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The U and G bits in IPv6 Interface Identifiers

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Document Type
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Author Sheng Jiang
Last updated 2013-02-01
Replaced by draft-ietf-6man-ug, draft-ietf-6man-ug, RFC 7136
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IESG IESG state I-D Exists
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6MAN                                                        B. Carpenter
Internet-Draft                                         Univ. of Auckland
Intended status: Informational                                  S. Jiang
Expires: August 4, 2013                     Huawei Technologies Co., Ltd
                                                        January 31, 2013

             The U and G bits in IPv6 Interface Identifiers


   The IPv6 addressing architecture defines a method by which the
   Universal and Group bits of an IEEE link-layer address are mapped
   into an IPv6 unicast interface identifier.  This document clarifies
   the status of those bits for interface identifiers that are not
   derived from an IEEE link-layer address.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 4, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Problem statement . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Proposed solution . . . . . . . . . . . . . . . . . . . . . . . 5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Change log [RFC Editor: Please remove]  . . . . . . . . . . . . 6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7

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1.  Introduction

   NOTE IN DRAFT: This version is in the form of a discussion document,
   but the proposal included uses normative language.  If the WG wishes
   to proceed with it, the authors suggest to make it a formal
   standards-track update of the IPv6 addressing architecture.

   According to the IPv6 addressing architecture [RFC4291], when an IPv6
   unicast Interface Identifier (IID) is formed on the basis of an IEEE
   EUI-64 address, usually itself expanded from a 48-bit MAC address, a
   particular format must be used:

     "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."

   The specification assumes that that the normal case is to transform
   an Ethernet-style address into an IID, preserving the semantics of
   two bits in particular:

   o  The "u" bit in an IEEE address is set to 0 to indicate universal
      scope (implying uniqueness) or to 1 to indicate local scope
      (without implying uniqueness).  In an IID this bit is inverted,
      i.e., 1 for universal scope and 0 for local scope.  According to
      [RFC5342], the reason for this was "to make it easier for network
      operators to type in local-scope identifiers".
   o  The "g" bit in an IEEE address is set to 1 to indicate group
      addressing (link-layer multicast).  This value is supposed to be
      preserved in an IID.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Problem statement

   Various new forms of IID have been defined or proposed, such as
   temporary addresses [RFC4941], Cryptographically Generated Addresses
   (CGAs) [RFC3972], stable privacy addresses
   [I-D.ietf-6man-stable-privacy-addresses], or mapped addresses
   [I-D.ietf-softwire-4rd].  In each case, the question of how to set
   and interpret the "u" and "g" bits has been debated.  For example,
   RFC 3972 specifies that they are zero in CGAs.

   NOTE IN DRAFT: Are there other examples we should include?  Are we
   sure that no IID format defines semantics for u/g?

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   The question underlying these repeated debates is: do these bits have
   any usefulness as currently defined?  Section 2.2.1 of RFC 5342
   discusses the mechanics of the bit allocations but does not explain
   the purpose or value of these bits in an IID.  There is an IANA
   registry for reserved IID values [RFC5453] but again there is no
   explanation of the purpose of the "u" and "g" bits.

   Another case where the "u" and "g" bits are specified is in the
   Reserved IPv6 Subnet Anycast Address format [RFC2526], which states
   that "for interface identifiers in EUI-64 format, the universal/local
   bit in the interface identifier MUST be set to 0" (i.e., local) and
   requires that "g" bit to be set to 1.  However, the text neither
   states nor implies any semantics for these bits in anycast addresses.

   There was a presumption when IPv6 was designed and the IID format was
   first specified that a universally unique IID might prove to be very
   useful, for example to contribute to solving the multihoming problem.
   Indeed, the addressing architecture [RFC4291] states this explicitly:

  "The use of the universal/local bit in the Modified EUI-64 format
   identifier is to allow development of future technology that can take
   advantage of interface identifiers with universal scope."

