[ppml] Proposed Policy: Adding an HD ratio choice for new IPv4 allocations

[Michael.Dillon at radianz.com]

> >  When this last came up I questioned why HD Ratio should be applied to
> >ISP allocations/assignments and received no responses. In fact, I even
> >challenged the application of HD Ratio to end-user address space on the
> >premise that the argument for HD Radio is based on the need to 
implement
> >a strict hierarchical numbering scheme. With dynamic routing protocols 
it
> >is not necessary to have a network hierarchically arranged strictly by
> >numeric address--subnets can appear in networks to which they are not
> >numerically related. 

To begin with, it's not hierarchy alone that creates the
problem. Hierarchy interacts with the bit-mask operations
used to divide network addresses and host addresses. These
bit-mask operations impose a power-of-2 structure on network
sizes and aggregate sizes. The hierarchy exacerbates the
problem because the power-of-2 rule applies at each level.

Suppose that I have a special product that requires 5
IP addresses per customer. I have 5 PoPs with 25 customers
at each PoP. Therefore, at each PoP I need 125 addresses for
a sum total of 625 addresses to cover the needs of all 
5 cities.

But IPv4 doesn't work that way. In fact, I have to give each
customer 8 addresses for a total of 200 addresses at a PoP.
That makes 1000 addresses in total, right? Wrong. I have to 
give each PoP 256 addresses, not 200. That makes for a total
of 1280 addresses. This is how hierarchy increases inefficiency
by compounding the "overhead" associated with the power-of-2
rule.

In a real network, each of the subnets will also have extra
addresses to accomodate growth because no network operator
can function by only growing their subnets by one customer
requirement at a time. The larger the network, the more this
overhead eats up addresses especially when you are trying 
to make sure that routes can be aggregated to keep the number
of individual routes low. In a small network you can ignore 
aggregation, give every customer a random block of addresses
and let dynamic routing sort out the mess. This will not
scale.

--Michael Dillon