Internet-Draft: draft-kunze-ark-14.txt                          J. Kunze
ARK Identifier Scheme                    University of California (UCOP)
Expires 24 January 2008                                 R. P. C. Rodgers
                                         US National Library of Medicine
                                                            24 July 2007

                  The ARK Persistent Identifier Scheme


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   The ARK (Archival Resource Key) naming scheme is designed to
   facilitate the high-quality and persistent identification of
   information objects. A founding principle of the ARK is that
   persistence is purely a matter of service and is neither inherent in
   an object nor conferred on it by a particular naming syntax. The best
   that an identifier can do is to lead users to the services that
   support persistence. The term ARK itself refers both to the scheme
   and to any single identifier that conforms to it.  An ARK has five

J. Kunze                                                        [Page 1]

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   an optional and mutable Name Mapping Authority Hostport, the "ark:"
   label, the Name Assigning Authority Number (NAAN), the assigned Name,
   and an optional and possibly mutable Qualifier supported by the NMA.
   The NAAN and Name together form the immutable persistent identifier
   for the object.  An ARK is a special kind of URL that connects users
   to three things: the named object, its metadata, and the provider's
   promise about its persistence. When entered into the location field
   of a Web browser, the ARK leads the user to the named object. That
   same ARK, followed by a single question mark ('?'), returns a brief
   metadata record that is both human- and machine-readable. When the
   ARK is followed by dual question marks ('??'), the returned metadata
   contains a commitment statement from the current provider.  Tools
   exist for minting, binding, and resolving ARKs.

J. Kunze                                                        [Page 2]

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1.  Introduction

   This document describes a scheme for the high-quality naming of
   information resources.  The scheme, called the Archival Resource Key
   (ARK), is well suited to long-term access and identification of any
   information resources that accommodate reasonably regular electronic
   description.  This includes digital documents, databases, software,
   and websites, as well as physical objects (books, bones, statues,
   etc.) and intangible objects (chemicals, diseases, vocabulary terms,
   performances).  Hereafter the term "object" refers to an information
   resource.  The term ARK itself refers both to the scheme and to any
   single identifier that conforms to it.  A reasonably concise and
   accessible overview and rationale for the scheme is available at

   Schemes for persistent identification of network-accessible objects
   are not new.  In the early 1990's, the design of the Uniform Resource
   Name [URNSYN] responded to the observed failure rate of URLs by
   articulating an indirect, non-hostname-based naming scheme and the
   need for responsible name management.  Meanwhile, promoters of the
   Digital Object Identifier [DOI] succeeded in building a community of
   providers around a mature software system [Handle] that supports name
   management.  The Persistent Uniform Resource Locator [PURL] was
   another scheme that has the unique advantage of working with
   unmodified web browsers.  ARKs represent an approach that attempts to
   build on the strengths and to avoid the weaknesses of the other

   A founding principle of the ARK is that persistence is purely a
   matter of service.  Persistence is neither inherent in an object nor
   conferred on it by a particular naming syntax.  Nor is the technique
   of name indirection - upon which URNs, Handles, DOIs, and PURLs are
   founded - of central importance.  Name indirection is an ancient and
   well-understood practice; new mechanisms for it keep appearing and
   distracting practitioner attention, with the Domain Name System [DNS]
   being a particularly dazzling and elegant example.  What is often
   forgotten is that maintenance of an indirection table is the
   overwhelming and unavoidable cost to the organization providing
   persistence, and the cost is equivalent across naming schemes.  That
   indirection has always been a native part of the web while being so
   lightly utilized for the persistence of web-based objects is an
   indication of how unsuited most organizations are to the task of
   table maintenance and to the overall challenge of digital permanence.

   Persistence is achieved through a provider's successful stewardship
   of objects and their identifiers.  The highest level of persistence
   will be reinforced by a provider's robust contingency, redundancy,
   and succession strategies.  It is further safeguarded to the extent
   that a provider's mission is shielded from marketplace and political
   instabilities.  These are by far the major challenges confronting
   persistence providers, and no identifier scheme has any direct impact

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   on them.  In fact, some schemes may be actual liabilities for
   persistence because they create short- and long-term dependencies for
   every object access on complex, special-purpose local and global
   infrastructures, parts of which are proprietary and all of which
   increase the carry-forward burden for the preservation community.  It
   is for this reason that the ARK scheme relies only on educated name
   assignment and light use of general-purpose infrastructures that the
   entire internet community needs (the DNS, web servers, and web
   browsers) and that one can reasonably expect many others to help
   carry forward into the technologically evolving future.

1.1.  Reasons to Use ARKs

   If no persistent identifier scheme contributes directly to
   persistence, why not just use URLs?  A particular URL may be as
   durable an identifier as it is possible to have, but nothing
   distinguishes it from an ordinary URL to the recipient who is
   wondering if it is suitable for long-term reference.  An ARK is just
   a URL, distinguished by its form, that provides some of the necessary
   conditions for credible persistence.  An ARK invites access to not
   one, but to three things:  to the object, to its metadata, and to a
   nuanced statement of commitment from the provider regarding the
   object.  Existence of the two extra services can be probed
   automatically by appending either `?' or `??' to the ARK.

   The form of the ARK also supports the natural separation of naming
   authorities into the original name assigning authority and the
   diverse multiple name mapping (or servicing) authorities that in
   succession and in parallel will take over custodial responsibilities
   from the original assigner for the large majority of a long-term
   object's archival lifetime.  The mapping authority, indicated by the
   hostname part of the URL that contains the ARK, serves to launch the
   ARK into cyberspace.  Should it ever fail (and there is no reason why
   a well-chosen hostname of a 100-year-old cultural memory institution
   shouldn't last as long as the DNS), that host name is considered
   disposeable and replaceable.  Again, the form of the ARK helps
   because it defines exactly how to recover the core immutable object
   identity, and several simple algorithms (based on the URN model) are
   defined for locating another mapping authority.

   There are tools to assist in generating ARKs and other identifiers,
   such as [NOID] and "uuidgen", both of which rely for uniqueness on
   human-maintained registries.  This document also contains some
   guidelines and considerations for managing namespaces and choosing
   hostnames wisely.

1.2.  Three Requirements of ARKs

   The first requirement of an ARK is to give users a link from an
   object to a promise of stewardship for it.  That promise is a multi-
   faceted covenant that binds the word of an identified service

J. Kunze             1.2. Three Requirements of ARKs            [Page 4]

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   provider to a specific set of responsibilities.  No one can tell if
   successful stewardship will take place because no one can predict the
   future.  Reasonable conjecture, however, may be based on past
   performance.  There must be a way to tie a promise of persistence to
   a provider's demonstrated or perceived ability - its reputation - in
   that arena.  Provider reputations would then rise and fall as
   promises are observed variously to be kept and broken.  This is
   perhaps the best way we have for gauging the strength of any
   persistence promise.  Note that over time, current providers have
   nothing to do with the intentions of the original assigners of names.

   The second requirement of an ARK is to give users a link from an
   object to a description of it.  The problem with a naked identifier
   is that without a description real identification is incomplete.
   Identifiers common today are relatively opaque, though some contain
   ad hoc clues that reflect brief life cycle periods such as the
   address of a short stay in a filesystem hierarchy.  Possession of
   both an identifier and an object is some improvement, but positive
   identification may still be uncertain since the object itself might
   not include a matching identifier or might not carry evidence obvious
   enough to reveal its identity without significant research.  In
   either case, what is called for is a record bearing witness to the
   identifier's association with the object, as supported by a recorded
   set of object characteristics.  This descriptive record is partly an
   identification "receipt" with which users and archivists can verify
   an object's identity after brief inspection and a plausible match
   with recorded characteristics such as title and size.

   The final requirement of an ARK is to give users a link to the object
   itself (or to a copy) if at all possible.  Persistent access is the
   central duty of an ARK.  Persistent identification plays a vital
   supporting role but, strictly speaking, it can be construed as no
   more than a record attesting to the original assignment of a never-
   reassigned identifier.  Object access may not be feasible for various
   reasons, such as catastrophic loss of the object, a licensing
   agreement that keeps an archive "dark" for a period of years, or when
   an object's own lack of tangible existence confuses normal concepts
   of access (e.g., a vocabulary term might be accessed through its
   definition).  In such cases the ARK's identification role assumes a
   much higher profile.  But attempts to simplify the persistence
   problem by decoupling access from identification and concentrating
   exclusively on the latter are of questionable utility.  A perfect
   system for assigning forever unique identifiers might be created, but
   if it did so without reducing access failure rates, no one would be
   interested.  The central issue - which may be summed up as the "HTTP
   404 Not Found" problem - would not have been addressed.

1.3.  Organizing Support for ARKs:  Our Stuff vs. Their Stuff

   An organization and the user community it serves can often be seen to
   struggle with two different areas of persistent identification: the

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   Our Stuff problem and the Their Stuff problem.  In the Our Stuff
   problem, we in the organization want our own objects to acquire
   persistent names.  Since we possess or control these objects, our
   organization tackles the Our Stuff problem directly.  Whether or not
   the objects are named by ARKs, our organization is the responsible
   party, so it can plan for, maintain, and make commitments about the

   In the Their Stuff problem, we in the organization want others'
   objects to acquire persistent names.  These are objects that we do
   not own or control, but some of which are critically important to us.
   But because they are beyond our influence as far as support is
   concerned, creating and maintaining persistent identifiers for Their
   Stuff is not especially purposeful or feasible for us to do.  There
   is little that we can do about someone else's stuff except encourage
   them to find or become providers of persistence services.

   Co-location of persistent access and identification services is
   natural.  Any organization that undertakes ongoing support of true
   persistent identification (which includes description) is well-served
   if it controls, owns, or otherwise has clear internal access to the
   identified objects, and this gives it an advantage if it wishes also
   to support persistent access to outsiders.  Conversely, persistent
   access to outsiders requires orderly internal collection management
   procedures that include monitoring, acquisition, verification, and
   change control over objects, which in turn requires object
   identifiers persistent enough to support auditable record keeping

   Although, organizing ARK services under one roof thus tends to make
   sense, object hosting can successfully be separated from name
   mapping.  An example is when a name mapping authority centrally
   provides uniform resolution services via a protocol gateway on behalf
   of organizations that host objects behind a variety of access
   protocols.  It is also reasonable to build value-added description
   services that rely on the underlying services of a set of mapping

   Supporting ARKs is not for every organization.  By requiring
   specific, revealed commitments to preservation, to object access, and
   to description, the bar for providing ARK services is higher than for
   some other identifier schemes.  On the other hand, it would be hard
   to grant credence to a persistence promise from an organization that
   could not muster the minimum ARK services.  Not that there isn't a
   business model for an ARK-like, description-only service built on top
   of another organization's full complement of ARK services.  For
   example, there might be competition at the description level for
   abstracting and indexing a body of scientific literature archived in
   a combination of open and fee-based repositories.  The description-
   only service would have no direct commitment to the objects, but
   would act as an intermediary, forwarding commitment statements from

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   object hosting services to requestors.

1.4.  Definition of Identifier

   An identifier is not a string of character data - an identifier is an
   association between a string of data and an object.  This abstraction
   is necessary because without it a string is just data.  It's nonsense
   to talk about a string's breaking, or about its being strong,
   maintained, and authentic.  But as a representative of an
   association, a string can do, metaphorically, the things that we
   expect of it.

   Without regard to whether an object is physical, digital, or
   conceptual, to identify it is to claim an association between it and
   a representative string, such as "Jane" or "ISBN 0596000278".  What
   gives a claim credibility is a set of verifiable assertions, or
   metadata, about the object, such as age, height, title, or number of
   pages.  In other words, the association is made manifest by a record
   (e.g., a cataloging or other metadata record) that vouches for it.

