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Internet-Draft                                                Ryan Moats
draft-ietf-urn-syntax-01.txt                                        AT&T
Expires in six months                                      November 1996

                               URN Syntax
                 Filename: draft-ietf-urn-syntax-01.txt

Status of This Memo

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   Uniform Resource Names (URNs) are intended to serve as persistent
   resource identifiers. This document sets forward the canonical syntax
   for URNs.  Support for both existing legacy and new namespaces is
   discussed. Requirements for URN presentation and transmission are
   presented.  Finally, there is a discussion of URN equivalence and how
   to determine it.

1. Introduction

   Uniform Resource Names (URNs) are intended to serve as persistent
   resource identifiers and are designed to make it easy to map other
   namespaces (which share the properties of URNs) into URN-space. The
   URN syntax therefore provides a means to encode character data in a
   form that can be sent in existing protocols, transcribed on most
   keyboards, etc.

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2. Syntax

   All URNs have the following syntax:

                    <URN> ::= ["urn:"] <NID> ":" <NSS>

   <NID> is the Namespace Identifier, and <NSS> is the Namespace
   Specific String.  The leading case-insensitive "urn:" sequence is
   currently optional, as no closure on its definite presence or absence
   has been reached.  The Namespace ID is used to determine the
   _syntactic_ interpretation of the Namespace Specific String (as
   discussed in [1]).

   RFC 1737 [2] presents additional requirements on URN encoding, which
   all have implications as far as limiting syntax.  On the other hand,
   the requirement to support existing legacy naming systems has the
   effect of broadening syntax.  Thus, we discuss the acceptable syntax
   for both the Namespace Identifier and the Namespace Specific String

2.1 Namespace Identifier Syntax

   The following is the syntax for the Namespace Identifier. To (a) be
   consistent with all potential resolution schemes and (b) not put any
   undue constraints on any potential resolution scheme, the syntax for
   the Namespace Identifier is:

   <NID>         ::= <letter> [ <let-hyp> ]

   <let-hyp>     ::= <letter> | "-" | <let-hyp>

   <letter>      ::= any one of the 52 alphabetic characters A through Z
                     in upper case and a through z in lower case

   This is slightly more restrictive that what is stated in RFC 1738 [4]
   (which allows the period "."). Further, the Namespace Identifier is
   case insensitive, so that "ISBN" and "isbn" refer to the same

   To avoid confusion with the optional "urn:" identifier, the NID "urn"
   is reserved and may not be used.

2.2 Namespace Specific String Syntax

   As required by 1737, there is a single canonical representation of
   the NSS portion of an URN.   The format of this single canonical form

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   <NSS>        ::= <URN chars>*

   <URN chars>  ::= <trans> | "%" <hex> <hex>

   <trans>      ::= <upper> | <lower> | <number> | <other>

   <hex>        ::= <number> | "A" | "B" | "C" | "D" | "E" | "F"

   <upper>      ::= "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" |
                    "I" | "J" | "K" | "L" | "M" | "N" | "O" | "P" |
                    "Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" |
                    "Y" | "Z"

   <lower>      ::= "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" |
                    "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" |
                    "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" |
                    "y" | "z"

   <number>     ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                    "8" | "9"

   <other>      ::= "(" | ")" | "+" | "
                    ":" | "=" | "?" | "@"

   Depending on the rules governing a namespace, valid identifiers in a
   namespace might contain characters that are not members of the URN
   character set above (<URN chars>).  Such strings MUST be translated
   into canonical NSS format before using them as protocol elements or
   otherwise passing them on to other applications. Translation is done
   by encoding each character outside the URN character set as a
   sequence of one to six octets using UTF-8 encoding, and the encoding
   of each of those octets as "%" followed by two characters from the
   <hex> character set above. The two characters give the hexadecimal
   representation of that octet.

