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The U and G bits in IPv6 Interface Identifiers

The information below is for an old version of the document.
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This is an older version of an Internet-Draft that was ultimately published as RFC 7136.
Author Sheng Jiang
Last updated 2013-03-29
Replaces draft-carpenter-6man-ug
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6MAN                                                     B. E. Carpenter
Internet-Draft                                         Univ. of Auckland
Updates: 4291 (if approved)                                     S. Jiang
Intended status: Standards Track            Huawei Technologies Co., Ltd
Expires: September 28, 2013                               March 28, 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, and updates RFC 4291

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 September 28, 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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Problem statement . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Usefulness of the U and G Bits  . . . . . . . . . . . . . . .   5
   4.  Clarification of Specifications . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Change log [RFC Editor: Please remove]  . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   According to the IPv6 addressing architecture [RFC4291], when a
   64-bit 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 information
   provided by 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".

      In an IID, this bit is in position 6, i.e., position 70 in the
      complete IPv6 address.

   o  The "g" bit in an IEEE address is set to 1 to indicate group
      addressing (link-layer multicast).  The value of this bit is
      preserved in an IID.

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      In an IID, this bit is in position 7, i.e., position 71 in the
      complete IPv6 address.

   This document discusses problems observed with the "u" and "g" bits
   as a result of the above requirements.  It then discusses the
   usefulness of the two bits, and updates RFC 4291 accordingly.

1.1.  Terminology

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

2.  Problem statement

   In addition to IIDs formed from IEEE EUI-64 addresses, various new
   forms of IID have been defined or proposed, such as temporary
   addresses [RFC4941], Cryptographically Generated Addresses (CGAs)
   [RFC3972], Hash-Based Addresses (HBAs) [RFC5535], stable privacy
   addresses [I-D.ietf-6man-stable-privacy-addresses], or mapped
   addresses for 4rd [I-D.ietf-softwire-4rd].  In each case, the
   question of how to set the "u" and "g" bits has to be decided.  For
   example, RFC 3972 specifies that they are both zero in CGAs, and the
   same applies to HBAs.  On the other hand, RFC 4941 specifies that "u"
   must be zero but leaves "g" variable.  The NAT64 addressing format
   [RFC6052] sets the whole byte containing "u" and "g" to zero.

   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.

   A common operational practice for well-known servers is to manually
   assign a small number as the IID, in which case "u" and "g" are both

   These cases illustrate that the statement quoted above from RFC 4291
   requiring "Modified EUI-64 format" is rather meaningless when applied
   to forms of IID that are not in fact based on an underlying EUI-64
   address.  In practice, the IETF has chosen to assign some 64-bit IIDs
   that have nothing to do with EUI-64.

   A particular case is that of /127 prefixes for point-to-point links
   between routers, as standardised by [RFC6164].  The addresses on
   these links are undoubtedly global unicast addresses, but they do not
   have a 64-bit IID.  The bits in the positions named "u" and "g" in

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   such an IID have no special significance and their values are not

   Each time a new IID format is proposed, the question arises whether
   these bits have any meaning.  Section 2.2.1 of RFC 5342 discusses the
   mechanics of the bit allocations but does not explain the purpose or
   usefulness 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.

   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.  Firstly,
   there are recurrent reports of manufacturers assigning the same MAC
   address to multiple devices.  Secondly, significant re-use of the
   same virtual MAC address is reported in virtual machine environments.
   Once transformed into IID format (with "u" = 1) these identifiers
   would purport to be universally unique but would in fact be
   ambiguous.  This has no known harmful effect as long as the
   replicated MAC addresses and IIDs are used on different layer 2
   links.  If they are used on the same link, of course there will be a
   problem, to be detected by duplicate address detection [RFC4862], but
   such a problem can usually only be resolved by human intervention.

   The conclusion from this is that the "u" bit is not a reliable
   indicator of universal uniqueness.

   We note that Identifier-Locator Network Protocol (ILNP), a
   multihoming solution that might be expected to benefit from
   universally unique IIDs in modified EUI-64 format, does not in fact
   rely on them.  ILNP uses its own format, defined as a Node Identifier
   [RFC6741].  ILNP has the constraint that a given Node Identifier must
   be unique within the context of a given Locator (i.e.  within a
   single given IPv6 subnetwork).  As we have just shown, the state of
   the "u" bit does not in any way guarantee such uniqueness, but
   duplicate address detection is available.

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   Thus, we can conclude that the value of the "u" bit in IIDs has no
   particular meaning.  In the case of an IID created from a MAC address
   according to RFC 4291, its value is determined by the MAC address,
   but that is all.