   In the complete absence of any testimony (metadata) regarding an
   association, a would-be identifier string is a meaningless sequence
   of characters.  To keep an externally visible but otherwise internal
   string from being perceived as an identifier by outsiders, for
   example, it suffices for an organization not to disclose the nature
   of its association.  For our immediate purpose, actual existence of
   an association record is more important than its authenticity or
   verifiability, which are outside the scope of this specification.

   It is a gift to the identification process if an object carries its
   own name as an inseparable part of itself, such as an identifier
   imprinted on the first page of a document or embedded in a data
   structure element of a digital document header.  In cases where the
   object is large, unwieldy, or unavailable (such as when licensing
   restrictions are in effect), a metadata record that includes the
   identifier string will usually suffice.  That record becomes a
   conveniently manipulable object surrogate, acting as both an
   association "receipt" and "declaration".

   Note that our definition of identifier extends the one in use for
   Uniform Resource Identifiers [URI].  The present document still
   sometimes (ab)uses the terms "ARK" and "identifier" as shorthand for
   the string part of an identifier, but the context should make the
   meaning clear.

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2.  ARK Anatomy

   An ARK is represented by a sequence of characters (a string) that
   contains the label, "ark:", optionally preceded by the beginning part
   of a URL.  Here is a diagrammed example.

         \___________________/ \__/ \___/ \______/ \____________/
           (replaceable)        |     |      |       Qualifier
                |         ARK Label   |      |    (NMA-supported)
                |                     |      |
      Name Mapping Authority          |    Name (NAA-assigned)
         Hostport (NMAH)              |
                           Name Assigning Authority Number (NAAN)

   The ARK syntax can be summarized,


   where the NMAH and Qualifier parts are in brackets to indicate that
   they are optional.

2.1.  The Name Mapping Authority Hostport (NMAH)

   Before the "ark:" label may appear an optional Name Mapping Authority
   Hostport (NMAH) that is a temporary address where ARK service
   requests may be sent.  It consists of "http://" (or any service
   specification valid for a URL) followed by an Internet hostname or
   hostport combination having the same format and semantics as the
   hostport part of a URL.  The most important thing about the NMAH is
   that it is "identity inert" from the point of view of object
   identification.  In other words, ARKs that differ only in the
   optional NMAH part identify the same object.  Thus, for example, the
   following three ARKs are synonyms for just one information object:


   Strictly speaking, in the realm of digital objects, these ARKs may
   lead over time to somewhat different or diverging instances of the
   originally named object.  In an ideal world, divergence of persistent
   objects is not desirable, but it is widely believed that digital
   preservation efforts will inevitably lead to alterations in some
   original objects (e.g, a format migration in order to preserve the
   ability to display a document).  If any of those objects are held
   redundantly in more than one organization (a common preservation
   strategy), chances are small that all holding organizations will
   perform the same precise transformations and all maintain the same

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   object metadata.  More significant divergence would be expected when
   the holding organizations serve different audiences or compete with
   each other.

   The NMAH part makes an ARK into an actionable URL.  As with many
   internet parameters, it is helpful to approach the NMAH being liberal
   in what you accept and conservative in what you propose.  From the
   recipient's point of view, the NMAH part should be treated as
   temporary, disposable, and replaceable.  From the NMA's point of
   view, it should be chosen with the greatest concern for longevity.  A
   carefully chosen NMAH should be at least as permanent as the
   providing organization's own hostname.  In the case of a national or
   university library, for example, there is no reason why the NMAH
   should not be considerably more permanent than soft-funded proxy
   hostnames such as,, and  In
   general and over time, however, it is not unexpected for an NMAH
   eventually to stop working and require replacement with the NMAH of a
   currently active service provider.

   This replacement relies on a mapping authority "resolver" discovery
   process, of which two alternate methods are outlined in a later
   section.  The ARK, URN, Handle, and DOI schemes all use a resolver
   discovery model that sooner or later requires matching the original
   assigning authority with a current provider servicing that
   authority's named objects; once found, the resolver at that provider
   performs what amounts to a redirect to a place where the object is
   currently held.  All the schemes rely on the ongoing functionality of
   currently mainstream technologies such as the Domain Name System
   [DNS] and web browsers.  The Handle and DOI schemes in addition
   require that the Handle protocol layer and global server grid be
   available at all times.

   The practice of prepending "http://" and an NMAH to an ARK is a way
   of creating an actionable identifier by a method that is itself
   temporary.  Assuming that infrastructure supporting [HTTP]
   information retrieval will no longer be available one day, ARKs will
   then have to be converted into new kinds of actionable identifiers.
   By that time, if ARKs see widespread use, web browsers would
   presumably evolve to perform this (currently simple) transformation

2.2.  The ARK Label Part - ark:

   The label part distinguishes an ARK from an ordinary identifier.  In
   a URL found in the wild, the string, "ark:/", indicates that the URL
   stands a reasonable chance of being an ARK.  If the context warrants,
   verification that it actually is an ARK can be done by testing it for
   existence of the three ARK services.

   Since nothing about an identifier syntax directly affects
   persistence, the "ark:" label (like "urn:", "doi:", and "hdl:")

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   cannot tell you whether the identifier is persistent or whether the
   object is available.  It does tell you that the original Name
   Assigning Authority (NAA) had some sort of hopes for it, but it
   doesn't tell you whether that NAA is still in existence, or whether a
   decade ago it ceased to have any responsibility for providing
   persistence, or whether it ever had any responsibility beyond naming.

   Only a current provider can say for certain what sort of commitment
   it intends, and the ARK label suggests that you can query the NMAH
   directly to find out exactly what kind of persistence is promised.
   Even if what is promised is impersistence (i.e., a short-term
   identifier), saying so is valuable information to the recipient.
   Thus an ARK is a high-functioning identifier in the sense that it
   provides access to the object, the metadata, and a commitment
   statement, even if the commitment is explicitly very weak.

2.3.  The Name Assigning Authority Number (NAAN)

   Recalling that the general form of the ARK is,


   the part of the ARK directly following the "ark:" is the Name
   Assigning Authority Number (NAAN) enclosed in `/' (slash) characters.
   This part is always required, as it identifies the organization that
   originally assigned the Name of the object.  It is used to discover a
   currently valid NMAH and to provide top-level partitioning of the
   space of all ARKs.  NAANs are registered in a manner similar to URN
   Namespaces, but they are pure numbers consisting of 5 digits or 9
   digits.  Thus, the first 100,000 registered NAAs fit compactly into
   the 5 digits, and if growth warrants, the next billion fit into the 9
   digit form.  In either case the fixed odd numbers of digits helps
   reduce the chances of finding a NAAN out of context and confusing it
   with nearby quantities such as 4-digit dates.

   The NAAN designates a top-level ARK namespace.  Once registered for a
   namespace, a NAAN is never re-registered.  It is possible, however,
   for there to be a succession of organizations that manage of an ARK

2.4.  The Name Part

   The part of the ARK just after the NAAN is the Name assigned by the
   NAA, and it is also required.  Semantic opaqueness in the Name part
   is strongly encouraged in order to reduce an ARK's vulnerability to
   era- and language-specific change.  Identifier strings containing
   linguistic fragments can create support difficulties down the road.
   No matter how appropriate or even meaningless they are today, such
   fragments may one day create confusion, give offense, or infringe on
   a trademark as the semantic environment around us and our communities

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   Names that look more or less like numbers avoid common problems that
   defeat persistence and international acceptance.  The use of digits
   is highly recommended.  Mixing in non-vowel alphabetic characters a
   couple at a time is a relatively safe and easy way to achieve a
   denser namespace (more possible names for a given length of the name
   string).  Such names have a chance of aging and traveling well.
   Tools exists that mint, bind, and resolve opaque identifiers, with or
   without check characters [NOID].  More on naming considerations is
   given in a subsequent section.

2.5.  The Qualifier Part

   The part of the ARK following the NAA-assigned Name is an optional
   Qualifier.  It is a string that extends the base ARK in order to
   create a kind of service entry point into the object named by the
   NAA.  At the discretion of the providing NMA, such a service entry
   point permits an ARK to support access to individual hierarchical
   components and subcomponents of an object, and to variants (versions,
   languages, formats) of components.  A Qualifier may be invented by
   the NAA or by any NMA servicing the object.

   In form, the Qualifier is a ComponentPath, or a VariantPath, or a
   ComponentPath followed by a VariantPath.  A VariantPath is introduced
   and subdivided by the reserved character `.', and a ComponentPath is
   introduced and subdivided by the reserved character `/'.  In this

   the string "/s3/f8" is a ComponentPath and the string ".05v.tiff" is
   a VariantPath.  The ARK Qualifier is a formalization of some
   currently mainstream URL syntax conventions.  This formalization
   specifically reserves meanings that permit recipients to make strong
   inferences about logical sub-object containment and equivalence based
   only on the form of the received identifiers; there is great
   efficiency in not having to inspect metadata records to discover such
   relationships.  NMAs are free not to disclose any of these
   relationships merely by avoiding the reserved characters above.
   Hierarchical components and variants are discussed further in the
   next two sections.

   The Qualifier, if present, differs from the Name in several important
   respects.  First, a Qualifier may have been assigned either by the
   NAA or later by the NMA.  The assignment of a Qualifier by an NMA
   effectively amounts to an act of publishing a service entry point
   within the conceptual object originally named by the NAA.  For our
   purposes, an ARK extended with a Qualifier assigned by an NMA will be
   called an NMA-qualified ARK.

   Second, a Qualifier assignment on the part of an NMA is made in
   fulfillment of its service obligations and may reflect changing

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   service expectations and technology requirements.  NMA-qualified ARKs
   could therefore be transient, even if the base, unqualified ARK is
   persistent.  For example, it would be reasonable for an NMA to
   support access to an image object through an actionable ARK that is
   considered persistent even if the experience of that access changes
   as linking, labeling, and presentation conventions evolve and as
   format and security standards are updated.  For an image "thumbnail",
   that NMA could also support an NMA-qualified ARK that is considered
   impersistent because the thumbnail will be replaced with higher
   resolution images as network bandwidth and CPU speeds increase.  At
   the same time, for an originally scanned, high-resolution master, the
   NMA could publish an NMA-qualfied ARK that is itself considered
   persistent.  Of course, the NMA must be able to return its separate
   commitments to unqualified, NAA-assigned ARKs, to NMA-qualified ARKs,
   and to any NAA-qualified ARKs that it supports.

   A third difference between a Qualifier and a Name concerns the
   semantic opaqueness constraint.  When an NMA-qualified ARK is to be
   used as a transient service entry point into a persistent object, the
   priority given to semantic opaqueness observed by the NAA in the Name
   part may be relaxed by the NMA in the Qualifier part.  If service
   priorities in the Qualifier take precedence over persistence, short-
   term usability considerations may recommend somewhat semantically
   laden Qualifier strings.

   Finally, not only is the set of Qualifiers supported by an NMA
   mutable, but different NMAs may support different Qualifier sets for
   the same NAA-identified object.  In this regard the NMAs act
   independently of each other and of the NAA.

   The next two sections describe how ARK syntax may be used to declare,
   or to avoid declaring, certain kinds of relatedness among qualified

2.5.1.  ARKs that Reveal Object Hierarchy

   An NAA or NMA may choose to reveal the presence of a hierarchical
   relationship between objects using the `/' (slash) character after
   the Name part of an ARK.  Some authorities will choose not to
   disclose this information, while others will go ahead and disclose so
   that manipulators of large sets of ARKs can infer object
   relationships by simple identifier inspection; for example, this
   makes it possible for a system to present a collapsed view of a large
   search result set.

   If the ARK contains an internal slash after the NAAN, the piece to
   its left indicates a containing object.  For example, publishing an
   ARK of the form,


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   is equivalent to publishing three ARKs,


   together with a declaration that the first object is contained in the
   second object, and that the second object is contained in the third.