   Namespaces MAY designate one or more characters from the URN
   character set as having special meaning for that namespace. If the
   namespace also uses that character in a literal sense as well, the
   character used in a literal sense must be encoded with "%" followed
   by the hexadecimal representation of that octet.  Therefore, the
   process of registering a namespace identifier shall include
   publication of a definition of which characters have a special
   meaning and how to encode these characters if used in a literal

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3. Support of existing legacy naming systems and new naming systems

   URN-aware applications MAY accept as input other resource identifiers
   from existing legacy namespaces.  If such identifiers contain
   characters that are not members of the URN character set specified in
   section 2.2, the identifier MUST be translated to canonical format as
   discussed in section 2.2.

   Some existing name spaces that have the properties of the URN-space
   contain some human-significant components, and these exist in a wide
   variety of languages.  However, URNs are NOT intended to convey
   information that is significant to humans.  While the translation
   rule in section 2.2 is provided for existing namespaces, new
   namespaces, as part of their registration documentation, MUST define
   a discipline for assigning new URNs that does not simplify the
   generation of human-significant names.

4. URN presentation and transport

   URN-aware applications MAY support "natural" display of URNs which
   contain characters encoded using "%" notation.  However, they MUST
   provide for display of URNs in canonical form (i.e. in a format
   suitable for transcription).

   URNs may only be transported in canonical format.

5. Equivalence in URNs

URNs are considered equivalent if they return the same resource.  For
various purposes, such as caching, a test is necessary to determine
equivalence without actually resolving the URNs and fetching/comparing
the underlying resources.  "Lexical equivalence" is a stricter condition
that the equivalence described above (functional equivalence).

5.1 Lexical Equivalence

   Lexical equivalence may be determined by comparing two URNs without
   making any network accesses. Two URNs are lexically equivalent if
   they are octet-by-octet equal after the following preprocessing

           1. drop any preceding "urn:" token
           2. normalize the case of the NID

   Some namespaces may define additional lexical equivalences, such as
   case-insensitivity of the NSS (or parts thereof).  Additional lexical
   equivalences MUST be documented as part of namespace registration,
   MUST always have the effect of eliminating some of the false
   negatives obtained by the procedure above, and MUST NEVER says that

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   two URNs are not equivalent if the procedure above says they are

5.2 Functional Equivalence

   Resolvers determine functional equivalence based on specific rules
   for the namespace.  Therefore, namespace registration must include
   documentation on how to determine functional equivalence for that

5.3 Examples

   The following URN comparisons highlight the difference between these
   types of equivalence:

     urn:isbn:1-23485-8-29, isbn:1-23485-8-29 are lexically equiv.
     urn:isbn:1-23485-8-29, ISBN:1-23485-8-29 are lexically equiv.
     urn:isbn:1-23485-8-29, isbn:123485829 are not lexically equiv.
        but may be functionally equivalent.

6. Security considerations

   Because of the number of potential namespaces, it must be restated
   that certain of the characters in the Namespace Specific String may
   have special meaning to certain namespace resolvers.  The process of
   registering a namespace identifier shall therefore include
   publication of a definition of which characters have a special

7. Acknowledgments

   Thanks to various members of the URN working group and <<your name
   here!!>> for comments on earlier drafts of this document.  This
   document is partially supported by the National Science Foundation.

8. References

   Request For Comments (RFC) and Internet Draft documents are available
   from <URL:ftp://ftp.internic.net> and numerous mirror sites.

         [1]         L. L. Daigle, P. Faltstrom, R. Iannella.  "A Frame-
                     work for the Assignment and Resolution of Uniform
                     Resource Names," Internet Draft (work in progress).
                     June 1996.

         [2]         K. Sollins, L. Masinter.  "Functional Requirements
                     for Uniform Resource Names," RFC 1737.  December

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         [3]         T. Berners-Lee. "Universal Resource Identifiers in
                     WWW," RFC 1630. June 1994.

         [4]         T. Berners-Lee, L. Masinter, M. McCahill. "Uniform
                     Resource Locators (URL)," RFC 1738.  December 1994.

9. Editor's address

   Ryan Moats
   15621 Drexel Circle
   Omaha, NE 68135-2358

   Phone:  +1 402 894-9456
   EMail:  jayhawk@ds.internic.net

                 This Internet Draft expires May 19, 1997.

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