Network Preservation Group J. Kunze
California Digital Library
R. Rodgers
US National Library of Medicine
May 22, 2008
The ARK Identifier Scheme
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Abstract
The ARK (Archival Resource Key) naming scheme is designed to
facilitate the high-quality and persistent identification of
information objects. A founding principle of the ARK is that
persistence is purely a matter of service and is neither inherent in
an object nor conferred on it by a particular naming syntax. The
best that an identifier can do is to lead users to the services that
support robust reference. The term ARK itself refers both to the
scheme and to any single identifier that conforms to it. An ARK has
five components:
[http://NMAH/]ark:/NAAN/Name[Qualifier]
an optional and mutable Name Mapping Authority Hostport (usually a
hostname), the "ark:" label, the Name Assigning Authority Number
(NAAN), the assigned Name, and an optional and possibly mutable
Qualifier supported by the NMA. The NAAN and Name together form the
immutable persistent identifier for the object independent of the URL
hostname. An ARK is a special kind of URL that connects users to
three things: the named object, its metadata, and the provider's
promise about its persistence. When entered into the location field
of a Web browser, the ARK leads the user to the named object. That
same ARK, inflected by appending a single question mark (`?'),
returns a brief metadata record that is both human- and machine-
readable. When the ARK is inflected by appending dual question marks
(`??'), the returned metadata contains a commitment statement from
the current provider. Tools exist for minting, binding, and
resolving ARKs.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Reasons to Use ARKs . . . . . . . . . . . . . . . . . . . 5
1.2. Three Requirements of ARKs . . . . . . . . . . . . . . . . 6
1.3. Organizing Support for ARKs: Our Stuff vs. Their Stuff . 7
1.4. Definition of Identifier . . . . . . . . . . . . . . . . . 8
2. ARK Anatomy . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1. The Name Mapping Authority Hostport (NMAH) . . . . . . . . 10
2.2. The ARK Label Part (ark:/) . . . . . . . . . . . . . . . . 11
2.3. The Name Assigning Authority Number (NAAN) . . . . . . . . 12
2.4. The Name Part . . . . . . . . . . . . . . . . . . . . . . 13
2.5. The Qualifier Part . . . . . . . . . . . . . . . . . . . . 13
2.5.1. ARKs that Reveal Object Hierarchy . . . . . . . . . . 15
2.5.2. ARKs that Reveal Object Variants . . . . . . . . . . . 16
2.6. Character Repertoires . . . . . . . . . . . . . . . . . . 17
2.7. Normalization and Lexical Equivalence . . . . . . . . . . 18
3. Naming Considerations . . . . . . . . . . . . . . . . . . . . 20
3.1. ARKS Embedded in Language . . . . . . . . . . . . . . . . 20
3.2. Objects Should Wear Their Identifiers . . . . . . . . . . 20
3.3. Names are Political, not Technological . . . . . . . . . . 21
3.4. Choosing a Hostname or NMA . . . . . . . . . . . . . . . . 21
3.5. Assigners of ARKs . . . . . . . . . . . . . . . . . . . . 23
3.6. NAAN Namespace Management . . . . . . . . . . . . . . . . 23
3.7. Sub-Object Naming . . . . . . . . . . . . . . . . . . . . 25
4. Finding a Name Mapping Authority . . . . . . . . . . . . . . . 26
4.1. Looking Up NMAHs in a Globally Accessible File . . . . . . 27
5. Generic ARK Service Definition . . . . . . . . . . . . . . . . 29
5.1. Generic ARK Access Service (access, location) . . . . . . 29
5.1.1. Generic Policy Service (permanence, naming, etc.) . . 29
5.1.2. Generic Description Service . . . . . . . . . . . . . 31
5.2. Overview of The HTTP URL Mapping Protocol (THUMP) . . . . 31
5.3. The Electronic Resource Citation (ERC) . . . . . . . . . . 34
5.4. Advice to Web Clients . . . . . . . . . . . . . . . . . . 36
5.5. Security Considerations . . . . . . . . . . . . . . . . . 37
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Appendix A. ARK Maintenance Agency . . . . . . . . . . . . . . . 40
Appendix B. Looking up NMAHs Distributed via DNS . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 44
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1. Introduction
This document describes a scheme for the high-quality naming of
information resources. The scheme, called the Archival Resource Key
(ARK), is well suited to long-term access and identification of any
information resources that accommodate reasonably regular electronic
description. This includes digital documents, databases, software,
and websites, as well as physical objects (books, bones, statues,
etc.) and intangible objects (chemicals, diseases, vocabulary terms,
performances). Hereafter the term "object" refers to an information
resource. The term ARK itself refers both to the scheme and to any
single identifier that conforms to it. A reasonably concise and
accessible overview and rationale for the scheme is available at
[ARK].
