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+Network Working Group                                         K. Sollins
+Request for Comments: 1737                                       MIT/LCS
+Category: Informational                                      L. Masinter
+                                                       Xerox Corporation
+                                                           December 1994
+
+
+           Functional Requirements for Uniform Resource Names
+
+Status of this Memo
+
+   This memo provides information for the Internet community.  This memo
+   does not specify an Internet standard of any kind.  Distribution of
+   this memo is unlimited.
+
+1.  Introduction
+
+   This document specifies a minimum set of requirements for a kind of
+   Internet resource identifier known as Uniform Resource Names (URNs).
+   URNs fit within a larger Internet information architecture, which in
+   turn is composed of, additionally, Uniform Resource Characteristics
+   (URCs), and Uniform Resource Locators (URLs).  URNs are used for
+   identification, URCs for including meta-information, and URLs for
+   locating or finding resources.  It is provided as a basis for
+   evaluating standards for URNs.  The discussions of this work have
+   occurred on the mailing list uri@bunyip.com and at the URI Working
+   Group sessions of the IETF.
+
+   The requirements described here are not necessarily exhaustive; for
+   example, there are several issues dealing with support for
+   replication of resources and with security that have been discussed;
+   however, the problems are not well enough understood at this time to
+   include specific requirements in those areas here.
+
+   Within the general area of distributed object systems design, there
+   are many concepts and designs that are discussed under the general
+   topic of "naming". The URN requirements here are for a facility that
+   addresses a different (and, in general, more stringent) set of needs
+   than are frequently the domain of general object naming.
+
+   The requirements for Uniform Resource Names fit within the overall
+   architecture of Uniform Resource Identification.  In order to build
+   applications in the most general case, the user must be able to
+   discover and identify the information, objects, or what we will call
+   in this architecture resources, on which the application is to
+   operate.  Beyond this statement, the URI architecture does not define
+   "resource."  As the network and interconnectivity grow, the ability
+   to make use of remote, perhaps independently managed, resources will
+
+
+
+Sollins & Masinter                                              [Page 1]
+
+RFC 1737        Requirements for Uniform Resource Names    December 1994
+
+
+   become more and more important.  This activity of discovering and
+   utilizing resources can be broken down into those activities where
+   one of the primary constraints is human utility and facility and
+   those in which human involvement is small or nonexistent.  Human
+   naming must have such characteristics as being both mnemonic and
+   short.  Humans, in contrast with computers, are good at heuristic
+   disambiguation and wide variability in structure.  In order for
+   computer and network based systems to support global naming and
+   access to resources that have perhaps an indeterminate lifetime, the
+   flexibility and attendant unreliability of human-friendly names
+   should be translated into a naming infrastructure more appropriate
+   for the underlying support system.  It is this underlying support
+   system that the Internet Information Infrastructure Architecture
+   (IIIA) is addressing.
+
+   Within the IIIA, several sorts of information about resources are
+   specified and divided among different sorts of structures, along
+   functional lines.  In order to access information, one must be able
+   to discover or identify the particular information desired,
+   determined both how and where it might be used or accessed.  The
+   partitioning of the functionality in this architecture is into
+   uniform resource names (URN), uniform resource characteristics (URC),
+   and uniform resource locators (URL).  A URN identifies a resource or
+   unit of information.  It may identify, for example, intellectual
+   content, a particular presentation of intellectual content, or
+   whatever a name assignment authority determines is a distinctly
+   namable entity.  A URL identifies the location or a container for an
+   instance of a resource identified by a URN.  The resource identified
+   by a URN may reside in one or more locations at any given time, may
+   move, or may not be available at all.  Of course, not all resources
+   will move during their lifetimes, and not all resources, although
+   identifiable and identified by a URN will be instantiated at any
+   given time.  As such a URL is identifying a place where a resource
+   may reside, or a container, as distinct from the resource itself
+   identified by the URN.  A URC is a set of meta-level information
+   about a resource.  Some examples of such meta-information are: owner,
+   encoding, access restrictions (perhaps for particular instances),
+   cost.
+
+   With this in mind, we can make the following statement:
+
+   o  The purpose or function of a URN is to provide a globally unique,
+      persistent identifier used for recognition, for access to
+      characteristics of the resource or for access to the resource
+      itself.
+
+
+
+
+
+
+Sollins & Masinter                                              [Page 2]
+
+RFC 1737        Requirements for Uniform Resource Names    December 1994
+
+
+   More specifically, there are two kinds of requirements on URNs:
+   requirements on the functional capabilities of URNs, and requirements
+   on the way URNs are encoded in data streams and written
+   communications.
