[sig-policy] Policy SIG Proposal - HD ratio for IPv4 allocations

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  • Subject: [sig-policy] Policy SIG Proposal - HD ratio for IPv4 allocations
  • From: APNIC Secretariat <secretariat at apnic dot net>
  • Date: Wed, 04 Aug 2004 15:25:03 +1000
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    • Dear All,

      Please find below a policy proposal for the forthcoming Policy SIG, to be presented at APNIC18 in Fiji.

      The ideas in this proposal were presented at APNIC16 as an informational item ("HD ratio for IPv4") on the agenda. You can find details of the presentation, transcripts of the discussions and minutes at:


      Your comments and feedback on this proposal are very much appreciated on this mailing list.

      Best wishes,
      ______________________________________________________________________ APNIC Secretariat <secretariat at apnic dot net>
      Asia Pacific Network Information Centre (APNIC) Tel: +61-7-3858-3100
      PO Box 2131 Milton, QLD 4064, Australia Fax: +61-7-3858-3199

      See you at APNIC 18 Nadi, Fiji, 31 August-3 September 2004 www.apnic.net/meetings ______________________________________________________________________


      prop-020-v001: Application of the HD ratio to IPv4

      Proposed by: Paul Wilson and Anne Lord, APNIC Secretariat
      Version: 1.0
      Date: 4 August 2004

      1 Summary

      Internet address space is managed hierarchically, by allocation from
      IANA to RIRs and from RIRs to LIRs (ISPs), and by assignment from LIRs
      to infrastructure and customer networks. At each level of allocation or
      assignment some address space may be reserved for future expansion
      and/or efficient aggregation. As more hierarchical levels are
      introduced, the overall efficiency of utilisation of the address space
      will decrease.

      The HD ratio (Host-Density ratio) has been proposed as a mechanism for
      measuring the utilisation of addresses within hierarchically-managed
      Internet address blocks [RFC 3194]. A given HD ratio value corresponds
      to a percentage utilisation which decreases as the size of the address
      space grows, thus allowing for the decreasing management efficiency
      which is described above.

      The HD ratio is used as the utilisation metric for address space under
      the current IPv6 management policy [ipv6-address-policy]. According to
      this policy, a block of IPv6 address space is considered to be utilised
      when its HD ratio reaches 0.80. This value is said to represent a
      conservative but manageable figure ("values of 80% or less correspond to
      comfortable trade-offs between pain and efficiency" [RFC 3194]).

      This document proposes the use of the HD ratio for measurement of IPv4
      utilisation, for the same purpose of determining when a given block of
      address space should be considered as fully utilised. The proposed value
      of the HD ratio for IPv4 is 0.96.

      2 Background and problem

      Under the current management framework for IPv4 address space
      [ipv4-address-policy] a block of IPv4 addresses is considered "utilised"
      when 80% of the addresses within the block have been allocated or
      assigned. This measure is applied equally for all address blocks,
      regardless of size.

      Current policies assume a hierarchical system of address space
      delegation (from IANA to RIRs to LIRs to customers, as described above),
      but they make no allowance for hierarchical management within allocated
      address space. For LIRs in particular, a hierarchical approach is often
      required for assignment of address space to service elements such as
      customer networks, individual PoPs, regionalised topologies, and even
      distinct ISP products. Small network infrastructures may require simple
      hierarchies, but large infrastructures can require several levels of
      address space subdivision. These levels of hierarchy are "hidden" in
      terms of recognition by the current RIR policy framework, and highly
      constrained by the 80% utilisation requirement. As a result, management
      of large blocks is often extremely difficult, requiring large internal
      routing tables and/or frequent renumbering of internal address blocks.

      One of the goals of the RIR system is to avoid unnecessary depletion of
      IPv4 address space, and the 80% utilisation requirement is justified on
      that basis. However address management policies must also be practical
      in terms of management overhead imposed. It may be argued that when
      large address spaces are involved, the "80% rule" imposes unreasonable
      management overheads on an LIR.

      A more reasonable approach should impose a more uniform degree of
      management overhead, rather than penalising the holders of large address
      blocks. This is achievable to some degree by basing utilisation
      requirements on the HD ratio rather than the fixed percentage-based
      measure which is in use today.