Schemes for persistent identification of network-accessible objects
are not new. In the early 1990's, the design of the Uniform Resource
Name [RFC2141] responded to the observed failure rate of URLs by
articulating an indirect, non-hostname-based naming scheme and the
need for responsible name management. Meanwhile, promoters of the
Digital Object Identifier [DOI] succeeded in building a community of
providers around a mature software system [Handle] that supports name
management. The Persistent Uniform Resource Locator [PURL] was
another scheme that had the advantage of working with unmodified web
browsers. ARKs represent an approach that attempts to build on the
strengths and to avoid the weaknesses of these schemes.
A founding principle of the ARK is that persistence is purely a
matter of service. Persistence is neither inherent in an object nor
conferred on it by a particular naming syntax. Nor is the technique
of name indirection -- upon which URNs, Handles, DOIs, and PURLs are
founded -- of central importance. Name indirection is an ancient and
well-understood practice; new mechanisms for it keep appearing and
distracting practitioner attention, with the Domain Name System (DNS)
[RFC1034] being a particularly dazzling and elegant example. What is
often forgotten is that maintenance of an indirection table is an
unavoidable cost to the organization providing persistence, and that
cost is equivalent across naming schemes. That indirection has
always been a native part of the web while being so lightly utilized
for the persistence of web-based objects indicates how unsuited most
organizations will probably be to the task of table maintenance and
to the much more fundamental challenge of keeping the objects
themselves viable.
Persistence is achieved through a provider's successful stewardship
of objects and their identifiers. The highest level of persistence
will be reinforced by a provider's robust contingency, redundancy,
and succession strategies. It is further safeguarded to the extent
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that a provider's mission is shielded from funding and political
instabilities. These are by far the major challenges confronting
persistence providers, and no identifier scheme has any direct impact
on them. In fact, some schemes may actually be liabilities for
persistence because they create short- and long-term dependencies for
every object access on complex, special-purpose infrastructures,
parts of which are proprietary and all of which increase the carry-
forward burden for the preservation community. It is for this reason
that the ARK scheme relies only on educated name assignment and light
use of general-purpose infrastructures that are maintained mostly by
the internet community at large (the DNS, web servers, and web
browsers).
1.1. Reasons to Use ARKs
If no persistent identifier scheme contributes directly to
persistence, why not just use URLs? A particular URL may be as
durable an identifier as it is possible to have, but nothing
distinguishes it from an ordinary URL to the recipient who is
wondering if it is suitable for long-term reference. An ARK embedded
in a URL provides some of the necessary conditions for credible
persistence, inviting access to not one, but to three things: to the
object, to its metadata, and to a nuanced statement of commitment
from the provider in question (the NMA, described below) regarding
the object. Existence of the two extra services can be probed
automatically by appending `?' and `??' to the ARK.
