Critical Comparison of URN Proposals

Mark Madsen (
Thu, 13 Jul 95 17:33:34 BST

From: Mark Madsen <>
Message-Id: <>
Subject: Critical Comparison of URN Proposals
Date: Thu, 13 Jul 95 17:33:34 BST

The attached is the preliminary version of a critique of the existing
URN proposals.  I wrote it in the hope that it would form the basis of
a discussion of those proposals in a top-down fashion.

Mark Madsen: <> <URL:>
Information Services Framework, The ANSA Project, APM Ltd., Castle Park,
Cambridge CB3 0RD, U.K.  <URL:>;  <>
Voice: +44-1223-568934; Reception: +44-1223-515010; Fax: +44-1223-359779

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IETF URI Working Group                                 Mark Madsen
Internet-Draft                                             APM Ltd
Expires: 18 January 1996                               13 July 1995

           A Critique of Existing URN Proposals


This document criticises existing URN (Uniform Resource Name)
proposals in the light of generality, extensibility, and general
futureproofing.  The idea is to draw upon the best characteristics of
the existing proposals so as to converge on an acceptably functional
and nonrestrictive draft specification for both URN syntax and
resolution schemes.

1. Introduction

The URI (Uniform Resource identifier) Working Group is currently
stalled on the issue of how to implement URNs.  There is no shortage
of proposals, but no single proposal has been perfected, and all
contain shortcomings which have prevented them gaining widespread
support from the members of the working group.

The aim of this document is to criticise the existing proposals in a
constructive manner, so as to draw out their positive qualities, and
then to develop a modular scheme for combining these best components
into new URN schemes.

Some specific architectural considerations are derived and discussed
in the following section.

2. General Issues

Requirements on URNs are discussed in [S&M].  These may be divided
into requirements on URNs themselves (global scope, location
independence, global uniqueness, and persistence), requirements on URN
schemes (scalability, legacy support, extensibility, independence of
naming authorities, resolvability), and requirements on URN encodings
(single encoding, simple comparison, transcribability,
transportability, parsability).

To these requirements could be added the necessity not to make any URN
scheme into a straitjacket for future Internet development.
Fulfillment of this requirement will clearly only be possible if the
issues relating to URN construction are separated from those relating
to resolution of URNs into other classes of Uniform Resources.

The next few sections discuss the existing proposals and documents in
the light of the considerations derived in this section.

3. Critiques of URN Proposals

The proposals are treated in chronological order.

3.1 The Path URN Scheme

This is described in internet draft draft-ietf-uri-urn-path-00.txt
"The Path URN Specification" by Daniel LaLiberte and Michael Shapiro.

The path scheme is based on the idea of an hierarchical name space,
with naming authorities being responsible for specified subtrees of
the name space.  The problem with this, as with any other hierarchical
scheme, is that of management of the levels near the root.  Experience
with DNS (Domain Naming Scheme) shows that this can be a serious
problem when weaknesses in the distribution of structure ar exposed by
use (as is presently the case with the .com subhierarchy).  Since
there are expected always to be many more resources in the Internet
than machines, the management problems can be expected to be severe.
Management issues are not addressed in the path scheme proposal.

The reason for the hierarchy built into the path scheme is that it
builds in the association between (sets of) naming authorities and
resolution services.  URN resolution is therefore an in-principle
terminating process, which is a positive point in favour of the
scheme.  In practice, failures will isolate entire subtrees of the
hierarchy from resolution.  DNS service failures are typically
softened by the practice of maintaining caches of all lookups: this
will not work as well for URN resolution because the Internet-wide
rate of creation and alteration of resources can reasonably be
expected to be many orders of magnitude greater than the addition of
new IP addresses.

A stronger criticism of the path scheme is that the hierarchical
process of resolution ties each resource's URN to a particular
resolution service in perpetuity.  Furthermore, the worst-case
scenario for resolution of a URN could involve as many resolution
services as there are components in the path of the URN under this

Tying URN construction and assignation to resolution in any way is a
stringent limit on the freedom of future Internet engineering to
exploit new technologies.  At the time of writing, trading technology
is becoming widespread.  Future traders could be badly hindered by
being constrained to resolve URNs in a path hierarchy.  This is not to
say that hierarchical schemes cannot or should not be built: in fact,
they may work better than any other type in certain circumstances and
for certain purposes.  However, it is easy to impose an hierarchical
resolution structure on a general namespace, so long as that namespace
is not itself tied to some other restrictive resolution scheme.

Many of the criticisms that apply to this scheme can be equally well
applied to some of the others discussed below.

3.2 The x-dns-2 Scheme

This is described in internet draft draft-ietf-uri-urn-x-dns-2-00.txt
"x-dns-2 URN Scheme" by Paul Hoffman and Ron Daniel.

This scheme is subject to the same criticism as the path scheme about
the way that the syntax incorporates the resolution service in the URN
itself.  However, this scheme will coexist reasonably well with other
schemes, because it encodes the "x-dns-2" scheme name in the URN as
well.  However, this means that resolvers unfamiliar with this scheme
convention may have difficulty in resolving such a URN.

