Network Working Group N. Walsh
Request for Comments: 3151 Sun Microsystems, Inc.
Category: Informational J. Cowan
Reuters Health Information
P. Grosso
Arbortext, Inc.
August 2001

A URN Namespace for Public Identifiers

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

This document describes a URN (Uniform Resource Name) namespace that
is designed to allow Public Identifiers to be eXPressed in URI
(Uniform Resource Identifiers) syntax.

1. IntrodUCtion

XML [1] external entities have two identifiers: a system identifier
and a public identifier. The system identifier is a URI, by
definition, but the public identifier is simply a string.

Historically, the system identifier of an external entity has been a
local, or system-specific identifier while the public identifier has
been a more global, persistent name.

Unfortunately, public identifiers do not fit neatly into the existing
web architecture because they are not legal URIs. Many new
specifications (XSLT, XML Schema, etc.) have the implicit or explicit
requirement that all external identifiers be URIs.

The purpose of this namespace is to allow public identifiers to be
encoded in URNs in a reliable, comparable way.

This document describes a scheme for representing public identifiers
as URNs by introducing a public identifier namespace, "publicid".

This namespace specification is for a formal namespace.

1.1 Public Identifiers

Any string which consists only of the public identifier characters
(defined by Production 13 of Extensible Markup Language (XML) 1.0
Second Edition [1]) is a legal public identifier.

In addition to the character set restriction, public identifiers must
be normalized by changing all strings of whitespace (the characters
#x20, #x9, #xD, and #xA) to single space characters (#x20), and
removing all leading and trailing whitespace.

In keeping with this specification's goal of allowing public
identifiers to be encoded in a reliable, comparable way, this
specification mandates that public identifiers be normalized before
encoding them into URNs. Throughout this specification, we assume
that normalization has already been performed.

1.2 Formal Public Identifiers

SGML [2] defines a restricted subset of public identifier called a
"Formal Public Identifier" (FPI).

FPIs are strings composed from the same range of characters as public
identifiers, but with an explicit internal structure. The structure
of Formal Public Identifiers is normatively described in SGML [2]; we
review it here for convenience.

Most Formal Public Identifiers consist of the following fields, in
this order: an owner identifier, a public text class, a public text
description, a public text language or public text designating
sequence, and an optional public text display version.

Owner identifiers may begin with "-//" or "+//"; otherwise "//" is
used to delimit fields in the FPI (with the exception of the public
text class which is delimited from the public text description by a
space).

In other Words, most FPIs look like this:

owner//class description//language//version

and most owners begin with "+//" or "-//", although they are not
required to. Here are some example FPIs:

+//IDN python.org//DTD XML Bookmark Exchange Language 1.0//EN//XML
-//OASIS//DTD DocBook XML V4.1.2//EN
-//ArborText::prod//DTD Help Navigation Document::19970708//EN
ISO/IEC 10179:1996//DTD DSSSL Architecture//EN
ISO 8879:1986//ENTITIES Added Latin 1//EN

This document describes an algorithm for encoding public identifiers
into URNs that explicitly allows the structured nature of formal
public identifiers to be preserved. However, an algorithm for
correctly identifying a Formal Public Identifier and determining the
various fields within it is out of scope for this document and not
necessary for the implementation of this URN namespace.

2. Specification Template

Namespace ID:

"publicid" requested.

Registration Information:

Registration Version Number: 1
Registration Date: 2001-05-08

Declared registrant of the namespace:

Norman Walsh
Sun Microsystems, Inc.
One Network Drive MS UBURO2-201
Burlington, MA
01803-0902

Norman.Walsh@East.Sun.COM

Declaration of structure:

The Namespace Specific String (NSS) for URNs in the "publicid"
namespace has the following structure:

urn:publicid:{transcribed-public-identifier}

Where:

{transcribed-public-identifier} is the text of the public
identifier transcribed according to the following rules:

- A space in the public identifier is transcribed as
"+". Whitespace normalization must be performed
before constructing a URN in the "publicid" namespace,
therefore adjacent "+" characters never occur in URNs
in this namespace.
- The sequence of characters "//" is transcribed as ":".
- The sequence of characters "::" is transcribed as ";".
- A literal "+" character is transcribed as "%2B".
- A literal ":" character (except in "::") is
transcribed as "%3A".
- A literal "/" character (except in "//") is
transcribed as "%2F".
- A literal ";" character is transcribed as "%3B".
- A literal "'" character is transcribed as "%27".
- A literal "?" character is transcribed as "%3F".
- A literal "#" character is transcribed as "%23".
- A literal "%" character is transcribed as "%25".

The special rules for "//" and "::" are designed to preserve the
structured nature of formal public identifiers without requiring the
translator to have special knowledge of FPI syntax.

The rules for "+", ":", "/", and ";" are required to preserve literal
occurrences of these characters in the 'publicid' URN namespace.

The remaining characters, " " (space), "'", "?", "#", and "%", are
the only other legal characters in public identifiers that cannot be
literally transcribed into a URN by the rules of RFC2141 [4] and RFC
2396 [5].

