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Versions: 00 01

Individual Submission                                          G. Huston
Internet-Draft                                                   Telstra
Expires: February 26, 2004                               August 28, 2003

 Commentary on  Distribution Mechanisms for Unique Local  IPv6 Unicast

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at http://

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   This Internet-Draft will expire on February 26, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   This memorandum examines the characteristics of Unique Local IPv6
   Unicast addresses, as well as the  requirements for address
   distribution mechanisms for this class of addresses. It is intended
   as a commentary on an Internet Draft currently under consideration in
   the IPv6 Working Group of the IETF.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Characteristics of Local Use Addresses . . . . . . . . . . . .  3
   3.  Locally Assigned Global IDs  . . . . . . . . . . . . . . . . .  5
   4.  Centrally Assigned Global IDs  . . . . . . . . . . . . . . . .  5
   5.  Local Use Address Distribution Mechanisms  . . . . . . . . . .  7
   5.1 Allocation Fees  . . . . . . . . . . . . . . . . . . . . . . .  7
   5.2 Allocation Period  . . . . . . . . . . . . . . . . . . . . . .  8
   5.3 Choice in Service Models . . . . . . . . . . . . . . . . . . .  8
   5.4 Recording Allocations  . . . . . . . . . . . . . . . . . . . .  9
   5.5 Reverse Mapping Local Use Addresses in ip6.arpa  . . . . . . .  9
   6.  Management Requirements for Local Use Addresses  . . . . . . . 10
   7.  Distribution Mechanisms  . . . . . . . . . . . . . . . . . . . 11
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   9.  Relationship with Existing Address Distribution Mechanisms . . 12
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 15
       Intellectual Property and Copyright Statements . . . . . . . . 16

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1. Introduction

   Current work within the IETF IPv6 working includes the drafting of a
   proposal to define part of the IPv6 unicast address space for local
   use. This is currently IETF work in progress being considered by the
   IPv6 Working Group, documented in an Internet draft,
   "draft-ietf-ipv6-unique-local-addr-00.txt" [1]. These addresses are
   intended for various forms of local communications and are not
   expected to be routable on the global Internet. The proposal refers
   to such addresses as "Unique Local IPv6 Unicast Addresses".

   There are a number of characteristics of such addresses that have
   been proposed in order to ensure that they can fulfill the role of a
   local-use address, and there are also a number of considerations
   relating to the distribution mechanisms for these addresses that
   distinguish them from globally routable unicast addresses. This
   document explores these intended characteristics in further detail as
   well as the associated distribution mechanisms.

2. Characteristics of Local Use Addresses

   The characteristics listed in the draft proposal for such addresses

      1.  Globally unique prefix.

      2.  Well known prefix to allow for easy filtering at site

      3.  Allows sites to be combined or privately interconnected
          without creating any address conflicts or require renumbering
          of interfaces using these prefixes.

      4.  Internet Service Provider independent and can be used for
          communications inside of a site without having any permanent
          or intermittent Internet connectivity.

      5.  If accidentally leaked outside of a site via routing or DNS,
          there is no conflict with any other addresses.

      6.  In practice, applications may treat these address like global
          scoped addresses.

   It could be argued that, strictly, the third and fifth
   characteristics are a  consequence of the first, as they can be all
   grouped under the overall characteristic of "use of a common unique

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   prefix". The second, forth and sixth characteristics commonly refer
   to unique use of a local address block drawn from the global unicast
   address pool.

   Restating this list of characteristics gives:

      1. Exclusive use of a common prefix drawn from the global unicast
      address space for all local use addresses.

      2. Unique assignment of local use address blocks from within the
      pool of addresses defined by this prefix.

   Section 3.1 of the Internet Draft proposal further refines the set of
   characteristics, by describing the address as a four part object:

         | 7 bits |  41 bits   |  16 bits  |          64 bits            |
         | prefix | global ID  | subnet ID |        interface ID         |


      prefix:   prefix to identify Local IPv6 unicast addresses.

      global ID:  global identifier used to create a globally unique

      subnet ID:  16-bit subnet ID is an identifier of a subnet within
      the site.

      interface ID:  64-bit Interface ID.

