______________________________________________________________________ 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) Where: 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 LIRs. 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/policy/ ipv6-address-policy.html [ipv4-address-policy] APNIC document: "Policies for IPv4 address space management in the Asia Pacific region" http://www.apnic.net/docs/ policy/add-manage-policy.html