Network Working Group                                        S. Krishnan
Request for Comments: 5453                                      Ericsson
Category: Standards Track                                  February 2009

                  Reserved IPv6 Interface Identifiers

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.

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Abstract

   Interface identifiers in IPv6 unicast addresses are used to identify
   interfaces on a link.  They are required to be unique within a
   subnet.  Several RFCs have specified interface identifiers or
   identifier ranges that have a special meaning attached to them.  An
   IPv6 node autoconfiguring an interface identifier in these ranges
   will encounter unexpected consequences.  Since there is no
   centralized repository for such reserved identifiers, this document
   aims to create one.

Table of Contents

   1. Introduction ....................................................2
      1.1. Applicability ..............................................2
      1.2. Requirements Notation ......................................3
   2. Issues with Reusing Reserved Interface Identifiers ..............3
      2.1. Possible Solutions .........................................3
   3. IANA Considerations .............................................3
   4. Acknowledgements ................................................4
   5. Security Considerations .........................................4
   6. References ......................................................5
      6.1. Normative References .......................................5
      6.2. Informative References .....................................5
   Appendix A. List of Potentially Affected RFCs ......................6

1.  Introduction

   An IPv6 unicast address is composed of two parts: a subnet prefix and
   an interface identifier (IID) that identifies a unique interface
   within the subnet prefix.  The structure of an IPv6 unicast address
   is depicted in "IPv6 Addressing Architecture" [RFC4291] and is
   replicated here for clarity.

   |          n bits               |           128-n bits            |
   +-------------------------------+---------------------------------+
   |       subnet prefix           |           interface ID          |
   +-------------------------------+---------------------------------+

                   Figure 1: IPv6 Unicast Address Format

   For all unicast addresses, except those that start with the binary
   value 000, Interface IDs are required to be 64 bits long and to be
   constructed in Modified EUI-64 format [RFC4291].  Examples of
   mechanisms that generate interface identifiers without a unique token
   include Cryptographically Generated Addresses [RFC3972], Privacy
   Addresses [RFC4941], Hash-Based Addresses [HBA], etc.  Non-unique
   interface identifiers can also be allocated using managed address
   assignment mechanisms like DHCPv6 (Dynamic Host Configuration
   Protocol for IPv6) [RFC3315].

1.1.  Applicability

   This document applies only to interface identifiers that are formed
   in the modified EUI-64 format as defined in Appendix A of [RFC4291].
   All other types of interface identifiers are out of its scope.

1.2.  Requirements Notation

   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].

2.  Issues with Reusing Reserved Interface Identifiers

   Let us assume a node comes up with an interface identifier that has
   been reserved for use in some other capacity, e.g., an IPv6 node that
   uses temporary IPv6 addresses [RFC4941] comes up with an IID of
   fdff:ffff:ffff:ffff.  This node will receive requests from all nodes
   that are requesting a service from a Mobile IPv6 home agent since the
   above-mentioned interface identifier has been reserved in [RFC2526]
   to serve as a MIPv6 home agent's anycast address.  At best, this is
   an annoyance to the node that came up with this address.  At worst,
   another node on the link would be denied service and may not look for
   other methods of acquiring a home agent.  Thus, such reserved
   interface identifiers MUST NOT be used for autonomous
   autoconfiguration or for managed address configuration.

2.1.  Possible Solutions

   There are two possible ways to go about avoiding usage of these
   reserved interface identifiers.  One of them would be to add a
   normative reference to each specification that reserves an interface
   identifier.  The other would be to create an IANA registry for such
   interface identifiers.  There are two disadvantages to the normative
   reference approach.  Firstly, this approach does not scale well
   because the number of such specifications that would need to be
   updated is large.  Secondly, the maturity level of the document
   reserving the IID might be lower than the one prohibited from using
   it; this will cause a downward reference problem.  Therefore, the
   better solution is to create an IANA registry for this purpose.

3.  IANA Considerations

   This document creates an IANA registry for reserved IPv6 interface
   identifiers.  Initial values for the reserved IPv6 interface
   identifiers are given below.

   +-----------------------------------------+-------------------------+
   |        Interface Identifier Range       |       Description       |
   +-----------------------------------------+-------------------------+
   |           0000:0000:0000:0000           |  Subnet-Router Anycast  |
   |                                         |        [RFC4291]        |
   |                                         |                         |
   | FDFF:FFFF:FFFF:FF80-FDFF:FFFF:FFFF:FFFF | Reserved Subnet Anycast |
   |                                         |    Addresses[RFC2526]   |
   +-----------------------------------------+-------------------------+

                       Table 1: Current Assignments

   It is possible that implementations might predate a specific
   assignment from this registry and hence not be cognizant of the
   reserved nature of the interface identifier.  Hence, future
   assignments from this registry are discouraged.  Future assignments,
   if any, are to be made through Standards Action [RFC5226].
   Assignments consist of a single interface identifier or a range of
   interface identifiers.

   NOTE: The address :: (all zeros in the interface identifier field) is
   used as the unspecified address and ::/0 is used as a default route
   indicator, as specified in [RFC5156].  These uses do not conflict
   with the reserved interface identifiers defined here, since the
   reserved identifiers defined in this document are used for avoiding
   conflicts with stateless address autoconfiguration that utilizes a
   64-bit prefix length.

4.  Acknowledgements

   The author would like to thank Alain Durand, Alex Petrescu, Bernie
   Volz, Bob Hinden, Christian Huitema, Fred Templin, Jordi Palet
   Martinez, Pekka Savola, Remi Denis-Courmount, Tim Enos, Ed
   Jankiewicz, Brian Carpenter, Alfred Hoenes, Jari Arkko, Pasi Eronen,
   Tim Polk, Lars Eggert, Derek Atkins, and Robert Sparks for reviewing
   this document and suggesting changes.

5.  Security Considerations

   By utilizing one of the reserved interface identifiers, an IPv6 node
   might receive requests that it is not authorized to receive.
   Information that creates or updates a registration in this registry
   needs to be authenticated and authorized by the IANA based on the
   instructions set forth by [RFC5226].

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
              Addresses", RFC 2526, March 1999.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

6.2.  Informative References

   [HBA]      Bagnulo, M., "Hash Based Addresses (HBA)", Work in
              Progress, October 2006.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.

   [RFC5156]  Blanchet, M., "Special-Use IPv6 Addresses", RFC 5156,
              April 2008.

Appendix A.  List of Potentially Affected RFCs

   Implementations of the following RFCs need to be aware of the
   reserved interface identifier ranges when they allocate new
   addresses.  Future revisions of these RFCs should ensure that this is
   either already sufficiently clear or that the text is amended to take
   this into account.

   o  RFC 2590 - Transmission of IPv6 Packets over Frame Relay Networks
      Specification

   o  RFC 3315 - Dynamic Host Configuration Protocol for IPv6 (DHCPv6)

   o  RFC 3972 - Cryptographically Generated Addresses (CGA)

   o  RFC 4489 - A Method for Generating Link-Scoped IPv6 Multicast
      Addresses

   o  RFC 4862 - IPv6 Stateless Address Autoconfiguration

   o  RFC 4941 - Privacy Extensions for Stateless Address
      Autoconfiguration in IPv6

   o  RFC 4982 - Support for Multiple Hash Algorithms in
      Cryptographically Generated Addresses (CGAs)

   o  RFC 5072 - IP Version 6 over PPP

Author's Address

   Suresh Krishnan
   Ericsson
   8400 Decarie Blvd.
   Town of Mount Royal, QC
   Canada

   Phone: +1 514 345 7900 x42871
   EMail: suresh.krishnan@ericsson.com