Network Working Group                                           J. Kunze
Internet-Draft                                California Digital Library
Intended status: Informational                                 E. Bermès
Expires: 10 February 2022               Bibliothèque nationale de France
                                                           9 August 2021


                       The ARK Identifier Scheme
                           draft-kunze-ark-28

Abstract

   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 robust reference.  The term ARK itself refers both to the
   scheme and to any single identifier that conforms to it.  An ARK has
   five components:

   [https://NMA/]ark:[/]NAAN/Name[Qualifiers]

   an optional and mutable Name Mapping Authority (usually a hostname),
   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 independent of the URL hostname.
   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, inflected
   by appending `?info', returns a metadata record that is both human-
   and machine-readable.  The returned record contains core metadata and
   a commitment statement from the current provider.  Tools exist for
   minting, binding, and resolving ARKs.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.




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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   This Internet-Draft will expire on 10 February 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Reasons to Use ARKs . . . . . . . . . . . . . . . . . . .   4
     1.2.  Three Requirements of ARKs  . . . . . . . . . . . . . . .   5
     1.3.  Organizing Support for ARKs: Our Stuff vs. Their Stuff  .   7
     1.4.  Definition of Identifier  . . . . . . . . . . . . . . . .   8
   2.  ARK Anatomy . . . . . . . . . . . . . . . . . . . . . . . . .   9
     2.1.  The Name Mapping Authority (NMA)  . . . . . . . . . . . .  11
     2.2.  The ARK Label Part (ark:) . . . . . . . . . . . . . . . .  12
     2.3.  The Name Assigning Authority Number (NAAN)  . . . . . . .  13
     2.4.  The Name Part . . . . . . . . . . . . . . . . . . . . . .  15
       2.4.1.  Optional: Shoulders . . . . . . . . . . . . . . . . .  16
     2.5.  The Qualifier Part  . . . . . . . . . . . . . . . . . . .  17
       2.5.1.  ARKs that Reveal Object Hierarchy . . . . . . . . . .  18
       2.5.2.  ARKs that Reveal Object Variants  . . . . . . . . . .  19
     2.6.  Character Repertoires . . . . . . . . . . . . . . . . . .  21
     2.7.  Normalization and Lexical Equivalence . . . . . . . . . .  22
     2.8.  Resolver Chains . . . . . . . . . . . . . . . . . . . . .  23
   3.  Naming Considerations . . . . . . . . . . . . . . . . . . . .  24
     3.1.  ARKS and Usability  . . . . . . . . . . . . . . . . . . .  24
     3.2.  Objects Should Wear Their Identifiers . . . . . . . . . .  24
     3.3.  Names are Political, not Technological  . . . . . . . . .  24
     3.4.  Choosing a Hostname or NMA  . . . . . . . . . . . . . . .  25
     3.5.  Assigners of ARKs . . . . . . . . . . . . . . . . . . . .  26
     3.6.  NAAN Namespace Management . . . . . . . . . . . . . . . .  27
     3.7.  Sub-Object Naming . . . . . . . . . . . . . . . . . . . .  28
   4.  Finding a Name Mapping Authority  . . . . . . . . . . . . . .  29
     4.1.  Looking Up NMAs in a Globally Accessible File . . . . . .  30
   5.  Generic ARK Service Definition  . . . . . . . . . . . . . . .  31



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     5.1.  Generic ARK Access Service (access, location) . . . . . .  31
       5.1.1.  Generic Policy Service (permanence, naming, etc.) . .  32
       5.1.2.  Generic Description Service . . . . . . . . . . . . .  34
     5.2.  Overview of The HTTP URL Mapping Protocol (THUMP) . . . .  34
     5.3.  The Electronic Resource Citation (ERC)  . . . . . . . . .  36
     5.4.  Advice to Web Clients . . . . . . . . . . . . . . . . . .  38
     5.5.  Enhancements and Related Specifications . . . . . . . . .  39
     5.6.  Security Considerations . . . . . . . . . . . . . . . . .  39
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  39
   Appendix A.  ARK Maintenance Agency: arks.org . . . . . . . . . .  42
   Appendix B.  Looking up NMAs Distributed via DNS  . . . . . . . .  42
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44

1.  Introduction

   [ Note about this transitional draft.  The ARKsInTheOpen.org
   Technical Working Group (https://wiki.duraspace.org/display/ARKs/
   Technical+Working+Group) is in the process of revising the ARK spec
   via a series of Internet-Drafts.  This draft contains many minor but
   noisy changes (lots of diffs but not much real change).  While the
   spec is in transition, new implementors should follow
   https://datatracker.ietf.org/doc/html/draft-kunze-ark-18. ]

   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
   [ARK].

   Schemes for persistent identification of network-accessible objects
   are not new.  In the early 1990's, the design of the Uniform Resource
   Name [RFC2141] 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 had the advantage of working with unmodified web
   browsers.  ARKs represent an approach that attempts to build on the
   strengths and to avoid the weaknesses of these schemes.




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   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)
   [RFC1034] being a particularly dazzling and elegant example.  What is
   often forgotten is that maintenance of an indirection table is an
   unavoidable cost to the organization providing persistence, and that
   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 indicates how unsuited most
   organizations will probably be to the task of table maintenance and
   to the much more fundamental challenge of keeping the objects
   themselves viable.

