Network Working Group                                           J. Kunze
Internet-Draft                                California Digital Library
Expires: October 7, 2013                                      R. Rodgers
                                            University of California San
                                                           April 5, 2013

                       The ARK Identifier Scheme


   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:


   an optional and mutable Name Mapping Authority Hostport (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 a single question mark (`?'),
   returns a brief metadata record that is both human- and machine-
   readable.  When the ARK is inflected by appending dual question marks
   (`??'), the returned metadata contains 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

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   working documents as Internet-Drafts.  The list of current Internet-
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   This Internet-Draft will expire on October 7, 2013.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Reasons to Use ARKs  . . . . . . . . . . . . . . . . . . .  5
     1.2.  Three Requirements of ARKs . . . . . . . . . . . . . . . .  6
     1.3.  Organizing Support for ARKs:  Our Stuff vs. Their Stuff  .  7
     1.4.  Definition of Identifier . . . . . . . . . . . . . . . . .  8
   2.  ARK Anatomy  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     2.1.  The Name Mapping Authority Hostport (NMAH) . . . . . . . . 10
     2.2.  The ARK Label Part (ark:/) . . . . . . . . . . . . . . . . 11
     2.3.  The Name Assigning Authority Number (NAAN) . . . . . . . . 12
     2.4.  The Name Part  . . . . . . . . . . . . . . . . . . . . . . 13
     2.5.  The Qualifier Part . . . . . . . . . . . . . . . . . . . . 13
       2.5.1.  ARKs that Reveal Object Hierarchy  . . . . . . . . . . 15
       2.5.2.  ARKs that Reveal Object Variants . . . . . . . . . . . 16
     2.6.  Character Repertoires  . . . . . . . . . . . . . . . . . . 17
     2.7.  Normalization and Lexical Equivalence  . . . . . . . . . . 18
   3.  Naming Considerations  . . . . . . . . . . . . . . . . . . . . 20
     3.1.  ARKS Embedded in Language  . . . . . . . . . . . . . . . . 20
     3.2.  Objects Should Wear Their Identifiers  . . . . . . . . . . 20
     3.3.  Names are Political, not Technological . . . . . . . . . . 21
     3.4.  Choosing a Hostname or NMA . . . . . . . . . . . . . . . . 21
     3.5.  Assigners of ARKs  . . . . . . . . . . . . . . . . . . . . 23
     3.6.  NAAN Namespace Management  . . . . . . . . . . . . . . . . 23
     3.7.  Sub-Object Naming  . . . . . . . . . . . . . . . . . . . . 25
   4.  Finding a Name Mapping Authority . . . . . . . . . . . . . . . 26
     4.1.  Looking Up NMAHs in a Globally Accessible File . . . . . . 27
   5.  Generic ARK Service Definition . . . . . . . . . . . . . . . . 29
     5.1.  Generic ARK Access Service (access, location)  . . . . . . 29
       5.1.1.  Generic Policy Service (permanence, naming, etc.)  . . 29
       5.1.2.  Generic Description Service  . . . . . . . . . . . . . 31
     5.2.  Overview of The HTTP URL Mapping Protocol (THUMP)  . . . . 31
     5.3.  The Electronic Resource Citation (ERC) . . . . . . . . . . 34
     5.4.  Advice to Web Clients  . . . . . . . . . . . . . . . . . . 36
     5.5.  Security Considerations  . . . . . . . . . . . . . . . . . 37
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
   Appendix A.  ARK Maintenance Agency  . . . . . . . . . . . . . . . 40
   Appendix B.  Looking up NMAHs Distributed via DNS  . . . . . . . . 41
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 44

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1.  Introduction

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

   Schemes for persistent identification of network-accessible objects
   are not new.  In the early 1990's, the design of the Uniform Resource
   Name [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.

   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

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   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

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 two extra services can be probed
   automatically by appending `?' and `??' to the ARK.

