[ppml] HD Ratio changes

Michael.Dillon at radianz.com Michael.Dillon at radianz.com
Tue Feb 10 09:23:04 EST 2004

>  When this last came up I questioned why HD Ratio should be applied to
>ISP allocations/assignments and received no responses.

With respect, I did say then, and I say again,
this is all detailed in the RFC for applying
the HD Ratio to IPv6 and in Paul Wilson's
discussion paper of last year. I admit that I
made some mistakes and posted the wrong URL for
one of those documents, but that has now been

> 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 
>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. 

Not everyone can afford the routing table overhead that
this entails. In fact, evading the use of hierarchy
makes this non-scalable and therefore non-usable for
larger networks. Hierarchy is the primary tool that
we have for making complex networks scalable. This is
a basic principle of physics that we cannot discard and
that we ignore at our peril. The larger your network becomes
the more important hierarchy becomes and the more overhead
is consumed because of hierarchy.

>that respect, the more people rely on dynamic routing to permit more
>flexible distribution of address space, the easier it is to manage 
>space and the more they can conserve (and the more they avoid the 
>driving HD Ratio).

We really cannot impose specific technical solutions 
as a matter of policy when there is no overriding policy
goal to justify it. In this case, the overriding policy 
goal which justified the 80% utilization rate is gone.
We no longer have a looming shortage of IPv4 addresses
because our existing stock will last 20 years. We have
a fully functional replacement for IPv4 which does not
have the same addressing constraints. And the large population
asian nations have shown that they will move towards the
IPv6 solution rather than increasing pressure on the IPv4
address space. Since the overriding policy goals have
now shifted, it is time to rationalize the policy to

> Regardless of the above, all of the references about the need for HD
>Ratio relate to end-user address assignment issues and don't seem to be
>relevant to allocating, or re-allocating (here we go with terminology
>again) blocks of addresses by ISP's. Frankly, I can't see how it would be
>relevant to that. 

The process of allocation, reallocation, and assignment
divides a large address block into a larger number of smaller
blocks. While it might be easy to get to 80% utilization
of 1 large block, as we divide into smaller blocks we
lose the effect of the law of large numbers. This means
that some of the small blocks will reach 80% before others
and we must start shifting addresses around robbing peter
to pay paul. If there are several levels to this hierarcy
of dividing, sub-dividing and sub-sub-diving, then we reach
a point at which the time period represented by 20% (100%-80%)
of an address pool is very short. That means that we could 
use up the 20% in a very short time because it is a fairly
small amount. In order to maintain a reasonable time buffer
we need to reserve more than 20% unused addresses from a
/23 even though 20% might be sufficient from a /17. The 
cumulative impact of all these buffers adds up to more than
20% overall when you add up all the /23s in a /17. That's
the hierarchical overhead.

>I'm, once again, concerned that this approach unnecessarily increases 
>and complexity and in doing so provides questionable relief.

This approach does not unnecessarily increase 
complexity because it is opt-in. If an organization is
satisfied with the status quo they can continue as before.
However, those organizations who need change can opt for
the slightly more complex calculation if the benefits outweigh
the added complexity.

As for waste, I do not consider hierarchical overhead 
to be waste. This overhead is part of the structure of
VLSM (Variable Length Subnet Masking) which is a fundamental
part of any modern IPv4 network. If you have a subnet with 5
interfaces on it and you assign 8 addresses to that subnet
then you are *NOT* wasting 3 addresses. Those 3 addresses
are part of the inevitable overhead of VLSM (and CIDR).

RFC 3194

Paul Wilson's discussion paper

Paul Wilson's slides

--Michael Dillon

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