   The length of the prefix + global ID part is 48 bits in length,
   allowing 16 bits for local assignation of subnet IDs and 64 bits for
   the interface ID. This allows for 2,199,023,255,552 assignable local
   use address blocks.

   There is a further characteristic of the address block defined in
   this section of the draft, namely:

      3. There is no internal structure within the global ID, and these
      global IDs cannot be aggregated in a routing context.

   The proposal splits this address pool into two halves: locally and
   centrally assigned prefixes. These will be considered in the
   following sections.

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3. Locally Assigned Global IDs

   One half of the Local Use address space, using the common prefix
   FD00::/8, is described as being "locally assigned". The proposal
   indicates that such locally assigned global IDs must be generated
   with a pseudo-random algorithm. The proposal notes that there is a
   very low  probability that the prefix will conflict with another
   locally generated prefix (section 11.2 of the draft proposal).
   Analysis of the probability involved here indicates that the
   probability of a collision in the space using a random draw function
   exceeds 0.5  after 1.24 million random draws. The general solution of
   the probability of a collision after d draws from n possible values
   is given by:

      P = 1 - ((n!) / ((n**d)((n-d)!)))

   Given that the value for n is 2.199,023,255,552, then the objective
   is to find the lowest value of d for which P is greater than or equal
   to 0.5. In this case the value for d  is some 1.24 million. This
   value is likely to be too small a value for any assured level of
   uniqueness, particularly if there is some consideration that no
   address conflicts would arise as a result of private interconnection.
   While the draft proposal asserts that collisions of locally assigned
   Global IDs "can be ignored for all practical purposes" (section
   11.2), the actual probability of a clash is one where there will a
   probable clash after 1.24 mission random draws. If this approach is
   used on a widespread basis then the risk of clashing Global IDs is
   far greater that the "theoretical" risk described in the proposal.
   Some further consideration should be given to this part of the

   It is observed  that this 'random draw' is an inadequate response to
   item 2 of the required characteristics for Local Use addresses. A
   probability of uniqueness is tangibly different to the property of
   assured uniqueness. If assurred uniqueness is an essential
   characteristic of all elements of this address space, then it is
   necessary to drop the random self-selection mechanism from the draft
   proposal, and that all Local-Use addresses be distributed in such a
   manner that uniqueness is assured in every case.

4. Centrally Assigned Global IDs

   The other half of the local use space is proposed in the draft to be
   "centrally assigned" using fixed size /48 blocks. This refines the
   second characteristic to read:

      2. Unique assignment of fixed size local use address blocks from
      within the pool of addresses defined by this prefix, using a

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      Global ID as the block prefix.

   The proposal notes that these assignments can be escrowed to resolve
   any disputes regarding duplicate assignments. It is noted that escrow
   is a specific solution to a more general characteristic, and the
   desired characteristic being defined here is:

      4. The assignment information must be recorded and stored in a
      reliable manner.

   The assignment function is described in the proposal as one that
   treats sequential allocations in a random fashion, and explicitly
   notes that they should not be assigned accordingly to any particular
   structure, and therefore they cannot be aggregated in a routing

      5.        Local Use Addresses are not intended to be passed within the
      global routing environment

   The complete list of characteristics of this Centrally Assigned Local
   Use IPv6 Unicast address space is:

      1.  Exclusive use of a common prefix drawn from the global unicast
          address space for all local use addresses.

      2.  Unique assignment of fixed size local use address blocks from
          within the pool of addresses defined by this prefix, using a
          Global ID as the block prefix.

      3.  There is no internal structure within the global ID, and these
          global IDs cannot be aggregated in a routing context.