   An IPv6 IID should not be created from a MAC group address, so the
   "g" bit will normally be zero, but this value also has no particular
   meaning.  Additionally, the "u" and the "g" bits are both meaningless
   in the format of an IPv6 multicast group ID [RFC3306], [RFC3307].

   None of the above implies that there is a problem with using the "u"
   and "g" bits in MAC addresses as part of the process of generating
   IIDs from MAC addresses, or with specifying their values in other
   methods of generating IIDs.  What it does imply is that, after an IID
   is generated by any method, no reliable deductions can be made from
   the state of the "u" and "g" bits; in other words, these bits have no
   useful semantics in an IID.

   Once this is recognised, we can avoid the problematic confusion
   caused by these bits each time that a new form of IID is proposed.

3.  Usefulness of the U and G Bits

   Given that the "u" and "g" bits do not have a reliable meaning in an
   IID, it is relevant to consider what usefulness they do have.

   If an IID is known or guessed to have been created according to RFC
   4291, it could be transformed back into a MAC address.  This can be
   very helpful during operational fault diagnosis.  For that reason,
   mapping the IEEE "u" and "g" bits into the IID has operational
   usefulness.  However, it should be stressed that "u" = "g" = 0 does
   not prove that an IID was formed from a MAC address; on the contrary,
   it might equally result from another method.  With other methods,
   there is no reverse transformation available.

   To the extent that each method of IID creation specifies the values
   of the "u" and "g" bits, and that each new method is carefully
   designed in the light of its predecessors, these bits tend to reduce
   the chances of duplicate IIDs.

4.  Clarification of Specifications

   This section describes clarifications to the IPv6 specifications that
   result from the above discussion.  Their aim is to reduce confusion
   while retaining the useful aspects of the "u" and "g" bits in IIDs.

   The EUI-64 to IID transformation defined in the IPv6 addressing
   architecture [RFC4291] MUST be used for all cases where an IPv6 IID

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   is derived from an IEEE MAC or EUI-64 address.  With any other form
   of link layer address, an equivalent transformation SHOULD be used.

   Specifications of other forms of 64-bit IID MUST specify how all 64
   bits are set, but need not treat the "u" and "g" bits in any special
   way.  A general semantic meaning for these bits MUST NOT be defined.
   However, the method of generating IIDs for specific link types may
   define some local significance for certain bits.

   In all cases, the bits in an IID have no general semantics; in other
   words, they have opaque values.  In fact, the whole IID should be
   viewed as opaque by third parties, except possibly in the local

   The following statement in section 2.5.1 of the IPv6 addressing
   architecture [RFC4291]:

   "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."

   is replaced by:

   "For all unicast addresses, except those that start with the binary
    value 000, Interface IDs are required to be 64 bits long. If derived
    from an IEEE MAC-layer address, they must be constructed in Modified
    EUI-64 format."

   The following statement in section 2.5.1 of the IPv6 addressing
   architecture [RFC4291] is obsoleted:

   "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."

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

5.  Security Considerations

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

6.  IANA Considerations

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   This document requests no immediate action by IANA.  However, the
   following should be noted when considering future proposed additions
   to the registry of reserved IID values, which requires Standards
   Action according to [RFC5453].  A reserved IID, or a range of
   reserved IIDs, will most likely specify values for both "u" and "g",
   since they are among the high-order bits.  At the present time, none
   of the known methods of generating IIDs will generate "u" = "g" = 1.
   Reserved IIDs with "u" = "g" = 1 are therefore unlikely to collide
   with automatically generated IIDs.

7.  Acknowledgements

   Valuable comments were received from Ran Atkinson, Remi Despres,
   Fernando Gont, Brian Haberman, Joel Halpern, Ray Hunter, Mark Smith,
   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].

8.  Change log [RFC Editor: Please remove]

   draft-ietf-6man-ug-00: first WG version, text clarified, added
   possibility of link-local significance, 2013-02-28.

   draft-carpenter-6man-ug-01: numerous clarifications following WG
   comments, discussed DAD, added new section on the usefulness of the u
   /g bits, expanded IANA considerations, set intended status,

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

9.  References

9.1.  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

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

9.2.  Informative References

              Gont, F., "A method for Generating Stable Privacy-Enhanced
              Addresses with IPv6 Stateless Address Autoconfiguration
              (SLAAC)", draft-ietf-6man-stable-privacy-addresses-05
              (work in progress), March 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.T., "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.

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

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

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

   [RFC5535]  Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535, June

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              October 2010.

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   [RFC6164]  Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,
              L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-
              Router Links", RFC 6164, April 2011.

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

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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