The form of the ARK also supports the natural separation of naming
authorities into the original name assigning authority and the
diverse multiple name mapping (or servicing) authorities that in
succession and in parallel will take over custodial responsibilities
from the original assigner (assuming the assigner ever held that
responsibility) for the large majority of a long-term object's
archival lifetime. The name mapping authority, indicated by the
hostname part of the URL that contains the ARK, serves to launch the
ARK into cyberspace. Should it ever fail (and there is no reason why
a well-chosen hostname for a 100-year-old cultural memory institution
shouldn't last as long as the DNS), that host name is considered
disposeable and replaceable. Again, the form of the ARK helps
because it defines exactly how to recover the core immutable object
identity, and simple algorithms (one based on the URN model) or even
by-hand internet query can be used for for locating another mapping
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
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hostnames with persistence in mind.
1.2. Three Requirements of ARKs
The first requirement of an ARK is to give users a link from an
object to a promise of stewardship for it. That promise is a multi-
faceted covenant that binds the word of an identified service
provider to a specific set of responsibilities. It is critical for
the promise to come from a current provider and almost irrelevant,
over a long period of time, what the original assigner's intentions
were. No one can tell if successful stewardship will take place
because no one can predict the future. Reasonable conjecture,
however, may be based on past performance. There must be a way to
tie a promise of persistence to a provider's demonstrated or
perceived ability -- its reputation -- in that arena. Provider
reputations would then rise and fall as promises are observed
variously to be kept and broken. This is perhaps the best way we
have for gauging the strength of any persistence promise.
The second requirement of an ARK is to give users a link from an
object to a description of it. The problem with a naked identifier
is that without a description real identification is incomplete.
Identifiers common today are relatively opaque, though some contain
ad hoc clues reflecting assertions that were briefly true, such as
where in a filesystem hierarchy an object lived during a short stay.
Possession of both an identifier and an object is some improvement,
but positive identification may still be uncertain since the object
itself might not include a matching identifier or might not carry
evidence obvious enough to reveal its identity without significant
research. In either case, what is called for is a record bearing
witness to the identifier's association with the object, as supported
by a recorded set of object characteristics. This descriptive record
is partly an identification "receipt" with which users and archivists
can verify an object's identity after brief inspection and a
plausible match with recorded characteristics such as title and size.
The final requirement of an ARK is to give users a link to the object
itself (or to a copy) if at all possible. Persistent access is the
central duty of an ARK. Persistent identification plays a vital
supporting role but, strictly speaking, it can be construed as no
more than a record attesting to the original assignment of a never-
reassigned identifier. Object access may not be feasible for various
reasons, such as a transient service outage, a catastrophic loss, a
licensing agreement that keeps an archive "dark" for a period of
years, or when an object's own lack of tangible existence confuses
normal concepts of access (e.g., a vocabulary term might be
"accessed" through its definition). In such cases the ARK's
identification role assumes a much higher profile. But attempts to
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simplify the persistence problem by decoupling access from
identification and concentrating exclusively on the latter are of
questionable utility. A perfect system for assigning forever unique
identifiers might be created, but if it did so without reducing
access failure rates, no one would be interested. The central issue
-- which may be summed up as the "HTTP 404 Not Found" problem --
would not have been addressed.
1.3. Organizing Support for ARKs: Our Stuff vs. Their Stuff
An organization and the user community it serves can often be seen to
struggle with two different areas of persistent identification: the
Our Stuff problem and the Their Stuff problem. In the Our Stuff
problem, we in the organization want our own objects to acquire
persistent names. Since we possess or control these objects, our
organization tackles the Our Stuff problem directly. Whether or not
the objects are named by ARKs, our organization is the responsible
party, so it can plan for, maintain, and make commitments about the
objects.
In the Their Stuff problem, we in the organization want others'
objects to acquire persistent names. These are objects that we do
not own or control, but some of which are critically important to us.
But because they are beyond our influence as far as support is
concerned, creating and maintaining persistent identifiers for Their
Stuff is not especially purposeful or feasible for us to engage in.
There is little that we can do about someone else's stuff except
encourage their uptake or adoption of persistence services.