Resolution in this scheme is handled by doing a standard DNS lookup to
find the resolution authority, and requesting direct resolution of the
URN from them.  Whether DNS can scale to cope with the number of
resolution requests that could conceivably be generated in such a
scenario seems doubtfuul.  This also exposes the fact that the scheme,
like the path scheme, relies on the naming authority and resolution
service being closely linked, an assumption which is unlikely to
remain true in the long term, as commercial and specialised resolution
services are set up.

A more serious criticism is that the scheme is tightly constrained in
two ways.  One is that it relies on being able to use HTTP headers.
This is a weakness, since one of the ways currently being considered
for improving HTTP's slowness is to change the ways in which headers
are dealt with.  Similarly, the scheme uses the HTTP status-line for
tracking resolution success and failure.  Thus the scheme is seen to
be too closely tied to HTTP to have a long lifetime [Madsen95].

3.3 The Handle Scheme

This is described in internet draft draft-ietf-uri-urn-handles-00.txt
"The Handle System" by William Harms and David Ely.

The handle scheme shares many characteristics with the aformentioned
URN schemes.  There is a namesapce hierarchy, in which there are extra
features, such as the ability for naming authorities to create
subsidiary naming authorities.  There is a global handle server, which
is distributed, and local handle servers, which gives a more flexible
model of how resolution may proceed.  However, these servers are again
both naming authorities and resolution services, with all the
limitations that implies.

The handle syntax itself seems too complicated for naming, and
sepcifies that the handles carry along with them sets of typed
structured information, and corresponding administrative information.
This is restrictive, and collides badly with the URI-WG's attempt to
provide a functionally clean repository for resource metadata in the
framework of URC schemes.

The proposal also considers such issues as handle administration tols,
access to services through firewalls, and cacheing of handles.  While
important, these issues are orthogonal to the construction of naming
scheme frameworks, and should be left to other documents.

3.4 The OCLC Scheme

This is slightly different from the preceding documents and is
described in internet draft draft-ietf-uri-urn-resolution-01.txt "URN
Services" by Keith Shafer, Eric Miller, Vincent Tkac and Stuart Weibel.

This scheme is the most flexible of those proposed.  It allows for the
existence of different naming authorities with different authority
levels, and for different resolution services.  It does provide for
the "author" of a resource to specify the resolution service to be
used for a given URN.  This is a drawback in two ways, firstly that it
restricts the freedom to generate new resolution services, and
secondly it leaves old URNs high and dry when the author-specified
resolution service overloads/fails/explodes.  It is also probably a
bad idea to assume that with each resource there can be associated an
author that is a single coherent entity.

It is interesting and useful that this proposal recommends a
resolution-request syntax, but it is important that this should be
seen only as a baseline, since different resolution services will
almost certainly wish to offer differentiating syntaxes and resolution
criteria.  The proposed URN sysntax itself consists simply of the
naming authority name paired with the opaque string assigned to the
resource by that authority.  This means that the naming authority can
still be queried for validation of a URN, but that it is not in
general expected to provide the resolution service itself.  

Another advantage of decoupling naming and resolution as seen in this
scheme is that for small URN collections issued by single naming
authorities, the optimisation of having those naming authorities offer
their own resolution service is simply arranged: other resolution
services simply each maintain a table of known naming authrities,
marking each according to whether they perform resolution or not.

4. Summary and Conclusions

None of the existing proposals is entirely satisfactory.  The most
promising framework from the point of view of extensibility and
future-proofness would be one constructed on the basis of the OCLC
scheme.  it is recommended that this be implemented and tested, along
with implementation specifications for the other schemes to show how
they compare mutually when built on top of this basic framework.

The result of following this recommendation will be a framework within
which multiple URN schemes can coexist, each offering their own
particular advantages in a given situation, while allowing naming
authorities to issue URNs appropriately.

5. Security Considerations

This document does not raise any security considerations beyond those
normally associated with URN storage and resolution.  Note, however,
that any URN scheme intended to allow secure resolution must provide
for encryption of transmitted URN data.  This point has been
emphasised by Mark Fisher in a sequence of messages to the URI-WG
mailing list.

6. Acknowledgements

The author is grateful to Andrew Herbert, Rob van der Linden, Nigel
Edwards and Owen Rees for discussions relating to the management and
resolution of URNs.  Thanks also go to those authors who have written
the internet drafts discussed in this document.

7. References

[Madsen95] Mark Madsen, "Decoupling the URL Scheme from the Transport
Protocol", June 1995.

[S&M] Karen Sollins and Larry Masinter, "Functional Requirements for
Uniform Resource Names", RFC 1737, December 1994.

8. Author's Address

Mark Madsen <>
Information Services Framework Group
Architecture Projects Management Ltd
Poseidon House, Castle Park
Cambridge CB3 0RD, UK

Telephone +44-1223-568934
Facsimile +44-1223-359779