   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 funding and political
   instabilities.  These are by far the major challenges confronting
   persistence providers, and no identifier scheme has any direct impact
   on them.  In fact, some schemes may actually be liabilities for
   persistence because they create short- and long-term dependencies for
   every object access on complex, special-purpose 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 are maintained mostly by
   the internet community at large (the DNS, web servers, and web
   browsers).

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 embedded
   in a URL provides some of the necessary conditions for credible
   persistence, inviting access to not one, but to three things: to the
   object, to its metadata, and to a nuanced statement of commitment
   from the provider in question (the NMA, described below) regarding
   the object.  Existence of the extra service can be probed
   automatically by appending `?info' to the ARK.




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   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 (assuming the assigner ever held that
   responsibility) for the large majority of a long-term object's
   archival lifetime.  The name 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 for 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 simple algorithms (one based on the URN model) or even
   by-hand internet query can be used for 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 with persistence in mind.

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
   provider to a specific set of responsibilities.  It is critical for
   the promise to come from a current provider and almost irrelevant,
   over a long period of time, what the original assigner's intentions
   were.  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.

   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 reflecting assertions that were briefly true, such as
   where in a filesystem hierarchy an object lived during a short stay.
   Possession of both an identifier and an object is some improvement,
   but positive identification may still be uncertain since the object



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   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 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 a transient service outage,
   a catastrophic loss, 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 crudely summed up as the "HTTP 404 Not
   Found" problem -- would not have been addressed.

   The central duty of an ARK is a high-quality experience of access and
   identification.  This means supporting reliable access during the
   period described in its stewardship promise and, failing that,
   supporting reliable access to a record describing the thing the ARK
   is associated with.

   ARK resolvers must support the `?info' inflection for requesting
   metadata.  Older versions of this specification distinguished between
   two minimal inflections: `?' (brief metadata) and `??' (more
   metadata).  While these older inflections are still reserved, because
   they have proven hard to recognize in some environments, supporting
   them is optional.











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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
   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
   objects.

   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 engage in.
   There is little that we can do about someone else's stuff except
   encourage their uptake or adoption 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
   practices.

   Although organizing ARK support 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
   authorities.

   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



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   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
   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".





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   Note that our definition of identifier extends the one in use for
   Uniform Resource Identifiers [RFC3986].  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.

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.

   ANATOMY OVERIEW
   ===============


        Resolver Service            Compact ARK
       __________________  ______________________________
      /                  \/                              \
      https://example.org/ark:12345/x6np1wh8k/c3/s5.v7.xsl
      .............................\________/.............
                 prefixes          Base Name    suffixes
      \__________________________________________________/
                          Mapping ARK

   When embedded in a URL, an ARK consists of a Compact ARK preceded by
   a Resolver Service.  The larger URL-based ARK is known as a Mapping
   ARK because it is ready to be mapped (resolved) to an information
   response (eg, a PDF or metadata).  A Mapping ARK is also know as a
   "fully qualified ARK".  The Resolver Service, which need not be
   limited to URLs in the future, maps the URL according to rules and
   abilities of an NMA (Name Mapping Authority).  The same URL string
   minus the Resolver Service component is known as a Compact ARK.  The
   Compact ARK is globally unique and may be resolvable via different
   Resolver Services over time (eg, when one archive succeeds another)
   or at the same time (eg, when one archive backs up another).

   At a high level, after the Label comes the NAAN (Name Assigning
   Authority Number) followed by the Name that it assigns to the
   identified thing.  The Name is a Base (or Base Name) that has
   Prefixes (NAAN, Label, possibly a Resolver Service) and optional
   Suffixes to identify Parts and Variant forms.  During resolution, a
   Resolver Service such as n2t.net may be able to deal with inflections
   query strings, and content negotiation.







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   ANATOMY DETAILS
   ===============
                           Base Compact Name   Qualifiers
                           _________________  ___________
                          /                 \/           \
      https://example.org/ark:12345/x6np1wh8k/c3/s5.v7.xsl
              \_________/ \__/\___/\_/\_____/\____/\_____/
                 NMA     Label NAAN |  Blade Parts Variants
                                  Shoulder
                              \_____________/
                                 Check Zone

   In a closer view, the Compact ARK consists of a Base Compact Name
   followed potentially by Qualifiers.  The Name (Base) often, but not
   necessarily, consists of a Shoulder (for subdividing a NAAN
   namespace) followed by a Blade.  If a check character is present in
   an ARK, by convention it is the right-most character of the Base
   Name, and will have been computed over the string of characters
   preceding it back to the beginning of the NAAN.  This string,
   including the check character itself, is the Check Zone.

   x

   Like the ARK itself, the NAAN "12345" and Shoulder "x6" have compact
   and fully qualified forms.

