I made the mistake of allowing my arm to be twisted into reviewing
draft-ietf-ipngwg-addr-arch-v3-10.txt last week, and was sad to find
what appears to be an ambiguity in some of text that deals with
subnet-scope multicast. Given that this document was already before
the IESG at the time I found this, I've been discussing this with our
AD, who brought in our WG chairs once he and I agreed that there might
be a problem here, but we felt that the discussion of what to do about
this really should take place out in the open on the WG mailing list.
So, what I said (after some initial clarifying discussion) was:
In the absence of a precise definition of the distinction between a
link and a subnet, it is unclear what a router should do with a packet
addressed to a transient multicast address with subnet-local scope.
Excerpting from the draft:
2 link-local scope
3 subnet-local scope
4 admin-local scope
...
link-local and site-local multicast scopes span the same
topological regions as the corresponding unicast scopes.
subnet-local scope is given a different and larger value
than link-local to enable possible support for subnets
that span multiple links.
admin-local scope is the smallest scope that must be
administratively configured, i.e., not automatically
derived from physical connectivity or other, non-
multicast-related configuration.
So subnet-local is a larger scope than link-local, and may be derived
automatically from physical connectivity (in some completely
unspecified way!). So what should a router do with that packet? To
forward or not to forward, that is the question.
One could address this concern by adding text (somewhere) to the
effect that, until such time as a standards track document specifies
how a router should determine what the subnet-scope boundaries are and
what a router should do with an otherwise valid packet addressed to a
multicast address with scop set to subnet-local, routers should handle
such packets precisely as they would if scop were set to link-local.
Or something like that.
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