INTERNET-DRAFT                                             Thomas Narten

                                                           June 16, 2005

          Issues Related to the Management of IPv6 Address Space


Status of this Memo

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   This document reviews the history of IPv6 address delegation and
   examines some of the technical implications of various allocation
   policies in terms of their impact on long-term address space
   utilization and aggregation. The purpose of this document is to
   stimulate discussion and foster improved understanding of the
   implications of various policies, so that appropriate policies are
   adopted that preserve the long-term scalability of IPv6.


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   Status of this Memo..........................................    1

   1.  Introduction.............................................    3

   2.  Definitions..............................................    3
      2.1.  Internet Registry (IR)..............................    3
      2.2.  Regional Internet Registry (RIR)....................    4
      2.3.  National Internet Registry (NIR)....................    4
      2.4.  Local Internet Registry (LIR).......................    4
      2.5.  Allocate............................................    4
      2.6.  Assign..............................................    4
      2.7.  Utilization.........................................    4
      2.8.  HD-Ratio............................................    5

   3.  Background...............................................    5
      3.1.  IPv6 Address Structure..............................    6
      3.2.  IPv6 Address Assignment History.....................    7
      3.3.  Goals of IPv6 Address Space Management..............    7
      3.4.  Utilization.........................................    8
      3.5.  Aggregation.........................................    9

   4.  How Much Address Space Do We Really Have?................   10
      4.1.  An example: Cable Modem/DSL Service in US...........   10
      4.2.  How Much Is Enough?.................................   11

   5.  On RIR->LIR->End User Assignment and Allocation Policies.   12
      5.1.  On the HD Ratio Metric..............................   12
      5.2.  On /48 Assignments to End Sites.....................   12
      5.3.  On the /64 Boundary.................................   15
      5.4.  Summing It All Up...................................   15

   6.  On Reservations and Aggregation..........................   17
      6.1.  On RIR Reservations.................................   17
      6.2.  On IANA->RIR Assignments............................   18

   7.  Security Considerations..................................   18

   8.  IANA Considerations......................................   18

   9.  Acknowledgments..........................................   18

   10.  References..............................................   18

   11.  Appendix A: HD-Ratio....................................   20

   12.  Author‚ÇÖs Address......................................   21

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1.  Introduction

   The assignment and allocation of global IPv6 unicast address space to
   ISPs and end-users is implemented under policies maintained and
   implemented by the Regional Internet Registries (RIRs) and IANA.  End
   users typically get IPv6 address space from their ISPs, who in turn
   obtain allocations from RIRs. RIRs in turn obtain address space from

   This document reviews the history of IPv6 address delegation and
   examines some of the technical implications of various allocation
   policies in terms of their impact on long-term address space
   utilization and aggregation. The purpose of this document is
   stimulate discussion and foster improved understanding of the
   implications of various policies, so that appropriate policies are
   adopted that preserve the long-term scalability of IPv6.

   Questions that this document attempts to shed light on include:

      - How large should the chunks be that IANA allocates to RIRs?

      - How much (if any) space should IANA hold in "reserve", to allow
        a RIR (or LIR) to "grow into" if/when it needs a further

      - What algorithms should RIRs use to manage their space, and at
        what point are they justified in asking IANA for more address

      - What are the long-term implications of these algorithms on
        overall IPv6 address space consumption, utilization and

2.  Definitions

   [Note: the following definitions are taken (and in some cases
   slightly modified) from the "IPv6 Address Allocation and Assignment
   Assignment Policy" document that was produced and adopted by the
   APNIC, ARIN and RIPE communities in 2002.]

2.1.  Internet Registry (IR)

   An Internet Registry (IR) is an organization that is responsible for
   distributing IP address space to its members or customers and for
   registering those distributions.  IRs are classified according to
   their primary function and territorial scope.

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2.2.  Regional Internet Registry (RIR)

   Regional Internet Registries (RIRs) are established and authorized by
   respective regional communities, and recognized by the IANA to serve
   and represent large geographical regions.  The primary role of RIRs
   is to manage and distribute public Internet address space within
   their respective regions.  Currently, there are five RIRs: AFRINIC,

2.3.  National Internet Registry (NIR)

   A National Internet Registry (NIR) primarily allocates address space
   to its members or constituents, which are generally LIRs organized at
   a national level.  NIRs exist mostly in the Asia Pacific region.

2.4.  Local Internet Registry (LIR)

   A Local Internet Registry (LIR) is an IR that primarily assigns
   address space to the users of the network services that it provides.
   LIRs are generally ISPs, whose customers are primarily end users or
   other ISPs.

