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

[神明達哉 <[e-mail]>]

At Sun, 9 Jul 2017 16:09:14 -0500,
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;

I'm not sure if we still want to continue wordsmith rather than either
accept the current text or give up advancing the address architecture,
but otherwise I largely like your text.  A few comments:

> 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.

I'd interpret the last two sentences as RFC5942 indicating:

  A prefix length associated with a manually configured address
  determines the on-link prefix associated with the manually
  configured address. (*)

If I'm correct, while I agree on (*) it's not obvious to me that
RFC5942 directly suggests this observation.  Some more explanation
would help here.

> 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.

I'm afraid the meaning of "associated with" is not very clear here.

> 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].

Personally I prefer this text over the one in rfc4291bis-09:

   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.

since IMO "a compromise between excessive rigidity and excessive
flexibility" is actually already provided in the existing spec and all
we need is a clarification rather than tweaking the definition and
listing awkward exceptions.  But I'm not sure if your version of text
is acceptable for "draft-bourbaki-6man-classless-ipv6" authors.
Hopefully this note clears any doubt from them:

> Note: the previous paragraph implies nothing about on-link determination
> which as discussed in section 2.1 is separate from subnet assignment in
> IPv6.

but we may need to explain it in more detail at the risk of sounding
too verbose.

--
JINMEI, Tatuya