Co-location of persistent access and identification services is
natural. Any organization that undertakes ongoing support of true
persistent identification (which includes description) is well-served
if it controls, owns, or otherwise has clear internal access to the
identified objects, and this gives it an advantage if it wishes also
to support persistent access to outsiders. Conversely, persistent
access to outsiders requires orderly internal collection management
procedures that include monitoring, acquisition, verification, and
change control over objects, which in turn requires object
identifiers persistent enough to support auditable record keeping
practices.
Although, organizing ARK services under one roof thus tends to make
sense, object hosting can successfully be separated from name
mapping. An example is when a name mapping authority centrally
provides uniform resolution services via a protocol gateway on behalf
of organizations that host objects behind a variety of access
protocols. It is also reasonable to build value-added description
services that rely on the underlying services of a set of mapping
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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.
Without regard to whether an object is physical, digital, or
conceptual, to identify it is to claim an association between it and
a representative string, such as "Jane" or "ISBN 0596000278". What
gives a claim credibility is a set of verifiable assertions, or
metadata, about the object, such as age, height, title, or number of
pages. In other words, the association is made manifest by a record
(e.g., a cataloging or other metadata record) that vouches for it.
In the complete absence of any testimony (metadata) regarding an
association, a would-be identifier string is a meaningless sequence
of characters. To keep an externally visible but otherwise internal
string from being perceived as an identifier by outsiders, for
example, it suffices for an organization not to disclose the nature
of its association. For our immediate purpose, actual existence of
an association record is more important than its authenticity or
verifiability, which are outside the scope of this specification.
It is a gift to the identification process if an object carries its
own name as an inseparable part of itself, such as an identifier
imprinted on the first page of a document or embedded in a data
structure element of a digital document header. In cases where the
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object is large, unwieldy, or unavailable (such as when licensing
restrictions are in effect), a metadata record that includes the
identifier string will usually suffice. That record becomes a
conveniently manipulable object surrogate, acting as both an
association "receipt" and "declaration".
Note that our definition of identifier extends the one in use for
Uniform Resource Identifiers [RFC3986]. The present document still
sometimes (ab)uses the terms "ARK" and "identifier" as shorthand for
the string part of an identifier, but the context should make the
meaning clear.
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2. ARK Anatomy
An ARK is represented by a sequence of characters (a string) that
contains the label, "ark:", optionally preceded by the beginning part
of a URL. Here is a diagrammed example.
http://example.org/ark:/12025/654xz321/s3/f8.05v.tiff
\________________/ \__/ \___/ \______/ \____________/
(replaceable) | | | Qualifier
| ARK Label | | (NMA-supported)
| | |
Name Mapping Authority | Name (NAA-assigned)
Hostport (NMAH) |
Name Assigning Authority Number (NAAN)
The ARK syntax can be summarized,
[http://NMAH/]ark:/NAAN/Name[Qualifier]
where the NMAH and Qualifier parts are in brackets to indicate that
they are optional.
2.1. The Name Mapping Authority Hostport (NMAH)
Before the "ark:" label may appear an optional Name Mapping Authority
Hostport (NMAH) that is a temporary address where ARK service
requests may be sent. It consists of "http://" (or any service
specification valid for a URL) followed by an Internet hostname or
hostport combination having the same format and semantics as the
hostport part of a URL. The most important thing about the NMAH is
that it is "identity inert" from the point of view of object
identification. In other words, ARKs that differ only in the
optional NMAH part identify the same object. Thus, for example, the
following three ARKs are synonyms for just one information object:
http://loc.gov/ark:/12025/654xz321
http://rutgers.edu/ark:/12025/654xz321
ark:/12025/654xz321
Strictly speaking, in the realm of digital objects, these ARKs may
lead over time to somewhat different or diverging instances of the
originally named object. In an ideal world, divergence of persistent
objects is not desirable, but it is widely believed that digital
preservation efforts will inevitably lead to alterations in some
original objects (e.g, a format migration in order to preserve the
ability to display a document). If any of those objects are held
redundantly in more than one organization (a common preservation
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strategy), chances are small that all holding organizations will
perform the same precise transformations and all maintain the same
object metadata. More significant divergence would be expected when
the holding organizations serve different audiences or compete with
each other.