2.5.  Allocate

   To allocate means to distribute address space to IRs for the purpose
   of subsequent distribution by them.

2.6.  Assign

   To assign means to delegate address space to an ISP or end-user, for
   specific use within the Internet infrastructure they operate.
   Assignments are made for specific documented purposes by specific
   organizations and are not sub-assigned to other parties.

2.7.  Utilization

   In IPv4, the utilization of a chunk of address space is defined as
   the ratio of the actual number of host assignments to the theoretical
   maximum number of host assignments. For example, a /24 in IPv4 can
   number 256 hosts, and if 200 have been assigned to hosts, the
   utilization would be 200/256 or 78%. In IPv4, a utilization ratio of
   80% is the threshold value used to justify the granting of additional
   address space. When an entity has used up 80% of its space, it is

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INTERNET-DRAFT                                             June 16, 2005

   eligible to receive more.

   Unlike IPv4, IPv6 is generally assigned to end sites in fixed amounts
   (e.g., in the /48 to /64 range per current address distribution
   policies).  The actual "utilization" of an IPv6 address block in
   absolute terms (i.e., counting numbers of assigned hosts) will be
   exceedingly low.  Thus, in IPv6, "utilization" is no longer the
   metric used for determining when additional address space can be
   obtained. Instead, an HD Metric is used, and it counts "links" or
   "subnets" rather than individual host assignments.

   In the context of IPv6, the term utilization refers to the allocation
   of address blocks to end sites, and not the number of addresses
   assigned within individual /48s at end sites.

2.8.  HD-Ratio

   The HD-Ratio is a way of measuring the efficiency of address
   assignment [RFC 3194].  It is an adaptation of the H-Ratio originally
   defined in [RFC1715] and is expressed as follows:

                     Log (number of allocated objects)
                HD = ------------------------------------------------
                     Log (maximum number of allocatable objects)

   where the objects are IPv6 end site assignments (e.g., /48s) assigned
   from an IPv6 prefix of a given size.

   The term "HD ratio" is somewhat misleading in the context of IPv6,
   since it is used to measure "site density" rather than "host

3.  Background

   The IETF has indicated that IANA may allocate IPv6 unicast address
   space out of the block 2000::/3, with the remaining 7/8ths of the
   address space to be held in reserve [RFC3513]. IANA, in turn, has
   been allocating small chunks of that address space to individual
   RIRs. The individual RIRs allocate addresses to ISPs (or more
   precisely, to LIRs), who then assign them to end sites. End sites are
   typically given assignments in the /48-/64 range, consistent with the
   recommendations given in [RFC 3177] and adopted RIR policy [RIR-

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INTERNET-DRAFT                                             June 16, 2005

3.1.  IPv6 Address Structure

   The general format of an IPv6 global unicast address is defined in
   RFC3513 [RFC3513]:

     |         64 - m bits    |   m bits  |       64 bits              |
     | global routing prefix  | subnet ID |       interface ID         |

                        IPv6 Address Structure

                                Figure 1

   The 64-bit IPv6 interface ID is an architectural boundary defined by
   Stateless Address Autoconfiguration [RFC2462].  Thus, in terms of
   RIR/IANA address allocation policy, the left-most 64 bits of an IPv6
   address are managed through the IANA/RIR processes.  Under the
   current IPv6 allocation policies, the value for ‚ÇÖm‚ÇÖ in the figure
   above is commonly assumed to be 16, such that the global routing
   prefix is 48 bits in length and the per-customer subnet ID is 16 bits
   in length.

   It is worth noting that the 16-bit subnet identifier is a policy
   choice, not an architectural one. Specifically, [RFC 3513bis] Section
   2.5 says:

      A slightly sophisticated host (but still rather simple) may
      additionally be aware of subnet prefix(es) for the link(s) it is
      attached to, where different addresses may have different values for

      |          n bits               |           128-n bits            |
      |       subnet prefix           |           interface ID          |

      Though a very simple router may have no knowledge of the internal
      structure of IPv6 unicast addresses, routers will more generally have
      knowledge of one or more of the hierarchical boundaries for the
      operation of routing protocols.  The known boundaries will differ
      from router to router, depending on what positions the router holds
      in the routing hierarchy.

      Except for the knowledge of the subnet boundary discussed in the
      previous paragraphs nodes should not make any assumptions about the
      structure of an IPv6 address.

