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Network Working Group                                       B. Carpenter
Internet-Draft                                         Univ. of Auckland
Intended status: Standards Track                            June 8, 2016
Expires: December 10, 2016

                    What does 'global' mean in IPv6?


   The word 'global' is used in two different ways in various
   IPv6-related RFCs and an IANA registry.  This document describes the
   resulting problem.

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Table of Contents

   1.  Problem description . . . . . . . . . . . . . . . . . . . . .   2
   2.  Possible fixes  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Problem description

   As defined in the IPv6 Addressing Architecture
   [I-D.ietf-6man-rfc4291bis], most of the IPv6 address space is
   reserved for Global Unicast addresses.  The high order bits of such
   addresses are named 'global routing prefix'.  However, the word
   'global' is not itself defined in the context of unicast addresses.

   One subset of Global Unicast address space is defined for Unique
   Local Addresses [RFC4193].  One can quarrel with something being
   called 'global' and 'local' at the same time, but RFC 4193 is

 This document defines an IPv6 unicast address format that is globally
 unique and is intended for local communications, usually inside of a
 site.  These addresses are not expected to be routable on the global
       - Globally unique prefix (with high probability of uniqueness).
       - In practice, applications may treat these addresses like global
         scoped addresses.
 By default, the scope of these addresses is global.  That is, they
 are not limited by ambiguity like the site-local addresses defined in
 [ADDARCH].  Rather, these prefixes are globally unique, and as such,
 their applicability is greater than site-local addresses.  Their
 limitation is in the routability of the prefixes, which is limited to
 a site and any explicit routing agreements with other sites to
 propagate them...

   In summary: ULAs are defined in these standards track documents as

   However, the IANA registry for special-purpose IPv6 addresses
   ipv6-special-registry.xhtml>, and the RFC that controls it [RFC6890]
   use the following definition:

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   o    Global - A boolean value indicating whether an IP datagram whose
        destination address is drawn from the allocated special-purpose
        address block is forwardable beyond a specified administrative

   It is evident, even from the last sentence quoted above from RFC
   4193, that ULAs do not meet this definition of 'global'.  As a
   result, they are marked in the registry with Global = False.  The
   registry also assigns them the property Forwardable = True, which is
   of course valid, but the fact remains that some RFCs say that ULAs
   are global, but RFC 6890 and the registry say that they are not.

   This inconsistency has consequences.  Of course, it is always
   possible for code that manipulates IPv6 addresses to determine with
   certainty that a given address is, or is not, a ULA.  But any code
   that uses the property 'global' from the IANA registry as a decision
   criterion might be wrong.

   As an example, consider the Python 'ipaddress' module
   ipaddress.html#ipaddress.IPv4Address.is_private>, which explicitly
   cites the IANA registry.  It provides the property 'is_global' which
   tests False for ULAs.  A reader of RFC 4193 would expect True.  The
   correct test in Python (apart from an explicit match with fc00::/7)
   is (is_private and not is_link_local).

2.  Possible fixes

   1.  Do nothing.

   2.  Change the registry entry for ULAs to Global=True (and update
       text and RFC 6890 accordingly).

   3.  That, plus rename the registry column from 'Global' to 'Global

   4.  Change the registry entry for ULAs to Global=Undefined (and
       update text and RFC 6890 accordingly).

   5.  Rename the registry column from 'Global' to 'Globally reachable'
       (and update text and RFC 6890 accordingly).

   6.  That, plus add a registry column for 'Global scope'.

   7.  Your suggestion goes here.

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3.  Security Considerations

   Misclassification of a ULA as non-global might cause it to be used
   for a purpose that should be limited to link-local addresses for
   security reasons.

4.  IANA Considerations

   If any changes are made as a result of this discussion, they will
   require IANA actions.

5.  Normative References

              Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", draft-ietf-6man-rfc4291bis-02 (work in
              progress), April 2016.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,

Author's Address

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

   Email: brian.e.carpenter@gmail.com

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