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.
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
prop-020-v001: Application of the HD ratio to IPv4
Proposed by: Paul Wilson and Anne Lord, APNIC Secretariat
Date: 4 August 2004
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
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.
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
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.
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