   Revealing the presence of hierarchy is completely up to the assigner
   (NMA or NAA).  It is hard enough to commit to one object's name, let
   alone to three objects' names and to a specific, ongoing relatedness
   among them.  Thus, regardless of whether hierarchy was present
   initially, the assigner, by not using slashes, reveals no shared
   inferences about hierarchical or other inter-relatedness in the
   following ARKs:


   Note that slashes around the ARK's NAAN (/12025/ in these examples)
   are not part of the ARK's Name and therefore do not indicate the
   existence of some sort of NAAN super object containing all objects in
   its namespace.  A slash must have at least one non-structural
   character (one that is neither a slash nor a period) on both sides in
   order for it to separate recognizable structural components.  So
   initial or final slashes may be removed, and double slashes may be
   converted into single slashes.

2.5.2.  ARKs that Reveal Object Variants

   An NAA or NMA may choose to reveal the possible presence of variant
   objects or object components using the `.' (period) character after
   the Name part of an ARK.  Some authorities will choose not to
   disclose this information, while others will go ahead and disclose so
   that manipulators of large sets of ARKs can infer object
   relationships by simple identifier inspection; for example, this
   makes it possible for a system to present a collapsed view of a large
   search result set.

   If the ARK contains an internal period after Name, the piece to its
   left is a base name and the piece to its right, and up to the end of
   the ARK or to the next period is a suffix.  A Name may have more than
   one suffix, for example,

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   There are two main rules.  First, if two ARKs share the same base
   name but have different suffixes, the corresponding objects were
   considered variants of each other (different formats, languages,
   versions, etc.) by the assigner (NMA or NAA).  Thus, the following
   ARKs are variants of each other:


   Second, publishing an ARK with a suffix implies the existence of at
   least one variant identified by the ARK without its suffix.  The ARK
   otherwise permits no further assumptions about what variants might
   exist.  So publishing the ARK,


   is equivalent to publishing the four ARKs,


   Revealing the possibility of variants is completely up to the
   assigner.  It is hard enough to commit to one object's name, let
   alone to multiple variants' names and to a specific, ongoing
   relatedness among them.  The assigner is the sole arbiter of what
   constitutes a variant within its namespace, and whether to reveal
   that kind of relatedness by using periods within its names.

   A period must have at least one non-structural character (one that is
   neither a slash nor a period) on both sides in order for it to
   separate recognizable structural components.  So initial or final
   periods may be removed, and adjacent periods may be converted into a
   single period.  Multiple suffixes should be arranged in sorted order
   (pure ASCII collating sequence) at the end of an ARK.

2.6.  Character Repertoires

   The Name and Qualifier parts are strings of visible ASCII characters
   and should be less than 128 bytes in length.  The length restriction
   keeps the ARK short enough to append ordinary ARK request strings
   without running into transport restrictions (e.g., within HTTP GET
   requests).  Characters may be letters, digits, or any of these six

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         =   #   *   +   @   _   $

   The following characters may also be used, but their meanings are

         %   -   .   /

   The characters `/' and `.' are ignored if either appears as the last
   character of an ARK.  If used internally, they allow a name assigner
   to reveal object hierarchy and object variants as previously

   Hyphens are considered to be insignificant and are always ignored in
   ARKs.  A `-' (hyphen) may appear in an ARK for readability, or it may
   have crept in during the formatting and wrapping of text, but it must
   be ignored in lexical comparisons.  As in a telephone number, hyphens
   have no meaning in an ARK.  It is always safe for an NMA that
   receives an ARK to remove any hyphens found in it.  As a result, like
   the NMAH, hyphens are "identity inert" in comparing ARKs for
   equivalence.  For example, the following ARKs are equivalent for
   purposes of comparison and ARK service access:


   The `%' character is reserved for %-encoding all other octets that
   would appear in the ARK string, in the same manner as for URIs [URI].
   A %-encoded octet consists of a `%' followed by two hex digits; for
   example, "%7d" stands in for `}'.  Lower case hex digits are
   preferred to reduce the chances of false acronym recognition; thus it
   is better to use "%acT" instead of "%ACT".  The character `%' itself
   must be represented using "%25".  As with URNs, %-encoding permits
   ARKs to support legacy namespaces (e.g., ISBN, ISSN, SICI) that have
   less restricted character repertoires [URNBIB].

2.7.  Normalization and Lexical Equivalence

   To determine if two or more ARKs identify the same object, the ARKs
   are compared for lexical equivalence after first being normalized.
   Since ARK strings may appear in various forms (e.g., having different
   NMAHs), normalizing them minimizes the chances that comparing two ARK
   strings for equality will fail unless they actually identify
   different objects.  In a specified-host ARK (one having an NMAH), the
   NMAH never participates in such comparisons.

   Normalization of an ARK for the purpose of octet-by-octet equality
   comparison with another ARK consists of four steps.  First, any upper
   case letters in the "ark:" label and the two characters following a
   `%' are converted to lower case.  The case of all other letters in

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   the ARK string must be preserved.  Second, any NMAH part is removed
   (everything from an initial "http://" up to the next slash) and all
   hyphens are removed.

   Third, structural characters (slash and period) are normalized.
   Initial and final occurrences are removed, and two structural
   characters in a row (e.g., // or ./) are replaced by the first
   character, iterating until each occurrence has at least one non-
   structural character on either side.  Finally, if there are any
   components with a period on the left and a slash on the right, either
   the component and the preceding period must be moved to the end of
   the Name part or the ARK must be thrown out as malformed.

   The fourth and final step is to arrange the suffixes in ASCII
   collating sequence (that is, to sort them) and to remove duplicate
   suffixes, if any.  It is also permissible to throw out ARKs for which
   the suffixes are not sorted.

   The resulting ARK string is now normalized.  Comparisons between
   normalized ARKs are case-sensitive, meaning that upper case letters
   are considered different from their lower case counterparts.

   To keep ARK string variation to a minimum, no reserved ARK characters
   should be %-encoded unless it is deliberately to conceal their
   reserved meanings.  No non-reserved ARK characters should ever be
   %-encoded.  Finally, no %-encoded character should ever appear in an
   ARK in its decoded form.

3.  Naming Considerations

   The most important threats faced by persistence providers include
   such things as funding loss, natural disaster, political and social
   upheaval, processing faults, and errors in human oversight.  There is
   nothing that an identifer scheme can do about such things.  Still, a
   few observed identifier failures and inconveniences can be traced
   back to naming practices that we now know to be less than optimal for

3.1.  ARKS Embedded in Language

   The ARK has different goals from the URI, so it has different
   character set requirements.  Because linguistic constructs imperil
   persistence, for ARKs non-ASCII character support is unimportant.
   ARKs and URIs share goals of transcribability and transportability
   within web documents, so characters are required to be visible, non-
   conflicting with HTML/XML syntax, and not subject to tampering during
   transmission across common transport gateways.  Add the goal of
   making an undelimited ARK recognizable in running prose, as in
   ark:/12025/=@_22*$, and certain punctuation characters (e.g., comma,
   period) end up being excluded from the ARK lest the end of a phrase
   or sentence be mistaken for part of the ARK.

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   This consideration has more direct effect on ARK usability in a
   natural language context than it has on ARK persistence.  The same is
   true of the rule preventing hyphens from having lexical significance.
   It is fine to publish ARKs with hyphens in them (e.g., such as the
   output of UUID/GUID generators), but the uniform treatment of hyphens
   as insignificant reduces the possibility of users transcribing
   identifiers that will have been broken through unpredictable
   hyphenation by word processors.  Any measure that reduces user
   irritation with an identifier will increase its chances of survival.

3.2.  Objects Should Wear Their Identifiers

   A valuable technique for provision of persistent objects is to try to
   arrange for the complete identifier to appear on, with, or near its
   retrieved object.  An object encountered at a moment in time when its
   discovery context has long since disappeared could then easily be
   traced back to its metadata, to alternate versions, to updates, etc.
   This has seen reasonable success, for example, in book publishing and
   software distribution.  An identifier string only has meaning when
   its association is known, and this a very sure, simple, and low-tech
   method of reminding everyone exactly what that association is.

3.3.  Names are Political, not Technological

   If persistence is the goal, a deliberate local strategy for
   systematic name assignment is crucial.  Names must be chosen with
   great care.  Poorly chosen and managed names will devastate any
   persistence strategy, and they do not discriminate by identifier
   scheme.  Whether a mistakenly re-assigned name is a URN, DOI, PURL,
   URL, or ARK, the damage - failed access and confusion - is not
   mitigated more in one scheme than in another.  Conversely, in-house
   efforts to manage names responsibly will go much further towards
   safeguarding persistence than any choice of naming scheme or name
   resolution technology.

   Branding (e.g., at the corporate or departmental level) is important
   for funding and visibility, but substrings representing brands and
   organizational names should be given a wide berth except when
   absolutely necessary in the hostname (the identity-inert) part of the
   ARK.  These substrings are not only unstable because organizations
   change frequently, but they are also dangerous because successor
   organizations often have political or legal reasons to actively
   suppress predecessor names and brands.  Any measure that reduces the
   chances of future political or legal pressure on an identifier will
   decrease the chances that our descendants will be obliged to
   deliberately break it.

3.4.  Choosing a Hostname or NMA

   Hostnames appearing in any identifier meant to be persistent must be
   chosen with extra care.  The tendency in hostname selection has

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   traditionally been to choose a token with recognizable attributes,
   such as a corporate brand, but that tendency wreaks havoc with
   persistence that is supposed to outlive brands, corporations, subject
   classifications, and natural language semantics (e.g., what did the
   three letters "gay" mean in 1958, 1978, and 1998?).  Today's
   recognized and correct attributes are tomorrow's stale or incorrect
   attributes.  In making hostnames (any names, actually) long-term
   persistent, it helps to eliminate recognizable attributes to the
   extent possible.  This affects selection of any name based on URLs,
   including PURLs and the explicitly disposable NMAHs.

   There is no excuse for a provider that manages its internal names
   impeccably not to exercise the same care in choosing what could be an
   exceptionally durable hostname, especially if it would form the
   prefix for all the provider's URL-based external names.  Registering
   an opaque hostname in the ".org" or ".net" domain would not be a bad
   start.  Another way is to publish your ARKs with an organizational
   domain name that will be mapped by DNS to an appropriate NMA host.
   This makes for shorter names with less branding vulnerability.

   It is a mistake to think that hostnames are inherently unstable.  If
   you require brand visibility, that may be a fact of life.  But things
   are easier if yours is the brand of long-lived cultural memory
   institution such as a national or university library or archive.
   Well-chosen hostnames from organizations that are sheltered from the
   direct effects of a volatile marketplace can easily provide longer-
   lived global resolvers than the domain names explicitly or implicitly
   used as starting points for global resolution by indirection-based
   persistent identifier schemes.  For example, it is hard to imagine
   circumstances under which the Library of Congress' domain name would
   disappear sooner than, say, "".

   For smaller libraries, archives, and preservation organizations,
   there is a natural concern about whether they will be able to keep
   their web servers and domain names in the face of uncertain funding.
   One option is to form or join a consortium [N2T] of like-minded
   organizations with the purpose of providing mutual preservation
   support.  The first goal of such a consortium would be to perpetually
   rent a hostname on which to establish a web server that simply
   redirects incoming member organization requests to the appropriate
   member server; using ARKs, for example, a 150-member consortium could
   run a very small server (24x7) that contained nothing more than 150
   rewrite rules in its configuration file.  Even more helpful would be
   additional consortial support for a member organization that was
   unable to continue providing services and needed to find a successor
   archival organization.  This would be a low-cost, low-tech way to
   publish ARKs (or URLs) under highly persistent hostnames.