      4.  The assignment information must be recorded and stored in a
          reliable manner.

      5.  Local Use Addresses are not intended to be passed within the
          global routing environment

   The potential for use of this address in end-to-end solutions
   relating to multi-homing is limited to the extent that this identity
   space is unstructured, so it cannot be used as a lookup key in any
   mapping system that maps identities into locators. If the intended
   use is through a sequence of mappings from domain name to identifier
   to current locator, then the last mapping (from identifier to
   locator) is not feasible in an unstructured identifier space. In this
   sense the role of such an address is limited to an assertion of a

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   fixed, globally unique label that can be used in conjunction with
   dynamic change of location-based address to provide some form of
   transport session resiliency in a multi-homed environment.

5. Local Use Address Distribution Mechanisms

   The proposal notes that:

      The requirements for centrally assigned global ID allocations are:

      *  Available to anyone in an unbiased manner.

      *  Permanent with no periodic fees.

      *  One time non-refundable allocation fee in the order of 10 Euros
         per allocation.

      *  The ownership of each individual allocation should be private,
         but should be escrowed.

   The unstated implication from the first requirement is that this is
   undertaken without consideration of the current or intended level of
   use of the address block, so that there are no qualifications
   regarding assignment of a Local Use Address block. The proposal also
   notes that such availability should include non-Internet access
   mechanisms as a desired additional mechanism.

   The second and third aspects of this proposed distribution mechanism
   describe the use of a one-time fee for a one-time service transaction
   that has enduring consequences.

5.1 Allocation Fees

   The first aspect here is the consideration of the allocation fee. The
   draft motivates this payment as a means of prevention of hoarding of
   blocks from within this pool by imposing a financial impost. While
   there are many forms of control over a distribution mechanism to
   prevent distortions such as hoarding, this pricing approach is seen
   as a lightweight and effective mechanism that has the potential to
   address the identified problem. However, there are some consequences
   of this aspect of the draft proposal that should be examined in
   further detail. The imposition of a charge without relation to
   service cost is seen in many regulatory regimes as an imposition that
   is likened to a monopoly rental or a form of taxation. Such forms of
   charges have no valid role, and should be avoided. It is more
   reasonable to allow the operator(s) of this distribution mechanism to
   be able to account for their costs in operating this service, and
   allow the operator to determine a service fee that is based on these

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   The operator needs to consider that if this is to be a one-time fee
   for an unbounded service (so called 'cemetery plot' fees), the fee
   should cover both the processing component and the subsequent record
   maintenance component of the service.

5.2 Allocation Period

   The proposal explicitly indicates that the allocation should be
   'permanent'. This implies that there is no concept of return of a
   Local Use prefix once it has been allocated from the central
   registry, and that there is no concept of a registry-recorded
   transfer of an allocation. The implication of this service model is
   that there is no form of reuse of blocks from this address space. The
   implicit assumption here is that for the entire useful lifetime of
   the technology, under all conceivable allocation demand scenarios,
   that there will be adequate available address space to continue to
   meet demand from the Local Use address pool. Without any form of
   periodic renewal or similar opportunity to alter the terms of use of
   this address space then, if exhaustion of the space is considered to
   be a potential risk, the observations made in 1994 regarding the
   possible outcomes of the  (then) IPv4 address allocation practices
   are once more relevant here:

      "It is perhaps a sad reflection of the conflict of short term
      objectives and longer term considerations that the evident short
      term motivations of ready and equitable access to the IPv4 address
      (which were the motivational factors in determining the current
      Internet address allocation policies) run the consequent risk of
      monopoly- based restrictive trade and barrier-based pricing as a
      longer term outcome of unallocated address space exhaustion." [2]

   Of course if there is a high degree of confidence that exhaustion of
   the Local Use address pool is not a remotely possible eventuality,
   then such address prefixes can be considered in the same terms as a
   single-use disposable facility, and these considerations are not
   directly relevant.

