Re: Thoughts on IPv6 Address Assignment to End Sites

[Brian E Carpenter <brian.e.carpenter@gmail.com>]

On 2008-08-02 10:23, Fred Baker wrote:

...
>>  This document updates and replaces RFC 3177.
> 
> If I understood Thomas correctly on Thursday, it does not replace 
> RFC 3177. It updates certain parts, perhaps just one, which for clarity 
> should be identified by section number, leaving the rest intact and in
> effect. Specifically, it addresses the comments regarding a /48 being 
> assigned to end sites. 

I just re-read 3177. It's really quite short and the notion of
a standard /48 allocation runs right through it. So I think it makes
more sense in the end to consider that it's completely replaced by
the new document. I'd be happier if the above sentence could read:

The IAB and IESG have agreed that this document replaces RFC 3177.

("updates and replaces" seems redundant.)

There's a paragraph in 3177 that starts:
  "It is not obvious, however, that all edge networks are likely to be
   recursively subnetted..."

I believe that paragraph, suitably tweaked, is worth rescuing and
adding to the draft.

<snip>

>>  itself (e.g., collapsing subnets). Hence renumbering a site into a
>>  prefix that has (at least) the same number of subnet bits is more
>>  straightforward, because only the top-level bits of the address need
>>  to change. A key goal of the RFC 3177 recommendations is to ensure
>>  that upon renumbering, one does not have to deal with renumbering
>>  into a smaller subnet size.
> 
> I have heard this argument, and I think it fails pretty dramatically. In
> my home, I have four subnets - a delegated prefix in my office, a prefix
> for the wired domain, and prefixes for two wireless domains (something
> about steel and water in the walls and their effect on 802.11
> frequencies). In two income homes, one could easily imagine a prefix for
> each office, and one could imagine a separate prefix for the
> grandmother's apartment or the pool. Following my reasoning about
> hexadecimal digits and human cognition - and your third point below -
> that would suggest that SOHO networks be allocated /60s, where SMBs
> might get /56s and larger enterprises /52s or /48s. If I am using five
> /64 prefixes out of a /48 allocated to me and am forced to renumber
> (gasp!) into a /60, it isn't so very hard. I am going to map the 13 bits
> of zero into a single bit of zero, and be done.
> 
> Where this argument makes sense is a network whose manager has used
> O(2^N) prefixes out of a larger allocation, and changes providers to one
> that gives him 2^(N-1) prefixes or less. Such an administration either
> intended to redesign its network or was incompetent in some way.

I don't quite agree about 'incompetent'. If a site was leased
a /48 by ISP A, and then decides to multihome to ISP B, and discovers
that B's pricing structure means that only a /56 is affordable,
then a lot depends on the approach taken to allocations under the /48.
For example, perhaps the IT department decided to use a /56 for each
major building on the company's campus. That was short-sighted perhaps,
but was it incompetent? That's a strong word.

The resulting recommendation to user sites is something like: don't
allocate subnets sparsely within your prefix, but concentrate
them. Or to say it another way: if your ISP gives you a /N prefix,
plan your subnets to fit inside a single /(N+4) or /(N+8), in case
you ever sign up with an ISP that hands out longer prefixes.

I suggest putting some such recommendation into the draft.

    Brian