Significance of IPv6 Interface Identifiers
RFC 7136

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Internet Engineering Task Force (IETF)                      B. Carpenter
Request for Comments: 7136                             Univ. of Auckland
Updates: 4291                                                   S. Jiang
Category: Standards Track                   Huawei Technologies Co., Ltd
ISSN: 2070-1721                                            February 2014

               Significance of IPv6 Interface Identifiers

Abstract

   The IPv6 addressing architecture includes a unicast interface
   identifier that is used in the creation of many IPv6 addresses.
   Interface identifiers are formed by a variety of methods.  This
   document clarifies that the bits in an interface identifier have no
   meaning and that the entire identifier should be treated as an opaque
   value.  In particular, RFC 4291 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
   that those two bits are significant only in the process of deriving
   interface identifiers from an IEEE link-layer address, and it updates
   RFC 4291 accordingly.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7136.

Carpenter & Jiang            Standards Track                    [Page 1]
RFC 7136                  IPv6 IID Significance            February 2014

Copyright Notice

   Copyright (c) 2014 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
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   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.  The Role of Duplicate Address Detection . . . . . . . . . . .   6
   5.  Clarification of Specifications . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8

1.  Introduction

   IPv6 unicast addresses consist of a prefix followed by an Interface
   Identifier (IID).  The IID is supposed to be unique on the links
   reached by routing to that prefix, giving an IPv6 address that is
   unique within the applicable scope (link local or global).  According
   to the IPv6 addressing architecture [RFC4291], when a 64-bit IPv6
   unicast 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.

   Thus, the specification assumes that the normal case is to transform
   an Ethernet-style address into an IID, but, in practice, there are
   various methods of forming such an IID.

Carpenter & Jiang            Standards Track                    [Page 2]
RFC 7136                  IPv6 IID Significance            February 2014

   The Modified EUI-64 format preserves the information provided by two
   particular bits in the MAC address:

   o  The "u/l" bit in a MAC address [IEEE802] is set to 0 to indicate
      universal scope (implying uniqueness) or to 1 to indicate local
      scope (without implying uniqueness).  In an IID formed from a MAC
      address, this bit is simply known as the "u" bit and its value is
      inverted, i.e., 1 for universal scope and 0 for local scope.