> > 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.
While I'm not sure that I agree with the terminology, since this is a change
from the common use of the word "subnet", and doesn't really agree with the
use of the term "subnet ID" in the addrarch, I now understand your distinction.
I don't think that we should overload the term "subnet" in this way, so
we should probably change one of these terms.
Using your terminology (I hope)...
A "subnet" is defined to be a scope that encompasses one or more links.
It can be as small as a single link, or as large as a whole site. The
definition of a "subnet" is not related to how network prefixes are
configured on those links and/or nodes. In particular, it has no special
relationship to the "subnet ID" of a unicast address.
In IPv6, we have the ability to send subnet-local multicast traffic,
but not subnet-local unicast traffic.
The point of "multi-link subnets" (let me know if I'm wrong) is to allow a
network prefix to span multiple links. So, really, we are talking about
"multi-link network prefixes", not subnets at all.
In order to make "multi-link network prefixes" work properly with
mechanisms such as DAD and RA, we need to forward certain link-local
multicast packets between those links. This makes "multi-link network
prefixes" more messy and complex than they would be if they'd been
planned into the architecture from the beginning.
Folks have discussed the fact that subnet-local multicast addresses should
really have been used for DAD, RA, etc. But, this doesn't necessarily make
sense. If you buy-in to the concept of "multi-link subnet prefixes", you'd
want these messages to span all of the links that are intended to share a
set of network prefixes. It isn't clear to me that this is the same set of
links that would comprise a single multicast subnet, but you could probably
define it that way, if desired.
But, even if it would help to use subnet-local multicast, we couldn't
change this now for two reasons:
1 - These mechanisms are already widely implemented using
link-local multicast addresss.
2 - We haven't defined the equivalent of a subnet-local all-nodes
multicast address and/or a subnet-local solicited-nodes
multicast address. So, we don't currently have the means
to reach the right sets of nodes at the subnet-local scope.
The addrarch, ND and autoconf RFCs currently make the architectural
assumption that a full unicast network prefix (site-local and/or global
prefix PLUS subnet ID) will only span a single link, which is, apparently,
in no way related to what scope a subnet-local multicast packet will span.
Do you agree?
So, really, the question is:
Do we want to modify the IPv6 architecture so that a single unicast
network prefix can span more than one link? By definition, a subnet
already spans multiple links.
Margaret
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