# I-D: How Roy would Implement URNs and URCs Today

Roy Fielding (fielding@beach.w3.org)
Sat, 08 Jul 1995 20:27:02 -0400

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    Message-Id: <199507090027.UAA11739@beach.w3.org>

    To: uri@bunyip.com

    Subject: I-D: How Roy would Implement URNs and URCs Today

    Date: Sat, 08 Jul 1995 20:27:02 -0400

    From: Roy Fielding <fielding@beach.w3.org>

I apparently missed the deadline by 30 minutes, so here is yet
another view of how URNs and URCs can be implemented. Mostly,
this is to remind people that there already exists an architecture
for the use of URNs, and unless there is a very compelling reason,
we shouldn't screw it up by attempting to standardize an
incompatible syntax.

It is short, so I'll just include the entire document.

....Roy T. Fielding Department of ICS, University of California, Irvine USA
<fielding@ics.uci.edu>
<URL:http://www.ics.uci.edu/dir/grad/Software/fielding>

=======================================================================
Uniform Resource Identifiers Working Group R. Fielding
INTERNET-DRAFT UC Irvine
Expires January 7, 1996 July 7, 1995

How Roy would Implement URNs and URCs Today
<draft-ietf-uri-roy-urn-urc-00.txt>

Status of this Memo

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as
``work in progress.''

To learn the current status of any Internet-Draft, please check
the ``1id-abstracts.txt'' listing contained in the Internet-
Drafts Shadow Directories on ftp.is.co.za (Africa),
nic.nordu.net (Europe), munnari.oz.au (Pacific Rim),
ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast).

Distribution of this document is unlimited. Please send comments
to the author, Roy T. Fielding <fielding@ics.uci.edu>, or to the
URI working group (URI-WG) of the Internet Engineering Task Force
(IETF) at <uri@bunyip.com>. Discussions of the group are archived at
<URL:http://www.acl.lanl.gov/URI/archive/uri-archive.index.html>.

This document has no formal status and should not be considered as
anything more than the opinions of the author. Although it is
hoped that someone will eventually implement these ideas, they are
nonetheless only ideas and are not intended as a standards track
document [which is why I have chosen such a strange title].

Abstract

This document describes how the author would implement Uniform
Resource Names (URNs) and Uniform Resource Characteristics (URCs),
such that the basic concepts and technology can be usable by today's
World-Wide Web clients and servers. It is intended to identify the
key ingredients which make the WWW extensible and open to the
introduction of URNs and URCs, and thereby steer the implementors
of URI technology toward more consistent solutions.

1. Introduction

The URI working group has been discussing the topic of Uniform
Resource Names (URNs) for over three years. Although the intentions
of those participating in the WG have always been good, and usually
constructive, the WG has failed to attain any consensus on how
a URN service can be implemented such that it satisfies everyone's
needs.

It is my opinion that this search for the "Holy Grail" of URNs is
both misguided and unnecessary. It is neither possible nor
appropriate for us to define a single URN service. Instead, the WG
should focus on the interfaces between clients, servers, and name
services, such that any reasonable form of naming service can be
introduced when they are available, and according to the needs of the
end users and content providers rather than those of the WG members.

The World-Wide Web already contains an architecture capable of
supporting the client and server interfaces necessary for URN
addressing, though these interfaces have rarely been defined as such.
This document is intended to remedy that situation. Furthermore,
it will attempt to identify how several URN services can be defined
and implemented today. Although these solutions will not solve
everyone's problems (including such issues as replication and
authentication of centralized name services), they do provide a
significant step forward and supply the infrastructure required by
all URN services.

This document assumes that the reader has knowledge of the basic
syntax of WWW Universal Resource Identifiers [1] and Uniform Resource
Locators (URLs) [2].

2. URI Syntax

The World-Wide Web architecture assumes that resource addresses are
identifiable by their scheme name. This applies to all URIs, not
just to what are commonly considered URLs today. A URI in absolute
form consists of

<scheme>:<scheme-specific-part>#<fragment>

where <scheme> contains only US-ASCII lowercase letters, digits, "+",
"-", or ".".

The scheme name identifies the handler routine which would be used
to resolve the address. Note that it does not necessarily define
the protocol to be used, although people commonly make that
assumption after seeing that the most common scheme names are
associated with preexisting Internet application protocols.

