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Network Working Group                                           N. WalshRequest for Comments: 3151                        Sun Microsystems, Inc.Category: Informational                                         J. Cowan                                              Reuters Health Information                                                               P. Grosso                                                         Arbortext, Inc.                                                             August 2001A URN Namespace for Public IdentifiersStatus 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:Walsh, et al.                Informational                      [Page 3]

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 ofRFC 2141 [4] andRFC2396 [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.Walsh, et al.                Informational                      [Page 4]

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      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 bothRFC 2141 andRFC 2396.   Validation mechanism:      None specified.   Scope:      Global3. 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 2001References   [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 2001Authors' 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.comWalsh, et al.                Informational                      [Page 8]

RFC 3151         A URN Namespace for Public Identifiers      August 2001Full 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   English.   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   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   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.Walsh, et al.                Informational                      [Page 9]