   +==========+=======+==============+================================+
   |   Form   |  Base | Compact Form |      Fully Qualified Form      |
   +==========+=======+==============+================================+
   |   NAAN   | 12345 |  ark:12345   | https://example.org/ark:12345  |
   +----------+-------+--------------+--------------------------------+
   | Shoulder |   x6  | ark:12345/x6 | https://example.org/ark:12345/ |
   |          |       |              |               x6               |
   +----------+-------+--------------+--------------------------------+

      Table 1: Examples of base, compact, and fully qualified forms

   x

   The ARK syntax can be summarized,

                  [https://NMA/]ark:[/]NAAN/Name[Qualifiers]









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   where the NMA, '/', and Qualifier parts are in brackets to indicate
   that they are optional.  The Base Compact Name is the substring
   comprising the "ark:" label, the NAAN and the assigned Name.  The
   Resolver Service is replaceable and makes the ARK actionable for a
   period of time.  Without the Resolver Service part, what remains is
   the Core Immutable Identity (the "persistible") part of the ARK.

2.1.  The Name Mapping Authority (NMA)

   Before the "ark:" label may appear an optional Name Mapping Authority
   (NMA) that is a temporary address where ARK service requests may be
   sent.  Preceded by a URI-type protocol designation such as
   "https://", it specifies a Resolver Service.  The NMA itself is an
   Internet hostname or host/port combination having the same format and
   semantics as the host/port part of a URL.  The most important thing
   about the NMA is that it is "identity inert" from the point of view
   of object identification.  In other words, ARKs that differ only in
   the optional NMA part identify the same object.  Thus, for example,
   the following three ARKs are synonyms for just one information
   object:

                    https://loc.gov/ark:12345/x54xz321
                https://rutgers.edu/ark:12345/x54xz321
                                    ark:12345/x54xz321

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

   The NMA part makes an ARK into an actionable URL.  As with many
   internet parameters, it is helpful to approach the NMA being liberal
   in what you accept and conservative in what you propose.  From the
   recipient's point of view, the NMA 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 NMA 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 NMA should not be



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   considerably more permanent than soft-funded proxy hostnames such as
   hdl.handle.net, dx.doi.org, and purl.org.  In general and over time,
   however, it is not unexpected for an NMA eventually to stop working
   and require replacement with the NMA 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
   [RFC1034] 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 "https://" and an NMA to an ARK is a way
   of creating an actionable identifier by a method that is itself
   temporary.  Assuming that infrastructure supporting [RFC2616]
   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
   automatically.

2.2.  The ARK Label Part (ark:)

   The label part distinguishes an ARK from an ordinary identifier.
   There is a new form of the label, "ark:", and an old form, "ark:/",
   both of which must be recognized in perpetuity.  Implementations
   should generate new ARKs in the new form (without the "/") and
   resolvers must always treat received ARKs as equivalent if they
   differ only in regard to new form versus old form labels.  Thus these
   two ARKs are equivalent:

                             ark:/12345/x54xz321
                              ark:12345/x54xz321

   In a URL found in the wild, the label 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.






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

                  [https://NMA/]ark:[/]NAAN/Name[Qualifiers]

   the part of the ARK directly following the "ark:" (or older "ark:/")
   label is the Name Assigning Authority Number (NAAN), up to but not
   including the next `/' (slash) character.  This part is always
   required, as it identifies the organization that originally assigned
   the Name of the object.  Typically the organization is an
   institution, a department, a laboratory, or any group that conducts a
   stable, policy-driven name assigning effort.  An organization may
   request a NAAN from the ARK Maintenance Agency [ARKagency] (described
   in Appendix A) by filling out the form [NAANrequest].

   For received ARKs, implementations must support a minimum NAAN length
   of 16 octets.  NAANs are opaque strings of one or more "betanumeric"
   characters, specifically,

       0123456789bcdfghjkmnpqrstvwxz












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   which consists of digits and consonants, minus the letter 'l'.
   Restricting NAANs to betanumerics (alphanumerics without vowels or
   'l') serves two goals.  It reduces the chances that words -- past,
   present, and future -- will appear in NAANs and carry unintended
   semantics.  It also helps usability by not mixing commonly confused
   characters ('0' and 'O', '1' and 'l') and by being compatible with
   strong transcription error detection (eg, the [NOID] check digit
   algorithm).  Since 2001, every assigned NAAN has consisted of exactly
   five digits.

   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 an ARK
   namespace.

   There are currently four NAANs available to all organizations.  An
   ARK bearing one of these NAANs carries a specific, immutable meaning
   that recipients can rely on for long term pragmatic benefit as
   described below.

   +==========+================================+==========+============+
   |  Shared  |     The immutable purpose,     |  Expect  |   OK for   |
   |   NAAN   |   meaning, or connotation of   |    to    | long term  |
   | meaning  |    ARKs bearing this NAAN.     | resolve? | reference? |
   +==========+================================+==========+============+
   |  12345   |   Example ARKs appearing in    |  maybe   |     no     |
   | examples |   documentation.  They might   |          |            |
   |          |  resolve, but no link checker  |          |            |
   |          |   need be concerned if they    |          |            |
   |          |   don't.  They should not be   |          |            |
   |          |   considered viable for long   |          |            |
   |          |        term reference.         |          |            |
   +----------+--------------------------------+----------+------------+
   |  99152   |      ARKs for controlled       |   yes    |    yes     |
   |  terms   |    vocabulary and ontology     |          |            |
   |          |    terms, such as metadata     |          |            |
   |          |  element names and pick-list   |          |            |
   |          |  values.  They should resolve  |          |            |
   |          |  to term definitions and are   |          |            |
   |          |     suitable for long term     |          |            |
   |          |           reference.           |          |            |
   +----------+--------------------------------+----------+------------+
   |  99166   |  ARKs for people, groups, and  |   yes    |    yes     |
   |  agents  |    institutions as "agents"    |          |            |
   |          |   (actors, such as creators,   |          |            |
   |          |   contributors, publishers,    |          |            |
   |          |    performers, etc).  They     |          |            |
   |          |    should resolve to agent     |          |            |



