"Charles E. Perkins" <[EMAIL PROTECTED]> writes:
> When the link-local addresses were being first discussed,
> I don't remember any discussion that suggested that a
> "link" should have a different extent than a "subnet".
Note: the term "subnet-local" has a specific meaning with regards to
multicast. Subnet-local != link-local (per addrarch).
For unicast, things are less clear. That is, for unicast, there is
link-local, site-local, and global. That's it. There is no
"subnet-local" definition per se (note that RFC 2460 does not define
the term "subnet"). My assumption has been that when folks talk about
scoping, they generally assume the scope boundaries for unicast and
multicast are the same (i.e., site-local multicast corresponds to a
site in unicast). This may not be a requirement per se, but I would
think that having two different notions of what the "site" boundaries
were would at best be confusing.
But looking at addr-arch, I see it contains the wording:
> Currently IPv6 continues the IPv4 model that a subnet prefix is
> associated with one link. Multiple subnet prefixes may be assigned
> to the same link.
So this could lead one to conclude that unicast "subnet" means
"link". And link has a *very* specific meaning (per 2460):
link - a communication facility or medium over which nodes can
communicate at the link layer, i.e., the layer
immediately below IPv6. Examples are Ethernets (simple
or bridged); PPP links; X.25, Frame Relay, or ATM
networks; and internet (or higher) layer "tunnels",
such as tunnels over IPv4 or IPv6 itself.
So, per the current specs, I conclude that in the case of unicast,
"subnet" is equivalent to "link". Personally, I much prefer that
"link" be used then because it's much more precise, and because
"subnet" in multicast terms is different than "subnet" in unicast
terms. This is confusing.
> Do you have examples of protocols that properly use "link-local"
> addresses instead of "subnet-local" protocols? It seems to me that
> DAD, multicast, router advertisements, and Mobile IP all need to
> have the "subnet-local" property, and I can't think of any
> network-layer use for the non-network-layer definition of
> "link-local".
I think we are in agreement so long as "subnet-local" ==
"link-local". Again, I think it would be better to stick with the
latter terminology, as it is more clearly defined, and at present, the
two terms seem to be equivalent. If you look at the revised addr-arch
document, however, it contains the words:
2.7 Multicast Addresses
...
subnet-local scope is given a different and larger value
than link-local to enable possible support for subnets
that span multiple links.
But this leaves open the question about whether "subnet" == "link" is
really still supposed to be the case with unicast addresses.
Thomas
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