On 11 Dec 2008, at 09:02, Scott Brim wrote:
I don't think I'm parsing this right, but in any case it's clear that
the ID is used to determine the appropriate network attachment point
to
deliver the packet to. That is, the forwarding function is using
the ID
as input -- to "locate" the destination.
The "forwarding function" never does (i.e. No layer-3 lookup
EVER uses the Identifier bits for anything).
It's called an identifier because it does not determine topological
location, but in this particular case -- delivering a packet -- the
only
function using this field is a forwarding function.
No. The last hop is not forwarding anything (i.e. at layer-3).
The last hop with IPv6 is usually bridging (i.e. at layer-2),
and is sometimes (RF links with simple or no link protocols)
purely broadcasting.
Further, ND is not bridging based on the ILNP ID -- IPv6 ND
bridges a packet based on the MAC address cached in the ND table.
The ID is an opaque index into that table, but the ID itself
is not used directly to either forward or bridge the packet.
There is no clear delineation here.
I think there is. Please see above.
Some tokens are used by forwarding on just the last hop.
I'd disagree.
Forwarding and bridging are different functions.
In IPv4 ARP on Ethernet, the entire address is used as an index
into the ARP table, but the last-hop isn't forwarded at all,
instead it is bridged based on the MAC address stored in the
ARP table.
Some are used for the last few hops.
All "forwarding" occurs in the middle, not on the first hop
or the last hop, as those are typically bridged and sometimes
broadcast. By contrast, forwarding is based quite directly
on the routing-prefix/Locator values.
It is difficult to say that
anything that appears in every packet is ever a pure "identifier" ...
but I assert that you don't need to. Our goal is to support
multihoming
and mobility while simultaneously solving routing and addressing
scaling
problems. To do that we need to make it possible for identification
functions to work independently of topological location. That doesn't
mean that individual fields or names ever need to be exclusively used
for identification, although some approaches do so -- just that they
can
be used for identification, independent of topology, when needed.
I think that the semantic overloading of the "address"
has created lots of issues for the Internet beyond those
of site multi-homing and mobility.
So I think it is important to have very crisp definitions all around
(forwarding vs bridging; identity vs location, et cetera) So I try
to be consistent and precise with my use of terms. However, other
folks' mileage differs on some/all of these issues, as is true with
many other issues discussed in the Routing RG. I think we'd all be
better off with some agreed upon crisp definitions for various terms,
but past attempts to do that in this RG have not succeeded.
Cheers,
Ran
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