Relevant ancillary documentation:

Extensible Markup Language (XML) Version 1.0 Second Edition [1]
Standard Generalized Markup Language (SGML) [2]
Registration procedures for public text owner identifiers [3]

Identifier uniqueness considerations:

The identifier uniqueness considerations for URNs in the
"publicid" namespace are the same as the identifier uniqueness
considerations for public identifiers. Formal Public Identifiers
with registered owner identifiers are required to be unique. For
unregistered owner identifiers and informal public identifiers,
they may or may not be unique. No enforcement policy can be
asserted.

Identifier persistence considerations:

The persistence of URNs in the "publicid" namespace is the same as
the persistence of the corresponding public identifier.

The "publicid" namespace is available for a wide range of uses; it
cannot be subjected to a uniform persistence policy. As a general
rule, formal public identifiers with registered owner identifiers
are more likely to be persistent than informal public identifiers
or formal public identifiers with unregistered owner identifiers.

One exception to this rule is the "IDN" scheme for producing a
registered owner identifier from a domain name. That scheme
contains at least all the weaknesses associated with the
persistence of domain names.

It is important to note that a properly registered owner
identifier can apply any policy desired to the portion of the
"publicid" URN namespace identified by that owner identifier.

Process of identifier assignment:

Identifiers in the "publicid" namespace are assigned by applying
the conversions described above to a public identifier. In order
to provide a URN in this namespace for a resource that does not
have a public identifier, one must be created (according to the
rules for creating public identifiers).

There is no requirement that a resource have only one public
identifier.

Process of identifier resolution:

Identifiers in the "publicid" namespace may be resolved by the
same policies and procedures as public identifiers. Public
identifiers can be resolved in many different ways. Many existing
systems provide facilities for resolving them by way of OASIS
TR9401 [6] Catalog files. Other systems resolve them by mapping
each component to a local pathname component. And some systems
simply "know about" a fixed set of public identifiers. In
addition, URNs in the 'publicid' namespace may be resolvable by
other mechanisms unique to URIs (such as caches).

Rules for Lexical Equivalence:

Whitespace normalization is performed before constructing a URN in
the "publicid" namespace, so URNs are lexically equivalent if and
only if they are lexically identical.

Conformance with URN Syntax:

No special considerations. URNs in this namespace conform to both
RFC2141 and RFC2396.

Validation mechanism:

None specified.

Scope:

Global

3. Examples

The following examples are not guaranteed to be real. They are
listed for pedagogical reasons only.

"ISO/IEC 10179:1996//DTD DSSSL Architecture//EN" becomes
"urn:publicid:ISO%2FIEC+10179%3A1996:DTD+DSSSL+Architecture:EN"

"ISO 8879:1986//ENTITIES Added Latin 1//EN" becomes
"urn:publicid:ISO+8879%3A1986:ENTITIES+Added+Latin+1:EN"

"-//OASIS//DTD DocBook XML V4.1.2//EN" becomes
"urn:publicid:-:OASIS:DTD+DocBook+XML+V4.1.2:EN"

"+//IDN example.org//DTD XML Bookmarks 1.0//EN//XML" becomes
"urn:publicid:%2B:IDN+example.org:DTD+XML+Bookmarks+1.0:EN:XML"

"-//ArborText::prod//DTD Help Document::19970708//EN" becomes
"urn:publicid:-:ArborText;prod:DTD+Help+Document;19970708:EN"

"foo" becomes
"urn:publicid:foo"

"3+3=6" becomes
"urn:publicid:3%2B3=6"

"-//Acme, Inc.//DTD Book Version 1.0" becomes
"urn:publicid:-:Acme,+Inc.:DTD+Book+Version+1.0"

4. Security Considerations

There are no additional security considerations other than those
normally associated with the use and resolution of URNs in
general.

References

[1] W3C, XML WG, "Extensible Markup Language (XML) 1.0 Second
Edition", February 1998, <http://www.w3.org/TR/REC-xml>.

[2] JTC 1, SC 34, "ISO 8879:1986 Information processing -- Text and
Office systems -- Standard Generalized Markup Language (SGML)",
1986.

[3] JTC 1, SC 34, "ISO/IEC 9070:1991 Information technology -- SGML
support facilities -- Registration procedures for public text
owner identifiers", 1991.

[4] Moats, R., "URN Syntax", RFC2141, May 1997.

[5] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC2396, August
1998.

[6] Grosso, P., "Entity Management: OASIS Technical Resolution
9401:1997 (Amendment 2 to TR 9401)", Sep 1997,
<http://www.oasis-open.org/Html/tr9401.html>.

Authors' Addresses

Norman Walsh
Sun Microsystems, Inc.
One Network Drive MS UBURO2-201
Burlington, MA 01803-0902
US

EMail: Norman.Walsh@East.Sun.COM

John Cowan
Reuters Health Information
45 West 36th St, 12th Floor
New York, NY 10018
US

EMail: jcowan@reutershealth.com

Paul Grosso
Arbortext, Inc.
1000 Victors Way
Ann Arbor, MI 48108-2744
US

EMail: pgrosso@arbortext.com

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