5.3 Choice in Service Models

   It is possible that clients of this allocation service want the
   choice between a single one-time permanent allocation (and a one-time
   service fee) and a defined period renewable service, where, at the
   end of the defined period the client has the choice of renewing the
   allocation or allowing it to lapse back to the pool. Given the
   central nature of the described distribution mechanism, allowing the
   client some choice in the form of service, rather than imposing a

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   single service model is seen as a reasonable measure.

   The model also proposes a single layer of distribution, where end
   clients interact with a proposed single central registry. Again this
   is an area where a different structure used for the distribution of
   many other forms of goods or resources, typically using some form of
   hierarchy in distribution with wholesale and retail roles. Such
   hierarchies often allow for a more efficient form of overall
   distribution than a single entity attempting to service a global
   consumer base. Current regulatory environments also look to
   competition as a means of ensuring that service regimes operate
   efficiently and that no single player can distort the price of the
   service through the imposition of monopoly rentals, artificial
   scarcity or selective servicing.

5.4 Recording Allocations

   The proposal indicates that information relating to the 'ownership'
   of each individual allocation be private. This is not an easily
   achieved outcome, given that 'ownership' is a public claim to the
   unique ability to access and exploit the resource. Furthermore, this
   implies that the resource itself is a form of property, and that
   property can be traded, swapped or otherwise disposed of at the
   discretion of the owner, inferring that the address block, is in some
   form, an asset of the holder. It is unclear that this interpretation
   of the status of an address is the actual intent of the proponents of
   this approach, and that other forms of expression of unique and
   enduring interest in the address resource may be more appropriate for
   this resource. This observation is made in the context of the
   characterization of the larger protocol address space as a public
   good that is distinguished from concepts of ownership or the
   inferring of aspects of property and asset into this resource.

5.5 Reverse Mapping Local Use Addresses in ip6.arpa

   It is unclear from the proposal whether Local Use Addresses could or
   should be entered into the ip6.arpa reverse mapping domain space. as
   a delegated domain.

   Locally assigned prefixes cannot be entered into this domain space
   because of the lack of a condition of assured uniqueness.

   The situation with respect to centrally assigned prefixes is not so
   clear. The considerations include:

   o  The potential size of the domain zone. Because of the lack of any
      structure beyond the 8th bit of the prefix, there is no ability to
      impose a hierarchy of zone files, and the reverse zone would need

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      to list all assigned local use prefixes and their delegation
      points. There are obvious implications in terms of the potential
      size of this zone file. and some consideration as to the
      efficiency of operation of a zone of such a potential size.

   o  The desired characteristic of Local Use prefixes where the
      "ownership" of the prefix is not public information. If the domain
      zone operator was distinct from the central registry operator,
      then the privacy of the address allocation information could
      preclude the domain operator from validating a delegation request
      for a Local Use address block.

   o  The potential use of these addresses in some classes of end-point
      identification may imply the need for an external entity, using
      the global DNS to be map from the local use identifier to a global
      use  address, and one way to perform this mapping in the DNS is to
      use the reverse domain to map from the end point local use address
      to a global DNS name, and then map forward from this name to a
      global address. Precluding local use addresses from the global DNS
      would preclude this form of mapping.

   For local use, a so called "two-faced" DNS can be configured to
   provide a local reverse mapping service for the local site.

6. Management Requirements for Local Use Addresses

   In summary, the characteristics of the management of this space is

   1.   Every applicant may obtain an address block in this prefix space
        without providing any form of justification to the registry

   2.   Every assigned Local-Use block is of the same size, namely a /

   3.   Each block is uniquely assigned to the applicant.

   4.   Each assignment is a randomly selected block from the entire
        remaining pool.

   5.   Each applicant may obtain an enduring assignment without further
        need to contact the registry or to pay further service fees
        (one-off service).

