[ppml] Proposed Policy: IPv6 HD ratio

Hi Leo.

> I have wondered why we don't use a flat measurement as we already
> do, simply relaxed to fit the additional v6 "free use of IP
> addresses".

One theory is that larger sites have multiple levels of hierarchy. At
the lower levels of the hierarchy, one can presumably flat route, in
which case high densities are achievable.

But if you have a hierarchy (with aggregation), each level in the
heirarchy will have a level of inherent inefficiency, and the total
inefficiency is multiplicative. E.g., 80% utilization  at level one combined
with 80% utilization at the next level up is only 64% utilization when
multiplied together.

One of the arguments being made for moving the HD ratio threshhold
upward is that a ratio of .8 is more in line with something like 12
levels of heirarchy in the network, a level that doesn't seem in line
with reality.

Thomas