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Versions: 00 RFC 3151

Network Working Group                                           N. Walsh
Internet-Draft                                    Sun Microsystems, Inc.
Expires: August 14, 2001                                        J. Cowan
                                              Reuters Health Information
                                                               P. Grosso
                                                         Arbortext, Inc.
                                                       February 13, 2001


                 A URN Namespace for Public Identifiers
                         draft-walsh-urn-publicid-00

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 14, 2001.

Copyright Notice

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

Abstract

   This document describes a URN namespace that is designed to allow
   Public Identifiers to be expressed in URI syntax.

1. Introduction

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



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   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.

   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.  But SGML[3]
   defines a restricted subset of public identifier called a "Formal
   Public Identifier" (FPI). For the purpose of this document, the
   significant difference between public identifiers and FPIs is that
   FPIs have internal structure and may have registered owner
   identifiers.

   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.

2. Specification Template

   Namespace ID:

     "publicid" requested.

   Registration Information:

     Registration Version Number: 1
     Registration Date: 2001-02-13

   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 purpose of this namespace is to allow public identifiers
     to be encoded in URNs in a reliable, comparable way. To that
     end, this document mandates that public identifiers be


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     normalized before encoding them into URNs. As described in ISO
     8879[3], a public identifier is normalized by removing all
     leading and trailing whitespace and replacing all remaining
     sequences of two or more whitespace characters with a single
     space.

     For public identifiers that are not FPIs, the Namespace
     Specific String (NSS) for URNs in the "publicid" namespace has
     the following structure:

       urn:publicid:{public-identifier-text}

     The character set of public identifiers is constrained by
     XML[1]. Most of the legal public identifier characters are
     also legal characters in URNs. Unless otherwise noted, the
     characters in the {public-identifier-text} are directly
     transcribed from the corresponding character in the public
     identifier. The following exceptions are made:

     +  Spaces in the public identifier are transcribed as "+"
        characters. Whitespace normalization must be performed
        before constructing a URN in the "publicid" namespace,
        therefore the sequence of characters "++" should never
        occur in such URNs.
     +  Literal "+" characters in the public identifier must be
        %-encoded.
     +  Literal ":" characters in the public identifier must be
        %-encoded.
     +  The reserved characters that may appear in public
        identifiers, "%", "/", "?", and "#", must be %-encoded.

     Formal Public Identifiers are a subset of public identifiers.
     They are strings composed from the same range of characters,
     but have an explicit internal structure.  The structure of
     Formal Public Identifiers is normatively described in SGML[3],
     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:


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       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

     An algorithm for correctly identifying a Formal Public
     Identifier and determining the various fields within it is out
     of scope for this document. We begin our discussion of the
     representation of FPIs in our URN namespace under the
     assumption that these steps have already been taken.

     The Namespace Specific String (NSS) for the URNs in the
     "publicid" namespace that represent Formal Public Identifiers
     have the following structure:

     urn:publicid:{owner-identifier}:{text-class}
                 :{text-description}:{language|designating-sequence}
                 {:display-version}?

     Where:

      {owner-identifier} is derived from the owner identifier in
        the FPI. Owner identifiers in FPIs have one of three forms:
        "+//" followed by a string, "-//" followed by a string, or
        a string that does not contain "//". The following rules
        apply to derive a URN {owner-identifier} from the owner
        identifier in an FPI:

        -  Owner identifiers that begin "+//" are transcribed into
           the URN {owner-identifier} by replacing "+//" with "+:"
           and transcribing the remaining string.
        -  Owner identifiers that begin "-//" are transcribed into
           the URN {owner-identifier} by replacing "-//" with "-:"
           and transcribing the remaining string.
        -  All other {owner-identifiers} are transcribed directly
           from the owner identifier in the FPI.

      {text-class} is the public text class from the FPI. The
        public text class of FPIs is constrained by SGML[3] to the
        following 13 strings: "CAPACITY", "CHARSET", "DOCUMENT",
        "DTD", "ELEMENTS", "ENTITIES", "LPD", "NONSGML",
        "NOTATION", "SHORTREF", "SUBDOC", "SYNTAX", or "TEXT". The
        "publicid" URN namespace explicitly relaxes this


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        constraint. Any string may be used.