   6.   Any service fee, if used, should be high enough to make massive
        seizure financially undesirable, yet low enough to make it
        readily accessible to individuals as well as corporate entities

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        on a global scale.

   7.   Any service fee, if used, should be clearly attributable to the
        costs associated with the provision of the service function for
        the lifetime of the provided service.

   8.   The service model is not restricted to a one-off assignment
        model, with the proviso that any other associated service models
        must have similar attributes of ease of accessibility.

   9.   The association of the assigned space and the identity of the
        applicant is not to be made public.

   10.  The assignment information is to be held in a way that is
        reliable and enduring.

7. Distribution Mechanisms

   Under the current arrangements, IANA is the IETF-selected registry
   for IPv4 global unicast and IPv6 global unicast address space, and
   the RIRs undertake the associated distribution function, using
   policies that have been developed by an open process within each

   A complete consideration of the various regulatory and logistical
   considerations is considered to be well beyond the appropriate scope
   of the Internet Engineering Task Force to undertake within the
   defined scope and mission, and a more general statement of intent
   would be more fitting in this context.

   An enumeration of the desired attributes of a distribution system is:

      The adopted distribution mechanism should be:

      *  efficient,

      *  fair,

      *  generally accessible and imposing no barrier to access,

      *  undertaken in a manner that preserves the desired
         characteristics of the Local Use address space,

      *  one that uses a fee structure that fairly reflects the costs of
         efficient service delivery mechanisms,

      *  one that allows a choice of service models where feasible,

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      *  one that prevents distortions of the distribution function
         through behaviours such as hoarding or selective reselling,

      *  one that does not place the operator(s) in contravention to
         various regulatory frameworks, and

      *  attuned to the long-term stable use of specific instances of
         this resource by consumers

8. IANA Considerations

   The Local Use Address draft proposes that:

      The IANA is instructed to allocate the FC00::/7 prefix for Unique
      Local IPv6 unicast addresses.

      The IANA is instructed to delegate, within a reasonable time, the
      prefix FC00::/8 to an allocation authority for Unique Local IPv6
      Unicast prefixes of length /48.  This allocation authority shall
      comply with the requirements described in section 3.2 of this
      document, including in particular the charging of a modest
      one-time fee, with any profit being used for the public good in
      connection with the Internet.

   It is noted that there are significant problems with this proposed
   approach to directions to IANA, particularly with the noted concept
   that this is a for-profit activity and IANA is, in effect, being
   directed to be in the position of selecting a global monopoly
   operator. The indeterminate nature of a fair, open and reasonable
   definition  of "the public good" is also a problem in the context of
   these instructions to IANA. Some of the lessons learned from DNS
   administration over the past decade would indicate that this is not a
   sensible  directive to pass to IANA, as it is unlikely to be
   reasonably implemented in this precise  form.

9. Relationship with Existing Address Distribution Mechanisms

   The Local Use proposal's desire to operate the address space without
   any form of discernable structure by having all block assignments be
   drawn from a random selection from across the entire managed space
   precludes the reuse of the current distribution mechanism of an IANA
   allocation to each of the RIRs to service their particular region. In
   the context of assuming that the RIRs undertake this function, the
   proposed mechanism would see FD00::/8 allocated to the RIRs and
   managed via a single registry maintained by the RIRs working
   together. Each RIR would lodge a "draw request" for a block from this
   registry in response to individual customer requests, and the

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   registry would respond with the selected block, using a random draw

   The potential areas of difference between the current RIR practice
   and the requirements here are:

   o  the absence of any form of justification for the allocation,

   o  a fixed size of allocation,

   o  the potential to make extensive use of automated mechanisms in the
      registry allocation function

   o  public reporting of allocations from this space only in summary
      form (no detailed reports, such as currently published via Whois

   o  consideration of adoption of a service model or models relating to
      the terms of the assignment.

   o  consideration of various forms of renewable allocations and the
      issue of whether permanent allocations are suitable for this
      intended role.

