Re: IPv6 Routing & ND vs. Addressing, (Was: Re: <draft-ietf-6man-rfc4291bis-09.txt>)

[Lorenzo Colitti <[e-mail]>]

On Mon, Jul 10, 2017 at 6:09 AM, David Farmer <[e-mail]> wrote:

> So, What should we say in RFC4291bis? Personally I'd prefer we eliminate
> the term subnet from RFC4291bis, but assuming that won't happen how about
> the following;
>
> Currently, IPv6 continues the IPv4 model in that a subnet prefix is
> associated with one link. Multiple subnet prefixes may be assigned to the
> same link.  The relationship between links and IPv6 subnet prefixes differs
> from the IPv4 model in that the prefixes used for on-link determination are
> separate and may differ from the assigned subnet prefixes. However, on-link
> prefixes identical with assigned subnet prefixes are recommended. A prefix
> length associated with a manually configured address determines the on-link
> prefix associated with the manually configured address. See [RFC5942] "The
> IPv6 Subnet Model: The Relationship between Links and Subnet Prefixes" for
> more details.
>
>
> Note: Any on-link prefixes longer than a covering assigned subnet prefix
> must be associated with the same link as the assigned subnet prefix. Also,
> any assigned subnet prefixes longer than a covering on-link prefix must be
> associated with the same link as the on-link prefix.
>
>
> ----
>
> IPv6 unicast routing is based on on-link prefixes of any valid length up
> to 128 [BCP198], which are not necessarily the same as the subnet prefixes
> assigned to a link.
>
>
> ----
>
> When forming or assigning unicast addresses, except those that start with
> the binary value 000, Interface IDs are required to be 64 bits long.  The
> rationale for using 64 bit Interface Identifiers can be found in [RFC7421].
>
> Note: the previous paragraph implies nothing about on-link determination
> which as discussed in section 2.1 is separate from subnet assignment in
> IPv6.
>
>
Thanks for putting this all together. I think it's the best assessment of
the current state of the standards that I've seen so far.

I'm not sure the text covers RFC6164 though; I'm not sure whether current
deployments of /127 meet the "Any on-link prefixes longer than a covering
assigned subnet prefix must be associated with the same link as the
assigned subnet prefix" text. I'd expect many networks to number entirely
separate links using /127s that are all drawn from one covering /64; at
least, I know that I've written addressing plans that do that and are still
in use.

Cheers,
Lorenzo