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   The use of a 16-bit subnet ID is a policy rather than architectural
   boundary, and the arguments for choosing a subnet ID of 16 are
   documented in the IAB/IESG recommendations given in [RFC3177].
   Although the RIR community adopted these recommendations, there have
   been on-going discussions within the RIR community as to whether
   giving a full 16 bits of subnet space to (essentially) all end sites
   is justified, necessary or prudent. The size of the subnet ID that is
   used by end sites has a significant impact on the overall utilization
   of IPv6 address space that can be achieved. For example, using a
   subnet ID of 8 bits (i.e., 256 values) would increase the effective
   size of the global routing prefix from 48 to 56 bits, an increase in
   size of two orders of magnitude.

3.2.  IPv6 Address Assignment History

   The earliest allocations from IANA to RIRs were made in July, 1999
   and consisted of /23 allocations [IAB-IANA98]. Out of these
   allocations, the RIRs began allocating /35s to LIRs. In 2001, [RFC
   3177] was developed, which called for assigning /48s to end sites in
   many cases. These recommendations were adopted by the RIRs in 2002
   [RIR-IPV6]. Simultaneously, it was recognized that if LIRs were
   assigning /48s, but were assigning them out of a /35, the size of the
   RIR->LIR allocation also needed to be increased. Both changes are
   described in [RIR-IPV6], and that policy is still largely in place

   From 1999 until December 2004, the size of the IANA->RIR allocation
   was a /23, an allocation size that had not been changed even though
   allocation policies had been significantly revised and updated since
   1999.  Moreover, in 2004, the RIRs began receiving requests from LIRs
   for (relatively) large amounts of address space that were justified
   under the in-place allocation policies, including requests for more
   than the default minimum size of /32. The combination of in-place
   allocation policies together with ISPs requesting IPv6 address
   allocations for numbering millions of end sites led to a need to
   revisit the default IANA->RIR allocation size, a topic under current
   discussion within the RIR community [RIR-IANA-IPV6].

   A detailed listing of all the IPv6 assignments that have been made
   can be found at [IPv6-STATS].

3.3.  Goals of IPv6 Address Space Management

   Two important goals in the management of address space are to ensure
   reasonable levels of utilization (so that we don‚ÇÖt exhaust the
   address space itself) as well as achieving good levels of

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INTERNET-DRAFT                                             June 16, 2005

   aggregation. In the current Internet routing architecture (i.e.,
   CIDR) aggregation is necessary in order to keep the size of the
   routing table manageable. By "size", we refer not just to the size of
   forwarding table (in terms of numbers of individual entries), but
   also the cost and overhead of sending routing updates (in terms of
   bandwidth needed), the processing power needed for the routing
   algorithms to converge in a timely fashion, and on the overall
   understandability and manageability of the system from an operational

3.4.  Utilization

   In IPv4, there has been much emphasis on maintaining high utilization
   of the address space. Indeed, IPv4 address allocation policies use an
   "80% utilization" rule. Sites and LIRs are eligible for additional
   address space when they can demonstrate usage of 80% of the space
   they have been assigned. Likewise, LIRs obtain more space from RIRs
   when their utilization reaches 80%.

   With IPv6, justification for obtaining address space is different in
   two important ways:

    1) Rather than use a "absolute percentage utilization" metric to
        indicate thresholds, an HD-ratio metric is used. Under the
        policy rules adopted in 2002, a site or LIR is eligible for more
        address space once it shows that the amount of address space it
        has used exceeds an HD ratio of .8.
    2) Rather than count "numbers of hosts assigned", the IPv6 HD ratio
        counts "assignments to end sites", typically /48 assignments;
        the actual number of devices assigned to those sites is not a

   Comparing just the HD-ratio metric to the 80% rule, an HD-ratio
   metric of .8 is significantly easier to satisfy than 80% rule,
   especially for LIRs (e.g., see [huston-hd-metric]).  As the table in
   Appendix A shows, for a /32 assignment, an LIR need only assign 7132
   /48s to reach the threshold, which corresponds to an absolute
   utilization of only 10.9%. Moreover, Appendix A also shows that the
   required level of utilization (in absolute percentage terms)
   decreases as the amount of address space the ratio is applied to
   increases. For example, one need only achieve a utilization of
   roughly 2-3% to justify allocations in the /20-/23 range, and this
   does not even consider that the IPv6 formula counts sites rather than
   host assignments.

   The amount of address space available in IPv6 is immensely larger
   than that available in IPv4. The IETF has intentionally chosen to use

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   address space relatively inefficiently (compared with IPv4) in order
   to simplify other aspects of IPv6. For example, lower utilization
   levels have the benefit of simplifying addressing at end sites (e.g.,
   through use of stateless address autoconfiguration [RFC 2462]); end
   sites get enough address space that they do not constantly have to
   renumber in order to maintain a high utilization, and get
   significantly more address space for a given sized-site than under
   the corresponding policies for IPv4 space.