The scheme handler routine may exist internal to the client
application (either hardcoded or within a modular library
architecture such as that found in libwww or libwww-perl),
or may be redirected to a proxy application via environment variables
or other user-configurable devices. This ability to extend the
addressing schemes of clients is one of the key features of WWW
technology.

In order to be successfully implemented within the current base of
WWW technology, the URN syntax must correspond to the basic URI
syntax as described above. That is, it must start with a scheme name
which identifies an appropriate resolver for that address (or allows
the client to identify that it has no resolver for that address).

3. Media Types

After an address is resolved and a retrieval action has been
accomplished through the appropriate scheme handler, a World-Wide Web
client will choose a second handler routine for the retrieved
document. The document handler is chosen according to that
document's Internet media type [5]. The media type is either
assigned by the transfer protocol or guessed by the client.

The document handler routine may exist internal to the client
application, or may be redirected to an external application via the
MIME mailcap facility. Although most handler routines are simply
viewers for the document content, others exist that control internal
events or prompt the user for additional input. This ability to
extend the behavior of clients is another one of the key features of
WWW technology.

4. URCs are Documents

The notion of Uniform Resource Characteristics (URCs) has been one
of the central issues in the debate about URN services. Simply put,
a URC is a set of characteristics regarding a named resource, in a
format that can be easily parsed, which identifies a set of locations
from which the named resource may be obtained. The URC can then be
used as the intermediate step between resolving a URN address and
determining the most appropriate location (from the perspective of
the client configuration) from which to retrieve the resource.

Proposals for the format of a URC have ranged from a simple list of
URLs to a hierarchical query language. In all cases, however, a URC
can be considered a document, and therefore should be assigned an
appropriate media type. Furthermore, since it is impossible for any
one group to define a single, all-encompassing format for URCs which
will satisfy the needs of all archivists and content providers, it
will be necessary to define a range of media types.

Note that this view of URCs already fits well with the WWW
architecture. If a URC is labelled as such, a WWW client can perform
location redirection as part of the document handler routine.
In other words, we can have URN -> URC -> URL indirection working
with only minor changes to existing clients.

Unfortunately, that's still not good enough. Current browsing clients
will default to "application/octet-stream" if they do not have a
handler routine installed for the indicated media type (usually
resulting in a prompt to save the document as a local file). In
practice, this has been a barrier to the wholesale introduction of
new media types. We need an implementation of URCs that will work
with all existing clients, because without that assurance, content
providers will be unwilling to use URCs as an intermediate step.

The solution is to start with an intermediate form of URC which
is a fixed variant of an already-universal media type: text/html.
This is outlined below in Section 6.

5. URI Resolution Architecture

But wait, there's more!

If a URC is identifiable as a document, then any document retrieval
action may result in an indirection. Therefore, we are no longer
talking about just URN resolution via URCs, but also URL redirection
via URCs (i.e., redirection of a single URL to multiple variants),
URN resolution to a single URL (i.e., minimal URCs), and URN
resolution directly to the named resource. As far as the client is
concerned, it is just using a URI to retrieve a resource. All of the
details of the resolution mechanism remain internal to the scheme
handler and the URN service provider, thereby removing the need for
the IETF to attempt to standardize any particular scheme, or any
particular URN service.

6. Graceful Introduction of URNs and URCs

Well, its not all just a bed of roses -- there are plenty of thorns
that need to be smoothed out in order to promulgate widespread
implementations of URNs and URCs over the existing WWW. The
following sections outline the steps I would take.

6.1. The ietf URI scheme

The first thing we need is a simple, but worthwhile, mechanism for
testing these ideas. I suggest that we should define a new URI
scheme called "ietf" -- it's purpose would be to provide a single
identifier for the replicated archives of the Internet Engineering
Taskforce. The format for this identifier is simply:

"ietf" ":" <existing-ietf-path>

For example, the identifier of RFC 1808 would become

ietf:/rfc/rfc1808.txt

and the one for this draft would be

ietf:/internet-drafts/draft-ietf-uri-roy-urn-urc-00.txt

The implementation of the scheme handler is a fairly straightforward
address replacement table and associated logic. For example, the
following could act as the configuration for my local client:

PREFIX REPLACEMENT AUTHORITATIVE?
ietf: file:/home/fielding/ietf No
ietf:/rfc/ ftp://ftp.ics.uci.edu/pub/ietf/rfc/ No
ietf:/rfc/ http://info.internet.isi.edu/in-notes/rfc/ No
ietf: http://ds.internic.net Yes
ietf: ftp://ds.internic.net Yes

The retrieval logic behind this table is also simple: try each of the
matching URI addresses (replacing the matching prefix with the
replacement string) until a good response is received, or until a
"not found" response is received from an authoritative location.