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   |          |  definitions and are suitable  |          |            |
   |          |    for long term reference.    |          |            |
   +----------+--------------------------------+----------+------------+
   |  99999   |  ARKs for test, development,   |  maybe   |     no     |
   | test ids |   or experimental purposes,    |          |            |
   |          |  often at scale.  They might   |          |            |
   |          |  resolve, but no link checker  |          |            |
   |          |   need be concerned if they    |          |            |
   |          |   don't.  They should not be   |          |            |
   |          |   considered viable for long   |          |            |
   |          |        term reference.         |          |            |
   +----------+--------------------------------+----------+------------+

            Table 2: Four NAANs shared across all organizations

   To make use of a shared NAAN, an organization has several options
   described in Section 2.4.1.

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
   evolves.

   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
   (eg, betanumerics) 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.  The absence of recognizable words makes typos harder
   to detect in opaque strings, so a common mitigation is to add a check
   character.  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.










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2.4.1.  Optional: Shoulders

   Just as an ARK namespace is subdivided by NAANs reserved for NAAs, it
   is generally advantageous for an NAA to subdivide its own NAAN
   namespace into "shoulders", where each shoulder is reserved for an
   internal department or unit.  Like the NAAN, which is a string of
   characters that follows the "ark:" label, a shoulder is a string of
   characters (starting with a "/") that extends the NAAN.  The base
   compact name assigned by the NAA consists of the NAAN, the shoulder,
   a final string known as the "blade".  (The shoulder plus blade
   terminology mirrors locksmith jargon describing the information-
   bearing parts of a key.)

   The blade string is chosen by the NAA such that the string created by
   concatenating the NAAN plus shoulder plus blade becomes the unique
   base object name.  Otherwise the blade may come from any source, for
   example, it might come from a counter, a timestamp, a [NOID] minter,
   a legacy 100-year-old accession number, etc.  If there is a check
   digit, it is expected to appear at the end of the blade and to be
   computed over the base compact name, which is generally the most
   important part of an ARK to make opaque.  In particular, check digits
   are not expected to cover qualifiers, which often name subobjects of
   a persistent object that are less stable and less opaquely named than
   the parent object (for example, ten years hence, the object's
   thumbnail image will be of a higher resolution and the OCR text file
   will be re-derived with improved algorithms.

   It is important not to use any delimiter between the shoulder string
   and blade string, especially not a "/" since it declares an object
   boundary (see the section on ARKs that reveal object hierarchy).

     ark:12345/x5wf6789/c2/s4.pdf       # correct primordinal shoulder
     ark:12345/x5/wf6789/c2/s4.pdf      # INCORRECT
                 ^ WRONG

   This little bit of discretion shields organizations from end users
   making inferences about expected levels of support based on
   recognizable shoulders.  To help in-house ARK administrators reliably
   know where the shoulder ends, it is recommended to use the "first-
   digit convention" so that shoulders are "primordinal".  A primordinal
   shoulder is a sequence of one or more betanumeric characters ending
   in a digit, as shown above.  This means that the shoulder is all
   consonant letters (often just one) after the NAAN and "/" up to and
   including the first digit encountered after the NAAN.  One property
   of primordinal shoulders is that there is an infinite number of them
   possible under any NAAN.





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   To help manage each namespace into the future, NAAs are encouraged to
   create shoulders, even if there is only one to start with.  If an
   organization wishes to create a shoulder under one of shared NAANs
   (99999, 12345, 99152, or 99166, described in Table 2), it should fill
   out the Shoulder Request Form [shoulderrequest].

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 Name 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
   example,

       https://example.org/ark:12345/x54xz321/s3/f8.v05.tiff

   the string "/s3/f8" is a ComponentPath and the string ".v05.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
   service expectations and technology requirements.  NMA-qualified ARKs



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   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
   ARKs.

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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                       ark:12345/x54/xz/321

   is equivalent to publishing three ARKs,

                       ark:12345/x54/xz/321
                       ark:12345/x54/xz
                       ark:12345/x54

   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:

                       ark:12345/x54_xz_321
                       ark:12345/x54_xz
                       ark:12345/x54xz321
                       ark:12345/x54xz
                       ark:12345/x54

   Note that slashes around the ARK's NAAN (/12345/ 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.  This makes it
   possible for a system to present a collapsed view of a large number
   of search result items without having to issue database queries in
   order to retrieve and analyze the inter-relatedness among all of
   those items.