   o  determining a fee schedule where the registry service is operated
      in a manner that is cost neutral to the membership.

   o  adoption of a transaction-based fee-for service model (as distinct
      from a membership service model)

   o  specific consideration relating to long term reliable storage of
      individual allocation information

   In this context, if the RIRs were to develop this as a supported
   process, then the areas of RIR liaison with the IETF would appear to
   be in understanding the role of coordinated RIR policies in this
   area, and the role of the IETF. As an example, the nomination of a
   fee schedule and a service model in the draft proposal would normally
   seen as prescribing  matters that would normally be determined by the
   RIRs through the adoption of policy proposals rather than a matter
   for the IETF to determine, while the consideration of permanent
   allocations would be a matter that would entail some substantive
   consideration by the IETF.

   On a purely pragmatic level there is no practical way that the IETF
   or the adopted distribution mechanism can totally prevent these
   address prefixes from leaking into the IPv6 global routing space.
   What is, or is not, carried in the routing space is largely a matter

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   of convention from within the operator community. If the decision is
   taken not to publish the details of individual Local Use unique
   allocations, then this would be a factor in determining whether or
   not blocks drawn from this space may be carried in the global routing
   system, but it would not absolutely prevent such use.

   The service model is again a relatively challenging concept. The
   original IPv4 address allocation system worked on a similar basis of
   enduring allocations, and this has proved to be problematic in terms
   of recovery of unused space in more recent times. While the draft
   proposal is explicit about attempting to prevent short term
   distortions such as hoarding, there is little doubt that any form of
   finite unmanaged resource will be placed under consumption pressures
   eventually. Attempting to set a global price that makes the resource
   generally accessible, while still attempting to make the price a
   deterrent to hoarding is not a completely reasonable exercise in
   global terms. What would be regarded as a trivially small fee within
   some economies would be seen as a prohibitively expensive price in
   other economies. More worryingly, the concept of an enduring
   assignment is that there is no opportunity to make any form of
   correction in later times to the extant assignments, and, as in IPv4,
   there is the distinct risk of giving early adopters a long term
   advantage that may not be enjoyed by later players who may be working
   under more restrictive allocation polices. A shorter term lease
   arrangement (such as 2 - 5 years) allows for regular renewal of the
   relationship with the registrar, allowing for assignment information
   to be updated to reflect the current state of the assignee, but would
   entail greater levels of registry activity. As this entire operation
   is intended to be sufficiently low in cost that it is generally
   accessible, and that the value here is not in routeable address
   space, but in the attribute of assurred uniqueness for the address
   space, the consideration of the level of registry activity is a
   critical one. It may be that the  distribution mechanism adopts both
   service models, allowing an enduring application to be undertaken at
   any time at one fee level, and a shorter identity-validated
   application and renewal to be undertaken on a biannual basis at a
   lower fee, This is obviously a matter for further consideration.

10. Security Considerations

   The considerations listed in the draft proposal are:

      Local IPv6 addresses do not provide any inherent security to the
      nodes that use them.  They may be used with filters at site
      boundaries to keep Local IPv6 traffic inside of the site, but this
      is no more or less secure than filtering any other type of global
      IPv6 unicast addresses.

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      Local IPv6 addresses do allow for address-based security
      mechanisms, including IPSEC, across end to end VPN connections.

   It is noted that in the latter case, where end to end VPN connections
   are being used, across local use address blocks there is a strong
   requirement for uniqueness of the Local Use address prefix.

11. Acknowledgements

   The author acknowledges the helpful comments of Alain Durand, Paul
   Wilson, Anne Lord and George Michaelson  in preparing this memo.


   [1]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
        Addresses", Internet-Drafts
        draft-ietf-ipv6-unique-local-addr-00.txt, August 2003, <http://

   [2]  Huston, G., "Observations on the Management of the Internet
        Address Space", RFC 1744, December 1994.

Author's Address

   Geoff Huston

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