Subject: RE: DAD vs. DIID

From: Margaret Wasserman <[email-masked]>
Date: Tue, 20 Aug 2002 16:41:05 -0400

At 03:17 PM 8/20/02 , Brian Zill wrote:
>Hi Charlie,
>
> > I haven't studied the multi-link subnet draft.  But, in order 
> > to be responsive to your note before taking that time...
>
>I guess we're just going to have to disagree about link-local vs.
>subnet-local.  These are two distinct scopes and should be treated as
>such by the architecture.  One shouldn't assume that because you can
>reach a node via a global address in the same subnet prefix as you that
>you can also reach it via a link-local address.  "DIID" breaks this by
>making the false assumption that these scopes are equivalent.

I agree that link-local and subnet-local would be different scopes.

What surprised me is the assumption that a subnet scope would be "larger"
than a link-local scope.  I've had experience running multiple subnets
on a single L2 link, but I have not had experience running a single
subnet across multiple L2 links.

So, while you indicate that a link-local address may not be able to 
reach all nodes on a subnet, isn't it also true that a subnet-local
address may not be able to reach all of the nodes on a link?

Margaret

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From: Robert Elz <[email-masked]>
Date: Wed, 21 Aug 2002 13:10:59 +0700

Date:        Tue, 20 Aug 2002 16:41:05 -0400
    From:        Margaret Wasserman <[email-masked]>
    Message-ID:  <[email-masked]>

  | What surprised me is the assumption that a subnet scope would be "larger"
  | than a link-local scope.  I've had experience running multiple subnets
  | on a single L2 link, but I have not had experience running a single
  | subnet across multiple L2 links.

Let me try to answer this a different way.   That is, while it is certainly
possible to run multiple subnets over an L2, this doesn't make them have
smaller scope than the L2.   The nodes that are only in one of the subnets
are that way by choice - they could be in any of them.   Any packet on the
link is available to all of the nodes connected to it (this is more or less
the canonical definition of the link layer).   So, you can't be connected to
a link and be in a smaller scope, unless you're looking inside L2 (or lower)
protocols, at L3 and above, the link is the basic unit.

  | So, while you indicate that a link-local address may not be able to 
  | reach all nodes on a subnet, isn't it also true that a subnet-local
  | address may not be able to reach all of the nodes on a link?

So, no, it is able to - though it may be that some of the nodes might
not choose to receive it.

On the other hand, a multi-link subnet (which as a concept takes some
grasping, I'm not yet sure I'm convinced of the need) binds multiple
links (unitary objects) into one L3 object.   Here it is clear that
it is possible to have internal divisions, and that one link would
be a smaller thing than than a subnet.

On the other hand, I'm not sure that I would treat the needs of
multi-link subnets as being compelling enough to prefer DAD over
DIID if there were other compelling arguments in favour of DIID.
That's because I'm not sure I see multi-link as all that compelling
without this extra issue.

But, here I don't see compelling arguments for DIID, if anything,
even without multi-link DAD seems like a better choice.   Given that,
then that DAD also doesn't shut the door on multi-link is just another
factor in its favour.

kre