   There are no obvious reasons why the organizations registering DNS
   names, URN Namespaces, and DOI publisher IDs should have among them
   one that is intrinsically more fallible than the next.  Moreover, it

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   is a misconception that the demise of DNS and of HTTP need adversely
   affect the persistence of URLs.  At such a time, certainly URLs from
   the present day might not then be actionable by our present-day
   mechanisms, but resolution systems for future non-actionable URLs are
   no harder to imagine than resolution systems for present-day non-
   actionable URNs and DOIs.  There is no more stable a namespace than
   one that is dead and frozen, and that would then characterize the
   space of names bearing the "http://" prefix.  It is useful to
   remember that just because hostnames have been carelessly chosen in
   their brief history does not mean that they are unsuitable in NMAHs
   (and URLs) intended for use in situations demanding the highest level
   of persistence available in the Internet environment.  A well-planned
   name assignment strategy is everything.

3.5.  Assigners of ARKs

   A Name Assigning Authority (NAA) is an organization that creates (or
   delegates creation of) long-term associations between identifiers and
   information objects.  Examples of NAAs include national libraries,
   national archives, and publishers.  An NAA may arrange with an
   external organization for identifier assignment.  The US Library of
   Congress, for example, allows OCLC (the Online Computer Library
   Center, a major world cataloger of books) to create associations
   between Library of Congress call numbers (LCCNs) and the books that
   OCLC processes.  A cataloging record is generated that testifies to
   each association, and the identifier is included by the publisher,
   for example, in the front matter of a book.

   An NAA does not so much create an identifier as create an
   association.  The NAA first draws an unused identifier string from
   its namespace, which is the set of all identifiers under its control.
   It then records the assignment of the identifier to an information
   object having sundry witnessed characteristics, such as a particular
   author and modification date.  A namespace is usually reserved for an
   NAA by agreement with recognized community organizations (such as
   IANA and ISO) that all names containing a particular string be under
   its control.  In the ARK an NAA is represented by the Name Assigning
   Authority Number (NAAN).

   The ARK namespace reserved for an NAA is the set of names bearing its
   particular NAAN.  For example, all strings beginning with
   "ark:/12025/" are under control of the NAA registered under 12025,
   which might be the National Library of Finland.  Because each NAA has
   a different NAAN, names from one namespace cannot conflict with those
   from another.  Each NAA is free to assign names from its namespace
   (or delegate assignment) according to its own policies.  These
   policies must be documented in a manner similar to the declarations
   required for URN Namespace registration [URNNID].

   To register for a NAAN, please read about the mapping authority
   discovery file in the next section and send email to

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3.6.  NAAN Namespace Management

   Every NAA must have a namespace management strategy.  A time-honored
   technique is to hierarchically partition a namespace into
   subnamespaces using prefixes that guarantee non-collision of names in
   different partition.  This practice is strongly encouraged for all
   NAAs, especially when subnamespace management will be delegated to
   other departments, units, or projects within an organization.  For
   example, with a NAAN that is assigned to a university and managed by
   its main library, care should be taken to reserve semantically opaque
   prefixes that will set aside large parts of the unused namespace for
   future assignments.  Prefix-based partition management is an
   important responsibility of the NAA.

   This sort of delegation by prefix is well-used in the formation of
   DNS names and ISBN identifiers.  An important difference is that in
   the former, the hierarchy is deliberately exposed and in the latter
   it is hidden.  Rather than using lexical boundary markers such as the
   period (`.') found in domain names, the ISBN uses a publisher prefix
   but doesn't disclose where the prefix ends and the publisher's
   assigned name begins.  This practice of non-disclosure, borrowed from
   the ISBN and ISSN schemes, is encouraged in assigning ARKs, because
   it reduces the visibility of an assertion that is probably not
   important now and may become a vulnerability later.

   Reasonable prefixes for assigned names usually consist of consonants
   and digits and are 1-5 characters in length.  For example, the
   constant prefix "x9t" might be delegated to a book digitization
   project that creates identifiers such as


   If longevity is the goal, it is important to keep the prefixes free
   of recognizable semantics; for example, using an acronym representing
   a project or a department is discouraged.  At the same time, you may
   wish to set aside a subnamespace for testing purposes under a prefix
   such as "fk..." that can serve as a visual clue and reminder to
   maintenance staff that this "fake" identifier was never published.

   There are other measures one can take to avoid user confusion,
   transcription errors, and the appearance of accidental semantics when
   creating identifiers.  If you are generating identifiers
   automatically, pure numeric identifiers are likeley to be
   semantically opaque enough, but it's probably useful to avoid leading
   zeroes because some users mistakenly treat them as optional, thinking
   (arithmetically) that they don't contribute to the "value" of the

   If you need lots of identifiers and you don't want them to get too
   long, you can mix digits with consonants (but avoid vowels since they
   might accidentally spell words) to get more identifiers without

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   increasing the string length.  In this case you may not want more
   than a two letters in a row because it reduces the chance of
   generating acronyms.  Generator tools such as [NOID] provide support
   for these sorts of identifiers, and can also add a computed check
   character as a guarantee against the most common transcription

3.7.  Sub-Object Naming

   As mentioned previously, semantically opaque identifiers are very
   useful for long-term naming of abstract objects, however, it may be
   appropriate to extend these names with less opaque extensions that
   reference contemporary service entry points (sub-objects) in support
   of the object.  Sub-object extensions beginning with a digit or
   underscore (`_') are reserved for the possibilty of developing a
   future registry of canonical service points (e.g., numeric references
   to versions, formats, languages, etc).

4.  Finding a Name Mapping Authority

   In order to derive an actionable identifier (these days, a URL) from
   an ARK, a hostport (hostname or hostname plus port combination) for a
   working Name Mapping Authority (NMA) must be found.  An NMA is a
   service that is able to respond to the three basic ARK service
   requests.  Relying on registration and client-side discovery, NMAs
   make known which NAAs' identifiers they are willing to service.

   Upon encountering an ARK, a user (or client software) looks inside it
   for the optional NMAH part (the hostport of the NMA's ARK service).
   If it contains an NMAH that is working, this NMAH discovery step may
   be skipped; the NMAH effectively uses the beginning of an ARK to
   cache the results of a prior mapping authority discovery process.  If
   a new NMAH needs to found, the client looks inside the ARK again for
   the NAAN (Name Assigning Authority Number).  Querying a global
   database, it then uses the NAAN to look up all current NMAHs that
   service ARKs issued by the identified NAA.  The global database is
   key, and two specific methods for querying it are given in this

   A third very promising method, called the Name-to-Thing [N2T]
   Resolver, is being explored.  It is a low-cost, highly stable,
   consortially maintained NMAH that simply exists to support actionable
   HTTP-based URLs for as long as HTTP is used.  One of its big
   advantages over the other two methods and the URN, Handle, DOI, and
   PURL methods, is that N2T addresses the namespace splitting problem.
   When objects maintained by one NMA are inherited by more than one
   successor NMA, until now one of those successors would be required to
   maintain forwarding tables on behalf of the other successors.

   In the interests of long-term persistence, however, ARK mechanisms
   are first defined in high-level, protocol-independent terms so that

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   mechanisms may evolve and be replaced over time without compromising
   fundamental service objectives.  Either or both specific methods
   given here may eventually be supplanted by better methods since, by
   design, the ARK scheme does not depend on a particular method, but
   only on having some method to locate an active NMAH.

   At the time of issuance, at least one NMAH for an ARK should be
   prepared to service it.  That NMA may or may not be administered by
   the Name Assigning Authority (NAA) that created it.  Consider the
   following hypothetical example of providing long-term access to a
   cancer research journal.  The publisher wishes to turn a profit and
   the National Library of Medicine wishes to preserve the scholarly
   record.  An agreement might be struck whereby the publisher would act
   as the NAA and the national library would archive the journal issue
   when it appears, but without providing direct access for the first
   six months.  During the first six months of peak commercial
   viability, the publisher would retain exclusive delivery rights and
   would charge access fees.  Again, by agreement, both the library and
   the publisher would act as NMAs, but during that initial period the
   library would redirect requests for issues less than six months old
   to the publisher.  At the end of the waiting period, the library
   would then begin servicing requests for issues older than six months
   by tapping directly into its own archives.  Meanwhile, the publisher
   might routinely redirect incoming requests for older issues to the
   library.  Long-term access is thereby preserved, and so is the
   commercial incentive to publish content.

   Although it will be common for an NAA also to run an NMA service, it
   is never a requirement.  Over time NAAs and NMAs will come and go.
   One NMA will succeed another, and there might be many NMAs serving
   the same ARKs simultaneously (e.g., as mirrors or as competitors).
   There might also be asymmetric but coordinated NMAs as in the
   library-publisher example above.

4.1.  Looking Up NMAHs in a Globally Accessible File

   This subsection describes a way to look up NMAHs using a simple name
   authority table represented as a plain text file.  For efficient
   access the file may be stored in a local filesystem, but it needs to
   be reloaded periodically to incorporate updates.  It is not expected
   that the size of the file or frequency of update should impose an
   undue maintenance or searching burden any time soon, for even
   primitive linear search of a file with ten-thousand NAAs is a
   subsecond operation on modern server machines.  The proposed file
   strategy is similar to the /etc/hosts file strategy that supported
   Internet host address lookup for a period of years before the advent
   of DNS.

   The name authority table file is updated on an ongoing basis and is
   available for copying over the internet from the California Digital
   Library at and from a

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   number of mirror sites.  The file contains comment lines (lines that
   begin with `#') explaining the format and giving the file's
   modification time, reloading address, and NAA registration
   instructions.  There is even a Perl script that processes the file
   embedded in the file's comments.  As of February 2006, currently
   registered Name Assigning Authorities are:

        12025            National Library of Medicine
        12026            Library of Congress
        12027            National Agriculture Library
        13030            California Digital Library
        13038            World Intellectual Property Organization
        20775            University of California San Diego
        29114            University of California San Francisco
        28722            University of California Berkeley
        21198            University of California Los Angeles
        15230            Rutgers University
        13960            Internet Archive
        64269            Digital Curation Centre
        62624            New York University
        67531            University of North Texas
        27927            Ithaka Electronic-Archiving Initiative
        12148            Bibliotheque nationale de France / National Library of France
        78319            Google
        88435            Princeton University
        78428            University of Washington
        89901            Archives of Region of Vastra Gotaland and City of Gothenburg, Sweden
        80444            Northwest Digital Archives
        25593            Emory University
        25031            University of Kansas
        17101            Centre for Ecology & Hydrology, UK
        65323            University of Calgary

   A snapshot of the name authority table file appears in an appendix.

4.2.  Looking up NMAHs Distributed via DNS

   This subsection introduces a method for looking up NMAHs that is
   based on the method for discovering URN resolvers described in
   [NAPTR].  It relies on querying the DNS system already installed in
   the background infrastructure of most networked computers.  A query
   is submitted to DNS asking for a list of resolvers that match a given
   NAAN.  DNS distributes the query to the particular DNS servers that
   can best provide the answer, unless the answer can be found more
   quickly in a local DNS cache as a side-effect of a recent query.
   Responses come back inside Name Authority Pointer (NAPTR) records.
   The normal result is one or more candidate NMAHs.

