Hi Dave,
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. 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. 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. 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. 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, but we shouldn't pretend that "subnet-local" multicast is possible, since it isn't. Margaret At 07:37 PM 8/20/02 , Dave Thaler wrote: > > -----Original Message----- > > From: Margaret Wasserman [mailto:[EMAIL PROTECTED]] >[...] > > 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? > >Since the boundary of a zone goes through a node, not through a link >(ref: scoped addr arch doc), then no it is not true. The "subnet-local" > >scope contains all interfaces on the link regardless of what addresses >are on those interfaces. > >However, there are no subnet-local unicast addresses (only multicast >ones) >so the question is moot in practice. The closest question you could >construct in practice is: "If a link has multiple subnet prefixes >assigned to it, and I source from a global address in one of them, and >the destination is a subnet-local multicast group, will another machine >on the same link, which does not have an address in the same prefix, >receive the packet." The answer is yes it will (so the answer to your >question is: no, not true). > >-Dave -------------------------------------------------------------------- IETF IPng Working Group Mailing List IPng Home Page: http://playground.sun.com/ipng FTP archive: ftp://playground.sun.com/pub/ipng Direct all administrative requests to [EMAIL PROTECTED] --------------------------------------------------------------------