   The form of the ARK also supports the natural separation of naming
   authorities into the original name assigning authority and the
   diverse multiple name mapping (or servicing) authorities that in
   succession and in parallel will take over custodial responsibilities
   from the original assigner (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

   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

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   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
   itself might not include a matching identifier or might not carry
   evidence obvious enough to reveal its identity without significant
   research.  In either case, what is called for is a record bearing
   witness to the identifier's association with the object, as supported
   by a recorded set of object characteristics.  This descriptive record
   is partly an identification "receipt" with which users and archivists
   can verify an object's identity after brief inspection and a
   plausible match with recorded characteristics such as title and size.

   The final requirement of an ARK is to give users a link to the object
   itself (or to a copy) if at all possible.  Persistent access is the
   central duty of an ARK.  Persistent identification plays a vital
   supporting role but, strictly speaking, it can be construed as no
   more than a record attesting to the original assignment of a never-
   reassigned identifier.  Object access may not be feasible for various
   reasons, such as 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

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   simplify the persistence problem by decoupling access from
   identification and concentrating exclusively on the latter are of
   questionable utility.  A perfect system for assigning forever unique
   identifiers might be created, but if it did so without reducing
   access failure rates, no one would be interested.  The central issue
   -- which may be summed up as the "HTTP 404 Not Found" problem --
   would not have been addressed.

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

   An organization and the user community it serves can often be seen to
   struggle with two different areas of persistent identification: the
   Our Stuff problem and the Their Stuff problem.  In the Our Stuff
   problem, we in the organization want our own objects to acquire
   persistent names.  Since we possess or control these objects, our
   organization tackles the Our Stuff problem directly.  Whether or not
   the objects are named by ARKs, our organization is the responsible
   party, so it can plan for, maintain, and make commitments about the

   In the Their Stuff problem, we in the organization want others'
   objects to acquire persistent names.  These are objects that we do
   not own or control, but some of which are critically important to us.
   But because they are beyond our influence as far as support is
   concerned, creating and maintaining persistent identifiers for Their
   Stuff is not especially purposeful or feasible for us to 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

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

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

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   object is large, unwieldy, or unavailable (such as when licensing
   restrictions are in effect), a metadata record that includes the
   identifier string will usually suffice.  That record becomes a
   conveniently manipulable object surrogate, acting as both an
   association "receipt" and "declaration".

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

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2.  ARK Anatomy

   An ARK is represented by a sequence of characters (a string) that
   contains the label, "ark:", optionally preceded by the beginning part
   of a URL.  Here is a diagrammed example.
       \________________/ \__/ \___/ \______/ \____________/
         (replaceable)     |     |      |       Qualifier
              |       ARK Label  |      |    (NMA-supported)
              |                  |      |
    Name Mapping Authority       |    Name (NAA-assigned)
       Hostport (NMAH)           |
                        Name Assigning Authority Number (NAAN)

   The ARK syntax can be summarized,


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

2.1.  The Name Mapping Authority Hostport (NMAH)

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


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

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

   This replacement relies on a mapping authority "resolver" discovery
   process, of which two alternate methods are outlined in a later
   section.  The ARK, URN, Handle, and DOI schemes all use a resolver
   discovery model that sooner or later requires matching the original
   assigning authority with a current provider servicing that
   authority's named objects; once found, the resolver at that provider
   performs what amounts to a redirect to a place where the object is
   currently held.  All the schemes rely on the ongoing functionality of
   currently mainstream technologies such as the Domain Name System
   [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 "http://" and an NMAH to an ARK is a way
   of creating an actionable identifier by a method that is itself
   temporary.  Assuming that infrastructure supporting [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

2.2.  The ARK Label Part (ark:/)

   The label part distinguishes an ARK from an ordinary identifier.  In
   a URL found in the wild, the string, "ark:/", indicates that the URL
   stands a reasonable chance of being an ARK.  If the context warrants,

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   verification that it actually is an ARK can be done by testing it for
   existence of the three ARK services.

   Since nothing about an identifier syntax directly affects
   persistence, the "ark:" label (like "urn:", "doi:", and "hdl:")
   cannot tell you whether the identifier is persistent or whether the
   object is available.  It does tell you that the original Name
   Assigning Authority (NAA) had some sort of hopes for it, but it
   doesn't tell you whether that NAA is still in existence, or whether a
   decade ago it ceased to have any responsibility for providing
   persistence, or whether it ever had any responsibility beyond naming.

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

2.3.  The Name Assigning Authority Number (NAAN)

   Recalling that the general form of the ARK is,


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

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

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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

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

2.5.  The Qualifier Part

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

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

   the string "/s3/f8" is a ComponentPath and the string ".05v.tiff" is
   a VariantPath.  The ARK Qualifier is a formalization of some
   currently mainstream URL syntax conventions.  This formalization
   specifically reserves meanings that permit recipients to make strong
   inferences about logical sub-object containment and equivalence based
   only on the form of the received identifiers; there is great
   efficiency in not having to inspect metadata records to discover such

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   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
   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

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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,


   is equivalent to publishing three ARKs,


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

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


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

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2.5.2.  ARKs that Reveal Object Variants

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

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



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


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


   is equivalent to publishing the four ARKs,


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

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

2.6.  Character Repertoires

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

       =   #   *   +   @   _   $

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

       %   -   .   /

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

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


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   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 hex
   digits; for example, "%7d" stands in for `}'.  Lower case hex digits
   are preferred to reduce the chances of false acronym recognition;
   thus it is better to use "%acT" instead of "%ACT".  The character `%'
   itself must be represented using "%25".  As with URNs, %-encoding
   permits ARKs to support legacy namespaces (e.g., ISBN, ISSN, SICI)
   that have less restricted character repertoires [RFC2288].

2.7.  Normalization and Lexical Equivalence

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

   Normalization of an ARK for the purpose of octet-by-octet equality
   comparison with another ARK consists of four steps.  First, any upper
   case letters in the "ark:" label and the two characters following a
   `%' are converted to lower case.  The case of all other letters in
   the ARK string must be preserved.  Second, any NMAH part is removed
   (everything from an initial "http://" up to the next slash) and all
   hyphens are removed.

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

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

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

   To keep ARK string variation to a minimum, no reserved ARK characters
   should be %-encoded unless it is deliberately to conceal their

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   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.

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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

3.1.  ARKS Embedded in Language

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

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

3.2.  Objects Should Wear Their Identifiers

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

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3.3.  Names are Political, not Technological

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

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

3.4.  Choosing a Hostname or NMA

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

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

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

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

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

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3.5.  Assigners of ARKs

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

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

   The ARK namespace reserved for an NAA is the set of names bearing its
   particular NAAN.  For example, all strings beginning with "ark:/
   12025/" are under control of the NAA registered under 12025, which
   might be the National Library of Finland.  Because each NAA has a
   different NAAN, names from one namespace cannot conflict with those
   from another.  Each NAA is free to assign names from its namespace
   (or delegate assignment) according to its own policies.  These
   policies must be documented in a manner similar to the declarations
   required for URN Namespace registration [RFC2611].

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

3.6.  NAAN Namespace Management

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

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   prefixes that will set aside large parts of the unused namespace for
   future assignments.  Prefix-based partition management is an
   important responsibility of the NAA.

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

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


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

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

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

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

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

   The global database is key, and ideally the lookup would be automatic
   and transparent to the user.  For this, the most promising method is
   probably the Name-to-Thing (N2T) Resolver [N2T] at  It is a
   proposed low-cost, highly reliable, consortially maintained NMAH that
   simply exists to support actionable HTTP-based URLs for as long as
   HTTP is used.  One of its big advantages over the other two methods
   and the URN, Handle, DOI, and PURL methods, is that N2T addresses the
   namespace splitting problem.  When objects maintained by one NMA are
   inherited by more than one successor NMA, until now one of those
   successors would be required to maintain forwarding tables on behalf
   of the other successors.

   There are two other ways to discover an NMAH, 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 NMAH.

   At the time of issuance, at least one NMAH for an ARK should be

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

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

4.1.  Looking Up NMAHs in a Globally Accessible File

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

   The name authority table file is updated on an ongoing basis and is
   available for copying over the internet from the California Digital
   Library at and from a
   number of mirror sites.  The file contains comment lines (lines that
   begin with `#') explaining the format and giving the file's
   modification time, reloading address, and NAA registration
   instructions.  There is even a Perl script that processes the file

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   embedded in the file's comments.  The currently registered Name
   Assigning Authorities are:

   12025   National Library of Medicine
   12026   Library of Congress
   12027   National Agriculture Library
   13030   California Digital Library
   13038   World Intellectual Property Organization
   20775   University of California San Diego
   29114   University of California San Francisco
   28722   University of California Berkeley
   21198   University of California Los Angeles
   15230   Rutgers University
   13960   Internet Archive
   64269   Digital Curation Centre
   62624   New York University
   67531   University of North Texas
   27927   Ithaka Electronic-Archiving Initiative
   12148   Bibliotheque nationale de France
              / National Library of France
   78319   Google
   88435   Princeton University
   78428   University of Washington
   89901   Archives of the Region of Vaestra Goetaland
              and City of Gothenburg, Sweden
   80444   Northwest Digital Archives
   25593   Emory University
   25031   University of Kansas
   17101   Centre for Ecology & Hydrology, UK
   65323   University of Calgary
   61001   University of Chicago
   52327   Bibliotheque et Archives Nationales du Quebec
              / National Libary and Archives of Quebec
   39331   National Szechenyi Library / National Library of Hungary
   26677   Library and Archives Canada / Bibliotheque et Archives Canada

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5.  Generic ARK Service Definition

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

5.1.  Generic ARK Access Service (access, location)

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

   o  a table of contents instead of a large complex document,

   o  a home page instead of an entire web site hierarchy,

   o  a rights clearance challenge before accessing protected data,

   o  directions for access to an offline object (e.g., a book),

   o  a description of an intangible object (a disease, an event), or

   o  an applet acting as "player" for a large multimedia object.

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

5.1.1.  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 should be covered: object
   permanence, object naming, object fragment addressing, and
   operational service support.

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

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

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

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

      Permanent: Unchanging Content: A commitment has been made to keep
      this resource permanently available.  Its identifier will always
      provide access to the resource.  Its content will not 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 the NMA who responds to requests for policy
   statements, it is useful for the NMA to be able to produce or
   summarize these historical NAA documents.  Naming policy may include
   the following aspects.

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      (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.

5.1.2.  Generic Description Service

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

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

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   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
   NMAH that is obtained by inspection and possibly lookup based on the
   ARK's NAAN.  The asker then sets up an HTTP session with the server
   system, sends a question via a THUMP request (contained within an
   HTTP request), receives an answer via a THUMP response (contained
   within an HTTP response), and closes the session.  That concludes the
   connected portion of the protocol.

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

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


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

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

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   the server's response (8-13).  The first and last lines (1 and 13)
   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 HTTP/1.1
       S: HTTP/1.1 200 OK
    5  S: Content-Type: text/plain
       S: THUMP-Status: 0.6 200 OK
       S: erc:
       S: who:    Lederberg, Joshua
   10  S: what:   Studies of Human Families for Genetic Linkage
       S: when:   1974
       S: where:
       S: [closes session]

   The first two server response lines (4-5) above are typical of HTTP.
   The next line (6) is peculiar to THUMP, and indicates the THUMP
   version and a normal return status.

   The balance of the response consists of a single metadata record
   (8-12) 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 is was
   created, and where the expression may be found.  This quartet of
   elements comes up again and again in ERCs.

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

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    1  C: [opens session]
       C: GET HTTP/1.1
       S: HTTP/1.1 200 OK
    5  S: Content-Type: text/plain
       S: THUMP-Status: 0.6 200 OK
       S: erc:
       S: who:    Lederberg, Joshua
   10  S: what:   Studies of Human Families for Genetic Linkage
       S: when:   1974
       S: where:
       S: erc-support:
       S: who:    USNLM
   15  S: what:   Permanent, Unchanging Content
       S: when:   20010421
       S: where:
       S: [closes session]

   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

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 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.

   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

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   specialized software tools.  The basic ERC needs to be succinct,
   transparent, and trivially parseable by software.

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

   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 location, some responsible person or party, and a
   date.  It doesn't matter what type of object it is, or whether one
   plans to read it, interact with it, smoke it, wear it, or navigate
   it.  Of course, this approach is flawed because uniformity of
   description for some object types requires more semantic contortion
   and sacrifice than for others.  That is why at the beginning of this
   document, the ARK was said to be suited to objects that accommodate
   reasonably regular electronic description.

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

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 = ""> Click Here <a>

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

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

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

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

   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.

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5.5.  Security Considerations

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

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

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6.  References

   [ANVL]     Kunze, J. and B. Kahle, "A Name-Value Language", 2008,

   [ARK]      Kunze, J., "Towards Electronic Persistence Using ARK
              Identifiers", IWAW/ECDL Annual Workshop Proceedings 3rd,
              August 2003,

              DCMI, "Kernel Metadata Working Group", 2001-2008,

   [DOI]      IDF, "The Digital Object Identifier (DOI) System",
              February 2001, <>.

   [ERC]      Kunze, J. and A. Turner, "Kernel Metadata and Electronic
              Resource Citations", October 2007,

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

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

   [N2T]      CDL, "Name-to-Thing Resolver", August 2006,

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

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

   [PURL]     Shafer, K., "Introduction to Persistent Uniform Resource
              Locators", 1996, <>.

   [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, RFC 854, May 1983.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

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   [RFC2141]  Moats, R., "URN Syntax", RFC 2141, May 1997.

   [RFC2288]  Lynch, C., Preston, C., and R. Jr, "Using Existing
              Bibliographic Identifiers as Uniform Resource Names",
              RFC 2288, February 1998.

   [RFC2611]  Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
              "URN Namespace Definition Mechanisms", BCP 33, RFC 2611,
              June 1999.

   [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, June 1999.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

   [RFC2915]  Mealling, M. and R. Daniel, "The Naming Authority Pointer
              (NAPTR) DNS Resource Record", RFC 2915, September 2000.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5013]  Kunze, J. and T. Baker, "The Dublin Core Metadata Element
              Set", RFC 5013, August 2007.

   [THUMP]    Gamiel, K. and J. Kunze, "The HTTP URL Mapping Protocol",
              August 2007,

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Appendix A.  ARK Maintenance Agency

   Production settings in which ARKs are used include the University of
   California, the National Library of France, the Internet Archive, and
   Portico, with maintenance based at the California Digital Library
   (CDL), housed at the University of California Office of the

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Appendix B.  Looking up NMAHs Distributed via DNS

   This subsection introduces an older method for looking up NMAHs 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 NMAHs.

   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
     ;; US Library of Congress
     ;;       order pref flags service regexp  replacement
      IN NAPTR  0     0   "h"  "ark"   "USLC"
      IN NAPTR  0     0   "h"  "ark"   "USLC"
      IN NAPTR  0     0   "h"  "ark"   "USLC"

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

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

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     ;; Digital Library Consortium
     ;;       order pref flags service regexp replacement
      IN NAPTR  0     0    ""  "ark"     ""

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

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

   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 NMAHs in their replacement fields.  Set them aside, if

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

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

   A Perl script that implements this algorithm is included here.

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

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

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

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

Kunze & Rodgers          Expires October 7, 2013               [Page 43]

Internet-Draft                     ARK                        April 2013

Authors' Addresses

   John A. Kunze
   California Digital Library
   415 20th St, 4th Floor
   Oakland, CA  94612


   R. P. C. Rodgers
   University of California San Francisco
   Box 0134, 185 Berry, China Basin, Lobby 6 290
   San Francisco, CA  94143-0134


Kunze & Rodgers          Expires October 7, 2013               [Page 44]