   Having said that, prudence calls for not being needlessly wasteful in
   terms of address space usage. It is hard to predict what the Internet
   will look like in 20-50 years, and poor decisions made now will
   almost certainly be very difficult to undo later.

3.5.  Aggregation

   One important goal for IPv6 is to achieve good aggregation of
   addresses over significantly longer-term time frames than in IPv4.
   Specifically, thought should be given to ensuring aggregation over
   time frames on the order of 10-20 years (and longer), rather than
   just 2 or 3.

   Routing in IPv6 is fundamentally equivalent to routing in IPv4,
   namely CIDR. Thus, the same pressures that have dominated IPv4
   routing can be expected in IPv6. Namely, the desire for "provider
   independent" (PI) space, the desire to multi-home, etc. Although the
   IETF‚ÇÖs multi6 and shim6 [shim6] efforts are developing a routing-
   infrastructure friendly (i.e., scalable) multi-homing solution, that
   work is still at a relatively early stage and is unlikely to result
   in widely-available, mature products for at least 5 years, even in
   optimistic scenarios. Moreover, it is unclear at the present time how
   broadly applicable the shim6 solution will ultimately be.  Thus, it
   would be foolish at the present time to assume that routing in IPv6
   will avoid any of the route scaling issues that continue to be a
   significant issue in IPv4.

   One consequence to the 80% rule as it has been applied to IPv4 is
   that it has contributed to significant fragmentation of the IPv4
   address space, which leads to an inability to aggregate routes as
   much as might be desired. To be fair, that is not a result of the 80%
   rule per se, rather, it results from a number of factors:

    - RIRs do not hold in reserve large amounts of address space
      adjacent to an existing allocation to allow subsequent requests
      for additional space to be satisfied from an adjacent block. That
      is, policies governing when RIRs can request additional address
      space from IANA do not favor achieving good aggregation by

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INTERNET-DRAFT                                             June 16, 2005

      allowing large amounts of space to be held in reserve. Indeed,
      achieving good levels of aggregation have simply not been
      considered important in practice and are not part of the adopted
      policies. Consequently, there are no incentives in the system for
      encouraging aggregation over long periods.

    - For various reasons (e.g., to get more optimal routing, traffic
      engineering, etc.), some LIRs deaggregate their allocations and
      advertise multiple longer prefixes. It should be noted that this
      type of fragmentation is reversible, since an LIR could fall back
      to advertising a single aggregate if necessary. That is, this type
      of fragmentation is an optimization, and isn‚ÇÖt required in order
      to maintain basic connectivity for all end sites.

    - Mergers and acquisitions results in administratively related sites
      being comprised of address space that is not aggregatable.

   To support aggregation over time frames of 10-20 years (and beyond),
   and to avoid reliving the fragmentation experience of IPv4, RIR
   address policies must support the holding of adequately-sized
   "reservations" of address space adjacent to address space that has
   been delegated.  Thus, the amount of space held in reserve by the
   RIRs, the manner in which it is held, and the time frame over which
   RIRs maintain that reserve are key policy questions with significant
   implications on long-term aggregation and fragmentation of the IPv6
   address space.

4.  How Much Address Space Do We Really Have?

4.1.  An example: Cable Modem/DSL Service in US

   In the hallway at a recent ARIN meeting, I was cornered by someone
   who had done a back-of-the envelope calculation that led him to
   believe the current policies needed adjustment. The argument went

      If I assign 4M /48‚ÇÖs of IPv6 (one to each cable modem on my
      network), according to the HD-ratio I am justified to obtain
      something around a /20 of IPv6 addresses.  In other words, I am
      justified in getting 268M /48‚ÇÖs even though I am only using 4M of
      them.  That would be enough for me to assign at least two for
      every household in the US (not just the 19M on my network).

      Now if all the cable providers (e.g., Comcast, Cox, Adelphia,
      Cablevision, Time-Warner, etc.) did the same for their networks;
      and each of the DSL companies made a similar move (SBC, Verizon,

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      Quest, etc.); perhaps we could easily see the broadband market in
      the US alone obtaining some 16 /20‚ÇÖs of IPv6 or a total of /16.
      There are only 8192 of those available in the current global
      unicast space of 2001::/3.

      Anyhow, you can see where this might lead...

4.2.  How Much Is Enough?

   At various times in the past, it has been asserted that we will
   "never run out of IPv6 addresses". For example, Wikipedia repeats the
   myth [WIKIPEDIA]:

        If the earth were made entirely out of 1 cubic millimetre grains
        of sand, then you could give a unique address to each grain in
        300 million planets the size of the earth.

