Internet DRAFT - draft-walsh-urn-publicid

draft-walsh-urn-publicid





Network Working Group                                           N. Walsh
Internet-Draft                                    Sun Microsystems, Inc.
Expires: November 6, 2001                                       J. Cowan
                                              Reuters Health Information
                                                               P. Grosso
                                                         Arbortext, Inc.
                                                             May 8, 2001


                 A URN Namespace for Public Identifiers
                         draft-walsh-urn-publicid-01

Status of this Memo

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

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   This Internet-Draft will expire on November 6, 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 system identifier
   and a public 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.

   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, #xD, and #xA, in this context) 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[3] 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[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. 



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   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"


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         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 RFC 2141[5] and RFC 2396[6]. 

   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: 



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         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[8] Catalog files. Other systems resolve them


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         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 they are lexically identical.

   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:+: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" 



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      "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 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]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
        Resource Identifiers (URI): Generic Syntax", RFC 2396, Aug 1998.

   [7]  Alvestrand, H., "Tags for the Identification of Languages", RFC
        3066, January 2001.

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









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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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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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