   In its full generality the [NAPTR] algorithm ambitiously accommodates
   a complex set of preferences, orderings, protocols, mapping services,
   regular expression rewriting rules, and DNS record types.  This

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   subsection proposes a drastic simplification of it for the special
   case of ARK mapping authority discovery.  The simplified algorithm is
   called Maptr.  It uses only one DNS record type (NAPTR) and restricts
   most of its field values to constants.  The following hypothetical
   excerpt from a DNS data file for the NAAN known as 12026 shows three
   example NAPTR records ready to use with the Maptr algorithm.
       ;; US Library of Congress
       ;;       order pref flags service regexp  replacement
        IN NAPTR  0     0   "h"  "ark"   "USLC"
        IN NAPTR  0     0   "h"  "ark"   "USLC"
        IN NAPTR  0     0   "h"  "ark"   "USLC"

   All the fields are held constant for Maptr except for the "flags",
   "regexp", and "replacement" fields.  The "service" field contains the
   constant value "ark" so that NAPTR records participating in the Maptr
   algorithm will not be confused with other NAPTR records.  The "order"
   and "pref" fields are held to 0 (zero) and otherwise ignored for now;
   the algorithm may evolve to use these fields for ranking decisions
   when usage patterns and local administrative needs are better

   When a Maptr query returns a record with a flags field of "h" (for
   hostport, a Maptr extension to the NAPTR flags), the replacement
   field contains the NMAH (hostport) of an ARK service provider.  When
   a query returns a record with a flags field of "" (the empty string),
   the client needs to submit a new query containing the domain name
   found in the replacement field.  This second sort of record exploits
   the distributed nature of DNS by redirecting the query to another
   domain name.  It looks like this.
       ;; Digital Library Consortium
       ;;       order pref flags service regexp replacement
        IN NAPTR  0     0    ""  "ark"     ""

   Here is the Maptr algorithm for ARK mapping authority discovery.  In
   it replace <NAAN> with the NAAN from the ARK for which an NMAH is

        (1) Initialize the DNS query:  type=NAPTR,

        (2) Submit the query to DNS and retrieve (NAPTR) records,
        discarding any record that does not have "ark" for the service

        (3) All remaining records with a flags fields of "h" contain
        candidate NMAHs in their replacement fields.  Set them aside, if

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        (4) Any record with an empty flags field ("") has a replacement
        field containing a new domain name to which a subsequent query
        should be redirected.  For each such record, set
        query=<replacement> then go to step (2).  When all such records
        have been recursively exhausted, go to step (5).

        (5) All redirected queries have been resolved and a set of
        candidate NMAHs has been accumulated from steps (3).  If there
        are zero NMAHs, exit - no mapping authority was found.  If there
        is one or more NMAH, choose one using any criteria you wish,
        then exit.

   A Perl script that implements this algorithm is included here.


     use Net::DNS;                 # include simple DNS package
     my $qtype = "NAPTR";               # initialize query type
     my $naa = shift;              # get NAAN script argument
     my $mad = new Net::DNS::Resolver;  # mapping authority discovery

     &maptr("$");      # call maptr - that's it

     sub maptr {                   # recursive maptr algorithm
          my $dname = shift;       # domain name as argument
          my ($rr, $order, $pref, $flags, $service, $regexp,
          my $query = $mad->query($dname, $qtype);
          return                   # non-productive query
               if (! $query || ! $query->answer);
          foreach $rr ($query->answer) {
               next           # skip records of wrong type
                    if ($rr->type ne $qtype);
               ($order, $pref, $flags, $service, $regexp,
                    $replacement) = split(/\s/, $rr->rdatastr);
               if ($flags eq "") {
                    &maptr($replacement);    # recurse
               } elsif ($flags eq "h") {
                    print "$replacement\n";  # candidate NMAH

   The global database thus distributed via DNS and the Maptr algorithm
   can easily be seen to mirror the contents of the Name Authority Table
   file described in the previous section.

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5.  Generic ARK Service Definition

   An ARK request's output is delivered information; examples include
   the object itself, a policy declaration (e.g., a promise of support),
   a descriptive metadata record, or an error message.  The experience
   of object delivery is expected to be an evolving mix of information
   that reflects changing service expectations and technology
   requirements; contemporary examples include such things as an object
   summary and component links formatted for human consumption.  ARK
   services must be couched in high-level, protocol-independent terms if
   persistence is to outlive today's networking infrastructural
   assumptions.  The high-level ARK service definitions listed below are
   followed in the next section by a concrete method (one of many
   possible methods) for delivering these services with today's

5.1.  Generic ARK Access Service (access, location)

   Returns (a copy of) the object or a redirect to the same, although a
   sensible object proxy may be substituted.  Examples of sensible
   substitutes include,

     - a table of contents instead of a large complex document,
     - a home page instead of an entire web site hierarchy,
     - a rights clearance challenge before accessing protected data,
     - directions for access to an offline object (e.g., a book),
     - a description of an intangible object (a disease, an event), or
     - an applet acting as "player" for a large multimedia object.

   May also return a discriminated list of alternate object locators.
   If access is denied, returns an explanation of the object's current
   (perhaps permanent) inaccessibility.

5.2.  Generic Policy Service (permanence, naming, etc.)

   Returns declarations of policy and support commitments for given
   ARKs.  Declarations are returned in either a structured metadata
   format or a human readable text format; sometimes one format may
   serve both purposes.  Policy subareas may be addressed in separate
   requests, but the following areas should should be covered:  object
   permanence, object naming, object fragment addressing, and
   operational service support.

   The permanence declaration for an object is a rating defined with
   respect to an identified permanence provider (guarantor), which will
   be the NMA.  It may include the following aspects.

        (a) "object availability" - whether and how access to the object
        is supported (e.g., online 24x7, or offline only),

        (b) "identifier validity" - under what conditions the identifier

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        will be or has been re-assigned,

        (c) "content invariance" - under what conditions the content of
        the object is subject to change, and

        (d) "change history" - access to corrections, migrations, and
        revisions, whether through links to the changed objects
        themselves or through a document summarizing the change history

   One approach to a permanence rating framework, conceived
   independently from ARKs, is given in [NLMPerm].  Under ongoing
   development and limited deployment at the US National Library of
   Medicine, it identifies the following "permanence levels":

        Not Guaranteed: No commitment has been made to retain this
        resource.  It could become unavailable at any time.  Its
        identifier could be changed.

        Permanent: Dynamic Content: A commitment has been made to keep
        this resource permanently available.  Its identifier will always
        provide access to the resource.  Its content could be revised or

        Permanent: Stable Content: A commitment has been made to keep
        this resource permanently available.  Its identifier will always
        provide access to the resource.  Its content is subject only to
        minor corrections or additions.

        Permanent: Unchanging Content: A commitment has been made to
        keep this resource permanently available.  Its identifier will
        always provide access to the resource.  Its content will not

   Naming policy for an object includes an historical description of the
   NAA's (and its successor NAA's) policies regarding differentiation of
   objects.  Since it the NMA who responds to requests for policy
   statements, it is useful for the NMA to be able to produce or
   summarize these historical NAA documents.  Naming policy may include
   the following aspects.

        (i) "similarity" - (or "unity") the limit, defined by the NAA,
        to the level of dissimilarity beyond which two similar objects
        warrant separate identifiers but before which they share one
        single identifier, and

        (ii) "granularity" - the limit, defined by the NAA, to the level
        of object subdivision beyond which sub-objects do not warrant
        separately assigned identifiers but before which sub-objects are
        assigned separate identifiers.

   Subnaming policy for an object describes the qualifiers that the NMA,

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   in fulfilling its ongoing and evolving service obligations, allows as
   extensions to an NAA-assigned ARK.  To the conceptual object that the
   NAA named with an ARK, the NMA may add component access points and
   derivatives (e.g., format migrations in aid of preservation) in order
   to provide both basic and value-added services.

   Addressing policy for an object includes a description of how, during
   access, object components (e.g., paragraphs, sections) or views
   (e.g., image conversions) may or may not be "addressed", in other
   words, how the NMA permits arguments or parameters to modify the
   object delivered as the result of an ARK request.  If supported,
   these sorts of operations would provide things like byte-ranged
   fragment delivery and open-ended format conversions, or any set of
   possible transformations that would be too numerous to list or to
   identify with separately assigned ARKs.

   Operational service support policy includes a description of general
   operational aspects of the NMA service, such as after-hours staffing
   and trouble reporting procedures.

5.3.  Generic Description Service

   Returns a description of the object.  Descriptions are returned in
   either a structured metadata format or a human readable text format;
   sometimes one format may serve both purposes.  A description must at
   a minimum answer the who, what, when, and where questions concerning
   an expression of the object.  Standalone descriptions should be
   accompanied by the modification date and source of the description
   itself.  May also return discriminated lists of ARKs that are related
   to the given ARK.

6.  Overview of The HTTP URL Mapping Protocol (THUMP)

   The HTTP URL Mapping Protocol (THUMP) is a way of taking a key (a
   kind of identifier) and asking such questions as, what information
   does this identify and how permanent is it?  [THUMP] is in fact one
   specific method under development for delivering ARK services.  The
   protocol runs over HTTP to exploit the web browser's current pre-
   eminence as user interface to the Internet.  THUMP is designed so
   that a person can enter ARK requests directly into the location field
   of current browser interfaces.  Because it runs over HTTP, THUMP can
   be simulated and tested within keyboard-based [TELNET] sessions.

   The asker (a person or client program) starts with an identifier,
   such as an ARK or a URL.  The identifier reveals to the asker (or
   allows the asker to infer) the Internet host name and port number of
   a server system that responds to questions.  Here, this is just the
   NMAH that is obtained by inspection and possibly lookup based on the
   ARK's NAAN.  The asker then sets up an HTTP session with the server
   system, sends a question via a THUMP request (contained within an
   HTTP request), receives an answer via a THUMP response (contained

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   within an HTTP response), and closes the session.  That concludes the
   connected portion of the protocol.

   A THUMP request is a string of characters beginning with a `?'
   (question mark) that is appended to the identifier string.  The
   resulting string is sent as an argument to HTTP's GET command.
   Request strings too long for GET may be sent using HTTP's POST
   command.  The three most common requests correspond to three
   degenerate special cases that keep the user's learning and typing
   burden low.  First, a simple key with no request at all is the same
   as an ordinary access request.  Thus a plain ARK entered into a
   browser's location field behaves much like a plain URL, and returns
   access to the primary identified object, for instance, an HTML

   The second special case is a minimal ARK description request string
   consisting of just "?".  For example, entering the string,


   into the browser's location field directly precipitates a request for
   a metadata record describing the object identified by
   ark:/12025/psbbantu.  The browser, unaware of THUMP, prepares and
   sends an HTTP GET request in the same manner as for a URL.  THUMP is
   designed so that the response (indicated by the returned HTTP content
   type) is normally displayed, whether the output is structured for
   machine processing (text/plain) or formatted for human consumption

   In the following example THUMP session, each line has been annotated
   to include a line number and whether it was the client or server that
   sent it.  Without going into much depth, the session has four pieces
   separated from each other by blank lines:  the client's piece (lines
   1-3), the server's HTTP/THUMP response headers (4-7), and the body of
   the server's response (8-17).  The first and last lines (1 and 17)
   correspond to the client's steps to start the TCP session and the
   server's steps to end it, respectively.

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      1  C: [opens session]
         C: GET HTTP/1.1
         S: HTTP/1.1 200 OK
      5  S: Content-Type: text/plain
         S: THUMP-Status: 0.1 200 OK
         S: |set: NLM | 12025/psbbantu? | 20030731
         S:         |
     10  S: here: 1 | 1 | 1
         S: erc:
         S: who:    Lederberg, Joshua
         S: what:   Studies of Human Families for Genetic Linkage
     15  S: when:   1974
         S: where:
         S: [closes session]

   The first two server response lines (4-5) above are typical of HTTP.
   The next line (6) is peculiar to THUMP, and indicates the THUMP
   version and a normal return status.  The balance of the response
   consists of a record set header (lines 8-10) and a single metadata
   record (12-16) that comprises the ARK description service response.
   The record set header identifies (8-9) who created the set, what its
   title is, when it was created, and where an automated process can
   access the set; it ends in a line (10) whose respective sub-elements
   indicate that here in this communication the recipient can expect to
   find 1 record, starting at the record numbered 1, from a set
   consisting of a total of 1 record (i.e., here is the entire set,
   consisting of exactly one record).

