RIPE Policy Proposal 2005-01

Internet address space is managed hierarchically:

• IANA allocates space to Regional Internet Registries (RIRs).
• RIRs allocate space to Local Internet Registries (LIRs).
• LIRs assign space to End Users.

At each level, some address space may be reserved for future expansion and/or efficient aggregation. As more hierarchical levels are introduced, the overall efficiency of the address space usage decreases.

The HD ratio (Host-Density ratio) is a way to measure address space usage [RFC 3194]. The HD ratio value can relate to a percentage of usage, which decreases as the amount of address space grows. This allows for the decreasing efficiency that occurs with more hierarchical levels.

The HD ratio is currently used to measure IPv6 address space usage [ipv6-address-policy]. The IPv6 Address Allocation and Assignment Policy considers a block of IPv6 address space to be ‘used’ when its HD ratio reaches 0.80. This is a manageable figure ("values of 80% or less correspond to comfortable trade-offs between pain and efficiency" [RFC 3194]).

This document proposes using the HD ratio to measure IPv4 usage. The proposed value of the HD ratio for IPv4 is 0.96.

a. Current: "An LIR may receive an additional allocation when about eighty percent (80%) of all the address space currently allocated to it is used in valid assignments or sub-allocations."

b. New: "An LIR may receive an additional allocation when its total allocated address space usage meets the HD-Ratio value of 0.96."

a. Background
The current document, “IPv4 Address Allocation and Assignment Policies for the RIPE NCC Service Region” [ipv4-address-policy], considers a block of IPv4 addresses to be ‘used’ when 80% of the addresses within the block have been sub-allocated or assigned. This is applied to all address blocks, regardless of size.

Current policies assume a hierarchical system of address space delegation. However, they do not make any 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 Points of Presence (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 not recognised by the current policy framework and are highly restricted by the "80% rule". As a result, managing large blocks is often 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. However, address management policies must also be practical in terms of how much management overhead they cause. When large amounts of address space are involved, the "80% rule" can result in more work for an LIR.

Basing usage on the HD ratio should lead to equal levels of management overhead across the board, rather than penalising the holders of large address blocks.

b. Impact
To see a rough estimation of the immediate impact of this proposal, an HD Ratio value of 0.96 was applied to the average amount of address space held by an LIR in the RIPE NCC Service Region. This showed that on average, LIRs would qualify for an additional allocation block when they have assigned or sub-allocated about 59% of their allocated address space.

c. Arguments supporting the proposal.
This proposal fairly takes into account addressing hierarchies used in large and extra-large registries and introduces a useful level of flexibility for those registries
The local Internet registries using the 80% criteria may continue to do so and will not be impacted by the new policy.
The RIPE NCC will provide support to minimise complicated calculations or administrative burden to LIRs.

d. Arguments opposing the proposal.
This proposal will have some limited impact on IPV4 address consumption.

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The HD ratio is calculated as follows [RFC 3194]:

Where:

Note: The current IPv4 policy considers addresses to be ‘used’ once they are assigned or sub-allocated by the LIR.

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We should decide an appropriate HD ratio value on a rational basis. To do this, we make certain assumptions about the number of "hidden" hierarchical levels involved in managing address blocks of various sizes. If we assume there is 80% usage at each level, we can easily calculate the overall usage.

The following table proposes a set of hierarchical levels which we can reasonably expect within different amounts of address space. If a usage of 80% is achieved at each hierarchical level, then the overall usage will be (0.80 to the power of "n"). It is then possible to calculate HD ratio values from this value.

Size range (prefix)	Level(n)	Utilisation (0.80**n)	HD ratio
/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 levels of hierarchy listed above are based on assumptions about the likely size and structure of LIRs holding address blocks of these sizes. A reasonable HD ratio value may be 0.96 (a round figure which occurs within most of these ranges) from the table above. The following table gives the usage requirements for IPv4 address blocks from /24 to /8 for this value.

IPv4 prefix	Addresses total	Addresses used	Util%
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, but the same calculations can be made for non-CIDR blocks.

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

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[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] RIPE NCC document: "IPv6 Address Allocation and Assignment Policy" http://www.ripe.net/ripe/docs/ipv6policy.html
[ipv4-address-policy] RIPE NCC document: "IPv4 Address Allocation and Assignment Policies for the RIPE NCC Service Region"
http://www.ripe.net/ripe/docs/ipv4-policies.html