Note that the first location points to my own personal archive -- the
place where I keep a copy of most of the specs I have referenced in
my past work (or anticipate referencing in the near future).
Clearly, I want to retrieve my local copy if I have one available.
The second address is also a local copy, but consisting of only RFCs
and maintained by others at UC Irvine working on Internet Mail and
network management issues. The ISI archive is also fairly close to
my (network and physical) location, but uses a slightly different
path and tends to be 1-2 days out-of-sync with the main IETF
archives, which are represented as the final two locations.

There are a couple of interesting features of this example which have
rarely been considered during past discussion of URN issues. The
first is that the table is particular to my own client setup. There
is no way for a centralized name service to know these details.
The second is that the table format could be generic to any URI which
can be resolved directly via some other URL (such as, for instance,
via the URL of a URN name service). Finally, note that the actual
protocol used to resolve the name is defined by the replacement URL,
and not by any decision of the WG.

6.2. The ietf URCs

The above example did not assume any changes to the existing IETF
archive namespace. However, we could get considerably more value
out of this scheme if partial name matching resulted in a URC.
For example, if the following name

ietf:/internet-drafts/draft-ietf-uri-roy-urn-urc

(note the missing "-00.txt") corresponded to a URC pointing to all
of the currently available format variants of this draft, then I
could avoid having to change references every time a new version is
placed in the archives. Similarly,

ietf:/internet-drafts/draft-ietf-uri

could point to a summary of all current drafts by the URI-WG, and

ietf:/rfc/rfc1521

could point to all format variants of RFC 1521.

6.3. The urc major media type

If URCs are to be given media types, we need to register them. MIME
provides four major types: text, application, multipart, message,
image, audio, and video [4]. However, it is clear that URCs do
not fit within any one of these categories, and that subtypes of URC
are desirable. Therefore, I suggest that we define a new major media
type called "urc".

RFC 1590 [5] states that "If a new fundamental top-level type is
needed, its specification must be published as an RFC or submitted in
a form suitable to become an RFC, and be subject to the Internet
standards process." We'll just put that on the to-do list.

6.4. The urc/html media type

The first URC format that must be defined is one which will not
adversely affect current WWW clients. Therefore, we need to define
a variant of HTML which will look like a menu on existing browsers,
and yet be machine recognizable as a URC by new browsers. We can
do this by using a fixed format and require a specific SGML DOCTYPE
declaration to appear as the first line of the URC document.
For starters, here is what one may look like:

<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML URC//EN">
<HTML><HEAD>
<TITLE>Available resources for ietf:/rfc/rfc1521</TITLE>
</HEAD><BODY>
<H1>ietf:/rfc/rfc1521</H1>
<DL COMPACT>
<DT>Title:
<DD>MIME (Multipurpose Internet Mail Extensions)
Part One: Mechanisms for Specifying and Describing the
Format of Internet Message Bodies
<DT>Author:
<DD>N. Borenstein
<DD>N. Freed
<DT>Date:
<DD>September 1993
<DT>Obsoletes:
<DD><A rel="obsoletes" href="ietf:/rfc/rfc1341">RFC 1341</A>
<DT>Updated-by:
<DD><A rev="updates" href="ietf:/rfc/rfc1590">RFC 1590</A>
</DL>
<MENU vary="location">
<LI>ftp.is.co.za (Africa)
<MENU vary="type">
<LI><A href="ftp://ftp.is.co.za/rfc/rfc1521.txt.gz">
gzip(text/plain), 20000 bytes</a>
<LI><A href="ftp://ftp.is.co.za/rfc/rfc1521.ps.gz">
gzip(application/postscript), 40000 bytes</A>
</MENU>
<LI>nic.nordu.net (Europe)
<MENU vary="type">
<LI><A href="ftp://nic.nordu.net/rfc/rfc1521.txt">
text/plain, 187424 bytes</a>
<LI><A href="ftp://nic.nordu.net/rfc/rfc1521.ps">
application/postscript, 393670 bytes</A>
</MENU>
<LI>munnari.oz.au (Pacific Rim)
<MENU vary="type">
<LI><A href="ftp://munnari.oz.au/rfc/rfc1521.txt">
text/plain, 187424 bytes</a>
<LI><A href="ftp://munnari.oz.au/rfc/rfc1521.ps">
application/postscript, 393670 bytes</A>
</MENU>
<LI>ds.internic.net (US East Coast)
<MENU vary="type">
<LI><A href="http://ds.internic.net/rfc/rfc1521.txt">
text/plain, 187424 bytes</a>
<LI><A href="http://ds.internic.net/rfc/rfc1521.ps">
application/postscript, 393670 bytes</A>
<LI><A href="ftp://ds.internic.net/rfc/rfc1521.txt">
text/plain, 187424 bytes</a>
<LI><A href="ftp://ds.internic.net/rfc/rfc1521.ps">
application/postscript, 393670 bytes</A>
</MENU>
<LI>ftp.isi.edu (US West Coast)
<MENU vary="type">
<LI><A href="ftp://ftp.isi.edu/rfc/rfc1521.txt">
text/plain, 187424 bytes</a>
<LI><A href="ftp://ftp.isi.edu/rfc/rfc1521.ps">
application/postscript, 393670 bytes</A>
</MENU>
</MENU>
</BODY></HTML>