   The returned record (12-16) is in the format of an Electronic
   Resource Citation [ERC], which is discussed in more detail in the
   next section.  For now, note that it contains four elements that
   answer the top priority questions regarding an expression of the
   object:  who played a major role in expressing it, what the
   expression was called, when is was created, and where the expression
   may be found.  This quartet of elements comes up again and again in

   The third degenerate special case of an ARK request (and no other
   cases will be described in this document) is the string "??",
   corresponding to a minimal permanence policy request.  It can be seen
   in use appended to an ARK (on line 2) in the example session that

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      1  C: [opens session]
         C: GET HTTP/1.1
         S: HTTP/1.1 200 OK
      5  S: Content-Type: text/plain
         S: THUMP-Status: 0.1 200 OK
         S: |set: NLM | 12025/psbbantu?? | 20030731
         S:         |
     10  S: here: 1 | 1 | 1
         S: erc:
         S: who:    Lederberg, Joshua
         S: what:   Studies of Human Families for Genetic Linkage
     15  S: when:   1974
         S: where:
         S: erc-support:
         S: who:    USNLM
         S: what:   Permanent, Unchanging Content
     20  S: when:   20010421
         S: where:
         S: [closes session]

   Again, a single metadata record (lines 12-21) is returned, but it
   consists of two segments.  The first segment (12-16) gives the same
   basic citation information as in the previous example.  It is
   returned in order to establish context for the persistence
   declaration in the second segment (17-21).

   Each segment in an ERC tells a different story relating to the
   object, so although the same four questions (elements) appear in
   each, the answers depend on the segment's story type.  While the
   first segment tells the story of an expression of the object, the
   second segment tells the story of the support commitment made to it:
   who made the commitment, what the nature of the commitment was, when
   it was made, and where a fuller explanation of the commitment may be

7.  Overview of Electronic Resource Citations (ERCs)

   An Electronic Resource Citation (or ERC, pronounced e-r-c) [ERC] is a
   kind of object description that uses Dublin Core Kernel [Kernel]
   metadata elements.  The ERC with Kernel elements provides a simple,
   compact, and printable record for holding data associated with an
   information resource.  By design, Kernel metadata balances the needs
   for expressive power, very simple machine processing, and direct
   human manipulation.

   A founding principle of Kernel metadata is that direct human contact
   with metadata will be a necessary and sufficient condition for the
   near term rapid development of metadata standards, systems, and

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   services.  Thus the machine-processable Kernel elements must only
   minimally strain people's ability to read, understand, change, and
   transmit ERCs without their relying on intermediation with
   specialized software tools.  The basic ERC needs to be succinct,
   transparent, and trivially parseable by software.

   In the current Internet, it is natural seriously to consider using
   XML as an exchange format because of predictions that it will obviate
   many ad hoc formats and programs, and unify much of the world's
   information under one reliable data structuring discipline that is
   easy to generate, verify, parse, and render.  It appears, however,
   that XML is still only catching on after years of standards work and
   implementation experience.  The reasons for it are unclear, but for
   now very simple XML interpretation is still out of reach.  Another
   important caution is that XML structures are hard on the eyeballs,
   taking up an amount of display (and page) space that significantly
   exceeds that of traditional formats.  Until these conflicts with ERC
   principle are resolved, XML is not a first choice for representing
   ERCs.  Borrowing instead from the data structuring format that
   underlies the successful spread of email and web services, the first
   ERC format uses [ANVL], which is based on email and HTTP headers
   [RFC822].  There is a naturalness to ANVL's label-colon-value format
   (seen in the previous section) that barely needs explanation to a
   person beginning to enter ERC metadata.

   Besides simplicity of ERC system implementation and data entry
   mechanics, ERC semantics (what the record and its constituent parts
   mean) must also be easy to explain.  ERC semantics are based on a
   reformulation and extension of the Dublin Core [DCORE] hypothesis,
   which suggests that the fifteen Dublin Core metadata elements have a
   key role to play in cross-domain resource description.  The ERC
   design recognizes that the Dublin Core's primary contribution is the
   international, interdisciplinary consensus that identified fifteen
   semantic buckets (element categories), regardless of how they are
   labeled.  The ERC then adds a definition for a record and some
   minimal compliance rules.  In pursuing the limits of simplicity, the
   ERC design combines and relabels some Dublin Core buckets to isolate
   a tiny kernel (subset) of four elements for basic cross-domain
   resource description.

   For the cross-domain kernel, the ERC uses the four basic elements -
   who, what, when, and where - to pretend that every object in the
   universe can have a uniform minimal description.  Each has a name or
   other identifier, a location, some responsible person or party, and a
   date.  It doesn't matter what type of object it is, or whether one
   plans to read it, interact with it, smoke it, wear it, or navigate
   it.  Of course, this approach is flawed because uniformity of
   description for some object types requires more semantic contortion
   and sacrifice than for others.  That is why at the beginning of this
   document, the ARK was said to be suited to objects that accommodate
   reasonably regular electronic description.

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   While insisting on uniformity at the most basic level provides
   powerful cross-domain leverage, the semantic sacrifice is great for
   many applications.  So the ERC also permits a semantically rich and
   nuanced description to co-exist in a record along with a basic
   description.  In that way both sophisticated and naive recipients of
   the record can extract the level of meaning from it that best suits
   their needs and abilities.  Key to unlocking the richer description
   is a controlled vocabulary of ERC record types (not explained in this
   document) that permit knowledgeable recipients to apply defined sets
   of additional assumptions to the record.

7.1.  ERC Syntax

   An ERC record is a sequence of metadata elements ending in a blank
   line.  An element consists of a label, a colon, and an optional
   value.  Here is an example of a record with five elements.

          who: Gibbon, Edward
          what: The Decline and Fall of the Roman Empire
          when: 1781

   A long value may be folded (continued) onto the next line by
   inserting a newline and indenting the next line.  A value can be thus
   folded across multiple lines.  Here are two example elements, each
   folded across four lines.

          who/created: University of California, San Francisco, AIDS
               Program at San Francisco General Hospital | University
               of California, San Francisco, Center for AIDS Prevention
                Heart Attack | Heart Failure
               | Heart

   An element value folded across several lines is treated as if the
   lines were joined together on one long line.  For example, the second
   element from the previous example is considered equivalent to

          what/Topic: Heart Attack | Heart Failure | Heart Diseases

   An element value may contain multiple values, each one separated from
   the next by a `|' (pipe) character.  The element from the previous
   example contains three values.

   For annotation purposes, any line beginning with a `#' (hash)
   character is treated as if it were not present; this is a "comment"
   line (a feature not available in email or HTTP headers).  For
   example, the following element is spread across four lines and

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   contains two values:

               Heart Attack
          #    | Heart Failure  -- hold off until next review cycle
               | Heart Diseases

7.2.  ERC Stories

   An ERC record is organized into one or more distinct segments, where
   where each segment tells a story about a different aspect of the
   information resource.  A segment boundary occurs whenever a segment
   label (an element beginning with "erc") is encountered.  The basic
   label "erc:" introduces the story of an object's expression (e.g.,
   its publication, installation, or performance).  The label "erc-
   about:" introduces the story of an object's content (what it is
   about) and "erc-support:" introduces the story of a support
   commitment made to it.  A story segment that concerns the ERC itself
   is introduced by the label "erc-from:".  It is an important segment
   that tells the story of the ERC's provenance.  Elements beginning
   with "erc" are reserved for segment labels and their associated story
   types.  From an earlier example, here is an ERC with two segments.

         who:    Lederberg, Joshua
         what:   Studies of Human Families for Genetic Linkage
         when:   1974
         who:    NIH/NLM/LHNCBC
         what:   Permanent, Unchanging Content
         # Note to ops staff:  date needs verification.
         when:   2001 04 21

   Segment stories are told according to journalistic tradition.  While
   any number of pertinent elements may appear in a segment, priority is
   placed on answering the questions who, what, when, and where at the
   beginning of each segment so that readers can make the most important
   selection or rejection decisions as soon as possible.  To make things
   simple, the listed ordering of the questions is maintained in each
   segment (as it happens most people who have been exposed to this
   story telling technique are already familiar with the above

   The four questions are answered by using corresponding element
   labels.  The four element labels can be re-used in each story
   segment, but their meaning changes depending on the segment (the
   story type) in which they appear.  In the example above, "who" is
   first used to name a document's author and subsequently used to name

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   the permanence guarantor (provider).  Similarly, "when" first lists
   the date of object creation and in the next segment lists the date of
   a commitment decision.  Four labels appearing across three segments
   effectively map to twelve semantically distinct elements.  Distinct
   element meanings are mapped to Dublin Core elements in a later

7.3.  The ERC Anchoring Story

   Each ERC contains an anchoring story.  It is usually the first
   segment labeled "erc:" and it concerns an "anchoring" expression of
   the object.  An "anchoring" expression is the one that a provider
   deemed the most suitable basic referent given the audience and
   application for which it produced the ERC.  If it sounds like the
   provider has great latitude in choosing its anchoring expression, it
   is because it does.  A typical anchoring story in an ERC for a born-
   digital document would be the story of the document's release on a
   web site; such a document would then be the anchoring expression.

   An anchoring story need not be the central descriptive goal of an ERC
   record.  For example, a museum provider may create an ERC for a
   digitized photograph of a painting but choose to anchor it in the
   story of the original painting instead of the story of the electronic
   likeness; although the ERC may through other segments prove to be
   centrally concerned with describing the electronic likeness, the
   provider may have chosen this particular anchoring story in order to
   make the ERC visible in a way that is most natural to patrons (who
   would find the Mona Lisa under da Vinci sooner than they would find
   it under the name of the person who snapped the photograph or scanned
   the image).  In another example, a provider that creates an ERC for a
   dramatic play as an abstract work has the task of describing a piece
   of intangible intellectual property.  To anchor this abstract object
   in the concrete world, if only through a derivative expression, it
   makes sense for the provider to choose a suitable printed edition of
   the play as the anchoring object expression (to describe in the
   anchoring story) of the ERC.

   The anchoring story has special rules designed to keep ERC processing
   simple and predictable.  Each of the four basic elements (who, what,
   when, and where) must be present, unless a best effort to supply it
   fails.  In the event of failure, the element still appears but a
   special value (described later) is used to explain the missing value.
   While the requirement that each of the four elements be present only
   applies to the anchoring story segment, as usual these elements
   appear at the beginning of the segment and may only be used in the
   prescribed order.  A minimal ERC would normally consist of just an
   anchoring story and the element quartet, as illustrated in the next

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         who:   National Research Council
         what:  The Digital Dilemma
         when:  2000

   A minimal ERC can be abbreviated so that it resembles a traditional
   compact bibliographic citation that is nonetheless completely machine
   processable.  The required elements and ordering makes it possible to
   eliminate the element labels, as shown here.

         erc: National Research Council | The Digital Dilemma | 2000

7.4.  ERC Elements

   As mentioned, the four basic ERC elements (who, what, when, and
   where) take on different specific meanings depending on the story
   segment in which they are used.  By appearing in each segment, albeit
   in different guises, the four elements serve as a valuable mnemonic
   device - a kind of checklist - for constructing minimal story
   segments from scratch.  Again, it is only in the anchoring segment
   that all four elements are mandatory.

   Here are some mappings between ERC elements and Dublin Core [DCORE]

          Segment     ERC Element     Equivalent Dublin Core Element
         ---------    -----------     ------------------------------
            erc          who          Creator/Contributor/Publisher
            erc          what                Title
            erc          when                Date
            erc          where               Identifier
         erc-about       who                  <none>
         erc-about       what                Subject
         erc-about       when                Coverage (temporal)
         erc-about       where               Coverage (spatial)

   The basic element labels may also be qualified to add nuances to the
   semantic categories that they identify.  Elements are qualified by
   appending a `/' (slash) and a qualifier term.  Often qualifier terms
   appear as the past tense form of a verb because it makes re-using
   qualifiers among elements easier.

         who/published:  ...
         when/published: ...
         where/published: ...

