Re: <draft-ietf-6man-rfc4291bis-09.txt>

[David Farmer <[e-mail]>]

On Wed, Jul 5, 2017 at 3:34 PM, Bob Hinden <[e-mail]> wrote:

> Hi David,
>
> > On Jul 3, 2017, at 1:22 PM, David Farmer <[e-mail]> wrote:
>
> > However, I still think the following paragraph is still too easily
> misunderstood to imply subnets must be /64 or 64 bits for both address
> generation and on-link determination.
> >
> >    Interface Identifiers are 64 bit long except if the first three bits
> >    of the address are 000, or when the addresses are manually
> >    configured, or by exceptions defined in standards track documents.
> >    The rationale for using 64 bit Interface Identifiers can be found in
> >
> >    [RFC7421].  An example of a standards track exception is [RFC6164]
> >    that standardises 127 bit prefixes on inter-router point-to-point
> >    links.
> >
> >
> > How about a note clarifying the intent of the this paragraph, something
> like this;
> >
> >       Note: While the previous paragraph does imply 64 bit subnet
> prefixes
> >       are typically assigned to most links. It does not imply anything
> >       about what portion, if any, of a subnet is considered to be
> on-link,
> >       see Section 2.1 for more discussion. However, Router Advertisements
> >       [RFC4861] specifying 64 bit on-link prefixes are typically
> >       configured on most links.
>
> Now that the text you refer to is in Section 2.4.1. "Interface
> Identifiers”, it is about Interface Identifiers, very little is said about
> prefixes (except that IIDs are required to be unique on a prefix, but even
> that is about IIDs).  I don’t think it is implying anything one way or
> about on-link properties.  I think that is covered pretty well in the new
> paragraph in Section 2.1 "Addressing Model” that includes the link to
> RFC5942.
>

As long as the others that who were insisting that the statement "IIDs MUST
be 64" means that subnets must be 64 bits for both address generation and
on-link determination are satisfied that this is not the case for on-link
determination based on what has been added to section 2.1, then I could
live without this.

However, I like the idea of having some kind of recommendation for the
on-link prefix.  I don't think there is guidance anywhere else as to what
the on-link prefix should normally be, at least explicitly, and the
addressing architecture seems like as good of a place as any to recommend
something in this regard.

Instead of directly recommending 64 bits, maybe in section 2.1 recommend
that in most cases on-link prefixes are configured the same as the subnet
prefixes they are associated with.  Combined this with 64 bit IIDs and
therefore 64 bit subnet prefixes, you get a recommendation for 64 bit
on-link prefixes in most cases.

While IIDs were the primary controversy, there were several other points
raised during the Last Call discussion, is there a plan to revisit any of
these as well?

Thanks.

-- 
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David Farmer               Email:[e-mail]
Networking & Telecommunication Services
Office of Information Technology
University of Minnesota
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Minneapolis, MN 55414-3029   Cell: 612-812-9952
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