Subject: [RAM] Re: Ramblings about "locator"

From: RJ Atkinson <[email-masked]>
Date: Thu, 14 Jun 2007 09:30:25 -0400

Earlier, Brian Carpenter wrote:
> 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.

A locator always has topological significance, IMHO.
However, a locator's topological significance is not
necessarily bound to a hierarchical bit-pattern
(though with IP routing the address does have that
sort of binding).

> 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.

	As someone who builds Ethernet switches these days,
my take is that the IEEE MAC address (either 802 or 1394)
in an Ethernet frame is an address (i.e. neither identifier,
nor locator).

A typical Ethernet switch simply has a table inside.  One feeds
in a destination MAC address and gets back which (bridge/switch)
interface(s) to send the frame out.  If the target MAC is not
in the table, one sends the frame out all interfaces.

{The claim that "an IEEE MAC is an Identifier" would be more
true if one considered Ethernet-II rather than IEEE 802.3.
Ethernet-II said there was one MAC address per node, while
802.3 has one MAC per interface.  Both definitions can work
fine with off-the-shelf bridging algorithms.}

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

I would phrase it differently.  A locator has location semantics,
but not identity semantics.  An identifier has identity semantics,
but not location semantics.  An address has both kinds of semantics.
So an MAC address in an Ethernet frame is an address.

And by the way, I view an EUI-64 (e.g. as used in O'Dell's
proposals) to be an Identifier, neither an address nor a locator.

Ran

_______________________________________________
RAM mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/ram

---

From: Dino Farinacci <[email-masked]>
Date: Thu, 14 Jun 2007 10:25:05 -0700

> in an Ethernet frame is an address (i.e. neither identifier,

I disagree. I think it is just as overloaded as an IP address. MAC  
addresses are used as serial numbers in many products. That is an ID  
if I ever thought there was one. And a MAC address is certainly used  
to find (that means "where", and "where" means location) an ethernet  
attached station in a L2 switched network.

And what about how MAC addresses are used in IS-IS and in IPv6  
stateless auto-configuration. In these cases, it's purely an ID.

And for the old guys, remember OSI and what an L1 area in IS-IS was  
used for? To route system-ids which were based on MAC addresses. So  
in this case, the MAC was a locator.

Dino

---

From: RJ Atkinson <[email-masked]>
Date: Thu, 14 Jun 2007 14:21:57 -0400

On  14 Jun 2007, at 13:25, Dino Farinacci wrote:
> I disagree. I think it is just as overloaded as an IP address.

My previous postings have said that things with overloaded
semantics (location sometimes, identity other times) are
called "addresses".

> MAC addresses are used as serial numbers in many products. That is  
> an ID if I ever thought there was one. And a MAC address is  
> certainly used to find (that means "where", and "where" means  
> location) an ethernet attached station in a L2 switched network.

Right.  Using my terminology proposal, that makes them "addresses"
(i.e. objects with mixed semantics, depending on use).

> in this case, the MAC was a locator.

So we agree, a MAC is an address -- hence has mixed semantics:
sometimes is used for identity and other times used for location.

Ran

---

From: "Joel M. Halpern" <[email-masked]>
Date: Thu, 14 Jun 2007 14:35:08 -0400

This seems wrong to me.
Given any identifier we care to define, someone can later come along 
and decide to "route" to that over some scope.
If that changes the "identifier" into an "address" or a "locator" 
then the terms do not refer to the name but to the combination on the 
name and the way it happens to be used, at the moment.  That seems wrong.

I am driven more and more to a notion along the lines suggested by 
Mr. Lear, that a locator is a naming string (binary or otherwise) 
that changes when you change location, and an identifier is a naming 
string that does not change when one changes location.  (Obviously, 
there are still edge cases.)  Or, to paraphrase Noel's description 
(and to mangle some work from John Day) a locator is where given 
several such things some notions of distance (closeness, relative 
topological placement) can be applied to the entities based on those locators.

Yours,
Joel

---

From: Dino Farinacci <[email-masked]>
Date: Thu, 14 Jun 2007 11:26:45 -0700

> sometimes is used for identity and other times used for location.

Yes agree what is an address. But to me, especially in this case, an  
address is both an ID and a Locator. Didn't look close enough to see  
if you described it that way.

Dino

---

From: Christian Vogt <[email-masked]>
Date: Thu, 14 Jun 2007 23:33:30 +0300

Joel M. Halpern wrote:
>                    [...] and an identifier is a naming string
> that does not change when one changes location.  

Yes.  This is actually implied by the Latin word "identificare", which
means "to make to resemble" -- e.g., a name with an entity.  It implies
the stability that we associate with identifiers.

A "locator" could possibly be defined as a "description of a location".

(I was thinking of defining it as an "identifier for a location", but
then found that a locator might actually be relative to the one using
it.  E.g., a source route consisting of "left's", "right's", and
"straight's" could be thought of as a locator, but it would lead to
different locations depending on from where it is being followed.)

Kind regards,
- Christian