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Request For Comments - RFC3151

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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.


   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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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

   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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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


   Declaration of structure:

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



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

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RFC 3151         A URN Namespace for Public Identifiers      August 2001

               -  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

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RFC 3151         A URN Namespace for Public Identifiers      August 2001

   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

   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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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.



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

      "ISO 8879:1986//ENTITIES Added Latin 1//EN" becomes

      "-//OASIS//DTD DocBook XML V4.1.2//EN" becomes

      "+//IDN example.org//DTD XML Bookmarks 1.0//EN//XML" becomes

      "-//ArborText::prod//DTD Help Document::19970708//EN" becomes

      "foo" becomes

      "3+3=6" becomes

      "-//Acme, Inc.//DTD Book Version 1.0" becomes

4. Security Considerations

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

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RFC 3151         A URN Namespace for Public Identifiers      August 2001


   [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)",

   [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

   [6]   Grosso, P., "Entity Management: OASIS Technical Resolution
         9401:1997 (Amendment 2 to TR 9401)", Sep 1997,

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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

   EMail: Norman.Walsh@East.Sun.COM

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

   EMail: jcowan@reutershealth.com

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

   EMail: pgrosso@arbortext.com

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RFC 3151         A URN Namespace for Public Identifiers      August 2001

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
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


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

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©2018 Martin Webb