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Re: IPv6 end user addressing
From: Owen DeLong <owen () delong com>
Date: Sat, 6 Aug 2011 09:36:33 -0700

On Aug 6, 2011, at 3:15 AM, Jeff Wheeler wrote:

On Sat, Aug 6, 2011 at 5:21 AM, Owen DeLong <owen () delong com> wrote:
At least don't make your life miserable by experimenting with too many different assignment sizes,
or advocate /64s or something, that's considered a design fault which will come back to you some day.
Read the RfCs and RIR policy discussions in the archives some years ago.

Note that in this thread, you advocate three things that are a little
tough to make work together:
* hierarchical addressing plan / routing
* nibble-aligned addressing plan
* minimum /48 per customer

Hasn't been hard so far.

If I were, for example, a hosting company with IPv6 terminated at the
layer-3 ToR switch, I would then use a /40 per rack of typical
"dedicated servers."  If you then want some bits to be a POP-locator
field for your hierarchical routing scheme, you are already forced to
request more than a /32.  The number of customers per layer-3 device
for typical end-user access networks was around the same into the
late-1990s/early-2000s, as ISPs had racks of Portmasters or whatever
box of choice for terminating dial-up.

I think we were talking about access customers. I don't see giving /48s
to individual dedicated servers as I don't see them having hierarchy
behind them. I would think that a /48 per TOR switch would be
reasonable in that case.

However, requesting more than a /32 is perfectly reasonable. In
the ARIN region, policy 2011-3.

Densities have changed, but this doesn't necessarily win you an
advantage when combining those three properties.  This is especially
true if you consider that density may change in a difficult-to-predict
manner in the future -- a BRAS box with a couple thousand customers
today might have three times as many in a couple of years (IPv6 is
supposed to help us avoid renumbering or injecting additional routes
into our network, right?)  As an access provider, if I shared your
view, I would be reserving a /36 or /32 per BRAS box.  If I then want
some additional bits for hierarchical routing ... I'm going to need a
pretty large address block for perhaps a pretty small number of
customers.  After all, my scheme, applying your logic, dictates that I
should use a /32 or perhaps a /28 per each POP or city (I need to plan
for several BRAS each), even if I don't have a lot of customers today!

Correct… I think a /36 per BRAS seems about right, but if you want
to put more than 3072 customers on a single BRAS and expect
technology to eventually support that, sure, go for a /32 per BRAS.
If you want to isolate your routing down to per BRAS (most providers
I'm aware of that have multiple BRAS have it set up so any customer
in a given POP may connect to any BRAS and they carry the
customer prefixes within the POP's routing table), then, I think a
/36 per BRAS is probably OK (up to 3072 customers at 75%
utilization, 4096 max), but, if you really think you will have 6,000
customer BRAS units, then, yes, a /32 would be correct.

However, I would be more likely to assign hierarchy per POP
instead. Figure out how many customers are likely in my
largest POP and allocate based on /48 per customer+growth
in that POP. For example, if my largest POP would serve 2,000
customer end-sites, I'd be looking at a /36 per POP. If the largest
POP served 3,073 customer end-sites or even 49,000 customer
end-sites, I'd plan on a /32 per POP.

Basically take the number of customer end-sites in your largest
POP, divide by 0.75 (keep 25% headroom minimum) and then
round up to the nearest nibble.

If you really think you will have more than 786,432 customer
sites served from your largest POP, then, I suppose a /28 per
POP is a reasonable request. That seems like pretty unlikely
density, even in 20 years. I realize that customer density in
urban areas does tend to increase, but, assuming a maximum
50% market penetration, serving a city the size of Philadelphia
out of a single POP still seems unlikely to me.

I think /56 is more sensible than /48, given the above, for most
end-users.  Either way, the users will be gaining a lot more
flexibility than they have with IPv4 today, where they probably get
just one IP address and have to pay a fee for any extras.  Giving the
typical end-user 8 fewer bits worth of address space allows the ISP
network more flexibility for hierarchical routing before they have to
go to their RIR and figure out how to justify an out-sized allocation.

Why? You point out that giving out /48s would require larger IPv6
allocations than /32, but, you don't give any reason to think this is

Let's look at the math. Let's assume a moderately large provider
expects to serve 45,000 customer end-sites out of their largest
POP (does anyone actually expect numbers this large in a
single POP)?

Now, let's say you have 50 POPs across your service region
and expect to triple in size over the next 5 years. That's 150
total POPs planned.

45,000 customer end sites with 25% minfree is 60,000
which, when rounded up to a nibble is 16 bits for 65,536.

150 POPs with 25% minfree is 200 which, when rounded
up to a nibble is 8 bits for 256 total POPs possible.

16+8 = 24, so, such a provider would need a /24 for their
access network.

There are 512 /24s in each of the /12s (what the IANA issues
to RIRs).

Also, if folks would stop thinking that every subnet should be a /64,
they will see that end-users, makers of set-top-gateways, or whatever,
can certainly address a whole lot of devices in a whole lot of subnets
even if the user is only given a /64.  Do we think DHCPv6 won't be the
most common way of assigning addresses on SOHO LANs, and that SLAAC
will be essential?  I, for one, think that some ISPs will be sick and


twisted enough to hand out /128s so they can continue charging for
more IP addresses; but certainly the makers of IPv6-enabled devices
will foresee that end-user LANs might not be /64 and include the
necessary functionality to work correctly with smaller subnets.

I think natural selection in the market will solve that problem relatively

Before you beat me to it, yes, we seem to have completely opposing
views on this subject.  I will change my mind when I can go to the RIR
and get a IPv6 /24 for a small ISP with a few POPs and a few tens of
thousands of customers.  Should RIR policy permit that sort of thing?

OK… Start changing your mind… Read 2011-3. It's been adopted
in the ARIN region. I'd also like to suggest you consider supporting
APNIC proposal 98 which is essentially the same policy and will be
discussed at the Busan meeting.


Attachment: smime.p7s

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