O - Other Configuration Flag | [RFC 4861] |
| 2 | H - Mobile IPv6 Home Agent Flag | [RFC 3775] |
| 3 | Prf - Router Selection Preferences | [RFC 4191] |
| 4 | Prf - Router Selection Preferences | [RFC 4191] |
| 5 | P - Neighbor Discovery Proxy Flag | [RFC 4389] |
| 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 [RFC 2434].
6. Security Considerations
This protocol shares the security issues of NDP that are documented
in the "Security Considerations" section of [RFC 4861].
The inclusion of additional optional bit fields provides a potential
covert channel that is useful for passing information.
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC 4861] 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
RFC 5175 IPv6 RA Flags Options March 2008
7.2. Informative References
[RFC 3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
[RFC 4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC 4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
Proxies (ND Proxy)", RFC 4389, April 2006.
Authors' Addresses
Brian Haberman (editor)
Johns Hopkins University Applied Physics Lab
11100 Johns Hopkins Road
Laurel, MD 20723-6099
USA
Phone: +1 443 778 1319
EMail: brian@innovationslab.net
Robert Hinden
Nokia
313 Fairchild Drive
Mountain View, CA 94043
USA
Phone: +1 650 625 2004
EMail: bob.hinden@nokia.com
Haberman & Hinden Standards Track PAGE 6
RFC 5175 IPv6 RA Flags Options March 2008
Full Copyright Statement
Copyright © The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Haberman & Hinden Standards Track PAGE 7
IPv6 Router Advertisement Flags Option
RFC TOTAL SIZE: 12463 bytes
PUBLICATION DATE: Wednesday, March 5th, 2008
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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