Internet Info 1997 December/Internet_Info_CD-ROM_Walnut_Creek_December_1997.iso/ietf/urn/urn-archives/urn-ietf.archive.9703/000022_owner-urn-ietf _Tue Mar 11 23:45:05 1997.msg

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Date: Tue, 11 Mar 1997 23:38:50 -0500
To: Daniel LaLiberte <[e-mail]>, [e-mail]
From: "Karen R. Sollins" <[e-mail]>
Subject: Re: [URN] semantic representation in URNs
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Reply-To: "Karen R. Sollins" <[e-mail]>
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Dan,

I am in agreement with you on a number of points.  I also was trying to be
objective and play devil's advocate, in order to encourage people to jump
in on whatever side they wanted (and you jumped in on both:-).  So, I'll
just extract parts of your message for response.


At 4:01 PM -0600 3/10/97, Daniel LaLiberte wrote:

...
>
> > Remember that one form of semantics is location information,
> > and if we allow others, we will have the problems in multiple
> > dimensions.
>
>Could you be more specific about this?  What are these multiple
>dimensions?  Are you saying that any kind of semantics ties down the
>identifier, as in it locates the identifier in some n-dimensional space?

Location is one form of semantic information; think of it as one dimension
in semantic space.  Another might be programming language in which
something is implemented or natural language in which it is expressed.
Consider that another dimension. Another might be some indication of
underlying algorithm on which the resource is based.  Again, another
dimension.  Of course the number of possibilities is unlimited.  Yes,
n-dimensional space.  If URNs can be assumed to contain semantic
information, services will be built on that assumption, taking advantage of
the embedded knowledge.  If that knowledge can become invalid just as
location information can become invalid, we have an n-dimensional problem
where in URLs we have a one-dimensional problem.  Seems undesirable to me,
but the question is whether we should actively discourage our "customers"
from shooting themselves in the foot in this particular way or not.

> > 3) The hierarchy embodied on URNs reflects namespace delegation.
>
>Delegation is not required, but hierarchy *allows* delegation.
>
> > To cause that to reflect other semantics as well will be very
> > difficult in the general case.
>
>I don't understand what you are thinking here.  Please explain.

What I was trying to say is that if the hierarchy reflects delegation then
using it to reflect something else as well (say, for example, derivation)
is extremely difficult because it is overloading the meaning of
hierarchical components.  Doing this in general probably becomes impossible.

>What is the end-to-end argument?

The reference is to:

Saltzer, J., Reed, D., and Clark, D.D., "End-to-End Arguments in System
Design", ACM Transactions on Computer Systems, Vol. 2, No. 4, November
1984, pp. 277-288.

This paper makes the argument that repeating functionality at different
layers of abstraction (particularly as exemplified in security mechanisms)
is a bad idea. It probably should be required reading for all systems and
protocol designers and implementers.  I strongly recommend it.

> > 5) Given the problems with character sets and internationalization,
> > what is the probability of expressing semantics that would be globally
> > meaningful.
>
>Is there really a problem here?  When would I need to refer to a document
>in chinese with a chinese URN?
>
>I don't follow character set discussions, but one of the PRO arguments
>is that we *need* to use international character sets to make URNs
>useful world-wide.

I don't follow the character set discussions either.  But, the problem I
was highlighting here was that, if there is an expectation that useful (to
whom- or what-ever) semantics is to be included in URNs then the
internationalization problem becomes much more complex than the character
set internationalization problem.  The names are no longer "just bit
strings"but instead include meaningful content.   Thus, given that we do
not have a universal language (and apparently not even a universal
character set), I just don't see how to deal.  But, maybe it simply isn't
an issue.  And we say that semantics can be included in whatever language
the creator of the URN chooses, and if the semantics cannot be understood
and useful in other contexts, tough luck.

>dan

            Karen