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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
documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups. Note that other groups may also
distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as ``work
in progress.''
To learn the current status of any Internet-Draft, please check
the ``1id-abstracts.txt'' listing contained in the Internet-
Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net
(Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East
Coast), or ftp.isi.edu (US West Coast).
Abstract
Uniform Resource Names (URNs) are intended to serve as persistent
resource identifiers. This document sets forward the canonical syntax
for URNs. A discussion of both existing legacy and new namespaces
and 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.
Therefore, the URN syntax 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 (phrases enclosed in quotes are
REQUIRED):
<URN> ::= "urn:" <NID> ":" <NSS>
where <NID> is the Namespace Identifier, and <NSS> is the Namespace
Specific String. The leading "urn:" sequence is case-insensitive.
The Namespace ID determines the _syntactic_ interpretation of the
Namespace Specific String (as discussed in [1]).
RFC 1630 [2] and RFC 1737 [3] each presents additional considerations
for URN encoding, which 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 [4] (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 RFC 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 an 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 future developments. It might be
used for denoting hierarchy to allow for relative URN processing, but
the WG has not yet reached consensus on this, so such developments
will be documented separately. Meanwhile, namespace developers
SHOULD NOT use an unencoded "/", but rather use %-encoding for "/"
("%2F").
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 an URN, MUST be %encoded:
<excluded> ::= octets 0-32 (0-20 hex) | "\" | """ | "#" | "&" | "<"
| ">" | "[" | "]" | "^" | "`" | "{" | "|" | "}" | octets 127-255 (7F-FF hex)
An 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 an
URN-namespace and fulfills the criteria of URN-namespaces MUST be
expressed in this syntax. If names in these namespaces contain
characters other than those defined for the URN character set, they
MUST be translated into canonical form as discussed in section 2.2.
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4. URN presentation and transport
The URN syntax defines the canonical format for URNs and all URN
transport and interchanges MUST take place in this format. Further,
all URN-aware applications MUST offer the option of displaying URNs
in this canonical form to allow for direct transcription (for example
by cut and paste techniques). Such applications MAY support display
of URNs in a more human-friendly form and may use a character set
that includes characters that aren't permitted in URN syntax as
defined in this RFC (that is, they may replace %-notation by
characters in some extended character set in display to humans).
5. Lexical Equivalence in URNs
For various purposes such as caching, it's often desirable to determine
if two URNs are the same without resolving them. The general purpose
means of doing so is by testing for "lexical equivalence" as defined
below.
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
Note that %-escaping MUST NOT be removed.
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. Examples of lexical equivalence
The following URN comparisons highlight the lexical equivalence
definitions:
1- URN:foo:a123/456
2- urn:foo:a123/456
3- urn:FOO:a123/456
4- urn:foo:A123/456
5- urn:foo:a123%2F456
6- URN:FOO:a123%2f456
URNs 1, 2, and 3 are all lexically equivalent. URN 4 is not
lexically equivalent any of the other URNs of the above set. URNs 5
and 6 are only lexically equivalent to each other.
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7. Functional Equivalence in URNs
Functional equivalence is determined by practice within a given
namespace and managed by resolvers for that namespeace. Namespace
registration must include guidance on how to determine functional
equivalence for that namespace, i.e. when two URNs are the identical
within a namespace.
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]
T. Berners-Lee, "Universal Resource Identifiers in WWW," RFC
1630, June 1994.
[3] K. Sollins and L. Masinter, "Functional Require-
ments for Uniform Resource Names," RFC 1737.
December 1994.
[4] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform
Resource Locators (URL)," Internet Draft (work in
progress), December 1996.
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11. Editor's address
Ryan Moats
AT&T
15621 Drexel Circle
Omaha, NE 68135-2358
USA
Phone: +1 402 894-9456
EMail: jayhawk@ds.internic.net
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>".
An URN MUST be considered an opaque URL by URL resolvers and either
passed (with the "urn:" tag) to an 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, an 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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