      {text-description} is the public text description transcribed
        from the FPI.

      {language} is the public text language transcribed from the
        FPI. The {language} codes used in "publicid" URNs should be
        drawn from RFC 3066[6].

      {designating-sequence} is the public text designating
        sequence transcribed from the FPI. Formal Public
        Identifiers that describe character sets may use the
        designating sequence (a string defined by ISO 2022[2]) to
        identify the character set.

      {display-version} is the public text display version
        transcribed from the FPI.

     Most of the legal public identifier characters are also legal
     characters in URNs. Unless otherwise noted, the characters in
     the {owner-identifier}, {text-class}, {text-description},
     {language}, {designating-sequence}, and {display-version} are
     directly transcribed from the corresponding character in the
     Formal Public Identifier. The following exceptions are made:

     +  Spaces in the FPI are transcribed as "+" characters.
        Whitespace normalization must be performed before
        constructing a URN in the "publicid" namespace, therefore
        the sequence of characters "++" should never occur in such
        URNs.
     +  Literal "+" characters in the FPI, except at the beginning
        of {owner-identifier}s for FPIs that have the "+//"-form of
        owner identifier, must be %-encoded. The "+" characters at
        the beginning of {owner-identifier}s for FPIs that have the
        "+//"-form of owner identifier, must not be %-encoded.
     +  The sequence "::" in the owner identifier or public text
        description is transcribed as "::"; all other uses of a
        literal ":" in the FPI must be %-encoded.
     +  The reserved characters that may appear in FPIs, "%", "/",
        "?", and "#", must be %-encoded.

     A small subset of Formal Public Identifiers cannot be
     represented by this namespace. An FPI cannot be represented if
     either of the following conditions applies:

     +  After transcription, the {owner-identifier}, {text-class},
        {text-description}, {language}, or {designating-sequence}
        would be empty. Allowing any of these fields to be empty
        could introduce ambiguous "::" sequences into the URN.


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     +  The FPI uses the optional unavailable text indicator
        defined in SGML[3] but rarely used in practice.

   Relevant ancillary documentation:

     Extensible Markup Language (XML) Version 1.0 Second Edition[1]
     Standard Generalized Markup Language (SGML)[3]
     Registration procedures for public text owner identifiers[4]

   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.

   Process of identifier assignment:

     Identifiers in the "publicid" namespace may be assigned by the
     same policies and procedures as public identifiers.

   Process of identifier resolution:

     Identifiers in the "publicid" namespace may be resolved by the
     same policies and procedures as public identifiers.


   Rules for Lexical Equivalence:

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

   Conformance with URN Syntax:

     No special considerations.

   Validation mechanism:

     None specified.



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   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 python.org//DTD XML Bookmark Exchange Language 1.0//EN//XML"
        becomes
        "urn:publicid:+:IDN+python.org:DTD:XML+Bookmark+Exchange+Language+1.0:EN:XML"


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


        "foo" becomes
        "urn:publicid:foo"

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

        "-//Acme, Inc.//DTD General Book Markup Version 1.0" becomes
        "urn:publicid:-%2F%2FAcme,+Inc.%2F%2FDTD+General+Book+Markup+Version+1.0"
        because it is not an FPI (it has no public text language or
        designating sequence).


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


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        Edition", February 1998,
        <http://www.w3.org/TR/REC-xml>.

   [2]  JTC 1, SC 2, "ISO (International Organization for
        Standardization) ISO 2022:1994 Information technology --
        Character code structure and extension techniques (fourth
        edition).", 1994.

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

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

   [5]  Moats, R., "URN Syntax", RFC 2141, May 1997.

   [6]  Alvestrand, H., "Tags for the Identification of Languages", RFC
        3066, January 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
   1700 Broadway, 31st Floor
   New York, NY  10019
   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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Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
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Acknowledgement

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



















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