This is only an example -- a complete definition (including BNF)
would be necessary for the format to be usable for automated
indirection.

7. Unfinished Business

I do not pretend to think that the suggestions identified by this
document will completely solve the URN problem. However, I am
certain that they will eventually be necessary in order to
successfully implement any URN scheme on the World-Wide Web.
Some of the outstanding problems are identified below, though
there are probably more.

7.1. Changes to HTML to support URNs

The HTML 2.0 specification [3] already defines an attribute of
anchors and link elements for containing a URN. However, no general
client supports it, and its not what we really want anyway. What we
need is a way to assign multiple URIs to a single hypertext anchor.
Fortunately, we don't need this right away, so it can be deferred
to the HTML WG for consideration later.

7.2. Name Persistence

One of the "requirements" identified for URNs is that they be
unique for all time (or at least a reasonable time such as to
make name collision impossible). This document completely
ignores that issue, as I think should any real implementation
of URNs. Name persistence is not something that technology can
guarantee, other than by the undesirable mechanism of assigning
a new name based on the location and time of creation. It is
quite possible that some URN schemes will have such persistence,
but it will be attained through the institutions responsible
for assigning the names and maintaining the resolution services,
not by constraining the syntax of names.

7.3. Sub-second Resolution

No constraints on resolution times are proposed, because they
are simply unnecessary. Nobody can determine the resolution time
for any particular user at any particular network (or, egads,
non-networked) site. People will use the quickest (or cheapest)
resolution available to them -- we do not need to define it in
advance, nor should we.
7.3. Security Considerations

No security considerations have been identified by this document.
This will require future work.

8. Acknowledgements

This paper is the result of over a year of thinking and only two
days of writing, so I have left some things out and have probably
failed to properly acknowledge all those who deserve to be.
Tim Berners-Lee is primarily responsible for the extensible
architecture of the World-Wide Web. I have discussed the issues
involved in URI indirection, and URCs as media types, with
Daniel LaLiberte several times, but he is not to blame for this
treatise. Larry Masinter has pointed out several times that the
WG is unable to "create" the institutions needed for true
persistence.

9. References

[1] T. Berners-Lee, "Universal Resource Identifiers in WWW:
A Unifying Syntax for the Expression of Names and Addresses of
Objects on the Network as used in the World-Wide Web", RFC 1630,
CERN, June 1994.

[2] T. Berners-Lee, L. Masinter, and M. McCahill, Editors,
"Uniform Resource Locators (URL)", RFC 1738, CERN,
Xerox Corporation, University of Minnesota, December 1994.

[3] T. Berners-Lee and D. Connolly, "HyperText Markup Language
Specification -- 2.0", Work in Progress, MIT/W3C,
June 1995. <URL:http://www.ics.uci.edu/pub/ietf/html/>

[4] N. Borenstein and N. Freed, "MIME (Multipurpose Internet Mail
Extensions): Mechanisms for Specifying and Describing the Format
of Internet Message Bodies", RFC 1521, Bellcore, Innosoft,
September 1993.

[5] J. Postel, "Media Type Registration Procedure", RFC 1590,
USC/ISI, March 1994.

10. Author's Address

Roy T. Fielding
Department of Information and Computer Science
University of California
Irvine, CA 92717-3425
U.S.A.

Tel: +1 (714) 824-4049
Fax: +1 (714) 824-4056
Email: fielding@ics.uci.edu

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