Subject: [RAM] Ramblings about "locator"

From: Brian E Carpenter <[email-masked]>
Date: Thu, 14 Jun 2007 10:50:52 +0200

I was thinking about why we're having trouble being crisp about
"locator".

Maybe part of it is that in some contexts, as Noel hinted, a
locator is loaded with topological significance. If the
network topology in a given region is a strict binary
hierarchy, then the locator may be very tightly mapped
to the topology (and is in effect a route). But in other
regions of the network where the topology is an arbitrary
graph, the same locator has no mapping to topology and needs
to be bound to a route by a routing protocol.

Another thought is that on a classical broadcast Ethernet,
an Ethernet address is a locator, but a rather strange one
in that nobody except the receiver knows where the located
interface is. But the same Ethernet address on a switched
Ethernet or in a spanning tree becomes much more
like a network-level locator; it's mapped to topology by
the switching or bridging mechanism.

Maybe the essential point is that a locator can at least in
principle be mapped to topology and an identifier can't.

    Brian

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From: <[email-masked]>
Date: Thu, 14 Jun 2007 14:38:03 +0300

>principle be mapped to topology and an identifier can't.

IMO an ethernet "address" is an identifier. It uniquely identifies the
Ethernet interface in the (name)space of all existing and future
ethernet interfaces.

A locator has some built in topological significance. It is conceivable
that any identifier could be used for a routing and forwarding in a flat
architecture, where there is no aggregation based on topology.

If we had no aggregation in IP addressing, i.e. if we had IP addresses
free of any topological significance (like the ethernet addresses), then
we would not have the current ID/loc split issue either. In a such
system we could do all the multi-homing, mobility, etc. we wanted with
more elaborate routing protocols.

To summarise the point in the ID/loc split discussion is not about
whether something can be used for routing and/or forwarding or not. The
issue is with the built-in topological significance that limits the use
of such "locator" to a somehow restricted topology, when we would want
to have somehow different topology (more dynamic, multi-homed, etc.)
instead. So the issue is about having identifiers free of network
topological significance. It is yet another discussion whether these
identifiers could/should have semantics on some other axis, e.g. whether
these identifiers should be somehow structured for organizational
(=administration/"ownership") reasons.

Regards,

	Jarno Rajahalme

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From: Marshall Eubanks <[email-masked]>
Date: Thu, 14 Jun 2007 08:46:57 -0400

Dear Brian;

> principle be mapped to topology and an identifier can't.

If in Ethernet what is essentially a random number (the MAC address)  
becomes
a locator then maybe the distinction will never be crisp.

Regards
Marshall

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From: Brian E Carpenter <[email-masked]>
Date: Thu, 14 Jun 2007 15:17:58 +0200

Marshall,

...
> a locator then maybe the distinction will never be crisp.

Well, that horrible thought crossed my mind. I think I disagree
with Jarno's comment, because as soon as an Ethernet address
gets into a spanning tree it has become a locator (at level 2).

     Brian

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From: Dino Farinacci <[email-masked]>
Date: Thu, 14 Jun 2007 10:26:58 -0700

> gets into a spanning tree it has become a locator (at level 2).

Even when it's not in a spanning tree. It is a locator when there are  
no physical loops as well as in TRILL.

Dino