[address-policy-wg] Interesting APNIC proposal
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Joao Luis Silva Damas
joao at psg.com
Thu Aug 14 11:08:28 CEST 2003
I have just noticed this quite interesting and well prepared policy proposal from APNIC regarding a different way of calculating address utilisation ratios. It seems of particular relevance in the cases of multiple levels of sub-allocation, a situation that has been the subject of debate in the RIPE region as well, therefore I believe it deserves consideration also in this region. The changes in address consumption derived from the proposal are addressed in section 5.2 The original posting on APNIC's mailing lists is available at http://www.apnic.net/mailing-lists/sig-policy/archive/2003/08/ msg00000.html Joao Damas ------------------------------------------------------------------------ Discussion Document Application of the HD-Ratio to IPv4 Prepared by: Paul Wilson, APNIC Secretariat Version: 1.0 Date: 7 August 2003 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 components and customer networks. Within each level of allocation or assignment some address space is generally reserved for future expansion and/or efficient aggregation. As more hierarchical levels are introduced into any address space, the overall efficiency of utilisation of that space (in terms of the total number of individual addresses actually used) will inevitably decrease. The HD Ratio (Host-Density Ratio) has been proposed as a practical 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 overall address management efficiency which is described above. The HD Ratio is currently used as the utilisation metric for IPv6 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 effectively 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 explores the possible 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. 2 Background and problem The current management framework for IPv4 address space relies on policies developed within each RIR community [ipv4- address-policy]. These policies dictate, effectively, that a given block of IPv4 addresses should be considered "utilised" when at least 80% of the addresses within the block have been allocated or assigned. This measure is applied equally for address blocks held by small or large LIRs, regardless of their size. In any case, it is deemed that once 80% of the space held by an LIR has been allocated or assigned, that LIR may request more address space from its appropriate RIR. 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 address management within an organisation's own 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 may be justifiable 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 attempt to impose a uniform degree of management overhead, rather than penalising the holders of large address blocks. This may be 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 A 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, referred to as the AD Ratio. 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 site addresses (/48s) which are utilised. In order to calculate the HD-Ratio for a given IPv6 block, it is necessary to know the size of that block, and number of host addresses which are in use. The current IPv6 policies allow for this by requiring registration of address assignments to the /48 level, however this degree of registration is not appropriate under IPv4 management policies. 3.2 Definition - the AD Ratio IPv4 address space utilisation is measured in terms of the number of allocations or assignments which have been made from a given address block, rather than the number of host addresses which are in use within the block. We therefore use the term "Allocation Density" for the measure of address space utilisation within an IPv4 block, rather than the term "Host Density" which represents host-address utilisation in IPv6. Consequently, we refer propose a new utilisation metric for IPv4, to be known as the "AD Ratio" (for Allocation or Assignment Density Ratio). The AD Ratio measures the number of addresses allocated or assigned from a given block of address space, as follows: AD = log(A)/log(S) Where: S is the size of the of the address block, and A is the number of addresses allocated or assigned from that block. 3.3 Selection of AD Ratio value The appropriate AD Ratio value for the purposes of this proposal 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. We then assume that at each level of this assumed hierarchy, the currently accepted 80% utilisation requirement is achieved, in order for the entire address space to be considered as fully utilised. The following table proposes a set of assumed hierarchical depths which may be reasonably expected within hierarchically-managed address spaces of certain sizes. At each delegation level an allocation density of 80% is assumed, so that within a hierarchy of "n" levels, the overall address space utilisation is calculated as 0.80 to the power of "n". Size range Depth Util AD 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 Note: the above AD Ratio values are derived directly from the formula above. For instance, a /20 block contains 4096 addresses, and 80% utilisation corresponds to 3276.8 addresses; therefore the corresponding AD Ratio is calculated as log(3276.8)/log(4096) = 0.973 The depths of address hierarchy listed above are notional approximations only, based on general assumptions about the likely size and structure of LIRs holding address blocks in the respective size ranges. From the table, a rational common AD ratio value may be determined as 0.966 (chosen as the most conservative value which is common to all of the listed ranges). For this value, the following table gives the percentage utilisation requirement for the full range of IPv4 address blocks (this is also derived directly from the AD ratio formula shown above). IPv4 Addresses Addresses Util% Prefix Total Utilised 32 1 1 100.00% 31 2 2 97.67% 30 4 4 95.40% 29 8 7 93.17% 28 16 15 91.00% 27 32 28 88.88% 26 64 56 86.81% 25 128 109 84.79% 24 256 212 82.82% 23 512 414 80.89% 22 1024 809 79.00% 21 2048 1580 77.16% 20 4096 3087 75.37% 19 8192 6030 73.61% 18 16384 11780 71.90% 17 32768 23010 70.22% 16 65536 44949 68.59% 15 131072 87804 66.99% 14 262144 171518 65.43% 13 524288 335046 63.90% 12 1048576 654485 62.42% 11 2097152 1278482 60.96% 10 4194304 2497408 59.54% 9 8388608 4878479 58.16% 8 16777216 9529704 56.80% 7 33554432 18615487 55.48% 6 67108864 36363809 54.19% 5 134217728 71033685 52.92% 4 268435456 138758413 51.69% 3 536870912 271053050 50.49% 2 1073741824 529479652 49.31% 1 2147483648 1034294583 48.16% 0 4294967296 2020408681 47.04% 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 If implemented at any particular level in the IP address delegation chain, this proposal would have a number of impacts on administrative processes at that level. It is not currently proposed to apply this proposal to the relationship between RIRs and IANA, therefore the implementation of the proposal at the RIR-LIR level is discussed there. 