Subject: [URN] Pre-release #3 of syntax -02 draft...

From: Ryan Moats <[email-masked]>
Date: Mon, 03 Feb 1997 11:05:24 -0600

Thanks to everyone for comments, and especially thanks to Leslie for
putting them together.  Attached is the pre-release #3 of 
the syntax draft (or is it #4?).

Changes from last pre-release are:

Addition of reserved character set and discussion section (2.3).
"%" and "/" are now moved to the reserved character set.  The
discussion of "%" and %-encoding now moved to this section.  Discussion
added for "/".

Any more comments, or can we go with this as -02?

Ryan

Internet-Draft                                                Ryan Moats
draft-ietf-urn-syntax-02.txt                                        AT&T
Expires in six months                                       January 1997

                               URN Syntax
                 Filename: draft-ietf-urn-syntax-02.txt

Status of This Memo

      This document is an Internet-Draft.  Internet-Drafts are working
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Abstract

   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
   required.  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
   separately.

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>         ::= <let-num> [ *<let-num-hyp> ]

   <let-num-hyp> ::= <upper> | <lower> | <number> | "-"

   <let-num>     ::= <upper> | <lower> | <number>

   <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"

   This is slightly more restrictive that what is stated in [3] (which

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   allows the characters "." and "+").  Further, the Namespace
   Identifier is case insensitive, so that "ISBN" and "isbn" refer to
   the same namespace.

   To avoid confusion with the "urn:" identifier, the NID "urn" is
   reserved and MUST 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
   follows:

   <NSS>         ::= 1*<URN chars>

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

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

   <hex>         ::= <number> | "A" | "B" | "C" | "D" | "E" | "F" |
                     "a" | "b" | "c" | "d" | "e" | "f"

   <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.

2.3 Reserved characters

   The remaining character set left to be discussed above is the
   reserved character set, which contains various characters reserved
   from normal use.  The reserved character set follows, with a
   discussion on the specifics of why each character is reserved.

   The reserved character set is:

   <reserved>    ::= "/" | "%"

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2.3.1 The "%" character

   The "%" character is reserved in the URN syntax for introducing the
   escape sequence for an octet.  Literal use of the "%" character in a
   namespace must be encoded using "%25" in URNs for that namespace.
   The presence of an "%" character in a URN MUST be followed by two
   characters from the <hex> character set.

   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 to that namespace.

2.3.2 The "/" character

   The "/" character is RESERVED for denoting relativity in the URN
   namespace (i.e. as part of the process of handling URNs).  A "/"
   should be %-encoded (i.e. %2F) if and only if:

         - in the particular namespace, "/" does not necessarily denote
           hierarchy (or other relative terms).
         - the designers of the namespace have determined that it is
           not desirable to expose that hierarchy _for_the_purposes_
           _of_describing_relationships_between_URNs (e.g., if some
           other hierarchy is desired)

2.4 Excluded characters

   The following list is included only for the sake of completeness.
   Any octets/characters on this list are explicitly NOT part of the URN
   character set, and if used in a URN, MUST be %encoded:

   <excluded> ::= octets 0-32 (0-20 hex) | "\" | """ | "#" | "&" | "<"
                  | ">" | "[" | "]" | "^" | "`" | "{" | "|" | "}" | octets 127-255 (7F-FF hex)

   A URN ends when an octet/character from the excluded character set
   (<excluded>) is encountered.  The character from the excluded
   character set is NOT part of the URN.

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

   Any namespace (existing or newly-devised) that is proposed as a URN-
   namespace and fulfills the criteria of URN-namespaces MUST be
   expressed in this syntax.  If names in these namespaces contain

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   characters other than those defined for the URN character set, they
   MUST be translated into canonical form as discussed in section 2.2.

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 MUST be transported in canonical format.

5. Lexical Equivalence in URNs

For various purposes, such as caching, it is necessary to determine
equivalence without actually resolving the URN. This done by testing for
"lexical equivalence". Two URNs are lexically equivalent if they are
octet-by-octet equal after the following preprocessing

        1. normalize the case of the leading "urn:" token
        2. normalize the case of the NID
        3. normalizing the case of any "%" escaping

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 say that two URNs are
not equivalent if the procedure above says they are equivalent.

6. Functional Equivalence in URNs

   Functional equivalence is determined by URN resolvers and is
   therefore outside the scope of this document.  Namespace registration
   MUST include documentation on how to determine functional equivalence
   for that namespace.

7. Examples of equivalence

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

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

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8. Security considerations

   This document specifies the syntax for URNs.  While some namespaces
   resolvers may assign special meaning to certain of the characters of
   the Namespace Specific String, any security consideration resulting
   from such assignment are outside the scope of this document.  It is
   strongly recommended that the process of registering a namespace
   identifier include any such considerations.

9. 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,
   Cooperative Agreement NCR-9218179.

10. References

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

         [1]         K. R. Sollins. "Requirements and a Framework for
                     URN Resolution Systems" Internet Draft (work in
                     progress).  November 1996.

         [2]         K. Sollins, L. Masinter.  "Functional Requirements
                     for Uniform Resource Names," RFC 1737.  December
                     1994.

         [3]         T. Berners-Lee, R. Fielding, L. Masinter. "Uniform
                     Resource Locators (URL)," Internet Draft (work in
                     progress).  December 1996.

11. Editor's address

   Ryan Moats
   AT&T
   15621 Drexel Circle
   Omaha, NE 68135-2358
   USA

   Phone:  +1 402 894-9456
   EMail:  [email-masked]

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Appendix A. Handling of URNs by URL resolvers/browsers.

   The URN syntax has been defined so that URNs can be used in places
   where URLs are expected.  A resolver that conforms to the current URL
   syntax specification [3] will extract a scheme value of "urn:"
   rather than a scheme value of "urn:<nid>".

   A URN MUST be considered an opaque URL by URL resolvers and either
   passed (with the "urn:" tag) to a URN resolver for resolution.  The
   URN resolver can either be an external resolver that the URL resolver
   knows of, or it can be functionality built-in to the URL resolver.

   To avoid confusion of users, a URL browser SHOULD display the com-
   plete URN (including the "urn:" tag) to ensure that there is no con-
   fusion between URN namespace identifiers and URL scheme identifiers.

                This Internet Draft expires July 31, 1997.

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