   These sorts of statements are made on the assumption that all of
   IPv6‚ÇÖs 128-bit address space can be use efficiently for addressing
   devices. But as discussed earlier, only 64-bits are available for
   addressing links, since 64 bits are used for an interface identifier.
   Or, stated differently, the above figure is only true if one believes
   that one can have single links, on which 2^64 hosts can be attached;
   in practice, the actual number of hosts connected to a single link is
   on the order of tens to thousands, with most links at the lower end
   of the range.

   A more realistic measure is to consider how many links, or end sites
   can be addressed. One way to frame the discussion is to ask the
   question: how many devices will a person or end site need to be able
   to assign addresses to?

   World population projections for the year 2050 estimate a world
   population of 9.2B persons [CENSUS]. Under the current address
   allocation policies (specifically, /48 to end sites and use of an HD-
   ratio threshold of .8), the following observation can be made.

   If we allow for one /48 per person, and assume there is only a single
   ISP for everyone, 9.2B /48s corresponds to a /7, or 1/128th of the
   entire available address space (and fully 1/16th of 2001::/3).
   Clearly, this does not indicate a need to panic or that IPv6 will run
   out of addresses. However, such numbers are also nowhere near
   "practically infinite".  A key policy question is whether the
   community is comfortable with a projection that can result in fully
   1% of the available address space used up in 50 years, given:

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      - the inherent uncertainties of predicting future address
        consumption rates (e.g., an appropriate number of devices per
        person, and indeed whether a per-person perspective is even the
        appropriate one).

      - the inherent uncertainty in the length of time that IPv6 will be
        a key part of the underlying infrastructure (i.e., 50 vs. 100
        vs. 200 years).

      - a long history of infamous "predictions", e.g., "640K ought to
        be enough for anybody." -  Bill Gates [GATES-640K]

5.  On RIR->LIR->End User Assignment and Allocation Policies

5.1.  On the HD Ratio Metric

   The current use of the HD Metric is documented in [RIR-IPV6]. An HD
   ratio of .8 is the threshold for justifying additional allocations.
   Now that we have some operational experience with using the HD-Ratio,
   we are seeing some (on the surface surprisingly) large allocations
   being made. They are "surprising" in the sense that people who look
   at specific examples feel that the current policies grant far more
   address space than a service provider actually needs to (comfortably)
   number their infrastructure and customer networks. Using the example
   from Section 4.1, a large cable/DSL provider could obtain a /20, even
   though that corresponds to a utilization of only 2.1% (In IPv4, 80%
   utilization would be required).

   The topic of various HD Ratio values and their implications are
   discussed in more detail in [huston-hd-ratio]. That work suggests
   that changing the HD ratio threshold from .8 to .94 reduces the
   overall address space consumption by an equivalent of 3 bits of
   address space over the next 50 years, without imposing hardship on
   ISPs/LIRs in numbering their networks. In addition, the work also
   suggests that a small number of "big" allocations accounts for a
   disproportionately large amount of total projected address space

5.2.  On /48 Assignments to End Sites

   Per existing policy, the RIRs assume that end site assignments are
   /48s, and thus utilization measurements are based on an HD-ratio that
   counts numbers of /48 assignments. Granting a /48 to an end site,
   allows a site to have up to 65,536 subnets. While that number may
   make sense for larger sites, it is hard to imagine a typical home

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INTERNET-DRAFT                                             June 16, 2005

   user requiring so much space. Indeed, the default end site assignment
   today is in practice the same for home users and larger businesses.

   Looking back at some of the original motivations behind the /48
   recommendation [RFC 3177], one overriding concern was to ensure that
   end sites could easily obtain sufficient address space without having
   to "jump through hoops" to do so. For example, if someone felt they
   needed more space, just the act of asking would at some level be
   sufficient justification.  As a comparison point, in IPv4, typical
   home users are given a single public IP address (even this is not
   always assured), but getting even a small number more is typically
   more expensive either in terms of the effort needed to obtain
   additional addresses, or in the actual monthly cost. (It should be
   noted that the "cost" of additional addresses cannot be justified by
   the actual cost of those addresses as charged by the RIRs, but the
   need for additional addresses is sometimes used to imply a different
   type or "higher grade" of service, for which some ISPs charge extra.)
   Thus, an important goal in IPv6 was to significantly change the
   default and minimal end site assignment, from "a single address" to
   "multiple networks".