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   If the ARK contains an internal period after the Name, the piece to
   the left of the first such period is a root 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,

                       ark:12345/x54.24
                       ark:12345/x4z/x54.24
                       ark:12345/x54.v18.fr.odf

   There are two main rules.  First, if two ARKs share the same root
   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:

                       ark:12345/x54.v18.fr.odf
                       ark:12345/x54.321xz
                       ark:12345/x54.44

   Second, publishing an ARK with a suffix implies the existence of at
   least one variant identified by the ARK without its suffix.  The ARK
   is otherwise silent about what additional variants might exist.  So
   publishing the ARK,

                       ark:12345/x54.v18.fr.odf

   is equivalent to publishing the four ARKs,

                       ark:12345/x54.v18.fr.odf
                       ark:12345/x54.v18.fr
                       ark:12345/x54.v18
                       ark:12345/x54

   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.






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2.6.  Character Repertoires

   The Name and Qualifier parts are strings of visible ASCII characters.
   For received ARKs, implementations must support a minimum length of
   255 octets for the string composed of the Base Name plus Qualifier.
   Implementations generating strings exceeding this length should
   understand that receiving implementations may not be able to index
   such ARKs properly.  Characters may be letters, digits, or any of
   these seven characters:

       =   ~   *   +   @   _   $

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

       %   -   .   /

   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
   described.

   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 NMA, hyphens are "identity inert" in comparing ARKs for
   equivalence.  For example, the following ARKs are equivalent for
   purposes of comparison and ARK service access:

                               ark:12345/x5-4-xz-321
      https://sneezy.dopey.com/ark:12345/x54--xz32-1
                               ark:12345/x54xz321

   The `%' character is reserved for %-encoding all other octets that
   would appear in the ARK string, in the same manner as for URIs
   [RFC3986].  A %-encoded octet consists of a `%' followed by two
   uppercase hex digits; for example, "%7D" stands in for `}'.
   Uppercase hex digits are preferred for compatibility with URI
   encoding conventions, especially useful when URL-based ARKs are
   compared for equivalence by ARK-unaware software systems; thus 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 [RFC2288].




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   Implementors should be prepared to normalize some common invalid
   characters that may be found in ARKs copy pasted from processed text.
   For example, when pasting an ARK that was broken during line
   wrapping, a user may inadvertently introduce newlines and spaces that
   should be removed, or a variety of dash-like (eg, en dash) Unicode
   characters that should be removed or converted to hyphens.

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
   NMAs), 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 NMA), the
   NMA never participates in such comparisons.  Normalization described
   here serves to define lexical equivalence but does not restrict how
   implementors normalize ARKs locally for storage.

   Normalization of a received ARK for the purpose of octet-by-octet
   equality comparison with another ARK consists of the following steps.

   1.   The NMA part (eg, everything from an initial "https://" up to
        the next slash), if present is removed.

   2.   Any URI query string is removed (everything from the first
        literal '?' to the end of the string).

   3.   The first case-insensitive match on "ark:/" or "ark:" is
        converted to "ark:" (replacing any uppercase letters and
        removing any terminal '/').

   4.   Any uppercase letters in the NAAN are converted to lowercase.

   5.   In the string that remains, the two characters following every
        occurrence of `%' are converted to lowercase.  The case of all
        other letters in the ARK string must be preserved.

   6.   All hyphens and common dash-like characters (eg, U+2010 to
        U+2015) are removed.

   7.   If normalization is being done as part of a resolution step, and
        if the end of the remaining string matches a known inflection,
        the inflection is noted and removed.







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   8.   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.

   9.   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 resulting ARK string is now normalized.  Comparisons between
   normalized ARKs are case-sensitive, meaning that uppercase letters
   are considered different from their lowercase 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.

2.8.  Resolver Chains

   Resolution is a computation, often multi-stage, that maps a client
   identifier to a response.  The response may be any "thing", such as a
   spreadsheet, a landing page, a metadata record, or a 404 Not Found.
   A single-stage retrieval of a web page is a resolution.  More
   interesting kinds of resolution involve forwarding (indirection) and/
   or proxying.

   On the web, forwarding is done with HTTP redirects.  In general ARK
   resolution on the web involves a chain of one or more redirects that
   ends with the web server, known as the Responder, that responds
   without redirecting.  The Responder might be a proxy and itself
   intiate a sub-resolution request chain unbeknownst to the original
   client, but that is out of scope here.  An ARK might have a Resource
   Responder that is a different host from the Metadata Responder.  The
   client starts resolution by contacting the NMA (server host) found in
   the original Mapping ARK URL.  This is known as the First Resolver.












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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
   persistence.

3.1.  ARKS and Usability

   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.

   Any measure that reduces user irritation with an identifier will
   increase its chances of survival.  This explains 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 (and their Unicode
   equivalents) as insignificant reduces the possibility of users
   transcribing identifiers that will have been broken through
   unpredictable hyphenation by word processors.

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



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   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
   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 NMAs.

   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



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   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, "handle.net".

   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 group of like-minded organizations
   with the purpose of providing mutual preservation support.  The first
   goal of such a group 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 group 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
   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://" or "https://" 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 NMAs (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



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   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:12345/" are under control of the NAA registered under 12345,
   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 [RFC2611].

   Organizations can request or update a NAAN by filling out the NAAN
   Request Form [NAANrequest].