+
+2. Requirements for functional capabilities
+
+   These are the requirements for URNs' functional capabilities:
+
+   o Global scope: A URN is a name with global scope which does not
+     imply a location.  It has the same meaning everywhere.
+
+   o Global uniqueness: The same URN will never be assigned to two
+     different resources.
+
+   o Persistence: It is intended that the lifetime of a URN be
+     permanent.  That is, the URN will be globally unique forever, and
+     may well be used as a reference to a resource well beyond the
+     lifetime of the resource it identifies or of any naming authority
+     involved in the assignment of its name.
+
+   o Scalability: URNs can be assigned to any resource that might
+     conceivably be available on the network, for hundreds of years.
+
+   o Legacy support: The scheme must permit the support of existing
+     legacy naming systems, insofar as they satisfy the other
+     requirements described here. For example, ISBN numbers, ISO
+     public identifiers, and UPC product codes seem to satisfy the
+     functional requirements, and allow an embedding that satisfies
+     the syntactic requirements described here.
+
+   o Extensibility: Any scheme for URNs must permit future extensions to
+     the scheme.
+
+   o Independence: It is solely the responsibility of a name issuing
+     authority to determine the conditions under which it will issue a
+     name.
+
+   o Resolution: A URN will not impede resolution (translation into a
+     URL, q.v.). To be more specific, for URNs that have corresponding
+     URLs, there must be some feasible mechanism to translate a URN to a
+     URL.
+
+3. Requirements for URN encoding
+
+   In addition to requirements on the functional elements of the URNs,
+   there are requirements for how they are encoded in a string:
+
+
+
+
+Sollins & Masinter                                              [Page 3]
+
+RFC 1737        Requirements for Uniform Resource Names    December 1994
+
+
+   o Single encoding: The encoding for presentation for people in clear
+     text, electronic mail and the like is the same as the encoding in
+     other transmissions.
+
+   o Simple comparison: A comparison algorithm for URNs is simple,
+     local, and deterministic. That is, there is a single algorithm for
+     comparing two URNs that does not require contacting any external
+     server, is well specified and simple.
+
+   o Human transcribability: For URNs to be easily transcribable by
+     humans without error, they should be short, use a minimum of
+     special characters, and be case insensitive. (There is no strong
+     requirement that it be easy for a human to generate or interpret a
+     URN; explicit human-accessible semantics of the names is not a
+     requirement.)  For this reason, URN comparison is insensitive to
+     case, and probably white space and some punctuation marks.
+
+   o Transport friendliness: A URN can be transported unmodified in the
+     common Internet protocols, such as TCP, SMTP, FTP, Telnet, etc., as
+     well as printed paper.
+
+   o Machine consumption: A URN can be parsed by a computer.
+
+   o Text recognition: The encoding of a URN should enhance the
+     ability to find and parse URNs in free text.
+
+4. Implications
+
+   For a URN specification to be acceptible, it must meet the previous
+   requirements.  We draw a set of conclusions, listed below, from those
+   requirements; a specification that satisfies the requirments without
+   meetings these conclusions is deemed acceptable, although unlikely to
+   occur.
+
+   o To satisfy the requirements of uniqueness and scalability, name
+     assignment is delegated to naming authorities, who may then assign
+     names directly or delegate that authority to sub-authorities.
+     Uniqueness is guaranteed by requiring each naming authority to
+     guarantee uniqueness.  The names of the naming authorities
+     themselves are persistent and globally unique and top level
+     authorities will be centrally registered.
+
+   o Naming authorities that support scalable naming are encouraged, but
+     not required.  Scalability implies that a scheme for devising names
+     may be scalable both at its terminators as well as within the
+     structure; e.g., in a hierarchical naming scheme, a naming
+     authority might have an extensible mechanism for adding new
+     sub-registries.
+
+
+
+Sollins & Masinter                                              [Page 4]
+
+RFC 1737        Requirements for Uniform Resource Names    December 1994
+
+
+   o It is strongly recommended that there be a mapping between the
+     names generated by each naming authority and URLs.  At any specific
+     time there will be zero or more URLs into which a particular URN
+     can be mapped.  The naming authority itself need not provide the
+     mapping from URN to URL.
+
+   o For URNs to be transcribable and transported in mail, it is
+     necessary to limit the character set usable in URNs, although there
+     is not yet consensus on what the limit might be.
+
+   In assigning names, a name assignment authority must abide by the
+   preceding constraints, as well as defining its own criteria for
+   determining the necessity or indication of a new name assignment.