   Another concern was that if a site changes ISPs and subsequently
   renumbers, renumbering from a larger set of "subnet bits" into a
   smaller set is particularly painful, as it it can require making
   changes to the network itself (e.g., collapsing links). In contrast,
   renumbering a site into a prefix that has the same number (or more)
   of subnet bits is easier, because only the top-level bits of the
   address need to change. Thus, another goal of the RFC 3177
   recommendation is to ensure that upon renumbering, one does not have
   to deal with renumbering into a smaller subnet size.

   The above concerns were met by the /48 recommendation, but could also
   be realized through a more conservative approach. For example, one
   can imagine "classes" of network types, with default sizes for each
   class. For example:

      - a PDA-type device with a low bandwidth WAN connection and one
        additional PAN connection - a single network or /64 assignment.
        A /64 assignment allows for the addressing of a number of hosts,
        each connected to the same PAN (personal area network, e.g.,
        Bluetooth) link as the device. This would be appropriate in
        deployments where the end device is not expected to provide
        connectivity for a larger site, but is intended to provide
        connectivity for the device and a small number of additional
        devices directly connected to the same PAN as the device.

      - home user - expected to have a small number of subnets, e.g.,
        less than 10 - a /56 (or /60) assignment

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INTERNET-DRAFT                                             June 16, 2005

      - small business/organization  - one having a small number of
        networks, e.g., less than 100  - a /56 assignment

      - large business/organization - an organization having more than
        100 subnets - a /48.

   A change in policy (such as above) would have a significant impact on
   address consumption projections and the expected longevity for IPv6.
   For example, changing the default assignment from a /48 to /56 (for
   the vast majority of end sites) would result in a savings of up to 8
   bits, reducing the "total projected address consumption" by (up to) 8
   bits or two orders of magnitude. (The exact amount of savings depends
   on the relative number of home users compared with the number of
   larger sites.)

   One can, of course, imagine a policy supporting the entire range of
   assignments between /48 and /64, depending on the size or type of end
   site. However, there are a number of arguments in favor of having a
   small number of classes of sizes:

      - It becomes easier for end users to identify an assignment size
        that is appropriate for them

      - It increases the likelihood that there will be agreement among
        ISPs on the appropriate assignment sizes for the various
        customer classes.

      - Having excess flexibility in selecting an appropriate assignment
        size for a given customer type can lead to different ISPs
        offering different assignment sizes to the same customer. This
        is undesirable because it may result in

           - an end site needing to renumber into a smaller subnet space
             when switching ISPs, or

           - it may lead to ISPs attempting to differentiate their
             service offerings by offering the most liberal address
             assignment policies (to attract customers), defeating the
             purpose of having multiple class assignment sizes.

      - The operational management of the reverse DNS tree is simplified
        if delegations are on nibble boundaries (e.g., /48, /52, /56,
        and /64).

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5.3.  On the /64 Boundary

   In theory, even more savings could be realized by reducing the size
   of the Interface identifier, i.e., making it smaller than 64 bits.
   While possible, this is the most difficult boundary to move in
   practice. Considerations include:

      - Stateless address autoconfiguration [RFC 2462] assumes Interface
        identifiers are fixed at 64 bits. Changing this would require
        revising that specification and devising a transition plan for
        migrating existing implementations from 64-bit identifiers to
        something shorter.

      - Stateless address autoconfiguration has been widely implemented,
        with additional implementations in the pipeline. Removing those
        implementations and replacing them with upgraded implementations
        will take many years.

      - it is unclear how one would transition to Interface identifiers
        of shorter length in the short term while preserving stateless
        address autoconfiguration.

      - one transition strategy might be to disable stateless address
        autoconfiguration completely (for generating addresses) and use
        DHCP to assign addresses. However, client implementation of DHC
        for address configuration is not mandatory in IPv6, and it is
        believed that few IPv6 devices actually implement the client
        portion of address configuration via DHC.

      - some existing IPv6 multicast standards assume that an IPv6
        routing prefix is no more than 64-bits in length, and include
        the 64-bit subnet prefix within an IPv6 multicast address

5.4.  Summing It All Up

   A fundamental question to ask is whether the Internet community is
   comfortable that the current allocation policies will result in an
   reasonable address consumption projections over the lifetime of IPv6,
   and if not where and how does it make sense to propose changes.

   It should be noted that some have argued:

        We‚ÇÖre currently only allocating address space out out of prefix
        001 and are holding the remaining 7/8ths of the available space
        in reserve.  If we later determine we‚ÇÖve been too liberal in our
        address usage, we can just implement more conservative policies

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INTERNET-DRAFT                                             June 16, 2005

        for the remainder of the space.