3.6.  NAAN Namespace Management

   Every NAA should have a namespace management strategy.  A classic
   hierarchical approach is to partition a NAAN namespace into
   subnamespaces known as "shoulders".  As explained in Section 2.4.1,
   each shoulder is a unique prefix that guarantees non-collision of
   names in different partitions.  This practice is strongly encouraged
   for all NAAs, especially when subnamespace management and assignment
   streams will be delegated to departments, units, or projects within
   an organization.  For example, with a NAAN that is assigned to a
   university and managed by its main library, the library should take
   care to reserve shoulders (semantically opaque shoulders being
   preferred) for distinct assignment streams.  Prefix-based partition
   management is typically an important responsibility of the NAA.

   This shoulder delegation approach plays out differently in two real-
   world examples: DNS names and ISBN identifiers.  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



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   disclose where the prefix ends and the publisher's assigned name
   begins.  This practice of non-disclosure, found in 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.

   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
   shoulder such as "fk9..." 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
   identifier.

   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
   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
   errors.  If used, it is recommended that the check character be
   appended to the original Base Compact Name string (ie, minus the
   check character), that original string having been the basis for
   computing the check character.

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).





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4.  Finding a Name Mapping Authority

   In order to derive an actionable identifier (these days, a URL) from
   an ARK, a 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 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 NMA part (the host part of the NMA's ARK service).
   If it contains an NMA that is working, this NMA discovery step may be
   skipped; the NMA effectively uses the beginning of an ARK to cache
   the results of a prior mapping authority discovery process.  If a new
   NMA 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 NMAs that service ARKs
   issued by the identified NAA.

   The global database is key, and ideally the lookup would be automatic
   and transparent to the user.  For this, the current most promising
   method is the Name-to-Thing (N2T) Resolver [N2T] at n2t.net.  It is a
   reliable, low-cost NMA supported by the ARK Alliance that primarily
   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.

   There are two other ways to discover an NMA, one of them described in
   a subsection below.  Another way, described in an appendix, is based
   on a simplification of the URN resolver discovery method, itself very
   similar in principle to the resolver discovery method used by Handles
   and DOIs.  None of these methods does more than what can be done with
   a very small, consortially maintained web server such as [N2T].

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






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   At the time of issuance, at least one NMA 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 NMAs in a Globally Accessible File

   This subsection describes a way to look up NMAs 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 a number of mirror sites
   [NAANregistry].  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.




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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
   technology.  Note that some services may be invoked in one operation,
   such as when an '?info' inflection returns both a description and a
   permanence declaration for an object.

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.












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5.1.1.  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 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
      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 persistence statements, conceived independently from
   ARKs, can be found at [PStatements], with ongoing work available at
   [ARKspecs].  An older approach to a permanence rating framework is
   given in [NLMPerm], which identified 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
      replaced.

      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.





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      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 change.

   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 is the NMA that 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,
   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.










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5.1.2.  Generic Description Service

   Returns a description of the object.  Descriptions are returned in a
   structured metadata format, a human-readable text format, or in one
   format that serves both purposes (such as human-readable HTML with
   embedded machine-readable metadata, or perhaps YAML).  A description
   must at a minimum answer the who, what, when, and where questions
   ("where" being the long-term identifier as opposed to a transient
   redirect target) 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.

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

   The HTTP URL Mapping Protocol (THUMP) is a way of taking a key (any
   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 via keyboard-based interactions [RFC0854].

   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
   NMA 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
   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 two most common requests correspond to two degenerate
   special cases.  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
   document.





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

           n2t.net/ark:67531/metadc107835?info

   into the browser's location field directly precipitates a request for
   a metadata record describing the object identified by ark:67531/
   metadc107835.  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 (text/
   html).  In addition to '?info', this specification reserves both '?'
   and '??' (originally older forms) for future use.

   The following example THUMP session assumes metadata being returned
   by a resolver (as server) to a browser client.  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 (4-8),
   and the body of the server's response (9-18).  The first and last
   lines (1 and 19) correspond to the client's steps to start the TCP
   session and the server's steps to end it, respectively.

    1  C: [opens session]
       C: GET https://n2t.net/ark:67531/metadc107835?info HTTP/1.1
       C:
       S: HTTP/1.1 200 OK
    5  S: Content-Type: text/plain
       S: THUMP-Status: 0.6 200 OK
       S: Link: </ark:67531/metadc107835> rel="describes";
       S:
       S: erc:
   10  S: who:   Austin, Larry
       S: what:  A Study of Rhythm in Bach's Orgelbüchlein
       S: when:  1952
       S: where: https://digital.library.unt.edu/ark:/67531/metadc107835
       S: erc-support:
   15  S: who:   University of North Texas Libraries
       S: what:  Permanent: Stable Content:
       S: when:  20081203
       S: where: https://digital.library.unt.edu/ark:/67531/
       S: [closes session]







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   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 final header line (7)
   asserts, for the benefit of recipients unfamiliar with ARK
   inflections, that the response describes the uninflected ARK.

   The balance of the response consists of a single metadata record
   (9-18) that comprises the ARK description service response.  The
   returned record is in the format of an Electronic Resource Citation
   [ERC], which is discussed in overview 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 it was
   created, and where the expression may be found (note that "where" is
   preferably a persistent, citable identifier rather than an unstable
   URL sometimes mistakenly referred to as a "location").  This quartet
   of elements comes up again and again in ERCs.  Lines 13-17 contain a
   minimal persistence statement.

