Internet-Draft Ryan Moats draft-ietf-urn-syntax-00.txt AT&T Expires in six months October 1996 URN Syntax Filename: draft-ietf-urn-syntax-00.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 presents the syntax for URNs. Support for existing legacy namespaces is discussed. URN transmission encoding requirements are presented. Finally, there is a discussion of URN equivalence and how to determine it. 1. 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 "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 ). RFC 1737  suggests 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. 1.1 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> | "-" <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  (which allows the period "."). Further, the Namespace Identifier is case insensitive, so that "ISBN" and "isbn" refer to the same namespace. To avoid confusion with the optional "urn:" identifier, the NID "urn" is reserved and may not be used. 1.2 Namespace Specific String Syntax Depending on the rules governing a namespace, valid identifiers in a namespace might contain characters that are reserved characters in URI syntax or non-printable ASCII characters. To accommodate the largest set of valid identifiers, the NSS portion of a URN shall use UTF-8 representation of ISO 10646 as its character set. Namespaces that do not currently use ISO 10646/UTF-8 are encouraged to migrate to it. Clients MUST be capable of %encoding the UTF-8 formatted NSS. %encoding, (as discussed in ) uses a percent sign "%" immediately followed by two hexadecimal digits (0-9, A-F) giving the binary code for that octet. The rules for %encoding presented in  apply with the following exceptions: 1.  states that occurrence of the '/' character in URIs must denote hierarchy, so that partial forms of a URI are possible. This restriction is unenforceable, and relative URLs do not have a scheme prefix, so we allow URNs to contain unescaped occurrences of the '/' character that do not denote hierarchy. 2. As an optimization when the transport between systems is known to be 8-bit-clean, clients MAY omit the %encoding on 8-bit characters but MUST still %encode the reserved characters below. For historic reasons, the characters "#" (%23), "?" (%3F), "%" (%25), "*" (%2A), "!" (%21), "<" (%3C), ">" (%3E), and '"' (%22), are reserved and must be %encoded. Thus client implementers should accept URNs from users in an unencoded form but must encode them before sending them to a resolver. URN resolvers MUST be capable of accepting URNs that have been %encoded for either 8-bit clean or 7-bit transports. %encoding is removed first, then UTF-8 decoding is performed. URN resolvers MUST return identical results from ANY legally encoded form of the URN. It should be noted that certain characters in the Namespace Specific String syntax may have special meaning in certain namespaces. 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 sense. 2. Support of existing legacy naming systems To allow for support existing legacy naming systems (as required by ), the Namespace Specific String shall be considered an "opaque string" in the sense of structure except as mentioned in Section 1. In addition, URN servers should be prepared to accept URNs that do not use ISO 10646/UTF-8 for those namespaces that currently use a different encoding. Note that this is not a general requirement on all resolvers, only resolvers that handle a namespace that is known not to use ISO 10646/UTF-8. 3. URN encoding for transmission Because the NSS of a URN is considered a series of octets of data, encoding URNs for transport is the responsibility of the transport mechanism and is not discussed here. Any mechanism that can handle arbitrary 8-bit data will successfully transport a URN. 4. Equivalence in URNs URNs are considered equivalent if they return the same result. 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). 4.1 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. remove any %encoding that might be present 2. drop any preceding "urn:" token 3. 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 two URNs are not equivalent if the procedure above says they are equivalent. 4.2 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 namespace. 4.3 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. 5. 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 meaning and how to encode these characters if used in a literal sense. 6. 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. 7. References Request For Comments (RFC) and Internet Draft documents are available from <URL:ftp://ftp.internic.net> and numerous mirror sites. L. L. Daigle, P. Faltstrom, R. Iannella. "A Framework for the Assignment and Resolution of Uniform Resource Names", Internet Draft (work in progress). June 1996. K. Sollins, L. Masinter. "Functional Requirements for Uniform Resource Names", RFC 1737. December 1994. T. Berners-Lee. "Universal Resource Identifiers in WWW", RFC 1630. June 1994. T. Berners-Lee, L. Masinter, M. McCahill. "Uniform Resource Locators (URL)", RFC 1738. December 1994. 8. Author's address Ryan Moats AT&T 15621 Drexel Circle Omaha, NE 68135-2358 USA Phone: +1 402 894-9456 EMail: email@example.com This Internet Draft expires April 1, 1997.