   There are a number of reasons to give pause to that line of thinking:

      - The Internet has become a critical component of the world‚ÇÖs
        infrastructure, and its overall stewardship is a matter of
        public concern. Indeed, IP addresses are a global public
        resource that must be managed prudently and fairly.

      - It is far easier in practice to (later) liberalize a more
        conservative allocation policy than it is to convert a more
        liberal policy to one that is more conservative. Indeed, the
        IPv4 experience, in which early adopters were able to obtain
        large amounts of address space (e.g., class A and B blocks)
        while later adopters got significantly less, has led to
        significant and continuing unhappiness about unfairness in IPv4
        address management.

      - Relatively small policy adjustments now would appear to
        significantly reduce the projected consumption of IPv6 address
        space, without causing significant hardship.

   In terms of ease of implementation, changing the HD ratio metric from
   .8 to something larger (e.g., .94) is the least painful. It has no
   impact on the assignments made to end sites and effects only the very
   largest of sites (at least in the short term). It would also be an
   easy value to change back downward (in the future), if needed. Making
   this type of change will reduce projected address space consumption
   my approximately one order of magnitude.

   Changing the default end site assignment sizes (e.g., moving from /48
   to /56 for SOHO sites) has the potential for realizing even larger
   savings, i.e., on the order of two orders of magnitude. However, some
   issues apply:

      - a (relatively) small number of assignments have already been
        made. What should be done with those?  How should they be
        counted?  Should an attempt be made to reclaim them?

      - The existing minimum RIR->LIR allocation size of /32 is based on
        the assumption that end sites will be getting /48s. If the
        default size is end site allocation is changed, should the
        default minimum RIR->LIR allocation size also be adjusted

   Finally, some of the issues relating to changing the size of the
   Interface identifier have been discussed in Section 5.3. In addition,

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   the above concerns related to changing the /48 boundary would also
   apply to changing the /64 boundary, and in all likelihood would be
   even more complex (e.g., to deal with transition issues).

   In summary, in deciding which of the above factors to change, it is
   important to find an adequate balance between obtaining reasonable
   utilization for the long term balanced against the desire to maximize
   aggregatability within the routing table and to limit the amount of
   forwarding table route fragmentation. In addition, because IPv6
   adddress space is a shared global resource, it must be managed in a
   prudent and fair manner.

6.  On Reservations and Aggregation

6.1.  On RIR Reservations

   In order to allow subsequent allocations from an LIR to expand the
   size of an existing address block allocation (as opposed to requiring
   the allocation of a different, non-aggregatable block), RIRs need to
   hold some space in reserve adjacent to an allocated block to allow
   for future growth. The reservation can be an explicit reservation, or
   it can be implicit if sparse allocation is employed [SPARSE].

   As an RIR allocates address blocks to LIRs, at some point it will
   need to go back to the IANA to obtain additional space. In order to
   allow for good aggregation over very long periods of time (e.g., 10
   years or more), it is important that sufficient space be held in
   reserve to allow for potential growth (i.e., subsequent adjacent
   allocations) over long time frames. That is, to allow for future
   aggregation, it would be better for RIRs to obtain additional space
   from IANA than to have them potentially fragment the address space by
   allocating space that would better be held in reserve for future
   adjacent allocations .

   An important policy question is how much space should be held for
   potential growth, the time period over which the space should be held
   in reserve, and how it is accounted for (e.g., when justifying the
   need for additional space from IANA). That is, what is the metric an
   RIR uses to justify going back to IANA for an additional address
   block. Does it count the reserved portion as being "allocated"? Does
   the RIR just use the same HD-ratio metric or a fixed percentage like

   At the present time, it is unclear how much IPv6 address space the
   RIRs are holding in reserve for subsequent allocations, but there is
   some evidence that it is not very much, e.g., on the order of a

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   single bit.

6.2.  On IANA->RIR Assignments

   A current proposal under discussion with the RIR community calls for
   having a /12 being the default IANA->RIR allocation unit [IANA-RIR-
   IPV6]. This size appears to be large enough to allow RIRs to
   implement sparse allocation or a related scheme and hold sufficient
   space in "reserve" for the time being.

   [XXX need to fill in/explore this space in more detail.]

   Note that it generally makes no sense for IANA to hold additional
   space in reserve when delegating space to an RIR, because an RIR will
   sub-allocate chunks to LIRs out its delegation. In order for a
   subsequent allocation for one of those LIRs to aggregate, free space
   must exist adjacent to the requesting LIR‚ÇÖs existing allocation. Any
   space held in reserve by IANA can only be adjacent to the left most
   or right most sub-delegated block.

