GENWiki

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.

Walsh, et al. Informational [Page 1] RFC 3151 A URN Namespace for Public Identifiers August 2001

 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:

Walsh, et al. Informational [Page 2] RFC 3151 A URN Namespace for Public Identifiers August 2001

 +//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:

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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 RFC 2141 [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.

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 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.

Walsh, et al. Informational [Page 5] RFC 3151 A URN Namespace for Public Identifiers August 2001

 Conformance with URN Syntax:

    No special considerations.  URNs in this namespace conform to both
    RFC 2141 and RFC 2396.

 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.

Walsh, et al. Informational [Page 6] RFC 3151 A URN Namespace for Public Identifiers August 2001

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", RFC 2141, May 1997.

 [5]   Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
       Resource Identifiers (URI): Generic Syntax", RFC 2396, 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>.

Walsh, et al. Informational [Page 7] RFC 3151 A URN Namespace for Public Identifiers August 2001

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

Walsh, et al. Informational [Page 8] RFC 3151 A URN Namespace for Public Identifiers August 2001

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

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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

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