Re: WG ACTION: 1 week continued discussion [LL70] DNAv4 normative or informative?

> I propose rewording your latest text as below. I hope that in editing your text I have made this
> section clearer. I have removed most use of "configured" and all use of
> "valid". I have added a little to make clear that these are transitional
> cases only. I have introduced and defined the term "operable" and made
> plain that there is no easy rule to determine operability. There is no
> need to define "valid" or anything extra in the terminology section at
> all.

The change from "valid" to "operable" is helpful, I think.  Comments below.

>    For these reasons, a host
>    SHOULD NOT have both a routable address and an IPv4 Link-Local address
>    configured on the same interface.

This advice no longer applies if one of those addresses is not "operable",
right?

>    When an operable routable address is
>    available on an interface, the host SHOULD NOT also assign an IPv4
>    Link-Local address on that interface. However, during transition (in
>    either direction) between using routable and IPv4 Link-Local addresses
>    both MAY be in use at once subject to these rules.

Should it say "these rules:"?  If so, perhaps the rules should be
bulleted.

>    The assignment of an IPv4 Link-Local address on an interface is based
>    solely on the state of the interface, and is independent of any other
>    protocols such as DHCP.  A host MUST NOT alter its behavior and use of
>    other protocols such as DHCP because the host has assigned an IPv4
>    Link-Local address to an interface.
>
>    The term "operable address" is used to mean an address which works
>    effectively for communication in the current network context (see below).
>    If a host finds that an interface that was previously configured with
>    an IPv4 Link- Local address now has an operable routable address
>    available the host MUST use the routable address when initiating new
>    communications, and MUST cease advertising the availability of the
>    IPv4 Link-Local address through whatever mechanisms that address had
>    been made known to others.  The host SHOULD continue to use the IPv4
>    Link-Local address for communications already underway, and MAY
>    continue to accept new communications addressed to the IPv4 Link-Local
>    address. Ways in which an operable routable address might become
>    available on an interface include:
>
>    * Manual configuration
>    * Address assignment through DHCP
>    * Roaming of the host to a network on which an existing address
>       becomes operable.

Does this have the same meaning as "a previously assigned address becomes
operable"?

Here's a proposed version, with changes:

1.9.  When to configure a Link-Local IPv4 address

   Having addresses of multiple different scopes assigned to an
   interface, with no adequate way to determine in what circumstances
   each address should be used, leads to complexity for applications and
   confusion for users.  A host with an address on a link can communicate
   with all other devices on that link, whether those devices use Link-
   Local addresses, or routable addresses.  For these reasons, a host
   SHOULD NOT have both an operable routable address and an IPv4
   Link-Local address configured on the same interface. The term
   "operable address" is used to mean an address which works
   effectively for communication in the current network context
   (see below).  When a operable routable address is available
   on an interface, the host SHOULD NOT also assign an IPv4
   Link-Local address on that interface.  However, during the
   transition (in either direction) between using routable and IPv4
   Link-Local addresses both MAY be in use at once subject to these rules:

   1. The assignment of an IPv4 Link-Local address on an interface is
      based solely on the state of the interface, and is independent of
      any other protocols such as DHCP.  A host MUST NOT alter its
      behavior and use of other protocols such as DHCP because the host
      has assigned an IPv4 Link-Local address to an interface.

   2. If a host finds that an interface that was previously configured
      with an IPv4 Link-Local address now has an operable routable
      address available, the host MUST use the routable address when
      initiating new communications, and MUST cease advertising the
      availability of the IPv4 Link-Local address through whatever
      mechanisms that address had been made known to others.  The host
      SHOULD continue to use the IPv4 Link-Local address for
      communications already underway, and MAY continue to accept new
      communications addressed to the IPv4 Link-Local address. Ways in
      which an operable routable address might become
      available on an interface include:

         * Manual configuration
         * Address assignment through DHCP
         * Roaming of the host to a network on which a previously
           assigned address becomes operable.

   3. If a host finds that an interface no longer has an operable routable
      address available, the host MAY identify a usable IPv4 Link-Local
      address (as described in section 2) and assign that address to the
      interface. Ways in which an operable routable address might cease to
      be available on an interface include:

         * Removal of the address from the interface through manual
           configuration
         * Expiration of the lease on the address assigned through DHCP
         * Roaming of the host to a new network on which the address is no
           longer operable.

   The determination by the system of whether an address is "operable" is
   not clear cut and many changes in the system context (e.g.
   router changes) may affect the operability of an address. In
   particular roaming of a host from one network to another is likely -
   but not certain - to change the operability of a configured address but
   detecting such a move is not always trivial.

   "Detection of Network Attachment (DNA) in IPv4" [DNAv4]
   provides further discussion of address assignment and operability
   determination.