7.  Security Considerations

8.  IANA Considerations

   This document makes no requests to IANA.

9.  Acknowledgments

   This work has been greatly facilitated and influenced by discussions
   held within the IAB ad-hoc IPv6 committee, hallway and public
   discussions at ARIN XV and RIPE 50 meetings, and by discussions with
   Geoff Huston in particular. It has also benefit from review comments
   from Andrew Dul, Geoff Huston, Lea Roberts and Leo Vegoda.

10.  References




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   [RFC 3194] The H-Density Ratio for Address Assignment Efficiency An
                         on the H ratio. A. Durand, C. Huitema. November

   [RFC 1715] The H Ratio for Address Assignment Efficiency. C. Huitema.
                         November 1994.

   [RFC 3513] Internet Protocol Version 6 (IPv6) Addressing
                    Architecture. R.  Hinden, S. Deering. April 2003.

   [RFC 3513bis] draft-ietf-ipv6-addr-arch-v4-04.txt (Approved by IESG,
                    awaiting publication in RFC Editor Queue.)

   [RFC 2462] IPv6 Stateless Address Autoconfiguration. S. Thomson, T.
                    Narten.  December 1998.

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

   [RFC 3177] IAB/IESG Recommendations on IPv6 Address Allocations to
                    Sites.  IAB, IESG. September 2001.

   [RFC 3956] Embedding the Rendezvous Point (RP) Address in an IPv6
                    Multicast Address. P. Savola, B. Haberman. November

                    Document ID: ripe-267, Date: 22 January 2003

                    (similar discussion can be found at the other RIRs
                    as well)

   [shim6] shim6 is not yet a WG, but will be soon (hopefully). mailing
                    list:, subscribe:
          , archive:

   [huston-hd-metric]  draft-huston-hd-metric-00.txt



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   [SPARSE] "IPv6 Address Space Management", Document ID: ripe-343,
                    Date: 22 February 2005, Paul Wilson, Raymond Plzak,
                    Axel Pawlik,

11.  Appendix A: HD-Ratio

   The following table provides equivalent absolute and percentage
   address utilization figures for IPv6 prefixes, corresponding to an
   HD-Ratio of 0.8

        P    48-P          Total /48s        Threshold      Util%

       48       0                   1                1     100.0%
       47       1                   2                2      87.1%
       46       2                   4                3      75.8%
       45       3                   8                5      66.0%
       44       4                  16                9      57.4%
       43       5                  32               16      50.0%
       42       6                  64               28      43.5%
       41       7                 128               49      37.9%
       40       8                 256               84      33.0%
       39       9                 512              147      28.7%
       38      10                1024              256      25.0%
       37      11                2048              446      21.8%
       36      12                4096              776      18.9%
       35      13                8192             1351      16.5%
       34      14               16384             2353      14.4%
       33      15               32768             4096      12.5%
       32      16               65536             7132      10.9%
       31      17              131072            12417       9.5%
       30      18              262144            21619       8.2%
       29      19              524288            37641       7.2%
       28      20             1048576            65536       6.3%
       27      21             2097152           114105       5.4%
       26      22             4194304           198668       4.7%
       25      23             8388608           345901       4.1%
       24      24            16777216           602249       3.6%
       23      25            33554432          1048576       3.1%
       22      26            67108864          1825677       2.7%
       21      27           134217728          3178688       2.4%
       20      28           268435456          5534417       2.1%
       19      29           536870912          9635980       1.8%
       18      30          1073741824         16777216       1.6%
       17      31          2147483648         29210830       1.4%
       16      32          4294967296         50859008       1.2%
       15      33          8589934592         88550677       1.0%

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       14      34         17179869184        154175683       0.9%
       13      35         34359738368        268435456       0.8%
       12      36         68719476736        467373275       0.7%
       11      37        137438953472        813744135       0.6%
       10      38        274877906944       1416810831       0.5%
       9       39        549755813888       2466810934       0.4%
       8       40       1099511627776       4294967296       0.4%
       7       41       2199023255552       7477972398       0.3%
       6       42       4398046511104      13019906166       0.3%
       5       43       8796093022208      22668973294       0.3%
       4       44      17592186044416      39468974941       0.2%

12.  Author‚ÇÖs Address

   Thomas Narten
   IBM Corporation
   3039 Cornwallis Ave.
   PO Box 12195 - BRQA/502
   Research Triangle Park, NC 27709-2195

   Phone: 919-254-7798

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Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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   This document and the information contained herein are provided on an

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