   Using past tense verbs for qualifiers also reminds providers and
   recipients that element values contain transient assertions that may

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   have been true once, but that tend to become less true over time.
   Recipients that don't understand the meaning of a qualifier can fall
   back onto the semantic category (bucket) designated by the
   unqualified element label.  Inevitably recipients (people and
   software) will have diverse abilities in understanding elements and

   Any number of other elements and qualifiers may be used in
   conjunction with the quartet of basic segment questions.  The only
   semantic requirement is that they pertain to the segment's story.
   Also, it is only the four basic elements that change meaning
   depending on their segment context.  All other elements have meaning
   independent of the segment in which they appear.  If an element label
   stripped of its qualifier is still not recognized by the recipient, a
   second fall back position is to ignore it and rely on the four basic

   Elements may be either Canonical, Provisional, or Local.  Canonical
   elements are officially recognized via a registry as part of the
   metadata vernacular.  All elements, qualifiers, and segment labels
   used in this document up until now belong to that vernacular.
   Provisional elements are also officially recognized via the registry,
   but have only been proposed for inclusion in the vernacular.  To be
   promoted to the vernacular, a provisional element passes through a
   vetting process during which its documentation must be in order and
   its community acceptance demonstrated.  Local elements are any
   elements not officially recognized in the registry.  The registry
   [Kernel] is a work in progress.

   Local elements can be immediately distinguishable from Canonical or
   Provisional elements because all terms that begin with an upper case
   letter are reserved for spontaneous local use.  No term beginning
   with an upper case letter will ever be assigned Canonical or
   Provisional status, so it should be safe to use such terms for local
   purposes.  Any recipient of external ERCs containing such terms will
   understand them to be part of the originating provider's local
   metadata dialect.  Here's an example ERC with three segments, one
   local element, and two local qualifiers.  The segment boundaries have
   been emphasized by comment lines (which, as before, are ignored by

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         who: Bullock, TH | Achimowicz, JZ | Duckrow, RB
                 | Spencer, SS | Iragui-Madoz, VJ
         what: Bicoherence of intracranial EEG in sleep,
                 wakefulness and seizures
         when: 1997 12 00
                 documents/disk0/00/00/01/22/index.html %}
         in: EEG Clin Neurophysiol | 1997 12 00 | v103, i6, p661-678
         IDcode: cog00000122
         # ---- new segment ----
         what/Subcategory: Bispectrum | Nonlinearity | Epilepsy
                 | Cooperativity | Subdural | Hippocampus | Higher moment
         # ---- new segment ----
         who: NIH/NLM/NCBI
         what: pm9546494
         when/Reviewed: 1998 04 18 021600

   The local element "IDcode" immediately precedes the "erc-about"
   segment, which itself contains an element with the local qualifier
   "Subcategory".  The second to last element also carries the local
   qualifier "Reviewed".  Finally, what might be a provisional element
   "in" appears near the end of the first segment.  It might have been
   proposed as a way to complete a citation for an object originally
   appearing inside another object (such as an article appearing in a
   journal or an encyclopedia).

7.5.  ERC Element Values

   ERC element values tend to be straightforward strings.  If the
   provider intends something special for an element, it will so
   indicate with markers at the beginning of its value string.  The
   markers are designed to be uncommon enough that they would not likely
   occur in normal data except by deliberate intent.  Markers can only
   occur near the beginning of a string, and once any octet of non-
   marker data has been encountered, no further marker processing is
   done for the element value.  In the absence of markers the string is
   considered pure data; this has been the case with all the examples
   seen thus far.  The fullest form of an element value with all three
   optional markers in place looks like this.

         VALUE =    [markup_flags]    (:ccode)    ,    DATA

   In processing, the first non-whitespace character of an ERC element
   value is examined.  An initial `[' is reserved to introduce a
   bracketed set of markup flags (not described in this document) that
   ends with `]'.  If ERC data is machine-generated, each value string
   may be preceded by "[]" to prevent any of its data from being

J. Kunze                 7.5. ERC Element Values               [Page 38]

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   mistaken for markup flags.  Once past the optional markup, the
   remaining value may optionally begin with a controlled code.  A
   controlled code always has the form "(:ccode)", for example,

         who: (:unkn) Anonymous
         what: (:791) Bee Stings

   Any string after such a code is taken to be an uncontrolled (e.g.,
   natural language) equivalent.  The code "unkn" indicates a
   conventional explanation for a missing value (stating that the value
   is unknown).  The remainder of the string makes an equivalent
   statement in a form that the provider deemed most suitable to its
   (probably human) audience.  The code "791" could be a fixed numeric
   topic identifier within an unspecified topic vocabulary.  Any code
   may be ignored by those that do not understand it.

   There are several codes to explain different ways in which a required
   element's value may go missing.

         (:unac)   temporarily inaccessible
         (:unal)   unallowed, suppressed intentionally
         (:unap)   not applicable, makes no sense
         (:unas)   value unassigned (e.g., Untitled)
         (:unav)   value unavailable indefinitely
         (:unkn)   unknown (e.g., Anonymous, Inconnue)
         (:etal)   too numerous to list (I<et alia>).
         (:none)   never had a value, never will
         (:null)   explicitly empty
         (:tba)    to be assigned or announced later

   Once past an optional controlled code, the remaining string value is
   subjected to one final test.  If the first next non-whitespace
   character is a `,' (comma), it indicates that the string value is
   "sort-friendly".  This means that the value is (a) laid out with an
   inverted word order useful for sorting items having comparably laid
   out element values (items might be the containing ERC records) and
   (b) that the value may contain other commas that indicate inversion
   points should it become necessary to recover the value in natural
   word order.  Typically, this feature is used to express Western-style
   personal names in family-name-given-name order.  It can also be used
   wherever natural word order might make sorting tricky, such as when
   data contains titles or corporate names.  Here are some example

J. Kunze                 7.5. ERC Element Values               [Page 39]

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         who:   ,  van Gogh, Vincent
         who:,Howell, III, PhD, 1922-1987, Thurston
         who:, Acme Rocket Factory, Inc., The
         who:, Mao Tse Tung
         who:, McCartney, Paul, Sir,
         what:, Health and Human Services, United States Government
                 Department of, The,

   There are rules to use in recovering a copy of the value in natural
   word order, if desired.  The above example strings have the following
   natural word order values, respectively.

         Vincent van Gogh
         Thurston Howell, III, PhD, 1922-1987
         The Acme Rocket Factory, Inc.
         Mao Tse Tung
         Sir Paul McCartney
         The United States Government Department of Health and Human Services

7.6.  ERC Element Encoding and Dates

   Some characters that need to appear in ERC element values might
   conflict with special characters used for structuring ERCs, so there
   needs to be a way to include them as literal characters that are
   protected from special interpretation.  This is accomplished through
   an encoding mechanism that resembles the %-encoding familiar to [URI]

   The ERC encoding mechanism also uses `%', but instead of taking two
   following hexadecimal digits, it takes one non-alphanumeric character
   or two alphabetic characters that cannot be mistaken for hex digits.
   It is designed not to be confused with normal web-style %-encoding.
   In particular it can be decoded without risking unintended decoding
   of normal %-encoded data (which would introduce errors).  Here are
   the one-character (non-alphanumeric) ERC encoding extensions.

         ERC       Purpose
         ---     ------------------------------------------------
         %!      decodes to the element separator `|'
         %%      decodes to a percent sign `%'
         %.      decodes to a comma `,'
         %_      a non-character used as syntax shim
         %{      a non-character that begins an expansion block
         %}      a non-character that ends an expansion block

   One particularly useful construct in ERC element values is the pair
   of special encoding markers ("%{" and "%}") that indicates a
   "expansion" block.  Whatever string of characters they enclose will
   be treated as if none of the contained whitespace (SPACEs, TABs,
   Newlines) were present.  This comes in handy for writing long, multi-

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   part URLs in a readable way.  For example, the value in

                    ? db = foo
                    & start = 1
                    & end = 5
                    & buf = 2
                    & query = foo + bar + zaf

   is decoded into an equivalent element, but with a correct and intact


   In a parting word about ERC element values, a commonly recurring
   value type is a date, possibly followed by a time.  ERC dates use the
   [TEMPER] format, taking on one of the following forms:

         1999                (four digit year)
         2000 12 29          (year, month, day)
         2000 12 29 235955   (year, month, day, hour, minute, second)

   In dates, all internal whitespace is squeezed out to achieve a
   normalized form suitable for lexical comparison and sorting.  This
   means that the following dates

         2000 12 29 235955           (recommended for readability)
         2000 12 29 23 59 55
         20001229 23 59 55
         20001229235955              (normalized date and time)

   are all equivalent.  The first form is recommended for readability.
   The last form (shortest and easiest to compute with) is the
   normalized form.  Hyphens and commas are reserved to create date
   ranges and lists, for example,

         1996-2000                   (a range of four years)
         1952, 1957, 1969            (a list of three years)
         1952, 1958-1967, 1985       (a mixed list of dates and ranges)
         20001229-20001231           (a range of three days)

7.7.  ERC Stub Records and Internal Support

   The ERC design introduces the concept of a "stub" record, which is an
   incomplete ERC record intended to be supplemented with additional
   elements before being released as a standalone ERC record.  A stub
   ERC record has no minimum required elements.  It is just a group of

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   elements that does not begin with "erc:" but otherwise conforms to
   the ERC record syntax.

   ERC stubs may be useful in supporting internal procedures using the
   ERC syntax.  Often they rely on the convenience and accuracy of
   automatically supplied elements, even the basic ones.  To be ready
   for external use, however, an ERC stub must be transformed into a
   complete ERC record having the usual required elements.  An ERC stub
   record can be convenient for metadata embedded in a document, where
   elements such as location, modification date, and size - which one
   would not omit from an externalized record - are omitted simply
   because they are much better supplied by a computation.  A separate
   local administrative procedure, not defined for ERC's in general,
   would effect the promotion of stubs into complete records.

   While the ERC is a general-purpose container for exchange of resource
   descriptions, it does not dictate how records must be internally
   stored, laid out, or assembled by data providers or recipients.
   Arbitrary internal descriptive frameworks can support ERCs simply by
   mapping (e.g., on demand) local records to the ERC container format
   and making them available for export.  Therefore, to support ERCs
   there is no need for a data provider to convert internal data to be
   stored in an ERC format.  On the other hand, any provider (such as
   one just getting started in the business of resource description) may
   choose to store and manipulate local data natively in the ERC format.

8.  Advice to Web Clients

   This section offers some advice to web client software developers.
   It is hard to write about because it tries to anticipate a series of
   events that might lead to native web browser support for ARKs.

   ARKs are envisaged to appear wherever durable object references are
   planned.  Library cataloging records, literature citations, and
   bibliographies are important examples.  In many of these places URLs
   (Uniform Resource Locators) currently stand in, and URNs, DOIs, and
   PURLs have been proposed as alternatives.

   The strings representing ARKs are also envisaged to appear in some of
   the places where URLs currently appear:  in hypertext links (where
   they are not normally shown to users) and in rendered text (displayed
   or printed).  Internet search engines, for example, tend to include
   both actionable and manifest links when listing each item found.  A
   normal HTML link for which the URL is not displayed looks like this.

          <a href = ""> Click Here <a>

   The same link with an ARK instead of a URL:

          <a href = "ark:/14697/b12345x"> Click Here <a>

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   Web browsers would in general require a small modification to
   recognize and convert this ARK, via mapping authority discovery, to
   the URL form.

          <a href = ""> Click Here <a>

   A browser that knows how to make that conversion could also
   automatically detect and replace a non-working NMAH.

   An NAA will typically make known the associations it creates by
   publishing them in catalogs, actively advertizing them, or simply
   leaving them on web sites for visitors (e.g., users, indexing
   spiders) to stumble across in browsing.