      3 Proposal

      In recognition of the problems outlined above, it is now proposed to
      consider replacing the current fixed percentage based utilisation
      requirement for IPv4 address space with an HD ratio based requirement.

      3.1 The HD ratio

      According to RFC3194, The HD ratio is calculated as follows:

      HD = log(U)/log(S)


      S is the size of the address block concerned, and
      U is the number of addresses which are utilised.

      Note: Under the current IPv4 policy framework, addresses are
      considered to be utilised once they are assigned or sub-allocated
      by the LIR.

      3.2 Selection of HD ratio value

      The appropriate HD ratio value should be decided on a rational
      basis. In order to do this, we make certain assumptions about the
      depth of "hidden" hierarchy involved in managing address blocks of
      various sizes. If we assume that 80% utilisation is achieved at
      each level of this assumed hierarchy, then the overall utilisation
      can be easily calculated.

      The following table proposes a set of hierarchical depths which may
      be reasonably expected within address spaces of given sizes. If 80%
      utilisation is achieved at each hierarchical level, then the
      overall utilisation will be (0.80 to the power of "n"); and from
      this value, corresponding HD ratio levels can then be calculated.

      Size range Depth Utilisation HD ratio
      (prefix) (n) (0.80**n) (calculated)
      ---------- ----- ----------- ------------

      /24 to /20 1 80% .960 to .973
      /20 to /16 1.5 72% .961 to .970
      /16 to /12 2 64% .960 to .968
      /12 to /8 2.5 57.2% .960 to .966
      /8 to /4 3 51.20% .960 to .966

      The depths of hierarchy listed above are based on simple
      assumptions about the likely size and structure of LIRs holding
      address blocks of these sizes. From the table, a rational HD ratio
      value may be chosen as 0.96 (a round figure which occurs within
      most of the above ranges). For this value, the following table
      gives the utilisation requirement for IPv4 address blocks from /24
      to /8.

      IPv4 Addresses Addresses Util%
      prefix total utilised
      ------ --------- --------- ------

      24 256 205 80.11%
      23 512 399 77.92%
      22 1024 776 75.79%
      21 2048 1510 73.71%
      20 4096 2937 71.70%
      19 8192 5713 69.74%
      18 16384 11113 67.83%
      17 32768 21619 65.98%
      16 65536 42055 64.17%
      15 131072 81811 62.42%
      14 262144 159147 60.71%
      13 524288 309590 59.05%
      12 1048576 602249 57.43%
      11 2097152 1171560 55.86%
      10 4194304 2279048 54.34%
      9 8388608 4433455 52.85%
      8 16777216 8624444 51.41%

      Note: This table provides values for CIDR blocks only, however for
      non-CIDR blocks the same calculations can be applied to produce
      equally meaningful results.

      4 Implementation

      This proposal will impact on procedures for allocation from APNIC to

      4.1 RIR-LIR procedures

      The impact of the proposal on the RIR-LIR administrative procedures
      would be to replace the current 80% utilisation requirement, with a
      0.96 HD ratio requirement.

      By way of examples, an LIR holding a total address space equal to a
      /16 would be able to receive more address space when they had
      allocated or assigned 64.17% of that space; while an LIR holding a
      /9 would be able to receive more space when they had allocated or
      assigned 52.85% of their address space.

      The HD ratio calculation is trivial, but slightly more complex than
      the existing 80% calculation. Some APNIC members may in some
      circumstances require extra assistance, however for those using
      MyAPNIC, the calculation would be automatic and require no
      additional effort.

      4.2 Implementation timeline

      If implemented, this policy could be effective within 3 months of the
      implementation date.

      5 References

      [RFC 3194] "The Host-Density ratio for address assignment efficiency: An
      update on the H ratio", A. Durand, C.Huitema, November 2001.

      [ipv6-address-policy] APNIC document: "IPv6 address allocation and
      assignment policy" http://www.apnic.net/docs/policy/

      [ipv4-address-policy] APNIC document: "Policies for IPv4 address space
      management in the Asia Pacific region" http://www.apnic.net/docs/