   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
   found.

5.3.  The Electronic Resource Citation (ERC)

   An Electronic Resource Citation (or ERC, pronounced e-r-c) [ERC] is a
   kind of object description that uses Dublin Core Kernel metadata
   elements [DCKernel].  The ERC with Kernel elements provides a simple,
   compact, and printable record for holding data associated with an
   information resource.  As originally designed [Kernel], Kernel
   metadata balances the needs for expressive power, very simple machine
   processing, and direct human manipulation.  The ERC sense of
   "citation" is not limited to the traditional referencing of a result
   or information fixed in time on a printed page, but to a more general
   kind of reference, both backward, to digital material that cannot be
   known to be fixed in time (true of virtually all online information),
   and forward, to material that is all the more valuable for improving
   or evolving over time.

   The previous section shows two limited examples of what is fully
   described elsewhere [ERC].  The rest of this short section provides
   some of the background and rationale for this record format.




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   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
   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.

   Borrowing from the data structuring format that underlies the
   successful spread of email and web services, the ERC format uses
   [ANVL], which is based on email and HTTP headers [RFC2822].  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.

   While ANVL elements are expected at the top level and don't
   themselves support hierarchy, the value of an ANVL element may be an
   arbitrary encoded hierarchy of JSON or XML.  Typically, the name of
   such an ANVL element ends in "json" or "xml", for example, "json" or
   "geojson".  Care should be taken to escape structural characters that
   appear in element names and values, specifically, line terminators
   (both newlines ("\n") and carriage returns ("\r")) and, in element
   names, colons (":").

   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 [RFC5013] 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 locator (a means to access it), 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



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   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.

   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.

5.4.  Advice to Web Clients

   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) are currently used, and inside some of
   those URLs are embedded URNs, Handles, and DOIs.  Unfortunately,
   there's no suggestion of a way to probe for extra services that would
   build confidence in those identifiers; in other words, there's no way
   to tell whether any of those identifiers is any better managed than
   the average URL.

   ARKs are also envisaged to appear in hypertext links (where they are
   not normally shown to users) and in rendered text (displayed or
   printed).  A normal HTML link for which the URL is not displayed
   looks like this.

   <a href = "https://example.org/index.htm"> Click Here <a>

   A URL with an embedded ARK invites access (via `?info') to extra
   services:

   <a href = "https://example.org/ark:14697/b12345x"> Click Here <a>

   Using the [N2T] resolver to provide identifier-scheme-agnostic
   protection against hostname instability, this ARK could be published
   as:

   <a href = "https://n2t.net/ark:14697/b12345x"> Click Here <a>







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   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.

5.5.  Enhancements and Related Specifications

   ARK services, data models, inflections, and applications continue to
   evolve.  Follow-on developments and specifications will be made
   available from the ARK Maintenance Agency [ARKspecs].

5.6.  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 host (NMA) 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.

6.  References

   [ANVL]     Kunze, J., Kahle, B., Masanes, J., and G. Mohr, "A Name-
              Value Language", 2005,
              <https://n2t.net/ark:/13030/c7x921j3h>.

   [ARK]      Kunze, J., "Towards Electronic Persistence Using ARK
              Identifiers", IWAW/ECDL Annual Workshop Proceedings 3rd,
              August 2003, <https://n2t.net/ark:/13030/c7n00zt1z>.

   [ARKagency]
              Alliance, A., "ARK Maintenance Agency", 2021,
              <https://arks.org>.

   [ARKspecs] Alliance, A., "ARK Maintenance Agency Specifications",
              2021, <https://arks.org/specs/>.




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   [DCKernel] Initiative, D. C. M., "Kernel Metadata Working Group",
              2001-2008, <https://dublincore.org/groups/kernel/>.

   [DOI]      Foundation, I. D., "The Digital Object Identifier (DOI)
              System", February 2001, <https://dx.doi.org/10.1000/203>.

   [ERC]      Kunze, J. and A. Turner, "Kernel Metadata and Electronic
              Resource Citations", October 2007,
              <https://n2t.net/ark:/13030/c7sn0141m>.

   [Handle]   Lannom, L., "Handle System Overview", ICSTI Forum No. 30,
              April 1999, <https://eric.ed.gov/?id=ED450775>.

   [Kernel]   Kunze, J., "A Metadata Kernel for Electronic Permanence",
              Journal of Digital Information Vol 2, Issue 2,
              ISSN 1368-7506, January 2002,
              <https://n2t.net/ark:/13030/c7rr1pm49>.

   [N2T]      Alliance, A., "Name-to-Thing Resolver", August 2006,
              <https://n2t.net>.

   [NAANregistry]
              ARKs.org, "NAAN Registry", 2019,
              <https://arks.org/e/pub/naan_registry.txt>.

   [NAANrequest]
              ARKs.org, "NAAN Request Form", 2018,
              <https://n2t.net/e/naan_request>.

   [NLMPerm]  Byrnes, M., "Permanence Levels and the Archives for NLM's
              Permanent Web Documents", March 2005,
              <https://www.nlm.nih.gov/pubs/techbull/ma05/
              ma05_archive.html>.