9.  Security Considerations

   The ARK naming scheme poses no direct risk to computers and networks.
   Implementors of ARK services need to be aware of security issues when
   querying networks and filesystems for Name Mapping Authority
   services, and the concomitant risks from spoofing and obtaining
   incorrect information.  These risks are no greater for ARK mapping
   authority discovery than for other kinds of service discovery.  For
   example, recipients of ARKs with a specified hostport (NMAH) should
   treat it like a URL and be aware that the identified ARK service may
   no longer be operational.

   Apart from mapping authority discovery, ARK clients and servers
   subject themselves to all the risks that accompany normal operation
   of the protocols underlying mapping services (e.g., HTTP, Z39.50).
   As specializations of such protocols, an ARK service may limit
   exposure to the usual risks.  Indeed, ARK services may enhance a kind
   of security by helping users identify long-term reliable references
   to information objects.

10.  Authors' Addresses

   John A. Kunze
   California Digital Library
   University of California, Office of the President
   415 20th St, 4th Floor
   Oakland, CA  94612-3550, USA

   Fax:   +1 510-893-5212

   R. P. C. Rodgers
   US National Library of Medicine
   8600 Rockville Pike, Bldg. 38A
   Bethesda, MD  20894, USA

   Fax:   +1 301-496-0673

J. Kunze                 10. Authors' Addresses                [Page 43]

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11.  References

   [ANVL]     J. Kunze, B. Kahle, et al, "A Name-Value Language", work
              in progress,

   [ARK]      J. Kunze, "Towards Electronic Persistence Using ARK
              Identifiers", Proceedings of the 3rd ECDL Workshop on Web
              Archives, August 2003, (PDF)

   [DCORE]    Dublin Core Metadata Initiative, "Dublin Core Metadata
              Element Set, Version 1.1:  Reference Description", July

   [DNS]      P.V. Mockapetris, "Domain Names - Concepts and
              Facilities", RFC 1034, November 1987.

   [DOI]      International DOI Foundation, "The Digital Object
              Identifier (DOI) System", February 2001,

   [ERC]      J. Kunze, "A Metadata Kernel for Electronic Permanence",
              Journal of Digital Information, Vol 2, Issue 2, January
              2002, ISSN 1368-7506, (PDF)

   [Handle]   L. Lannom, "Handle System Overview", ICSTI Forum, No. 30,
              April 1999,

   [HTTP]     R. Fielding, et al, "Hypertext Transfer Protocol --
              HTTP/1.1", RFC 2616, June 1999.

   [Kernel]   Dublin Core Metadata Initiative, "Kernel Metadata Working

   [MD5]      R. Rivest, "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [N2T]      CDL, "Name-to-Thing Resolover", work in progress, August

   [NAPTR]    M. Mealling, Daniel, R., "The Naming Authority Pointer
              (NAPTR) DNS Resource Record", RFC 2915, September 2000.

   [NLMPerm]  M. Byrnes, "Defining NLM's Commitment to the Permanence of
              Electronic Information", ARL 212:8-9, October 2000,

J. Kunze                     11. References                    [Page 44]

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   [NOID]     J. Kunze, "Nice Opaque Identifiers", February 2005,

   [PURL]     K. Shafer, et al, "Introduction to Persistent Uniform
              Resource Locators", 1996,

   [RFC822]   D. Crocker, "Standard for the format of ARPA Internet text
              messages", RFC 822, August 1982.

   [TELNET]   J. Postel, J.K. Reynolds, "Telnet Protocol Specification",
              RFC 854, May 1983.

   [TEMPER]   J. Kunze, "Temporal Enumerated Ranges", work in progress,

   [THUMP]    K. Gamiel, J. Kunze, "The HTTP URL Mapping Protocol", work
              in progress,

   [URI]      T. Berners-Lee, et al, "Uniform Resource Identifiers
              (URI): Generic Syntax", RFC 2396, August 1998.

   [URNBIB]   C. Lynch, et al, "Using Existing Bibliographic Identifiers
              as Uniform Resource Names", RFC 2288, February 1998.

   [URNSYN]   R. Moats, "URN Syntax", RFC 2141, May 1997.

   [URNNID]   L. Daigle, et al, "URN Namespace Definition Mechanisms",
              RFC 2611, June 1999.

12.  Appendix:  ARK Implementations

   Currently, the primary implementation activity is at the California
   Digital Library (CDL),

   housed at the University of California Office of the President, where
   over 200,000 ARKs have been assigned to objects that the CDL owns or
   controls.  Some experimentation in ARKs is taking place at JSTOR, the
   Digital Curation Centre, WIPO and at the University of California's
   San Diego, San Francisco, and Berkeley campuses.

   The US National Library of Medicine (NLM) also has an experimental,
   prototype ARK service under development.  It is being made available
   for purposes of demonstrating various aspects of the ARK system, but
   is subject to temporary or permanent withdrawal (without notice)
   depending upon the circumstances of the small research group
   responsible for making it available.  It is described at:

J. Kunze           12. Appendix:  ARK Implementations          [Page 45]

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   Comments and feedback may be addressed to

13.  Appendix:  Current ARK Name Authority Table

   This appendix contains a copy of the Name Authority Table (a file) at
   the time of writing.  It may be loaded into a local filesystem (e.g.,
   /etc/natab) for use in mapping NAAs (Name Assigning Authorities) to
   NMAHs (Name Mapping Authority Hostports).  It contains Perl code that
   can be copied into a standalone script that processes the table (as a
   file).  Because this is still a proposed file, none of the values in
   it are real.

J. Kunze              13. Appendix:  ARK /etc/natab            [Page 46]

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     # Name Assigning Authority / Name Mapping Authority Lookup Table
     #    Last change:   2007.06.05
     #       Reload from:
     #       Mirrored at:
     #       To register:
     #       Process with:  Perl script at end of this file (optional)
     # Each NAA appears at the beginning of a line with the NAA Number
     # first, a colon, and an ARK or URL to a statement of naming policy
     # (see for an example).
     # All the NMA hostports that service an NAA are listed, one per
     # line, indented, after the corresponding NAA line.
     #       National Library of Medicine
     #       Library of Congress
     #       National Agriculture Library
     #       California Digital Library
     #       World Intellectual Property Organization
     #       University of California San Diego
     #       University of California San Francisco
     #       University of California Berkeley
     #       University of California Los Angeles

J. Kunze              13. Appendix:  ARK /etc/natab            [Page 47]

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     #       Rutgers University
     #       Internet Archive
     #       Digital Curation Centre
     #       New York University
     #       University of North Texas
     #       Ithaka Electronic-Archiving Initiative
     #       Bibliothque nationale de France / National Library of France
     #       Princeton University
     #       University of Washington
     #       Archives of Region of Vstra Gtaland and City of Gothenburg, Sweden
     #       Northwest Digital Archives
     #       Emory University
     #       University of Kansas

J. Kunze              13. Appendix:  ARK /etc/natab            [Page 48]

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     #       Google
     #    Centre for Ecology & Hydrology, UK
     #    University of Calgary
     #12345: reserved for examples
     #--- end of data ---
     # The following Perl script takes an NAA as argument and outputs
     # the NMAs in this file listed under any matching NAA.
     # my $naa = shift;
     # while (<>) {
     #       next if (! /^$naa:/);
     #       while (<>) {
     #               last if (! /^[#\s]./);
     #               print "$1\n" if (/^\s+(\S+)/);
     #       }
     # }
     # Create a g/t/nroff-safe version of this table with the UNIX command,
     #       expand natab | sed 's/\\/\\\e/g' > natab.roff
     # end of file

14.  Copyright Notice

   Copyright (C) The IETF Trust (2007).  This document is subject to the
   rights, licenses and restrictions contained in BCP 78, and except as
   set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an

Expires 24 January 2008

J. Kunze                  14. Copyright Notice                 [Page 49]

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                           Table of Contents

Status of this Document  . . . . . . . . . . . . . . . . . . . . . .   1
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   1
1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .   3
1.1.  Reasons to Use ARKs  . . . . . . . . . . . . . . . . . . . . .   4
1.2.  Three Requirements of ARKs . . . . . . . . . . . . . . . . . .   4
1.3.  Organizing Support for ARKs:  Our Stuff vs. Their Stuff  . . .   5
1.4.  Definition of Identifier . . . . . . . . . . . . . . . . . . .   7
2.  ARK Anatomy  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
2.1.  The Name Mapping Authority Hostport (NMAH) . . . . . . . . . .   8
2.2.  The ARK Label Part - ark:  . . . . . . . . . . . . . . . . . .   9
2.3.  The Name Assigning Authority Number (NAAN) . . . . . . . . . .  10
2.4.  The Name Part  . . . . . . . . . . . . . . . . . . . . . . . .  10
2.5.  The Qualifier Part . . . . . . . . . . . . . . . . . . . . . .  11
2.5.1.  ARKs that Reveal Object Hierarchy  . . . . . . . . . . . . .  12
2.5.2.  ARKs that Reveal Object Variants . . . . . . . . . . . . . .  13
2.6.  Character Repertoires  . . . . . . . . . . . . . . . . . . . .  14
2.7.  Normalization and Lexical Equivalence  . . . . . . . . . . . .  15
3.  Naming Considerations  . . . . . . . . . . . . . . . . . . . . .  16
3.1.  ARKS Embedded in Language  . . . . . . . . . . . . . . . . . .  16
3.2.  Objects Should Wear Their Identifiers  . . . . . . . . . . . .  17
3.3.  Names are Political, not Technological . . . . . . . . . . . .  17
3.4.  Choosing a Hostname or NMA . . . . . . . . . . . . . . . . . .  17
3.5.  Assigners of ARKs  . . . . . . . . . . . . . . . . . . . . . .  19
3.6.  NAAN Namespace Management  . . . . . . . . . . . . . . . . . .  20
3.7.  Sub-Object Naming  . . . . . . . . . . . . . . . . . . . . . .  21
4.  Finding a Name Mapping Authority . . . . . . . . . . . . . . . .  21
4.1.  Looking Up NMAHs in a Globally Accessible File . . . . . . . .  22
4.2.  Looking up NMAHs Distributed via DNS . . . . . . . . . . . . .  23
5.  Generic ARK Service Definition . . . . . . . . . . . . . . . . .  26
5.1.  Generic ARK Access Service (access, location)  . . . . . . . .  26
5.2.  Generic Policy Service (permanence, naming, etc.)  . . . . . .  26
5.3.  Generic Description Service  . . . . . . . . . . . . . . . . .  28
6.  Overview of The HTTP URL Mapping Protocol (THUMP)  . . . . . . .  28
7.  Overview of Electronic Resource Citations (ERCs) . . . . . . . .  31
7.1.  ERC Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  33
7.2.  ERC Stories  . . . . . . . . . . . . . . . . . . . . . . . . .  34
7.3.  The ERC Anchoring Story  . . . . . . . . . . . . . . . . . . .  35
7.4.  ERC Elements . . . . . . . . . . . . . . . . . . . . . . . . .  36
7.5.  ERC Element Values . . . . . . . . . . . . . . . . . . . . . .  38
7.6.  ERC Element Encoding and Dates . . . . . . . . . . . . . . . .  40
7.7.  ERC Stub Records and Internal Support  . . . . . . . . . . . .  41
8.  Advice to Web Clients  . . . . . . . . . . . . . . . . . . . . .  42
9.  Security Considerations  . . . . . . . . . . . . . . . . . . . .  43
10.  Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . .  43
11.  References  . . . . . . . . . . . . . . . . . . . . . . . . . .  44
12.  Appendix:  ARK Implementations  . . . . . . . . . . . . . . . .  45
13.  Appendix:  Current ARK Name Authority Table . . . . . . . . . .  46
14.  Copyright Notice  . . . . . . . . . . . . . . . . . . . . . . .  49

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