   However, this has not so far proved to be the case.  Also, there is
   evidence from the field that IEEE MAC addresses with "u" = 0 are
   sometime incorrectly assigned to multiple MAC interfaces.  Once
   transformed into IID format (with "u" = 1) these identifiers would
   purport to be universally unique but would in fact be ambiguous.
   Also, ILNP, the currently specified multihoming solution that might
   be expected to benefit from universally unique IIDs in modified
   EUI-64 format does not in fact rely on them; it uses its own format,
   defined as a Node Identifier [RFC6741].  We can conclude that the "u"
   bit in IIDs has no semantic value.  In the case of an IID created
   from a MAC address according to RFC 4941, its value is determined by
   the MAC address, but that is all.

   The "g" bit in an IID has no meaning in IPv6.  If an IID is for some
   reason created from a MAC group address, the bit will be set, but
   that is all.  Both the "u" and the "g" bit are meaningless in the
   format of an IPv6 multicast group ID [RFC3306], [RFC3307].

   The problem caused by the above is the confusion and distraction
   caused each time that a new form of IID is proposed.  Since the bits
   concerned appear to have no useful semantics, this is wasteful.

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3.  Proposed solution

   It should be noted that IIDs known or guessed to have been created
   according to RFC 4941 could be transformed back into MAC addresses,
   for example during fault diagnosis.  For that reason, keeping the "u"
   and "g" bits in the IID has operational value.  Therefore, the EUI-64
   to IPv6 IID transformation defined in RFC 4941 MUST be used for all
   cases where an IID is derived from a MAC address.

   However, for all forms of IID that are not derived from an EUI-64 MAC
   address (or an equivalent form of link-layer address), it is not
   required to set the "u" and "g" bits in any particular way.  These
   bits have no semantics in an IID.  Specifications of other forms of
   IID MUST specify how they should be set, without defining any
   semantics for them.

   The statement about future technology quoted above from RFC 4941 is

   As far as is known, no existing implementation will be affected by
   these changes.  The benefit is that future design discussions are

4.  Security Considerations

   No new security exposures or issues are raised by this document.

5.  IANA Considerations

   This document requests no immediate action by IANA.  However, in
   considering future proposed additions to the registry of reserved IID
   values [RFC5453], no special consideration is needed of the "u" and
   "g" bits, since they have no special meaning.

6.  Acknowledgements

   Valuable comments were received from ... and other participants in
   the 6MAN working group.

   Brian Carpenter was a visitor at the Computer Laboratory, Cambridge
   University during part of this work.

   This document was produced using the xml2rfc tool [RFC2629].

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7.  Change log [RFC Editor: Please remove]

   draft-carpenter-6man-ug-00: original version, 2013-01-31.

8.  References

8.1.  Normative References

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

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

   [RFC5342]  Eastlake, D., "IANA Considerations and IETF Protocol Usage
              for IEEE 802 Parameters", BCP 141, RFC 5342,
              September 2008.

   [RFC5453]  Krishnan, S., "Reserved IPv6 Interface Identifiers",
              RFC 5453, February 2009.

8.2.  Informative References

              Gont, F., "A method for Generating Stable Privacy-Enhanced
              Addresses with IPv6 Stateless Address Autoconfiguration
              (SLAAC)", draft-ietf-6man-stable-privacy-addresses-03
              (work in progress), January 2013.

              Jiang, S., Despres, R., Penno, R., Lee, Y., Chen, G., and
              M. Chen, "IPv4 Residual Deployment via IPv6 - a Stateless
              Solution (4rd)", draft-ietf-softwire-4rd-04 (work in
              progress), October 2012.

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

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [RFC3306]  Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
              Multicast Addresses", RFC 3306, August 2002.

   [RFC3307]  Haberman, B., "Allocation Guidelines for IPv6 Multicast
              Addresses", RFC 3307, August 2002.

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   [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.

   [RFC6741]  Atkinson,, RJ., "Identifier-Locator Network Protocol
              (ILNP) Engineering Considerations", RFC 6741,
              November 2012.

Authors' Addresses

   Brian Carpenter
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland,   1142
   New Zealand


   Sheng Jiang
   Huawei Technologies Co., Ltd
   Q14, Huawei Campus
   No.156 Beiqing Road
   Hai-Dian District, Beijing  100095
   P.R. China


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