Re: IPv6 addresses really are scarce after all

Thomas,

A few additions to your description of how we got to where we are nowemail.

RFC3177, where the /48 recommendation was made, used the H ratioanalysis to explain why a /48 was acceptable. However the IETF didnot make any recommendation to the RIRs that the H ratio (currentversion is now called HD ratio) should be used by the RIRs in theirallocation process nor what specific HD ratio be used. These choiceswere made the RIRs when they developed their IPv6 allocation policies.

In my view the potential problems you describe have more to do withthe specific HD ratio the RIRs choose to use as opposed to the /48.It's my understanding the HD-ratio value that the RIRs are now usingis much more conservative than before. This should, as I understandit, avoid the overall problems you describe (e.g., cable modem, DLS,etc.).

While I agree that 64K subnets is a lot of subnets for a home user,my reluctance to formally change the /48 recommendation is that itwill lead to much more restrictive allocation policies. Just likethe one that started this thread:

* /64 - Site needing only a single subnet.
* /60 - Site with 2-3 subnets initially.
* /56 - Site with 4-7 subnets initially.
* /52 - Site with 8-15 subnets initially.
* /48 - Site with 16+ subnets initially.

If this is what we would expect to get by changing the RFC3177 /48recommendation, then in my view is not a good idea.


Bob




On Aug 24, 2007, at 2:34 PM, ext Thomas Narten wrote:

Some background and history.

The IETF gave it's view on where the boundary for IPv6 blocks to end
sites should be (i.e., the /48 recommendation) in RFC 3177 back in the
2000-2001 time frame.

At that time, there were plenty of folk in the IETF that thought the
IETF could/should set address policy, notwithstanding fact that the
RIRs were doing that for IPv4.

RFC 3177 was a way for the IETF to make a recommendation to the RIRs,
without stepping on RIR toes (too much). Note that it is only a
recommendation, and the RFC is informational.

The fact is, the /48 boundary is _NOT_ architectural, and saying it is
doesn't make it so. I challenge anyone to find a standards track
document that relies on /48 being part of the architecture. (And you
might want to have a look at
http://tools.ietf.org/id/draft-narten-ipv6-3177bis-48boundary-02.txt
as it does mention a couple of related things.)

What the appropriate size of an assignment for an end site should be
is really more of an operational issue than architectural one. If a
site gets too little, and needs to get more later (maybe at the cost
of renumbering) that is an operational issue. And the idea that we can
give out "enough" address space to a site so that it doesn't have to
renumber (ever) is pretty silly. After all, most end sites will need
to renumber when they switch providers. Surely, we hope/expect a large
number of such provider switches to take place over a 5 or 10 year
period anyway?

The RIRs adopted the "/48 to everyone" in its initial stab at IPv6
policy back in 2002. Even at that time, there was much screaming and
gnashing of teeth from the operator community saying "wasteful",
"excessive", "really stupid", etc. So, even though all the RIRs
adopted the policy anyway, the unhappiness with /48 (as a general
recommendation) didn't go away, and was repeated often at subsequent
RIR meetings...

Then, 2-3 years ago, RIPE made some really really big IPv6 allocations
to a few ISPs. This raised eyebrows, because it raised the question
that if so much space could legitimately be given out (and it was much
much more than the operators would have been given for the same number
of IPv4 customers), it was conceivable to imagine scenarios in 50-100
years where the IPv6 free pool was signficantly used. As one Cable
Modem operator explained it to me:

      If I assign 4M /48's of IPv6 (one to each cable modem on my
      network), according to the HD-ratio I am justified to obtain
      something around a /20 of IPv6 addresses.  In other words, I am

justified in getting 268M /48's even though I am only using4M of

      them.  That would be enough for me to assign at least two for
      every household in the US (not just the 19M on my network).

      Now if all the cable providers (e.g., Comcast, Cox, Adelphia,
      Cablevision, Time-Warner, etc.) did the same for their networks;
      and each of the DSL companies made a similar move (SBC, Verizon,

Quest, etc.); perhaps we could easily see the broadbandmarket in

      the US alone obtaining some 16 /20's of IPv6 or a total of /16.
      There are only 8192 of those available in the current global
      unicast space of 2001::/3.

      Anyhow, you can see where this might lead...

This led to a bunch of discussions and policy proposals in the RIRs,
culminating with the adoption in ARIN, RIPE & APNIC of recommendations
that adjusted the HD ratio thresholds and moved away from "/48 for
everyone", making it an ISP (or LIR in RIR terminology) choice. The
community feedback was that LIRs were smart enough to make reasonable
assignments based on actual customer need.

If one does the math, giving every home user a /56 instead of a /48
provides almost two orders of magnitude more headroom in terms of
address usage. And at what cost? Surely, everyone will agree that
giving a /56 to home sites is more than enough space for the foresable
future! That's enough for 256 subnets per home site! That's an
incredible amount of address space!

(I have a really hard time reconciling the previous paragraph with the
subject of this thread.)

It is worth noting that there are no _firm_ recomendations in the
current RIR policies (in the sense of saying you must give home users
a /56) , because in discussions at ARIN and APNIC in particular, there
was pushback from the operations folk on being to prescriptive. They
argued that they are in the best position to decide reasonable
assignment sizes and wouldn't support more rigid guidelines. That
leads me to doubt that the specific propoasl that was mentioned that
started this thread will actually get much traction within ARIN, but
that is not my problem. :-)

Folk should note that a policy proposal submitted to an RIR is not
unlike an individual submission "submitted" to the IETF.  Best to
treat it as such. YMMV.

I find the fact that RFC 3177 has not been revised to reflect the
reality of today is a bit disapointing. Even in some of the messages
of this thread, I sense denial about the proper/actual role the IETF
plays in setting address policy. I see some of the same people arguing
here that /48 should not be changed, despite having made the same
arguments on RIR lists and having lost... There is water under the
bridge here folk...

The reality is that "/48 for everyone" has been overcome by events. If
you don't like that, take it to the RIRs.

And, FWIW, I was one of those that pushed for the changes.  As one who
originally supported of the /48 recommendation in RFC 3177, I think it
was a mistake. Giving a /48 to every home user by default is simply
not managing the address space prudently. Home users will do more than
fine with a /56.

Thomas

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