Subject: Re: DAD vs. DIID

From: [email-masked]
Date: Fri, 9 Aug 2002 15:10:37 +0200

Hi,

On Mon, 5 Aug 2002, Margaret Wasserman wrote
in "Re: Thoughts on draft-savola-ipv6-127-prefixlen-04.txt":

> In the Yokohama meeting, Steve Deering lead a
> discussion on the uniqueness of IIDs, and there
> was a pretty big majority of people in the room
> who thought that we shouldn't even require IIDs
> to be unique on a link.  Addresses need to be
> unique (obviously), but not necessarily the
> IIDs from which they are created.  The discussion
> will be documented in the minutes, and we'll offer
> a chance for people on the list to comment before
> we make any final decision, of course.

I went through the minutes and presentation of
Yokohama meeting about "DAD vs. DIID Discussion".

Maybe I am missing something here, but, if the
Interface Identifier is not unique anymore on a
link, what is it then supposed to identify and
be used for ?

Also, how would then the Link Local address be formed ?

Thanks for any clarification there,
Robert

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From: Margaret Wasserman <[email-masked]>
Date: Fri, 09 Aug 2002 09:58:44 -0400

Hi Robert,

>link, what is it then supposed to identify and

The suggestion doesn't change the purpose of an IID, just
the scope of its uniqueness.

In the current (pre-change) addressing architecture, an IID
is unique on the link.  So, within the scope of a given link,
the IID can be used to identify a particular interface (and,
by extension, a given node).  

This change would modify the scope of that uniqueness to be
a subnet prefix, not the whole link.  So, within a set of
interfaces using a particular subnet prefix, the IID would
identify a particular interface.  Most links will have
multiple subnet prefixes in use, because they will have a 
link-local prefix and a global prefix.  Some links may have
many subnet prefixes in use (link-local, one or more 
site-local subnets and one or more global subnets).

Have you seen the slides that Steve Deering presented on
this subject?  If they aren't already up on the IETF
proceedings page, they should be shortly.  They give an 
example of what this means.

Although the wording is fairly arcane in order to be 
exactly correct, the real change comes down to this:

         OLD:    IIDs are required to be unique on a link.
         
         NEW:    Only complete addresses are required to 
                 be unique on a link.

Relaxing the restriction to the second case makes the use of
privacy addresses easier, it doesn't require any major changes
to implementations (we are already doing Duplicate _Address_
Detection, not Duplicate IID Detection), and it meets the
requirements to uniquely identify a node for communication.

A side-effect of this change is that we will need to update
the Address Autoconfiguration spec to require the use of DAD
on all addresses, and not allow the currently documented
optimization to only perform DAD on the link-local address.

I can only think of two cases where the same IID would get used
for different hosts on the same link:

         - If some hosts on the link do not configure global 
                 addresses, but others do.

         - If there are two (or more) subnets running over the
                 same link, and not all of the hosts on the 
                 link are members of both (or all) subnets.

>Also, how would then the Link Local address be formed ?

Not all IIDs are used to create link-local addresses.  An
interface is required to have at least one link-local address,
and the IID used to create that address must be different than
the IIDs used to create link-local addresses on other interfaces
on the same link (as required by the per-subnet uniqueness clause, 
and as checked by DAD).  But it is not necessary, for example, to 
create link-local addresses with the randomly generated IIDs used 
to create privacy addresses. 

It is important for folks to understand what we are changing
here, so please let me know if any of the above is confusing
or unclear.

Margaret