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

From: [email-masked] (Noel Chiappa)
Date: Thu, 14 Jun 2007 09:42:12 -0400 (EDT)

> From: Brian E Carpenter <[email-masked]>

    > I was thinking about why we're having trouble being crisp about
    > "locator".
    > Maybe part of it is that in some contexts .. a locator is loaded with
    > topological significance.
    > ...
    > Another thought is that on a classical broadcast Ethernet, an Ethernet
    > address is a 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.

Sorry, this is confused. You are confusing two (maybe three) different
properties (see my message from yesterday):

- What is being named (an interface, etc)
- What level the name is at (internetwork, or in the case of the Ethernet
	address, network)
- Location sensitivity (i.e. some information about where the thing is is
	encoded *directly* into the name)

"Locator" means different things to different people (for instance, in GSE it
names a subnetwork, not an interface), but AFAIK almost everyone agrees it
has the last property.

Here's a simple test to see if something has this property: can you look at
two *different* names from that namespace, and tell *purely from the names*
whether they are in some way 'close' to each other, in locational terms, or
not? If not, if you have no idea whether the names are close or far, then the
names are *not* locators.

Ethernet addresses say *nothing* about *where* an interface is, you can't
tell from looking at two what their locational relational status is, so they
can't be locators.

Also, be careful with the term/concept "mapping". You can (in some designs)
map an 'endpoint identifier' a la JNC (which is absolutely not
location-sensitive) into some other name, the latter giving you its location.
However, that doesn't mean that because the 'endpoint identifier' can "in
principle be mapped to topology" that it's a locator.

A locator contains, *directly encoded into the name*, information about the
location is - and the 'are these two close' test is a simple test for it.

	Noel

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From: [email-masked] (Noel Chiappa)
Date: Thu, 14 Jun 2007 12:55:50 -0400 (EDT)

> From: RJ Atkinson <[email-masked]>

    > 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).

I was going to assume that what you mean by "identifier" here is 'a name that
simply identifies something, without saying anything else about the thing'
(which is what I thought you were saying in your previous draft taxomony
message). However, that's clearly not the definition you're using, because
under that definition, an IEEE MAC address *is* an identifier; it calls out
one specific interface, without saying anything else about it (e.g. where it
is).

(Technically, I know, it's not true that it saying nothing else, because it
call tell you who the manufacturer is; yet another semantic axis. But I think
we can ignore that axis for the moment.)

So I am forced to conclude that you must mean something else by "identifier";
I was thinking that perhaps you implicitly mean that an "identifier" names a
transport-level entity - but I don't think you mean that either.

This is an example of why I don't like the term "identifier". (To me, all
names "identify" *something*; the only question is *what* does it identify,
and what are the interesting properties of that name.)

And as for using "address" - that term is so hopelessly damaged at this point
(except for specific modifier instances like 'IPvN address') that I have no
idea what it means (although I like your earlier suggestion that it be defined
to be 'a name with the mixed semantics of an ''internetwork locator'' and a
''transport identifier'' ', which is what an 'IPvN address' is) - but if we
use that definition, I absolutely cannot see how an IEEE MAC address can be an
"address".

    > {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.

The issue of whether the namespace allows more than one name to be bound to a
single object is yet another independent axis which one can use to describe a
namespace.

It is not one that I had considered important in deciding whether a particular
namespace as a "locator", "identifier", or whatever. Your comment indicates that
you do think it is important?

    > A locator has location semantics, but not identity semantics.

And what exactly are "identity semantics"?

Reading that term de novo, I would assume that it simply means 'a name that
identifies an object'; or, from the mathmetical roots, perhaps 'a namespace
where names have a one-one correspondence to objects'.

    > 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.

What do you mean by "location semantics"?

Do you mean 'a name that includes some information about where the thing is
is, encoded *directly* into the name'? I don't think you can, because an
Ethernet address doesn't have the property, but by your proposed definition it
has "location semantics".

Do you "specifies a location"?

Also, just because a name identifies a thing which is at a particular location
(e.g.  an interface), that does not mean that the name has 'location
semantics'. To me, there has to be *information* in the name (so that e.g. it
can pass the 'are these two close' test).

    Noel

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From: [email-masked] (Noel Chiappa)
Date: Thu, 14 Jun 2007 15:34:52 -0400 (EDT)

> From: "Joel M. Halpern" <[email-masked]>

    > 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.

Exactly.

    > 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.

Good point.

    > a locator is a naming string (binary or otherwise) that changes when
    > you change location

Exactly.

    > an identifier is a naming string that does not change when one changes
    > location.

This seems like a plausible definition to me. (It leaves out any notion of
*what* is being named, but that's OK with me.) All these two terms ("locator"
and "identifier") would now signify would be choices along that one
'location' axis which I have previously discussed.

    > 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.

Exactly.

	Noel

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From: [email-masked] (Noel Chiappa)
Date: Thu, 14 Jun 2007 17:17:12 -0400 (EDT)

> since the IEEE MAC address is used in a lookup table .. to forward the
    > frame/packet, therefore the MAC address must have location semantics
    > and so could not be a pure Identifier.

Actually, I think I probably said something as follows:

  If we define 'locator' is a 'internetwork-level location-sensitive name for
  an interface', then one obvious design option for a 'full' locator, for the
  typical Ethernet interface, is some sort of hierarchical name for the
  physical network (i.e. subnet), with the MAC address appended.

In other words, an IEEE MAC address is *part* of a locator (if you are
defining 'locator' to be 'internetwork-level location-sensitive name for an
interface').

Yes, if you plug that interface in somewhere else, the locator changes - but
the low-order part (the MAC address) will be the same. Similarly, because to
me a 'locator' is the name of an interface, the MAC address (or something
that maps to it, as in IPv4) would have to be part of the locator.

Does that make sense?

    > I am pretty confused at this point about what your perspective is. I
    > gather your thinking has evolved since this precise question last came
    > up.

No, not really. The things I think have changed are that i) I'm more
sensitive now to understanding how all these terms (e.g. "locator") all mean
different things to different people, and ii) I'm slowly becoming more
precise (with all these different axes of name attributes: locationality,
level, object, etc) about the taxonomy of names.

	Noel

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From: [email-masked]
Date: Thu, 14 Jun 2007 17:38:04 EDT

I looked for the word "ramble" in my English-German dictionary  and found:
 
planlos reden = talking without any plan
 
All I have learnt from the current discussion is the meaning of this  word.
 
Heiner