+
+5. Other considerations
+
+   There are three issues about which this document has intentionally
+   not taken a position, because it is believed that these are issues to
+   be decided by local determination or other services within an
+   information infrastructure.  These issues are equality of resources,
+   reflection of visible semantics in a URN, and name resolution.
+
+   One of the ways in which naming authorities, the assigners of names,
+   may choose to make themselves distinctive is by the algorithms by
+   which they distinguish or do not distinguish resources from each
+   other.  For example, a publisher may choose to distinguish among
+   multiple printings of a book, in which minor spelling and
+   typographical mistakes have been made, but a library may prefer not
+   to make that distinction.  Furthermore, no one algorithm for testing
+   for equality is likely to applicable to all sorts of information.
+   For example, an algorithm based on testing the equality of two books
+   is unlikely to be useful when testing the equality of two
+   spreadsheets.  Thus, although this document requires that any
+   particular naming authority use one algorithm for determining whether
+   two resources it is comparing are the same or different, each naming
+   authority can use a different such algorithm and a naming authority
+   may restrict the set of resources it chooses to identify in any way
+   at all.
+
+   A naming authority will also have some algorithm for actually
+   choosing a name within its namespace.  It may have an algorithm that
+   actually embeds in some way some knowledge about the resource.  In
+   turn, that embedding may or may not be made public, and may or may
+   not be visible to potential clients.  For example, an unreflective
+   URN, simply provides monotonically increasing serial numbers for
+   resources.  This conveys nothing other than the identity determined
+   by the equality testing algorithm and an ordering of name assignment
+   by this server.  It carries no information about the resource itself.
+
+
+
+Sollins & Masinter                                              [Page 5]
+
+RFC 1737        Requirements for Uniform Resource Names    December 1994
+
+
+   An MD5 of the resource at some point, in and of itself may be
+   reflective of its contents, and, in fact, the naming authority may be
+   perfectly willing to publish the fact that it is using MD5, but if
+   the resource is mutable, it still will be the case that any potential
+   client cannot do much with the URN other than check for equality.
+   If, in contrast, a URN scheme has much in common with the assignment
+   ISBN numbers, the algorithm for assigning them is public and by
+   knowing it, given a particular ISBN number, one can learn something
+   more about the resource in question.  This full range of
+   possibilities is allowed according to this requirements document,
+   although it is intended that naming authorities be discouraged from
+   making accessible to clients semantic information about the resource,
+   on the assumption that that may change with time and therefore it is
+   unwise to encourage people in any way to depend on that semantics
+   being valid.
+
+   Last, this document intentionally does not address the problem of
+   name resolution, other than to recommend that for each naming
+   authority a name translation mechanism exist.  Naming authorities
+   assign names, while resolvers or location services of some sort
+   assist or provide URN to URL mapping.  There may be one or many such
+   services for the resources named by a particular naming authority.
+   It may also be the case that there are generic ones providing service
+   for many resources of differing naming authorities.  Some may be
+   authoritative and others not.  Some may be highly reliable or highly
+   available or highly responsive to updates or highly focussed by other
+   criteria such as subject matter.  Of course, it is also possible that
+   some naming authorities will also act as resolvers for the resources
+   they have named.  This document supports and encourages third party
+   and distributed services in this area, and therefore intentionally
+   makes no statements about requirements of URNs or naming authorities
+   on resolvers.
+
+Security Considerations
+
+   Applications that require translation from names to locations, and
+   the resources themselves may require the resources to be
+   authenticated. It seems generally that the information about the
+   authentication of either the name or the resource to which it refers
+   should be carried by separate information passed along with the URN
+   rather than in the URN itself.
+
+
+
+
+
+
+
+
+
+
+Sollins & Masinter                                              [Page 6]
+
+RFC 1737        Requirements for Uniform Resource Names    December 1994
+
+
+Authors' Addresses
+
+   Larry Masinter
+   Xerox Palo Alto Research Center
+   3333 Coyote Hill Road
+   Palo Alto, CA 94304
+
+   Phone: (415) 812-4365
+   Fax:   (415) 812-4333
+   EMail: masinter@parc.xerox.com
+
+
+   Karen Sollins
+   MIT Laboratory for Computer Science
+   545 Technology Square
+   Cambridge, MA 02139
+
+   Voice: (617) 253-6006
+   Phone: (617) 253-2673
+   EMail: sollins@lcs.mit.edu
+
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+Sollins & Masinter                                              [Page 7]
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