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.







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  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:



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  +//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

     [email protected]

  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.



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  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.




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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>.





























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Authors' Addresses

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

  EMail: [email protected]


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

  EMail: [email protected]


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

  EMail: [email protected]
























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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
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
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  followed, or as required to translate it into languages other than
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  The limited permissions granted above are perpetual and will not be
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  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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  HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
  MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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



















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