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 0.966 AD 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 68.59% of that space; while an LIR holding a /9 would be able to receive more space when they had allocated or assigned 58.16% of their address space. For blocks smaller than /22, it is proposed that the current 80% requirement be used, in order that this new policy does not impose tighter utilisation requirements than were previously imposed. The AD-Ratio calculation is trivial, but certainly more complex and less intuitive than the existing 80% calculation. Some APNIC members may in some circumstances require extra assistance, however for those using the MyAPNIC service, the calculation would be automatic and therefore no additional effort would be involved. 4.2 Assignment procedures In order to support consistent calculation of an LIR's AD Ratio, it would be preferable for each LIR to register infrastructure assignments in the same way as customer assignments. This could be done by whois update via email, or via the MyAPNIC service. It would not be necessary to register individual hardware components or subnets, but rather only the independent infrastructure blocks which are designated by the LIR, and which can be justified on the same basis as customer assignments. Such registrations would be publicly visible as normal whois records, unless database changes were implemented to specifically hide them. 4.3 Implementation timeline If implemented, this policy could be effective within 3 months of the implementation date. 5 Impacts 5.1 Administrative Impact The primary administrative impact of this proposal is to ease the administrative address management burden experienced by LIRs, especially those with large address space holdings. The proposal recognises the need to manage address space hierarchically within an LIR service infrastructure, and makes allowance for it through the use of the AD ratio for assessment of address utilisation. This proposal would impose a small administrative cost on LIRs. In the first place, an LIR's internal systems (manual or automated) would need to incorporate a new method of calculating address space utilisation (and especially when determining the point at which the LIR may request more space from APNIC). In the second place, an LIR would need to register infrastructure assignments in the same way as customer assignments, which would impose additional administrative cost. In both cases, LIRs using the MyAPNIC service would experience a small extra cost because these changes can be automated within that system. The administrative impact on internal systems of the APNIC Secretariat is also relatively small. APNIC hostmaster processes can be tailored easily to accommodate a changed method of calculating address space utilisation; and the whois database can certainly accommodate an increased number of registrations. 5.2 Address Space Consumption Because this proposal lowers the utilisation requirement for IPv4 address blocks, it would certainly increase the rate of deployment of IP addresses. In analysing this impact, we can identify two separate factors contributing to increased consumption: firstly, an initial impact resulting from increased "wastage" of deployed address space; and secondly, on ongoing impact as utilisation requirements continue to fall for individual LIRs' growing address holdings. 5.2.1 Initial impact The initial impact on address consumption can be estimated by calculating for each APNIC LIR the difference between the current 80% utilisation, and the AD-ratio-determined utilisation requirement. This calculation will indicate the amount of extra "wasted" address space which would result from the proposed policy change. Total LIRs in sample 788 Total address space held 4.17 (/8 blocks) Utilised addresses (80%) 3.32 Utilised addresses (AD 0.966) 2.53* Extra "wasted" space 0.81 Extra "wastage" as % of total 19% * This figure is calculated from the sample of 788 LIRs, according to actual address space holdings These figures show that by reducing the address space utilisation requirement from 80% to AD 0.966, an additional 0.81 blocks are consumed out of a total 4.17 blocks allocated. This corresponds to an additional of 19% of the total allocated address space. It should be noted that this assessment indicates a theoretical impact in terms of increased address consumption, assuming all deployed address space is actually utilised. The actual impact will be less than this due to underutilisation of address space; and furthermore the impact will not take place at one time, but progressively as part of ongoing address space allocations. 5.2.2 Ongoing impact The ongoing impact on address consumption can be estimated by distributing additional address space to the same set of LIRs, in proportion to their existing address space holdings. For the purposes of this analysis, an additional /8 block is distributed to the same set of 788 LIRs, in proportion to their existing address holdings. Initial address space held 4.17 (/8 blocks) Additional space allocated 1.00 Total address space now held 5.17 Utilised addresses (AD 0.966) 3.11* Additional addresses utilised 0.58* Utilised addresses (80%) 0.80 Extra "wasted" space 0.22 Extra "wastage" as % of allocation 22% * These figures are calculated from the same sample of 788 LIRs, assuming a proportional distribution of an additional /8 block These figures show that after an additional /8 block is distributed and utilised, 58% of that block would actually be utilised, rather than 80%. Therefore up to 22% of that block would be "wasted" by the use of AD 0.966 in place of 80% as the utilisation measure, resulting potentially in an increased consumption rate of up to 22%. Again, this calculation is theoretical only, and assumes that all address space which has been distributed will be utilised. 5.2.3 Conclusions on address consumption The above analysis indicates that the adoption of this proposal would cause an initial additional consumption of up to 19% of address space allocated. In APNIC's case, a total of 8.07 /8 blocks have been allocated (as of 1 July 2003), so up to an additional 1.53 blocks would eventually be consumed as a result of the change. The analysis also indicates that this proposal would cause an overall 22% increase in the rate of address consumption. In APNIC's case, a total of 1.90 /8 blocks per year are currently being allocated (in the 12 months to 1 July 2003), and this rate would therefore rise to 2.32 blocks per year. The assumptions on which the above analysis is made include: firstly, that the 788 LIRs in the sample are representative of all LIRs in the region; and secondly, that a consistent rate of growth will be experienced by all LIRs in the region. 6 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/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)
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