   [NOID]     Kunze, J., "Nice Opaque Identifiers", April 2006,
              <https://metacpan.org/pod/distribution/Noid/noid>.

   [PStatements]
              Kunze, J., "Persistence statements: describing digital
              stickiness", October 2016,
              <https://n2t.net/ark:/13030/c7833mx7t>.

   [PURL]     Shafer, K., "Introduction to Persistent Uniform Resource
              Locators", 1996,
              <https://www.internetsociety.org/inet96/proceedings/a4/
              a4_1.htm>.





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   [shoulderrequest]
              ARKs.org, "Shoulder Request Form", 2021,
              <https://n2t.net/e/shoulder_request>.

   [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, RFC 854, DOI 10.17487/RFC0854, May
              1983, <https://www.rfc-editor.org/info/rfc854>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC2141]  Moats, R., "URN Syntax", RFC 2141, DOI 10.17487/RFC2141,
              May 1997, <https://www.rfc-editor.org/info/rfc2141>.

   [RFC2288]  Lynch, C., Preston, C., and R. Daniel, "Using Existing
              Bibliographic Identifiers as Uniform Resource Names",
              RFC 2288, DOI 10.17487/RFC2288, February 1998,
              <https://www.rfc-editor.org/info/rfc2288>.

   [RFC2611]  Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
              "URN Namespace Definition Mechanisms", BCP 33, RFC 2611,
              DOI 10.17487/RFC2611, June 1999,
              <https://www.rfc-editor.org/info/rfc2611>.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,
              <https://www.rfc-editor.org/info/rfc2616>.

   [RFC2822]  Resnick, P., Ed., "Internet Message Format", RFC 2822,
              DOI 10.17487/RFC2822, April 2001,
              <https://www.rfc-editor.org/info/rfc2822>.

   [RFC2915]  Mealling, M. and R. Daniel, "The Naming Authority Pointer
              (NAPTR) DNS Resource Record", RFC 2915,
              DOI 10.17487/RFC2915, September 2000,
              <https://www.rfc-editor.org/info/rfc2915>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5013]  Kunze, J. and T. Baker, "The Dublin Core Metadata Element
              Set", RFC 5013, DOI 10.17487/RFC5013, August 2007,
              <https://www.rfc-editor.org/info/rfc5013>.



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   [THUMP]    Gamiel, K. and J. Kunze, "The HTTP URL Mapping Protocol",
              August 2007, <https://www.ietf.org/archive/id/draft-kunze-
              thump-03.txt>.

Appendix A.  ARK Maintenance Agency: arks.org

   The ARK Maintenance Agency [ARKagency] at arks.org has several
   functions.

   *  To manage the registry of organizations that will be assigning
      ARKs.  Organizations can request or update a NAAN by filling out
      the NAAN Request Form [NAANrequest].

   *  To be a clearinghouse for information about ARKs, such as best
      practices, introductory documentation, tutorials, community
      forums, etc.  These supplemental resources help ARK implementor in
      high-level applications across different sectors and disciplines,
      and with a variety of metadata standards.

   *  To be a locus of discussion about future versions of the ARK
      specification.

Appendix B.  Looking up NMAs Distributed via DNS

   This subsection introduces an older method for looking up NMAs that
   is based on the method for discovering URN resolvers described in
   [RFC2915].  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 NMAs.

   In its full generality the [RFC2915] algorithm ambitiously
   accommodates a complex set of preferences, orderings, protocols,
   mapping services, regular expression rewriting rules, and DNS record
   types.  This 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.






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     12026.ark.arpa.
     ;; US Library of Congress
     ;;       order pref flags service regexp  replacement
      IN NAPTR  0     0   "h"  "ark"   "USLC"  lhc.nlm.nih.gov:8080
      IN NAPTR  0     0   "h"  "ark"   "USLC"  foobar.zaf.org
      IN NAPTR  0     0   "h"  "ark"   "USLC"  sneezy.dopey.com

   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
   understood.

   When a Maptr query returns a record with a flags field of "h" (for
   host, a Maptr extension to the NAPTR flags), the replacement field
   contains the NMA (host) 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.

     12345.ark.arpa.
     ;; Digital Library Consortium
     ;;       order pref flags service regexp replacement
      IN NAPTR  0     0    ""  "ark"     ""   dlc.spct.org.

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

   1.  Initialize the DNS query: type=NAPTR, query=<NAAN>.ark.arpa.

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

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

   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).



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

   A Perl script that implements this algorithm is included here.

   #!/depot/bin/perl

   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("$naa.ark.arpa");                # call maptr - that's it

   sub maptr {                             # recursive maptr algorithm
           my $dname = shift;              # domain name as argument
           my ($rr, $order, $pref, $flags, $service, $regexp,
                   $replacement);
           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 NMA
                   }
           }
   }

   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.

Authors' Addresses

   John A. Kunze
   California Digital Library
   1111 Franklin Street
   Oakland, CA 94607
   United States of America




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   Email: jak@ucop.edu


   Emmanuelle Bermès
   Bibliothèque nationale de France
   Quai François Mauriac
   75706 Paris
   France

   Email: emmanuelle.bermes@bnf.fr









































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