Subject: RE: DAD vs. DIID

From: "Dave Thaler" <[email-masked]>
Date: Wed, 21 Aug 2002 19:31:26 -0700

> From: Margaret Wasserman [mailto:[email-masked]]
[...]
> I'm not talking about the limited way in which the "subnet-local"
> multicast scope works today, but about the definition of a subnet.
> 
> The word "subnet" means a group of nodes that share a common global
> prefix, including subnet ID.  The fact that we don't have a multicast
> mechanism to successfully reach this group of nodes doesn't change
> the definition of "subnet".
> 
> So, a subnet may include all of the nodes on a single link -- this
will
> be the most common case, where all of the nodes are generating
addresses
> from a single set of RAs.
> 
> A subnet may also include a subset of nodes on a single link, if there
> is more than one prefix in use on the link, and not all nodes use all
> of the prefixes.

All of them have interfaces in the subnet scope zone, regardless of
whether or not they use addresses in the subnet prefix.

> Iff we adopt the multi-link subnet proposal, a subnet may also include
> a set of nodes that share a subnet prefix across multiple links.
Still
> without any requirement that the subnet contain all of the nodes on
> each link.

But still true that all of those nodes have interfaces in the subnet
scope zone regardless of what addresses they have.  

> I agree that it would be quite tricky to use an IPv6 multicast address
> to reach a particular subnet, given how multicast addresses are
> constructed.  There is no way for a given node to determine whether
> a subnet-local multicast is intended for any of its subnets.  

I disagree.
Nodes don't have subnets, nodes have interfaces and addresses.
There is a way for a given node to determine whether a subnet-local
multicast is intended for any of its interfaces, which is that
it's intended for it if the node has joined that subnet-local multicast
address on a given interface.

> Besides,
> as Pekka pointed out, none of the nodes on the link will have joined
> the subnet-local multicast group, so the traffic actually won't reach
> any of them.

Eh?  It will reach all of them, and will be received by the IPv6 layer
as long as it has joined the group.  If it hasn't joined the group, then
it's not supposed to be received by that node.  That's why it's called
multicast, not broadcast.

> IPv4 did have a concept of subnet-local broadcast (i.e.
128.224.4.255),
> but the ability to address all of the nodes on a single subnet is
> apparently lacking in IPv6.  I don't consider this a big problem, 

Here we agree in concept, but I would nitpick with your terminology.
IPv4 does not have a concept of "subnet-local" (as defined by the scoped
Addrarch) broadcast, it's really network-prefix-local broadcast, and
IPv6
doesn't have such a thing.  

> but
> we shouldn't pretend that "subnet-local" multicast is possible, since
> it isn't.

"Subnet-local" multicast is indeed possible.
"Network-prefix-local" multicast is not possible.

-Dave

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From: Pekka Savola <[email-masked]>
Date: Thu, 22 Aug 2002 10:33:42 +0300 (EEST)

On Wed, 21 Aug 2002, Dave Thaler wrote:
> Eh?  It will reach all of them, 
               ^^^^^^^^^^^^^^^^^
> and will be received by the IPv6 layer

Please compare this to your message a few days back (and remember the 
context of all-nodes -multicast addresses):

>> 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?
[...]
> The answer is yes it will (so the answer to your
> question is: no, not true).

If all nodes in the subnet have not joined the subnet-local multicast 
group, all nodes in the subnet will not be reachable through it.

There's some bad miscommunication here.

Therefore if you specify "all-nodes-in-subnet" multicast address, it is
completely useless for that purpose unless you create some very dirty
hacks.

-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords