Internet DRAFT - draft-kunze-ark
draft-kunze-ark
Network Working Group J. Kunze
Internet-Draft Ronin Institute
Intended status: Informational E. Bermès
Expires: 9 May 2024 École nationale des Chartes
6 November 2023
The ARK Identifier Scheme
draft-kunze-ark-38
Abstract
The ARK (Archival Resource Key) naming scheme is designed to
facilitate the high-quality and persistent identification of
information objects. The label "ark:" marks the start of a core ARK
identifier that can be made actionable by prepending the beginning of
a URL. Meant to be usable after today's networking technologies
become obsolete, that core should be recognizable in the future as a
globally unique ARK independent of the URL hostname, HTTP, etc. A
founding principle of ARKs is that persistence is purely a matter of
service and neither inherent in an object nor conferred on it by a
particular naming syntax. The best any identifier can do is lead
users to services that support robust reference. A full-functioning
ARK leads the user to the identified object and, with the "?info"
inflection appended, returns a metadata record and a commitment
statement that is both human- and machine-readable. Tools exist for
minting, binding, and resolving ARKs.
Responsibility for this Document
The ARK Alliance Technical Working Group [ARKAtech] is responsible
for the content of this Internet Draft. The group homepage lists
monthly meeting notes and agendas starting from March 2019.
Revisions of the spec are maintained on github at [ARKdrafts].
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at https://arks-
org.github.io/arkspec/draft-ark-spec.html. Status information for
this document may be found at https://datatracker.ietf.org/doc/draft-
kunze-ark/.
Source for this draft and an issue tracker can be found at
https://github.com/arks-org/arkspec.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1. The Name Mapping Authority (NMA) . . . . . . . . . . . . 11
2.2. The ARK Label Part (ark:) . . . . . . . . . . . . . . . . 13
2.3. The Name Assigning Authority Number (NAAN) . . . . . . . 14
2.4. The Name Part . . . . . . . . . . . . . . . . . . . . . . 16
2.4.1. Optional: Shoulders . . . . . . . . . . . . . . . . . 16
2.5. The Qualifier Part . . . . . . . . . . . . . . . . . . . 17
2.5.1. ARKs that Reveal Object Hierarchy . . . . . . . . . . 19
2.5.2. ARKs that Reveal Object Variants . . . . . . . . . . 20
3. ARK Processing . . . . . . . . . . . . . . . . . . . . . . . 21
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3.1. Character Repertoires . . . . . . . . . . . . . . . . . . 21
3.2. Normalization and Lexical Equivalence . . . . . . . . . . 23
3.3. Resolver Chains and Roles . . . . . . . . . . . . . . . . 24
3.4. Finding a Resolver Service . . . . . . . . . . . . . . . 25
4. Naming Considerations . . . . . . . . . . . . . . . . . . . . 27
4.1. ARKS and Usability . . . . . . . . . . . . . . . . . . . 27
4.2. Objects Should Wear Their Identifiers . . . . . . . . . . 27
4.3. Names are Political, not Technological . . . . . . . . . 28
4.4. Choosing a Hostname or NMA . . . . . . . . . . . . . . . 28
4.5. Assigners of ARKs . . . . . . . . . . . . . . . . . . . . 30
4.6. NAAN Namespace Management . . . . . . . . . . . . . . . . 30
4.7. Sub-Object Naming . . . . . . . . . . . . . . . . . . . . 32
5. Generic ARK Service Definition . . . . . . . . . . . . . . . 32
5.1. Generic ARK Access Service (access, location) . . . . . . 32
5.1.1. Generic Policy Service (permanence, naming, etc.) . . 33
5.1.2. Generic Description Service . . . . . . . . . . . . . 35
5.2. Overview of The HTTP URL Mapping Protocol (THUMP) . . . . 35
5.3. The Electronic Resource Citation (ERC) . . . . . . . . . 37
5.4. Advice to Web Clients . . . . . . . . . . . . . . . . . . 39
5.5. Enhancements and Related Specifications . . . . . . . . . 40
5.6. Security Considerations . . . . . . . . . . . . . . . . . 40
6. Informative References . . . . . . . . . . . . . . . . . . . 40
Appendix A. ARK Maintenance Agency: arks.org . . . . . . . . . . 43
Appendix B. Looking up NMAs Distributed via DNS . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 46
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
[ARK].
Schemes for persistent identification of network-accessible objects
are not new. In the early 1990's, the design of the Uniform Resource
Name [RFC2141] responded to the observed failure rate of URLs by
articulating an indirect, non-hostname-based naming scheme and the
need for responsible name management. Meanwhile, promoters of the
Digital Object Identifier [DOI] succeeded in building a community of
providers around a mature software system [Handle] that supports name
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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. For example,
like URNs, ARKs have an internal label ("ark:") to help them be
recognizable as globally unique identifiers in a post-HTTP Internet.
Unlike DOIs and Handles, ARKs can be created without centralized fee-
based infrastructures. ARK resolvers can take advantage of advanced
resolution features such as content negotiation (like DOIs) and
suffix passthrough [SPT] (similar to PURL partial redirects). Like
PURLs, ARKs openly embrace URLs as the best current choice for
actionability.
A founding principle of the ARK is that persistence is purely a
matter of service. Persistence is neither inherent in an object nor
conferred on it by a particular naming syntax. Nor is the technique
of name indirection -- upon which URNs, Handles, DOIs, and PURLs are
founded -- of central importance. Name indirection is an ancient and
well-understood practice; new mechanisms for it keep appearing and
distracting practitioner attention, with the Domain Name System (DNS)
[RFC1034] being a particularly dazzling and elegant example. What is
often forgotten is that maintenance of an indirection table is an
unavoidable cost to the organization providing persistence, and that
cost is equivalent across naming schemes. That indirection has
always been a native part of the web while being so lightly utilized
for the persistence of web-based objects indicates how unsuited most
organizations will probably be to the task of table maintenance and
to the much more fundamental challenge of keeping the objects
themselves viable.
Persistence is achieved through a provider's successful stewardship
of objects and their identifiers. The highest level of persistence
will be reinforced by a provider's robust contingency, redundancy,
and succession strategies. It is further safeguarded to the extent
that a provider's mission is shielded from funding and political
instabilities. These are by far the major challenges confronting
persistence providers, and no identifier scheme has any direct impact
on them. In fact, some schemes may actually be liabilities for
persistence because they create short- and long-term dependencies for
every object access on complex, special-purpose infrastructures,
parts of which are proprietary and all of which increase the carry-
forward burden for the preservation community. It is for this reason
that the ARK scheme relies only on educated name assignment and light
use of general-purpose infrastructures that are maintained mostly by
the Internet community at large (the DNS, web servers, and web
browsers).
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As purely a matter of service, persistence is difficult, not known to
be commercially attractive, and likely to be undertaken by only a
small fraction of content providers that have preservation in their
mission. This vision runs counter to some early predictions that
technology-backed persistent identifiers would somehow become
ubiquitous. On the plus side, persistent identifier solutions should
not need to be "internet scale".
1.1. Reasons to Use ARKs
If no persistent identifier scheme contributes directly to
persistence, why not just use URLs? A particular URL may be as
durable an identifier as it is possible to have, but nothing
distinguishes it from an ordinary URL to the recipient who is
wondering if it is suitable for long-term reference. An ARK embedded
in a URL provides some of the necessary conditions for credible
persistence, inviting access to not one, but to three things: to the
object, to its metadata, and to a nuanced statement of commitment
from the provider in question (the NMA, described below) regarding
the object. Existence of the extra service can be probed
automatically by appending "?info" to the ARK.
The form of the ARK also supports the natural separation of naming
authorities into the original name assigning authority and the
diverse multiple name mapping (or servicing) authorities that in
succession and in parallel will take over custodial responsibilities
from the original assigner (assuming the assigner ever held that
responsibility) for the large majority of a long-term object's
archival lifetime. The name mapping authority, indicated by the
hostname part of the URL that contains the ARK, serves to launch the
ARK into cyberspace. Should it ever fail (and there is no reason why
a well-chosen hostname for a 100-year-old cultural memory institution
shouldn't last as long as the DNS), that host name is considered
disposeable and replaceable. Again, the form of the ARK helps
because it defines exactly how to recover the core immutable object
identity, and simple algorithms (one based on the URN model) or even
by-hand Internet query can be used for for locating another mapping
authority.
There are tools to assist in generating ARKs and other identifiers,
such as [NOID] and "uuidgen", both of which rely for uniqueness on
human-maintained registries. This document also contains some
guidelines and considerations for managing namespaces and choosing
hostnames with persistence in mind.
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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 identification
plays a vital supporting role but, strictly speaking, it can be
construed as no more than a record attesting to the original
assignment of a never-reassigned identifier. Object access may not
be feasible for various reasons, such as a transient service outage,
a catastrophic loss, a licensing agreement that keeps an archive
"dark" for a period of years, or when an object's own lack of
tangible existence confuses normal concepts of access (e.g., a
vocabulary term might be "accessed" through its definition). In such
cases the ARK's identification role assumes a much higher profile.
But attempts to simplify the persistence problem by decoupling access
from identification and concentrating exclusively on the latter are
of questionable utility. A perfect system for assigning forever
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unique identifiers might be created, but if it did so without
reducing access failure rates, no one would be interested. The
central issue -- which may be crudely summed up as the "HTTP 404 Not
Found" problem -- would not have been addressed.
The central duty of an ARK is a high-quality experience of access and
identification. This means supporting reliable access during the
period described in its stewardship promise and, failing that,
supporting reliable access to a record describing the thing the ARK
is associated with.
ARK resolvers must support the "?info" inflection for requesting
metadata. Older versions of this specification distinguished between
two minimal inflections: '?' (brief metadata) and '??' (more
metadata). While these older inflections are still reserved, because
they have proven hard to recognize in some environments, supporting
them is optional.
1.3. Organizing Support for ARKs: Our Stuff vs. Their Stuff
An organization and the user community it serves can often be seen to
struggle with two different areas of persistent identification: the
Our Stuff problem and the Their Stuff problem. In the Our Stuff
problem, we in the organization want our own objects to acquire
persistent names. Since we possess or control these objects, our
organization tackles the Our Stuff problem directly. Whether or not
the objects are named by ARKs, our organization is the responsible
party, so it can plan for, maintain, and make commitments about the
objects.
In the Their Stuff problem, we in the organization want others'
objects to acquire persistent names. These are objects that we do
not own or control, but some of which are critically important to us.
But because they are beyond our influence as far as support is
concerned, creating and maintaining persistent identifiers for Their
Stuff is not especially purposeful or feasible for us to engage in.
There is little that we can do about someone else's stuff except
encourage their uptake or adoption of persistence services.
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Co-location of persistent access and identification services is
natural. Any organization that undertakes ongoing support of true
persistent identification (which includes description) is well-served
if it controls, owns, or otherwise has clear internal access to the
identified objects, and this gives it an advantage if it wishes also
to support persistent access to outsiders. Conversely, persistent
access to outsiders requires orderly internal collection management
procedures that include monitoring, acquisition, verification, and
change control over objects, which in turn requires object
identifiers persistent enough to support auditable record keeping
practices.
Although organizing ARK support under one roof thus tends to make
sense, object hosting can successfully be separated from name
mapping. An example is when a name mapping authority centrally
provides uniform resolution services via a protocol gateway on behalf
of organizations that host objects behind a variety of access
protocols. It is also reasonable to build value-added description
services that rely on the underlying services of a set of mapping
authorities.
Supporting ARKs is not for every organization. By requiring
specific, revealed commitments to preservation, to object access, and
to description, the bar for providing ARK services is higher than for
some other identifier schemes. On the other hand, it would be hard
to grant credence to a persistence promise from an organization that
could not muster the minimum ARK services. Not that there isn't a
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.
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Without regard to whether an object is physical, digital, or
conceptual, to identify it is to claim an association between it and
a representative string, such as "Jane" or "ISBN 0596000278". What
gives a claim credibility is a set of verifiable assertions, or
metadata, about the object, such as age, height, title, or number of
pages. In other words, the association is made manifest by a record
(e.g., a cataloging or other metadata record) that vouches for it.
In the complete absence of any testimony (metadata) regarding an
association, a would-be identifier string is a meaningless sequence
of characters. To keep an externally visible but otherwise internal
string from being perceived as an identifier by outsiders, for
example, it suffices for an organization not to disclose the nature
of its association. For our immediate purpose, actual existence of
an association record is more important than its authenticity or
verifiability, which are outside the scope of this specification.
It is a gift to the identification process if an object carries its
own name as an inseparable part of itself, such as an identifier
imprinted on the first page of a document or embedded in a data
structure element of a digital document header. In cases where the
object is large, unwieldy, or unavailable (such as when licensing
restrictions are in effect), a metadata record that includes the
identifier string will usually suffice. That record becomes a
conveniently manipulable object surrogate, acting as both an
association "receipt" and "declaration".
Note that our definition of identifier extends the one in use for
Uniform Resource Identifiers [RFC3986]. The present document still
sometimes (ab)uses the terms "ARK" and "identifier" as shorthand for
the string part of an identifier, but the context should make the
meaning clear.
2. ARK Anatomy
An ARK is represented by a sequence of characters (a string) that
contains the Label, "ark:", optionally preceded by the beginning part
of a URL. Here is a diagrammed example.
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ANATOMY OVERVIEW
================
Resolver Service Compact ARK
__________________ ______________________________
/ \/ \
https://example.org/ark:12345/x6np1wh8k/c3/s5.v7.xsl
\___________________________/\________/\___________/
Prefixes Base Name Suffixes
\__________________________________________________/
Mapping ARK
When embedded in a URL, an ARK consists of a Compact ARK preceded by
a Resolver Service. The larger URL-based ARK is known as a Mapping
ARK because it is ready to be mapped (resolved) to an information
response (eg, a PDF or metadata). A Mapping ARK is also know as a
"fully qualified ARK". The Resolver Service, which need not be
limited to URLs in the future, maps the URL according to rules and
abilities of an NMA (Name Mapping Authority). The same URL string
minus the Resolver Service component is known as a Compact ARK. The
Compact ARK is globally unique and may be resolvable via different
Resolver Services over time (eg, when one archive succeeds another)
or at the same time (eg, when one archive backs up another).
At a high level, after the Label comes the NAAN (Name Assigning
Authority Number) followed by the Name that it assigns to the
identified thing. The Base Name has Prefixes (NAAN, Label, possibly
a Resolver Service) and optional Suffixes to identify Parts and
Variant forms. During resolution, a Resolver Service such as n2t.net
may be able to deal with inflections query strings, and content
negotiation.
ANATOMY DETAILS
===============
Base Compact Name Qualifiers
_________________ ___________
/ \/ \
https://example.org/ark:12345/x6np1wh8k/c3/s5.v7.xsl
\_________/ \__/\___/\_/\_____/\____/\_____/
NMA Label NAAN | Blade Parts Variants
Shoulder
\_____________/
Check Zone
In a closer view, the Compact ARK consists of a Base Compact Name
followed potentially by Qualifiers. The Base Name often, but not
necessarily, consists of a Shoulder (for subdividing a NAAN
namespace) followed by a Blade. If a check character is present in
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an ARK, by convention it is the right-most character of the Base
Name, and will have been computed over the string of characters
preceding it back to the beginning of the NAAN. This string,
including the check character itself, is the Check Zone.
Like the ARK itself, the NAAN "12345" and Shoulder "x6" have compact
and fully qualified forms.
+==========+=======+==============+================================+
| Form | Base | Compact Form | Fully Qualified Form |
+==========+=======+==============+================================+
| NAAN | 12345 | ark:12345 | https://example.org/ark:12345 |
+----------+-------+--------------+--------------------------------+
| Shoulder | x6 | ark:12345/x6 | https://example.org/ark:12345/ |
| | | | x6 |
+----------+-------+--------------+--------------------------------+
Table 1: Example base, compact, and fully qualified form components.
The ARK syntax can be summarized,
[https://NMA/]ark:[/]NAAN/Name[Qualifiers]
where the NMA, '/', and Qualifier parts are in brackets to indicate
that they are optional. The Base Compact Name is the substring
comprising the "ark:" label, the NAAN and the assigned Name. The
Resolver Service is replaceable and makes the ARK actionable for a
period of time. Without the Resolver Service part, what remains is
the Core Immutable Identity (the "persistible") part of the ARK.
2.1. The Name Mapping Authority (NMA)
Before the "ark:" label may appear an optional Name Mapping Authority
(NMA) that is a temporary address where ARK service requests may be
sent. Preceded by a URI-type protocol designation such as
"https://", it specifies a Resolver Service. The NMA itself is an
Internet hostname or host/port combination, optionally followed by
URI-type path components, all ending in a '/'. The hostname has the
same format and semantics as the host/port part of a URL. In any
optional path that follows it, the path is considered to end with the
'/' in the first occurrence of "/ark:".
The most important thing about the NMA is that it is "identity inert"
from the point of view of object identification. In other words,
ARKs that differ only in the optional NMA part identify the same
object. Thus, for example, the following three ARKs are synonyms for
just one information object:
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http://example.org/rslvr/ark:12345/x6np1wh8k
https://example.com/ark:12345/x6np1wh8k
ark:12345/x6np1wh8k
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. It can be argued that divergence of
persistent objects is not desirable, but it is widely believed that
digital preservation efforts will inevitably lead to alterations in
some original objects (e.g, a format migration in order to preserve
the ability to display a document). If any of those objects are held
redundantly in more than one organization (a common preservation
strategy), chances are small that all holding organizations will
perform the same precise transformations and all maintain the same
object metadata. More significant divergence would be expected when
the holding organizations serve different audiences or compete with
each other.
The NMA part makes an ARK into an actionable URL. As with many
Internet parameters, it is helpful to approach the NMA being liberal
in what you accept and conservative in what you propose. From the
recipient's point of view, the NMA part should be treated as
temporary, disposable, and replaceable. From the NMA's point of
view, it should be chosen with the greatest concern for longevity. A
carefully chosen NMA should be at least as permanent as the providing
organization's own hostname. In the case of a national or university
library, for example, there is no reason why the NMA could not be
considerably more permanent than soft-funded proxy hostnames such as
hdl.handle.net, dx.doi.org, and purl.org. In general and over time,
however, it is not unexpected for an NMA eventually to stop working
and require replacement with the NMA of a currently active service
provider.
This replacement relies on a mapping authority "resolver" discovery
process, of which two alternate methods are outlined in a later
section. The ARK, URN, Handle, and DOI schemes all use a resolver
discovery model that sooner or later requires matching the original
assigning authority with a current provider servicing that
authority's named objects; once found, the resolver at that provider
performs what amounts to a redirect to a place where the object is
currently held. All the schemes rely on the ongoing functionality of
currently mainstream technologies such as the Domain Name System
[RFC1034] and web browsers. The Handle and DOI schemes in addition
require that the Handle protocol layer and global server grid be
available at all times.
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The practice of prepending "https://" and an NMA to an ARK is a way
of creating an actionable identifier by a method that is itself
temporary. Assuming that infrastructure supporting [RFC2616]
information retrieval will no longer be available one day, ARKs will
then have to be converted into new kinds of actionable identifiers.
By that time, if ARKs see widespread use, web browsers would
presumably evolve to perform this (currently simple) transformation
automatically.
2.2. The ARK Label Part (ark:)
The label part distinguishes an ARK from an ordinary identifier.
There is a new form of the label, "ark:", and an old form, "ark:/",
both of which must be recognized in perpetuity. Implementations
should generate new ARKs in the new form (without the "/") and
resolvers must always treat received ARKs as equivalent if they
differ only in regard to new form versus old form labels. Thus these
two ARKs are equivalent:
ark:/12345/x6np1wh8k
ark:12345/x6np1wh8k
In a URL found in the wild, the label indicates that the URL stands a
reasonable chance of being an ARK. If the context warrants,
verification that it actually is an ARK can be done by testing it for
existence of the three ARK services.
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.
An NAA identifies an autonomous assignment stream for a set of
objects as well as a reference to help locate a resolver for them.
Often, NAA policies and practices reflect an organization
(department, project, data center, periodical, etc.) in which it is
embedded. An organization may have more than one NAA, for example, a
publisher may have a distinct NAA for each of its three journals.
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Only a current provider can say for certain what sort of commitment
it intends, and the ARK label suggests that you can query the NMA
directly to find out exactly what kind of persistence is promised.
Even if what is promised is impersistence (i.e., a short-term
identifier), saying so is valuable information to the recipient.
Thus an ARK is a high-functioning identifier in the sense that it
provides access to the object, the metadata, and a commitment
statement, even if the commitment is explicitly very weak.
2.3. The Name Assigning Authority Number (NAAN)
Recalling that the general form of the ARK is,
[https://NMA/]ark:[/]NAAN/Name[Qualifiers]
the part of the ARK directly following the "ark:" (or older "ark:/")
label is the Name Assigning Authority Number (NAAN), up to but not
including the next '/' (slash) character. This part is always
required, as it identifies the organization that originally assigned
the Name of the object. Typically the organization is an
institution, a department, a laboratory, or any group that conducts a
stable, policy-driven name assigning effort. An organization may
request a NAAN from the ARK Maintenance Agency [ARKagency] (described
in Appendix A) by filling out the form [NAANrequest].
For received ARKs, implementations must support a minimum NAAN length
of 16 octets. NAANs are opaque strings of one or more "betanumeric"
characters, specifically,
0123456789bcdfghjkmnpqrstvwxz
which consists of digits and consonants, minus the letter 'l'.
Restricting NAANs to betanumerics (alphanumerics without vowels or
'l') serves two goals. It reduces the chances that words -- past,
present, and future -- will appear in NAANs and carry unintended
semantics. It also helps usability by not mixing commonly confused
characters ('0' and 'O', '1' and 'l') and by being compatible with
strong transcription error detection (eg, the [NOID] check digit
algorithm). Since 2001, every assigned NAAN has consisted of exactly
five digits.
The NAAN designates a top-level ARK namespace. Once registered for a
namespace, a NAAN is never re-registered. It is possible, however,
for there to be a succession of organizations that manage an ARK
namespace.
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There are currently four NAANs available for assignment on reserved
shoulders (see the Shoulder section) by all organizations. An ARK
bearing one of these NAANs carries a specific, immutable meaning that
recipients can rely on for long term pragmatic benefit as described
below.
+==========+================================+==========+============+
| Shared | The immutable purpose, | Expect | OK for |
| NAAN | meaning, or connotation of | to | long term |
| meaning | ARKs bearing this NAAN. | resolve? | reference? |
+==========+================================+==========+============+
| 12345 | Example ARKs appearing in | maybe | no |
| examples | documentation. They might | | |
| | resolve, but link checkers | | |
| | usually need not be concerned | | |
| | if they don't. They should | | |
| | not be considered viable for | | |
| | long term reference. | | |
+----------+--------------------------------+----------+------------+
| 99152 | ARKs for controlled | yes | yes |
| terms | vocabulary and ontology | | |
| | terms, such as metadata | | |
| | element names and pick-list | | |
| | values. They should resolve | | |
| | to term definitions and are | | |
| | suitable for long term | | |
| | reference. | | |
+----------+--------------------------------+----------+------------+
| 99166 | ARKs for people, groups, and | yes | yes |
| agents | institutions as "agents" | | |
| | (actors, such as creators, | | |
| | contributors, publishers, | | |
| | performers, etc). They | | |
| | should resolve to agent | | |
| | definitions and are suitable | | |
| | for long term reference. | | |
+----------+--------------------------------+----------+------------+
| 99999 | ARKs for test, development, | maybe | no |
| test ids | or experimental purposes, | | |
| | often at scale. They might | | |
| | resolve, but link checkers | | |
| | usually need not be concerned | | |
| | if they don't. They should | | |
| | not be considered viable for | | |
| | long term reference. | | |
+----------+--------------------------------+----------+------------+
Table 2: Four NAANs shared across all ARK-assigning organizations.
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To make use of a shared NAAN, an organization has several options
described in Section 2.4.1.
2.4. The Name Part
The part of the ARK just after the NAAN is the Name assigned by the
NAA, and it is also required. Semantic opaqueness in the Name part
is strongly encouraged in order to reduce an ARK's vulnerability to
era- and language-specific change. Identifier strings containing
linguistic fragments can create support difficulties down the road.
No matter how appropriate or even meaningless they are today, such
fragments may one day create confusion, give offense, or infringe on
a trademark as the semantic environment around us and our communities
evolves.
Names that look more or less like numbers avoid common problems that
defeat persistence and international acceptance. The use of digits
is highly recommended. Mixing in non-vowel alphabetic characters
(eg, betanumerics) a couple at a time is a relatively safe and easy
way to achieve a denser namespace (more possible names for a given
length of the name string). Such names have a chance of aging and
traveling well. The absence of recognizable words makes typos harder
to detect in opaque strings, so a common mitigation is to add a check
character. Tools exists that mint, bind, and resolve opaque
identifiers, with or without check characters [NOID]. More on naming
considerations is given in a subsequent section.
2.4.1. Optional: Shoulders
Just as an ARK namespace is subdivided by NAANs reserved for NAAs, it
is generally advantageous for an NAA to subdivide its own NAAN
namespace into "shoulders", where each shoulder is reserved for an
internal department or unit. Like the NAAN, which is a string of
characters that follows the "ark:" label, a shoulder is a string of
characters (starting with a "/") that extends the NAAN. The base
compact name assigned by the NAA consists of the NAAN, the shoulder,
a final string known as the "blade". (The shoulder plus blade
terminology mirrors locksmith jargon describing the information-
bearing parts of a key.)
The blade string is chosen by the NAA such that the string created by
concatenating the NAAN plus shoulder plus blade becomes the unique
base object name. Otherwise the blade may come from any source, for
example, it might come from a counter, a timestamp, a [NOID] minter,
a legacy 100-year-old accession number, etc. If there is a check
digit, it is expected to appear at the end of the blade and to be
computed over the base compact name minus the label part (see Check
Zone), which is generally the most important part of an ARK to make
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opaque. In particular, check digits are not expected to cover
qualifiers, which often name subobjects of a persistent object that
are less stable and less opaquely named than the parent object (for
example, ten years hence, the object's thumbnail image will be of a
higher resolution and the OCR text file will be re-derived with
improved algorithms.
It is important not to use any delimiter between the shoulder string
and blade string, especially not a "/" since it declares an object
boundary (see the section on ARKs that reveal object hierarchy).
ark:12345/x6np1wh8k/c2/s4.pdf # correct primordinal shoulder
ark:12345/x6/np1wh8k/c2/s4.pdf # INCORRECT
^ WRONG
This little bit of discretion shields organizations from end users
making inferences about expected levels of support based on
recognizable shoulders. To help in-house ARK administrators reliably
know where the shoulder ends, it is recommended to use the "first-
digit convention" so that shoulders are "primordinal". A primordinal
shoulder is a sequence of one or more betanumeric characters ending
in a digit, as shown above. This means that the shoulder is all
consonant letters (often just one) after the NAAN and "/" up to and
including the first digit encountered after the NAAN. One property
of primordinal shoulders is that there is an infinite number of them
possible under any NAAN.
To help manage each namespace into the future, NAAs are encouraged to
create shoulders, even if there is only one to start with. If an
organization wishes to create a shoulder under one of shared NAANs
(99999, 12345, 99152, or 99166, described in Table 2), it should fill
out the Shoulder Request Form [shoulderrequest].
2.5. The Qualifier Part
The part of the ARK following the NAA-assigned Name is an optional
Qualifier. It is a string that extends the Base Name in order to
create a kind of service entry point into the object named by the
NAA. At the discretion of the providing NMA, such a service entry
point permits an ARK to support access to individual hierarchical
components and subcomponents of an object, and to variants (versions,
languages, formats) of components. A Qualifier may be invented by
the NAA or by any NMA servicing the object.
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In form, the Qualifier is a ComponentPath, or a VariantPath, or a
ComponentPath followed by a VariantPath. A VariantPath is introduced
and subdivided by the reserved character '.', and a ComponentPath is
introduced and subdivided by the reserved character '/'. In this
example,
https://example.org/ark:12345/x6np1wh8k/c3/s5.v7.xsl
the string "/c3/s5" is a ComponentPath and the string ".v7.xsl" is a
VariantPath. The ARK Qualifier is a formalization of some currently
mainstream URL syntax conventions. This formalization specifically
reserves meanings that permit recipients to make strong inferences
about logical sub-object containment and equivalence based only on
the form of the received identifiers; there is great efficiency in
not having to inspect metadata records to discover such
relationships. NMAs are free not to disclose any of these
relationships merely by avoiding the reserved characters above.
Hierarchical components and variants are discussed further in the
next two sections.
The Qualifier, if present, differs from the Name in several important
respects. First, a Qualifier may have been assigned either by the
NAA or later by the NMA. The assignment of a Qualifier by an NMA
effectively amounts to an act of publishing a service entry point
within the conceptual object originally named by the NAA. For our
purposes, an ARK extended with a Qualifier assigned by an NMA will be
called an NMA-qualified ARK.
Second, a Qualifier assignment on the part of an NMA is made in
fulfillment of its service obligations and may reflect changing
service expectations and technology requirements. NMA-qualified ARKs
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.
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A third difference between a Qualifier and a Name concerns the
semantic opaqueness constraint. When an NMA-qualified ARK is to be
used as a transient service entry point into a persistent object, the
priority given to semantic opaqueness observed by the NAA in the Name
part may be relaxed by the NMA in the Qualifier part. If service
priorities in the Qualifier take precedence over persistence, short-
term usability considerations may recommend somewhat semantically
laden Qualifier strings.
Finally, not only is the set of Qualifiers supported by an NMA
mutable, but different NMAs may support different Qualifier sets for
the same NAA-identified object. In this regard the NMAs act
independently of each other and of the NAA.
The next two sections describe how ARK syntax may be used to declare,
or to avoid declaring, certain kinds of relatedness among qualified
ARKs.
2.5.1. ARKs that Reveal Object Hierarchy
An NAA or NMA may choose to reveal the presence of a hierarchical
relationship between objects using the '/' (slash) character after
the Name part of an ARK. Some authorities will choose not to
disclose this information, while others will go ahead and disclose so
that manipulators of large sets of ARKs can infer object
relationships by simple identifier inspection; for example, this
makes it possible for a system to present a collapsed view of a large
search result set.
If the ARK contains an internal slash after the NAAN, the piece to
its left indicates a containing object. For example, publishing an
ARK of the form,
ark:12345/x54/xz/321
is equivalent to publishing three ARKs,
ark:12345/x54/xz/321
ark:12345/x54/xz
ark:12345/x54
together with a declaration that the first object is contained in the
second object, and that the second object is contained in the third.
Revealing the presence of hierarchy is completely up to the assigner
(NMA or NAA). It is hard enough to commit to one object's name, let
alone to three objects' names and to a specific, ongoing relatedness
among them. Thus, regardless of whether hierarchy was present
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initially, the assigner, by not using slashes, reveals no shared
inferences about hierarchical or other inter-relatedness in the
following ARKs:
ark:12345/x54_xz_321
ark:12345/x54_xz
ark:12345/x54xz321
ark:12345/x54xz
ark:12345/x54
Note that slashes around the ARK's NAAN (/12345/ in these examples)
are not part of the ARK's Name and therefore do not indicate the
existence of some sort of NAAN super object containing all objects in
its namespace. A slash must have at least one non-structural
character (one that is neither a slash nor a period) on both sides in
order for it to separate recognizable structural components. So
initial or final slashes may be removed, and double slashes may be
converted into single slashes.
2.5.2. ARKs that Reveal Object Variants
An NAA or NMA may choose to reveal the possible presence of variant
objects or object components using the '.' (period) character after
the Name part of an ARK. Some authorities will choose not to
disclose this information, while others will go ahead and disclose so
that manipulators of large sets of ARKs can infer object
relationships by simple identifier inspection. This makes it
possible for a system to present a collapsed view of a large number
of search result items without having to issue database queries in
order to retrieve and analyze the inter-relatedness among all of
those items.
If the ARK contains an internal period after the Name, the piece to
the left of the first such period is a root name and the piece to its
right, and up to the end of the ARK or to the next period is a
suffix. A Name may have more than one suffix, for example,
ark:12345/x54.24
ark:12345/x4z/x54.24
ark:12345/x54.v18.fr.odf
There are two main rules. First, if two ARKs share the same root
name but have different suffixes, the corresponding objects were
considered variants of each other (different formats, languages,
versions, etc.) by the assigner (NMA or NAA). Thus, the following
ARKs are variants of each other:
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ark:12345/x54.v18.fr.odf
ark:12345/x54.321xz
ark:12345/x54.44
Second, publishing an ARK with a suffix implies the existence of at
least one variant identified by the ARK without its suffix. The ARK
is otherwise silent about what additional variants might exist. So
publishing the ARK,
ark:12345/x54.v18.fr.odf
is equivalent to publishing the four ARKs,
ark:12345/x54.v18.fr.odf
ark:12345/x54.v18.fr
ark:12345/x54.v18
ark:12345/x54
Revealing the possibility of variants is completely up to the
assigner. It is hard enough to commit to one object's name, let
alone to multiple variants' names and to a specific, ongoing
relatedness among them. The assigner is the sole arbiter of what
constitutes a variant within its namespace, and whether to reveal
that kind of relatedness by using periods within its names.
A period must have at least one non-structural character (one that is
neither a slash nor a period) on both sides in order for it to
separate recognizable structural components. So initial or final
periods may be removed, and adjacent periods may be converted into a
single period.
3. ARK Processing
3.1. Character Repertoires
The Name and Qualifier parts are strings of visible ASCII characters.
For received ARKs, implementations must support a minimum length of
255 octets for the string composed of the Base Name plus Qualifier.
Implementations generating strings exceeding this length should
understand that receiving implementations may not be able to index
such ARKs properly. Characters may be letters, digits, or any of
these seven characters:
= ~ * + @ _ $
The following characters may also be used, but their meanings are
reserved:
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% - . /
The characters '/' and '.' are ignored if either appears as the last
character of an ARK. If used internally, they allow a name assigner
to reveal object hierarchy and object variants as previously
described.
Hyphens are considered to be insignificant and are always ignored in
ARKs. A '-' (hyphen) may appear in an ARK for readability, or it may
have crept in during the formatting and wrapping of text, but it must
be ignored in lexical comparisons. As in a telephone number, hyphens
have no meaning in an ARK. It is always safe for an NMA that
receives an ARK to remove any hyphens found in it. As a result, like
the NMA, hyphens are "identity inert" in comparing ARKs for
equivalence. For example, the following ARKs are equivalent for
purposes of comparison and ARK service access:
ark:12345/x5-4-xz-321
https://sneezy.dopey.com/ark:12345/x54--xz32-1
ark:12345/x54xz321
The '%' character is reserved for %-encoding all other octets that
would appear in the ARK string, in the same manner as for URIs
[RFC3986]. A %-encoded octet consists of a '%' followed by two
uppercase hex digits; for example, "%7D" stands in for '}'.
Uppercase hex digits are preferred for compatibility with URI
encoding conventions, especially useful when URL-based ARKs are
compared for equivalence by ARK-unaware software systems; thus use
"%ACT" instead of "%acT". The character '%' itself must be
represented using "%25". As with URNs, %-encoding permits ARKs to
support legacy namespaces (e.g., ISBN, ISSN, SICI) that have less
restricted character repertoires [RFC2288].
Implementors should be prepared to normalize some common invalid
characters that may be found in ARKs copy pasted from processed text.
For example, when pasting an ARK that was broken during line
wrapping, a user may inadvertently propagate newlines, spaces,
hyphens, and hyphen-like characters (eg, U+2010 to U+2015) that were
introduced by the publisher. The normalization strategy is up to the
implementor and may include converting hyphen-like characters to
hyphens and removing whitespace.
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3.2. Normalization and Lexical Equivalence
To determine if two or more ARKs identify the same object, the ARKs
are compared for lexical equivalence after first being normalized.
Since ARK strings may appear in various forms (e.g., having different
NMAs), normalizing them minimizes the chances that comparing two ARK
strings for equality will fail unless they actually identify
different objects. In a specified-host ARK (one having an NMA), the
NMA never participates in such comparisons. Normalization described
here serves to define lexical equivalence but does not restrict how
implementors normalize ARKs locally for storage.
Normalization of a received ARK for the purpose of octet-by-octet
equality comparison with another ARK consists of the following steps.
1. The NMA part (eg, everything from an initial "https://" up to the
first occurrence of "/ark:"), if present is removed.
2. Any URI query string is removed (everything from the first
literal '?' to the end of the string).
3. The first case-insensitive match on "ark:/" or "ark:" is
converted to "ark:" (replacing any uppercase letters and removing
any terminal '/').
4. Any uppercase letters in the NAAN are converted to lowercase.
5. In the string that remains, the two characters following every
occurrence of '%' are converted to uppercase. The case of all
other letters in the ARK string must be preserved.
6. All hyphens are removed. Implementors should be aware that non-
ASCII hyphen-like characters (eg, U+2010 to U+2015) may arrive in
the place of hyphens and, if they wish, remove them.
7. If normalization is being done as part of a resolution step, and
if the end of the remaining string matches a known inflection,
the inflection is noted and removed.
8. Structural characters (slash and period) are normalized: initial
and final occurrences are removed, and two structural characters
in a row (e.g., // or ./) are replaced by the first character,
iterating until each occurrence has at least one non-structural
character on either side.
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9. If there are any components with a period on the left and a slash
on the right, either the component and the preceding period must
be moved to the end of the Name part or the ARK must be thrown
out as malformed.
The resulting ARK string is now normalized. Comparisons between
normalized ARKs are case-sensitive, meaning that uppercase letters
are considered different from their lowercase counterparts.
To keep ARK string variation to a minimum, no reserved ARK characters
should be %-encoded unless it is deliberately to conceal their
reserved meanings. No non-reserved ARK characters should ever be
%-encoded. Finally, no %-encoded character should ever appear in an
ARK in its decoded form.
3.3. Resolver Chains and Roles
To resolve a Compact ARK (ie, an ARK beginning "ark:") it must
initially be promoted to a Mapping ARK so that it becomes actionable.
On the web, this means finding a suitable web Resolver Service to
prepend to the compact form of the identifier in order to convert it
to a URL (cf [CURIE]). (This is more or less true for any type of
identifier not already in URL form.)
The identifier's Resolver Service is the first point of contact in
the resolution process (eg, the NMA in a typical URL). It can be
seen as the "first resolver" because resolution may involve multiple
redirections via a chain of resolvers before a resolution response is
returned by the last resolver (the "responder"). The chain is as
long as the number of redirections. In particular, when the first
resolver is also the last resolver, the chain has zero length. Most
ARKs using N2T.net as the first resolver will be redirected to a
second resolver listed in the record for a given ARK's NAAN. For
example, an ARK bearing the NAAN 12148 (BnF) and the NMA n2t.net (as
its first resolver) could be redirected to a second resolver,
ark.bnf.fr. Whether n2t.net or ark.bnf.fr will be the first resolver
depends on what NMA appears in the ARK at the time of resolution.
Currently, BnF ARKs are published with the BnF's NMA (ark.bnf.fr), so
most BnF ARKs will not start with n2t.net.
Resolution in general can be seen as a multi-stage computation that
maps a client identifier to some sort of response. On the web, each
resolver in the chain is an HTTP server; even if the "responder"
(last resolver) is a proxy server that intiates a non-web sub-
resolution process, that is invisible to the original client and out
of scope for this discussion. A web resolution response may take on
a variety of forms, including the return of a landing page, or a
metadata record, or a web-based 404 Not Found message. A given
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response, as well as the specific chain of resolvers traversed,
depends not only on the identifier, but also on such things as the
time, location, credentials, and technical platform of the client
initiating resolution.
Also, for a given identifier, the "responder" (last resolver) for an
object request may be different from the responder for a metadata
request. While maintenance of objects and their metadata are often
co-located in one organization, for technical reasons it is not
uncommon that requests for objects and metadata are forwarded to
different responders. To add credibility to a persistence promise,
it can be useful to maintain a secondary copy of object metadata at
an external and publicly visible resolver. For example, N2T.net was
originally designed to store a secondary copy of metadata for many
millions of identifiers.
3.4. Finding a Resolver Service
In order to derive an actionable identifier (these days, a URL) from
an ARK, a Resolver Service must be found. On the web, the Resolver
Service consists of a URI scheme and an NMA, where the NMA is a host
or host/port combination, optionally followed by URI-type path
components, all ending in a '/'. The Resolver Service is expected to
respond to basic ARK service requests. An NMA may provide mapping
services for more than one NAAN.
Upon encountering an ARK, a user (or client software) determines if
it is a Mapping ARK (ie, it is a URL beginning with a Resolver
Service). If the Resolver Service is working, this discovery step
likely can be skipped assuming the URL correctly identifies a working
resolver. If a new Resolver Service needs to be 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 Resolver Services that service ARKs issued by the
identified NAA. This NAAN-to-NMA resolver discovery method is common
(cf URN, Handle, DOI) but does not address the namespace splitting
problem, which is when a portion of a NAAN space originally
maintained entirely by one NMA is taken on by a second NMA; now the
NAAN alone cannot reveal which NMA (resolver) to choose.
The global database is key, and ideally the lookup would be automatic
and transparent to the user. For this, the current mainstream method
is the Name-to-Thing (N2T) Resolver [N2T] at n2t.net. It is based on
a plain text [NAANregistry] database containing explanatory comments,
so it can also be directly inspected by users, for example, to
manually find a Resolver Service. N2T is a reliable, low-cost
Resolver Service provided by the ARK Alliance primarily to support
actionable HTTP-based URLs for as long as HTTP is used. N2T scales
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to store and resolve over 100 million individual identifiers and
their metadata, and has played a valuable role in persistent
identification by keeping a redundant copy of identifier metadata.
Because it has the option of storing redirection information for
individual identifiers rather than just at the name assigning
authority level, N2T can deal with namespace splitting; when a
portion of a NAAN space maintained by one NMA is taken on by a second
NMA, N2T can rely on individual identifier redirects for that portion
and a single NAAN-based rule for the remainder.
An appendix describes an historical way to discover an NMA based on a
simplification of the URN resolver discovery method, itself very
similar in principle to the resolver discovery method used by Handles
and DOIs. None of these methods does more than what can be done with
a very small, consortially maintained web server such as [N2T].
In the interests of long-term persistence, however, ARK mechanisms
are first defined in high-level, protocol-independent terms so that
mechanisms may evolve and be replaced over time without compromising
fundamental service objectives. Either or both specific methods
given here may eventually be supplanted by better methods since, by
design, the ARK scheme does not depend on a particular method, but
only on having some method to locate an active NMA.
At the time of issuance, at least one NMA for an ARK should be
prepared to service it. That NMA may or may not be administered by
the Name Assigning Authority (NAA) that created it. Consider the
following hypothetical example of providing long-term access to a
cancer research journal. The publisher wishes to turn a profit and a
national library 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
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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. Naming Considerations
The most important threats faced by persistence providers include
such things as funding loss, natural disaster, political and social
upheaval, processing faults, and errors in human oversight. There is
nothing that an identifer scheme can do about such things. Still, a
few observed identifier failures and inconveniences can be traced
back to naming practices that we now know to be less than optimal for
persistence.
4.1. ARKS and Usability
Because linguistic constructs imperil persistence, for ARKs non-ASCII
character support is not a priority. ARKs and URIs share goals of
transcribability and transportability within web documents, so
characters are required to be visible, non-conflicting with HTML/XML
syntax, and not subject to tampering during transmission across
common transport gateways.
Any measure that reduces user irritation with an identifier will
increase its chances of survival. This explains the rule preventing
hyphens from having lexical significance. It is fine to publish ARKs
with hyphens in them (e.g., such as the output of UUID/GUID
generators), but the uniform treatment of hyphens (and their Unicode
equivalents) as insignificant reduces the possibility of users
transcribing identifiers that will have been broken through
unpredictable hyphenation by word processors.
4.2. 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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4.3. 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.
4.4. Choosing a Hostname or NMA
Hostnames appearing in any identifier meant to be persistent must be
chosen with extra care. The tendency in hostname selection has
traditionally been to choose a token with recognizable attributes,
such as a corporate brand, but that tendency wreaks havoc with
persistence that is supposed to outlive brands, corporations, subject
classifications, and natural language semantics (e.g., what did the
three letters "gay" mean in 1958, 1978, and 1998?). Today's
recognized and correct attributes are tomorrow's stale or incorrect
attributes. In making hostnames (any names, actually) long-term
persistent, it helps to eliminate recognizable attributes to the
extent possible. This affects selection of any name based on URLs,
including PURLs and the explicitly disposable NMAs.
There is no excuse for a provider that manages its internal names
impeccably not to exercise the same care in choosing what could be an
exceptionally durable hostname, especially if it would form the
prefix for all the provider's URL-based external names. Registering
an opaque hostname in the ".org" or ".net" domain would not be a bad
start. Another way is to publish your ARKs with an organizational
domain name that will be mapped by DNS to an appropriate NMA host.
This makes for shorter names with less branding vulnerability.
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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, "handle.net".
For smaller libraries, archives, and preservation organizations,
there is a natural concern about whether they will be able to keep
their web servers and domain names in the face of uncertain funding.
One option is to form or join a group of like-minded organizations
with the purpose of providing mutual preservation support. The first
goal of such a group would be to perpetually rent a hostname on which
to establish a web server that simply redirects incoming member
organization requests to the appropriate member server; using ARKs,
for example, a 150-member group could run a very small server (24x7)
that contained nothing more than 150 rewrite rules in its
configuration file. Even more helpful would be additional consortial
support for a member organization that was unable to continue
providing services and needed to find a successor archival
organization. This would be a low-cost, low-tech way to publish ARKs
(or URLs) under highly persistent hostnames.
There are no obvious reasons why the organizations registering DNS
names, URN Namespaces, and DOI publisher IDs should have among them
one that is intrinsically more fallible than the next. Moreover, it
is a misconception that the demise of DNS and of HTTP need adversely
affect the persistence of URLs. At such a time, certainly URLs from
the present day might not then be actionable by our present-day
mechanisms, but resolution systems for future non-actionable URLs are
no harder to imagine than resolution systems for present-day non-
actionable URNs and DOIs. There is no more stable a namespace than
one that is dead and frozen, and that would then characterize the
space of names bearing the "http://" or "https://" prefix. It is
useful to remember that just because hostnames have been carelessly
chosen in their brief history does not mean that they are unsuitable
in NMAs (and URLs) intended for use in situations demanding the
highest level of persistence available in the Internet environment.
A well-planned name assignment strategy is everything.
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4.5. 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:12345/" are under control of the NAA registered under 12345,
which might be the National Library of Finland. Because each NAA has
a different NAAN, names from one namespace cannot conflict with those
from another. Each NAA is free to assign names from its namespace
(or delegate assignment) according to its own policies. These
policies must be documented in a manner similar to the declarations
required for URN Namespace registration [RFC2611].
Organizations can request or update a NAAN by filling out the NAAN
Request Form [NAANrequest].
4.6. NAAN Namespace Management
Every NAA should have a namespace management strategy. A classic
hierarchical approach is to partition a NAAN namespace into
subnamespaces known as "shoulders". As explained in Section 2.4.1,
each shoulder is a unique prefix that guarantees non-collision of
names in different partitions. This practice is strongly encouraged
for all NAAs, especially when subnamespace management and assignment
streams will be delegated to departments, units, or projects within
an organization. For example, with a NAAN that is assigned to a
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university and managed by its main library, the library should take
care to reserve shoulders (semantically opaque shoulders being
preferred) for distinct assignment streams. Prefix-based partition
management is typically an important responsibility of the NAA.
This shoulder delegation approach plays out differently in two real-
world examples: DNS names and ISBN identifiers. In the former, the
hierarchy is deliberately exposed and in the latter it is hidden.
Rather than using lexical boundary markers such as the period ('.')
found in domain names, the ISBN uses a publisher prefix but doesn't
disclose where the prefix ends and the publisher's assigned name
begins. This practice of non-disclosure, found in the ISBN and ISSN
schemes, is encouraged in assigning ARKs because it reduces the
visibility of an assertion that is probably not important now and may
become a vulnerability later.
If longevity is the goal, it is important to keep the prefixes free
of recognizable semantics; for example, using an acronym representing
a project or a department is discouraged. At the same time, you may
wish to set aside a subnamespace for testing purposes under a
shoulder such as "fk9..." that can serve as a visual clue and
reminder to maintenance staff that this "fake" identifier was never
published.
There are other measures one can take to avoid user confusion,
transcription errors, and the appearance of accidental semantics when
creating identifiers. If you are generating identifiers
automatically, pure numeric identifiers are likeley to be
semantically opaque enough, but it's probably useful to avoid leading
zeroes because some users mistakenly treat them as optional, thinking
(arithmetically) that they don't contribute to the "value" of the
identifier.
If you need lots of identifiers and you don't want them to get too
long, you can mix digits with consonants (but avoid vowels since they
might accidentally spell words) to get more identifiers without
increasing the string length. In this case you may not want more
than a two letters in a row because it reduces the chance of
generating acronyms. Generator tools such as [NOID] provide support
for these sorts of identifiers, and can also add a computed check
character as a guarantee against the most common transcription
errors. If used, it is recommended that the check character be
appended to the original Base Compact Name string (ie, minus the
check character), that original string having been the basis for
computing the check character.
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4.7. 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).
5. Generic ARK Service Definition
An ARK request's output is delivered information; examples include
the object itself, a policy declaration (e.g., a promise of support),
a descriptive metadata record, or an error message. The experience
of object delivery is expected to be an evolving mix of information
that reflects changing service expectations and technology
requirements; contemporary examples include such things as an object
summary and component links formatted for human consumption. ARK
services must be couched in high-level, protocol-independent terms if
persistence is to outlive today's networking infrastructural
assumptions. The high-level ARK service definitions listed below are
followed in the next section by a concrete method (one of many
possible methods) for delivering these services with today's
technology. Note that some services may be invoked in one operation,
such as when an "?info" inflection returns both a description and a
permanence declaration for an object.
5.1. Generic ARK Access Service (access, location)
Returns (a copy of) the object or a redirect to the same, although a
sensible object proxy may be substituted. Examples of sensible
substitutes include,
* a table of contents instead of a large complex document,
* a home page instead of an entire web site hierarchy,
* a rights clearance challenge before accessing protected data,
* directions for access to an offline object (e.g., a book),
* a description of an intangible object (a disease, an event), or
* an applet acting as "player" for a large multimedia object.
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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 be covered: object
permanence, object naming, object fragment addressing, and
operational service support.
The permanence declaration for an object is a rating defined with
respect to an identified permanence provider (guarantor), which will
be the NMA. It may include the following aspects.
1. "object availability" -- whether and how access to the object is
supported (e.g., online 24x7, or offline only),
2. "identifier validity" -- under what conditions the identifier
will be or has been re-assigned,
3. "content invariance" -- under what conditions the content of the
object is subject to change, and
4. "change history" -- access to corrections, migrations, and
revisions, whether through links to the changed objects
themselves or through a document summarizing the change history
One approach to persistence statements, conceived independently from
ARKs, can be found at [PStatements], with ongoing work available at
[ARKspecs]. An older approach to a permanence rating framework is
given in [NLMPerm], which identified the following "permanence
levels":
Not Guaranteed: No commitment has been made to retain this
resource. It could become unavailable at any time. Its
identifier could be changed.
Permanent: Dynamic Content: A commitment has been made to keep
this resource permanently available. Its identifier will always
provide access to the resource. Its content could be revised or
replaced.
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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 is the NMA that responds to requests for policy
statements, it is useful for the NMA to be able to produce or
summarize these historical NAA documents. Naming policy may include
the following aspects.
1. "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
2. "granularity" -- the limit, defined by the NAA, to the level of
object subdivision beyond which sub-objects do not warrant
separately assigned identifiers but before which sub-objects are
assigned separate identifiers.
Subnaming policy for an object describes the qualifiers that the NMA,
in fulfilling its ongoing and evolving service obligations, allows as
extensions to an NAA-assigned ARK. To the conceptual object that the
NAA named with an ARK, the NMA may add component access points and
derivatives (e.g., format migrations in aid of preservation) in order
to provide both basic and value-added services.
Addressing policy for an object includes a description of how, during
access, object components (e.g., paragraphs, sections) or views
(e.g., image conversions) may or may not be "addressed", in other
words, how the NMA permits arguments or parameters to modify the
object delivered as the result of an ARK request. If supported,
these sorts of operations would provide things like byte-ranged
fragment delivery and open-ended format conversions, or any set of
possible transformations that would be too numerous to list or to
identify with separately assigned ARKs.
Operational service support policy includes a description of general
operational aspects of the NMA service, such as after-hours staffing
and trouble reporting procedures.
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5.1.2. Generic Description Service
Returns a description of the object. Descriptions are returned in a
structured metadata format, a human-readable text format, or in one
format that serves both purposes (such as human-readable HTML with
embedded machine-readable metadata, or perhaps YAML). A description
must at a minimum answer the who, what, when, and where questions
("where" being the long-term identifier as opposed to a transient
redirect target) concerning an expression of the object. Standalone
descriptions should be accompanied by the modification date and
source of the description itself. May also return discriminated
lists of ARKs that are related to the given ARK.
5.2. Overview of The HTTP URL Mapping Protocol (THUMP)
The HTTP URL Mapping Protocol (THUMP) is a way of taking a key (any
identifier) and asking such questions as, what information does this
identify and how permanent is it? [THUMP] is in fact one specific
method under development for delivering ARK services. The protocol
runs over HTTP to exploit the web browser's current pre-eminence as
user interface to the Internet. THUMP is designed so that a person
can enter ARK requests directly into the location field of current
browser interfaces. Because it runs over HTTP, THUMP can be
simulated and tested via keyboard-based interactions [RFC0854].
The asker (a person or client program) starts with an identifier,
such as an ARK or a URL. The identifier reveals to the asker (or
allows the asker to infer) the Internet host name and port number of
a server system that responds to questions. Here, this is just the
NMA that is obtained by inspection and possibly lookup based on the
ARK's NAAN. The asker then sets up an HTTP session with the server
system, sends a question via a THUMP request (contained within an
HTTP request), receives an answer via a THUMP response (contained
within an HTTP response), and closes the session. That concludes the
connected portion of the protocol.
A THUMP request is a string of characters beginning with a '?'
(question mark) that is appended to the identifier string. The
resulting string is sent as an argument to HTTP's GET command.
Request strings too long for GET may be sent using HTTP's POST
command. The two most common requests correspond to two degenerate
special cases. First, a simple key with no request at all is the
same as an ordinary access request. Thus a plain ARK entered into a
browser's location field behaves much like a plain URL, and returns
access to the primary identified object, for instance, an HTML
document.
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The second special case is a minimal ARK description request string
consisting of just "?info". For example, entering the string,
n2t.net/ark:67531/metadc107835?info
into the browser's location field directly precipitates a request for
a metadata record describing the object identified by ark:67531/
metadc107835. The browser, unaware of THUMP, prepares and sends an
HTTP GET request in the same manner as for a URL. THUMP is designed
so that the response (indicated by the returned HTTP content type) is
normally displayed, whether the output is structured for machine
processing (text/plain) or formatted for human consumption (text/
html). In addition to "?info", this specification reserves both '?'
and '??' (originally older forms) for future use.
The following example THUMP session assumes metadata being returned
by a resolver (as server) to a browser client. Each line has been
annotated to include a line number and whether it was the client or
server that sent it. Without going into much depth, the session has
four pieces separated from each other by blank lines: the client's
piece (lines 1-3), the server's HTTP/THUMP response headers (4-8),
and the body of the server's response (9-18). The first and last
lines (1 and 19) correspond to the client's steps to start the TCP
session and the server's steps to end it, respectively.
1 C: [opens session]
C: GET https://n2t.net/ark:67531/metadc107835?info HTTP/1.1
C:
S: HTTP/1.1 200 OK
5 S: Content-Type: text/plain
S: THUMP-Status: 0.6 200 OK
S: Link: </ark:67531/metadc107835> rel="describes";
S:
S: erc:
10 S: who: Austin, Larry
S: what: A Study of Rhythm in Bach's Orgelbüchlein
S: when: 1952
S: where: https://digital.library.unt.edu/ark:/67531/metadc107835
S: erc-support:
15 S: who: University of North Texas Libraries
S: what: Permanent: Stable Content:
S: when: 20081203
S: where: https://digital.library.unt.edu/ark:/67531/
S: [closes session]
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The first two server response lines (4-5) above are typical of HTTP.
The next line (6) is peculiar to THUMP, and indicates the THUMP
version and a normal return status. The final header line (7)
asserts, for the benefit of recipients unfamiliar with ARK
inflections, that the response describes the uninflected ARK.
The balance of the response consists of a single metadata record
(9-18) that comprises the ARK description service response. The
returned record is in the format of an Electronic Resource Citation
[ERC], which is discussed in overview in the next section. For now,
note that it contains four elements that answer the top priority
questions regarding an expression of the object: who played a major
role in expressing it, what the expression was called, when it was
created, and where the expression may be found (note that "where" is
preferably a persistent, citable identifier rather than an unstable
URL sometimes mistakenly referred to as a "location"). This quartet
of elements comes up again and again in ERCs. Lines 13-17 contain a
minimal persistence statement.
Each segment in an ERC tells a different story relating to the
object, so although the same four questions (elements) appear in
each, the answers depend on the segment's story type. While the
first segment tells the story of an expression of the object, the
second segment tells the story of the support commitment made to it:
who made the commitment, what the nature of the commitment was, when
it was made, and where a fuller explanation of the commitment may be
found.
5.3. The Electronic Resource Citation (ERC)
An Electronic Resource Citation (or ERC, pronounced e-r-c) [ERC] is a
kind of object description that uses Dublin Core Kernel metadata
elements [DCKernel]. The ERC with Kernel elements provides a simple,
compact, and printable record for holding data associated with an
information resource. As originally designed [Kernel], Kernel
metadata balances the needs for expressive power, very simple machine
processing, and direct human manipulation. The ERC sense of
"citation" is not limited to the traditional referencing of a result
or information fixed in time on a printed page, but to a more general
kind of reference, both backward, to digital material that cannot be
known to be fixed in time (true of virtually all online information),
and forward, to material that is all the more valuable for improving
or evolving over time.
The previous section shows two limited examples of what is fully
described elsewhere [ERC]. The rest of this short section provides
some of the background and rationale for this record format.
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A founding principle of Kernel metadata is that direct human contact
with metadata will be a necessary and sufficient condition for the
near term rapid development of metadata standards, systems, and
services. Thus the machine-processable Kernel elements must only
minimally strain people's ability to read, understand, change, and
transmit ERCs without their relying on intermediation with
specialized software tools. The basic ERC needs to be succinct,
transparent, and trivially parseable by software.
Borrowing from the data structuring format that underlies the
successful spread of email and web services, the ERC format uses
[ANVL], which is based on email and HTTP headers [RFC2822]. There is
a naturalness to ANVL's label-colon-value format (seen in the
previous section) that barely needs explanation to a person beginning
to enter ERC metadata.
While ANVL elements are expected at the top level and don't
themselves support hierarchy, the value of an ANVL element may be an
arbitrary encoded hierarchy of JSON or XML. Typically, the name of
such an ANVL element ends in "json" or "xml", for example, "json" or
"geojson". Care should be taken to escape structural characters that
appear in element names and values, specifically, line terminators
(both newlines ("\n") and carriage returns ("\r")) and, in element
names, colons (":").
Besides simplicity of ERC system implementation and data entry
mechanics, ERC semantics (what the record and its constituent parts
mean) must also be easy to explain. ERC semantics are based on a
reformulation and extension of the Dublin Core [RFC5013] hypothesis,
which suggests that the fifteen Dublin Core metadata elements have a
key role to play in cross-domain resource description. The ERC
design recognizes that the Dublin Core's primary contribution is the
international, interdisciplinary consensus that identified fifteen
semantic buckets (element categories), regardless of how they are
labeled. The ERC then adds a definition for a record and some
minimal compliance rules. In pursuing the limits of simplicity, the
ERC design combines and relabels some Dublin Core buckets to isolate
a tiny kernel (subset) of four elements for basic cross-domain
resource description.
For the cross-domain kernel, the ERC uses the four basic elements --
who, what, when, and where -- to pretend that every object in the
universe can have a uniform minimal description. Each has a name or
other identifier, a locator (a means to access it), some responsible
person or party, and a date. It doesn't matter what type of object
it is, or whether one plans to read it, interact with it, smoke it,
wear it, or navigate it. Of course, this approach is flawed because
uniformity of description for some object types requires more
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semantic contortion and sacrifice than for others. That is why at
the beginning of this document, the ARK was said to be suited to
objects that accommodate reasonably regular electronic description.
While insisting on uniformity at the most basic level provides
powerful cross-domain leverage, the semantic sacrifice is great for
many applications. So the ERC also permits a semantically rich and
nuanced description to co-exist in a record along with a basic
description. In that way both sophisticated and naive recipients of
the record can extract the level of meaning from it that best suits
their needs and abilities. Key to unlocking the richer description
is a controlled vocabulary of ERC record types (not explained in this
document) that permit knowledgeable recipients to apply defined sets
of additional assumptions to the record.
5.4. Advice to Web Clients
ARKs are envisaged to appear wherever durable object references are
planned. Library cataloging records, literature citations, and
bibliographies are important examples. In many of these places URLs
(Uniform Resource Locators) are currently used, and inside some of
those URLs are embedded URNs, Handles, and DOIs. Unfortunately,
there's no suggestion of a way to probe for extra services that would
build confidence in those identifiers; in other words, there's no way
to tell whether any of those identifiers is any better managed than
the average URL.
ARKs are also envisaged to appear in hypertext links (where they are
not normally shown to users) and in rendered text (displayed or
printed). A normal HTML link for which the URL is not displayed
looks like this.
<a href = "https://example.org/index.htm"> Click Here <a>
A URL with an embedded ARK invites access (via "?info") to extra
services:
<a href = "https://example.org/ark:14697/b12345x"> Click Here <a>
Using the [N2T] resolver to provide identifier-scheme-agnostic
protection against hostname instability, this ARK could be published
as:
<a href = "https://n2t.net/ark:14697/b12345x"> Click Here <a>
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An NAA will typically make known the associations it creates by
publishing them in catalogs, actively advertizing them, or simply
leaving them on web sites for visitors (e.g., users, indexing
spiders) to stumble across in browsing.
5.5. Enhancements and Related Specifications
ARK services, data models, inflections, and applications continue to
evolve. Follow-on developments and specifications will be made
available from the ARK Maintenance Agency [ARKspecs].
5.6. Security Considerations
The ARK naming scheme poses no direct risk to computers and networks.
Implementors of ARK services need to be aware of security issues when
querying networks and filesystems for Name Mapping Authority
services, and the concomitant risks from spoofing and obtaining
incorrect information. These risks are no greater for ARK mapping
authority discovery than for other kinds of service discovery. For
example, recipients of ARKs with a specified NMA should treat it like
a URL and be aware that the identified ARK service may no longer be
operational.
Apart from mapping authority discovery, ARK clients and servers
subject themselves to all the risks that accompany normal operation
of the protocols underlying mapping services (e.g., HTTP). As
specializations of such protocols, an ARK service may limit exposure
to the usual risks. Indeed, ARK services may enhance a kind of
security by helping users identify long-term reliable references to
information objects.
6. Informative References
[ANVL] Kunze, J., Kahle, B., Masanes, J., and G. Mohr, "A Name-
Value Language", 2005,
<https://n2t.net/ark:/13030/c7x921j3h>.
[ARK] Kunze, J., "Towards Electronic Persistence Using ARK
Identifiers", IWAW/ECDL Annual Workshop Proceedings , 3
August 2003, <https://n2t.net/ark:/13030/c7n00zt1z>.
[ARKagency]
ARK Alliance, "ARK Maintenance Agency", 2021,
<https://arks.org>.
[ARKAtech] ARK Alliance, "ARK Alliance Technical Working Group",
2022, <https://wiki.lyrasis.org/display/ARKs/
Technical+Working+Group>.
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[ARKdrafts]
ARK Alliance, "ARK Drafts Repository", 2022,
<https://github.com/arks-org/arkspec>.
[ARKspecs] ARK Alliance, "ARK Maintenance Agency Specifications",
2021, <https://arks.org/specs/>.
[CURIE] W3C, "CURIE Syntax 1.0", December 2010,
<https://www.w3.org/TR/2010/NOTE-curie-20101216/>.
[DCKernel] Dublin Core Metadata Initiative, "Kernel Metadata Working
Group", 20012008, <https://dublincore.org/groups/kernel/>.
[DOI] I. D. Foundation, "The Digital Object Identifier (DOI)
System", February 2001, <https://doi.org/10.1000/203>.
[ERC] Kunze, J. and A. Turner, "Kernel Metadata and Electronic
Resource Citations", October 2007,
<https://n2t.net/ark:/13030/c7sn0141m>.
[Handle] Lannom, L., "Handle System Overview", ICSTI Forum No. 30 ,
April 1999, <https://eric.ed.gov/?id=ED450775>.
[Kernel] Kunze, J., "A Metadata Kernel for Electronic Permanence",
Journal of Digital Information Vol 2, Issue 2, ISSN
1368-7506 , January 2002,
<https://n2t.net/ark:/13030/c7rr1pm49>.
[N2T] ARK Alliance, "Name-to-Thing Resolver", August 2006,
<https://n2t.net>.
[NAANregistry]
ARKs.org, "NAAN Registry", 2019,
<https://n2t.net/e/pub/naan_registry.txt>.
[NAANrequest]
ARKs.org, "NAAN Request Form", 2018,
<https://n2t.net/e/naan_request>.
[NLMPerm] Byrnes, M., "Permanence Levels and the Archives for NLM's
Permanent Web Documents", March 2005,
<https://www.nlm.nih.gov/pubs/techbull/ma05/
ma05_archive.html>.
[NOID] Kunze, J., "Nice Opaque Identifiers", April 2006,
<https://metacpan.org/dist/Noid/view/noid>.
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[PStatements]
Kunze, J., "Persistence statements: describing digital
stickiness", October 2016,
<https://n2t.net/ark:/13030/c7833mx7t>.
[PURL] Shafer, K., "Introduction to Persistent Uniform Resource
Locators", 1996,
<https://www.internetsociety.org/inet96/proceedings/a4/
a4_1.htm>.
[RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, DOI 10.17487/RFC0854, May
1983, <https://www.rfc-editor.org/rfc/rfc854>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/rfc/rfc1034>.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, DOI 10.17487/RFC2141,
May 1997, <https://www.rfc-editor.org/rfc/rfc2141>.
[RFC2288] Lynch, C., Preston, C., and R. Daniel, "Using Existing
Bibliographic Identifiers as Uniform Resource Names",
RFC 2288, DOI 10.17487/RFC2288, February 1998,
<https://www.rfc-editor.org/rfc/rfc2288>.
[RFC2611] Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
"URN Namespace Definition Mechanisms", BCP 33, RFC 2611,
DOI 10.17487/RFC2611, June 1999,
<https://www.rfc-editor.org/rfc/rfc2611>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999,
<https://www.rfc-editor.org/rfc/rfc2616>.
[RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822,
DOI 10.17487/RFC2822, April 2001,
<https://www.rfc-editor.org/rfc/rfc2822>.
[RFC2915] Mealling, M. and R. Daniel, "The Naming Authority Pointer
(NAPTR) DNS Resource Record", RFC 2915,
DOI 10.17487/RFC2915, September 2000,
<https://www.rfc-editor.org/rfc/rfc2915>.
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[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[RFC5013] Kunze, J. and T. Baker, "The Dublin Core Metadata Element
Set", RFC 5013, DOI 10.17487/RFC5013, August 2007,
<https://www.rfc-editor.org/rfc/rfc5013>.
[shoulderrequest]
ARKs.org, "Shoulder Request Form", 2021,
<https://n2t.net/e/shoulder_request>.
[SPT] Kunze, J., "What is Suffix Passthrough?", May 2021,
<http://n2t.net/e/suffix_passthrough.html>.
[THUMP] Gamiel, K. and J. Kunze, "The HTTP URL Mapping Protocol",
August 2007, <https://www.ietf.org/archive/id/draft-kunze-
thump-03.txt>.
Appendix A. ARK Maintenance Agency: arks.org
The ARK Maintenance Agency [ARKagency] at arks.org has several
functions.
* To manage the registry of organizations that will be assigning
ARKs. Organizations can request or update a NAAN by filling out
the NAAN Request Form [NAANrequest].
* To be a clearinghouse for information about ARKs, such as best
practices, introductory documentation, tutorials, community
forums, etc. These supplemental resources help ARK implementors
in high-level applications across different sectors and
disciplines, and with a variety of metadata standards.
* To be a locus of discussion about future versions of the ARK
specification.
Appendix B. Looking up NMAs Distributed via DNS
This subsection introduces an older method for looking up NMAs that
is based on the method for discovering URN resolvers described in
[RFC2915]. It relies on querying the DNS system for Name Authority
Pointer (NAPTR) records that mirror the contents of the plain text
[NAANregistry] database. 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
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side-effect of a recent query. Responses come back inside NAPTR
records. The normal result is one or more candidate NMAs.
In its full generality the [RFC2915] algorithm ambitiously
accommodates a complex set of preferences, orderings, protocols,
mapping services, regular expression rewriting rules, and DNS record
types. This subsection proposes a drastic simplification of it for
the special case of ARK mapping authority discovery. The simplified
algorithm is called Maptr. It uses only one DNS record type (NAPTR)
and restricts most of its field values to constants. The following
hypothetical excerpt from a DNS data file for the NAAN known as 12026
shows three example NAPTR records ready to use with the Maptr
algorithm.
12026.ark.arpa.
;; US Library of Congress
;; order pref flags service regexp replacement
IN NAPTR 0 0 "h" "ark" "USLC" lhc.nlm.nih.gov:8080
IN NAPTR 0 0 "h" "ark" "USLC" foobar.zaf.org
IN NAPTR 0 0 "h" "ark" "USLC" sneezy.dopey.com
All the fields are held constant for Maptr except for the "flags",
"regexp", and "replacement" fields. The "service" field contains the
constant value "ark" so that NAPTR records participating in the Maptr
algorithm will not be confused with other NAPTR records. The "order"
and "pref" fields are held to 0 (zero) and otherwise ignored for now;
the algorithm may evolve to use these fields for ranking decisions
when usage patterns and local administrative needs are better
understood.
When a Maptr query returns a record with a flags field of "h" (for
host, a Maptr extension to the NAPTR flags), the replacement field
contains the NMA (host) of an ARK service provider. When a query
returns a record with a flags field of "" (the empty string), the
client needs to submit a new query containing the domain name found
in the replacement field. This second sort of record exploits the
distributed nature of DNS by redirecting the query to another domain
name. It looks like this.
12345.ark.arpa.
;; Digital Library Consortium
;; order pref flags service regexp replacement
IN NAPTR 0 0 "" "ark" "" dlc.spct.org.
Here is the Maptr algorithm for ARK mapping authority discovery. In
it replace <NAAN> with the NAAN from the ARK for which an NMA is
sought.
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1. Initialize the DNS query: type=NAPTR, query=<NAAN>.ark.arpa.
2. Submit the query to DNS and retrieve (NAPTR) records, discarding
any record that does not have "ark" for the service field.
3. All remaining records with a flags fields of "h" contain
candidate NMAs in their replacement fields. Set them aside, if
any.
4. Any record with an empty flags field ("") has a replacement field
containing a new domain name to which a subsequent query should
be redirected. For each such record, set query=<replacement>
then go to step (2). When all such records have been recursively
exhausted, go to step (5).
5. All redirected queries have been resolved and a set of candidate
NMAs has been accumulated from steps (3). If there are zero
NMAs, exit -- no mapping authority was found. If there is one or
more NMA, choose one using any criteria you wish, then exit.
A Perl script that implements this algorithm is included here.
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#!/usr/bin/env perl
use Net::DNS; # include simple DNS package
my $qtype = "NAPTR"; # initialize query type
my $naa = shift; # get NAAN script argument
my $mad = new Net::DNS::Resolver; # mapping authority discovery
&maptr("$naa.ark.arpa"); # call maptr - that's it
sub maptr { # recursive maptr algorithm
my $dname = shift; # domain name as argument
my ($rr, $order, $pref, $flags, $service, $regexp,
$replacement);
my $query = $mad->query($dname, $qtype);
return # non-productive query
if (! $query || ! $query->answer);
foreach $rr ($query->answer) {
next # skip records of wrong type
if ($rr->type ne $qtype);
($order, $pref, $flags, $service, $regexp,
$replacement) = split(/\s/, $rr->rdatastr);
if ($flags eq "") {
&maptr($replacement); # recurse
} elsif ($flags eq "h") {
print "$replacement\n"; # candidate NMA
}
}
}
Authors' Addresses
John A. Kunze
Ronin Institute
United States of America
Email: jakkbl@gmail.com
Emmanuelle Bermès
École nationale des Chartes
65 Rue de Richelieu
75002 Paris
France
Email: emmanuelle.bermes@chartes.psl.eu
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