The NMAH part makes an ARK into an actionable URL. As with many
internet parameters, it is helpful to approach the NMAH being liberal
in what you accept and conservative in what you propose. From the
recipient's point of view, the NMAH part should be treated as
temporary, disposable, and replaceable. From the NMA's point of
view, it should be chosen with the greatest concern for longevity. A
carefully chosen NMAH should be at least as permanent as the
providing organization's own hostname. In the case of a national or
university library, for example, there is no reason why the NMAH
should not be considerably more permanent than soft-funded proxy
hostnames such as hdl.handle.net, dx.doi.org, and purl.org. In
general and over time, however, it is not unexpected for an NMAH
eventually to stop working and require replacement with the NMAH of a
currently active service provider.
This replacement relies on a mapping authority "resolver" discovery
process, of which two alternate methods are outlined in a later
section. The ARK, URN, Handle, and DOI schemes all use a resolver
discovery model that sooner or later requires matching the original
assigning authority with a current provider servicing that
authority's named objects; once found, the resolver at that provider
performs what amounts to a redirect to a place where the object is
currently held. All the schemes rely on the ongoing functionality of
currently mainstream technologies such as the Domain Name System
[RFC1034] and web browsers. The Handle and DOI schemes in addition
require that the Handle protocol layer and global server grid be
available at all times.
The practice of prepending "http://" and an NMAH to an ARK is a way
of creating an actionable identifier by a method that is itself
temporary. Assuming that infrastructure supporting [RFC2616]
information retrieval will no longer be available one day, ARKs will
then have to be converted into new kinds of actionable identifiers.
By that time, if ARKs see widespread use, web browsers would
presumably evolve to perform this (currently simple) transformation
automatically.
2.2. The ARK Label Part (ark:/)
The label part distinguishes an ARK from an ordinary identifier. In
a URL found in the wild, the string, "ark:/", indicates that the URL
stands a reasonable chance of being an ARK. If the context warrants,
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verification that it actually is an ARK can be done by testing it for
existence of the three ARK services.
Since nothing about an identifier syntax directly affects
persistence, the "ark:" label (like "urn:", "doi:", and "hdl:")
cannot tell you whether the identifier is persistent or whether the
object is available. It does tell you that the original Name
Assigning Authority (NAA) had some sort of hopes for it, but it
doesn't tell you whether that NAA is still in existence, or whether a
decade ago it ceased to have any responsibility for providing
persistence, or whether it ever had any responsibility beyond naming.
Only a current provider can say for certain what sort of commitment
it intends, and the ARK label suggests that you can query the NMAH
directly to find out exactly what kind of persistence is promised.
Even if what is promised is impersistence (i.e., a short-term
identifier), saying so is valuable information to the recipient.
Thus an ARK is a high-functioning identifier in the sense that it
provides access to the object, the metadata, and a commitment
statement, even if the commitment is explicitly very weak.
2.3. The Name Assigning Authority Number (NAAN)
Recalling that the general form of the ARK is,
[http://NMAH/]ark:/NAAN/Name[Qualifier]
the part of the ARK directly following the "ark:" is the Name
Assigning Authority Number (NAAN) enclosed in `/' (slash) characters.
This part is always required, as it identifies the organization that
originally assigned the Name of the object. It is used to discover a
currently valid NMAH and to provide top-level partitioning of the
space of all ARKs. NAANs are registered in a manner similar to URN
Namespaces, but they are pure numbers consisting of 5 digits or 9
digits. Thus, the first 100,000 registered NAAs fit compactly into
the 5 digits, and if growth warrants, the next billion fit into the 9
digit form. In either case the fixed odd numbers of digits helps
reduce the chances of finding a NAAN out of context and confusing it
with nearby quantities such as 4-digit dates.
The NAAN designates a top-level ARK namespace. Once registered for a
namespace, a NAAN is never re-registered. It is possible, however,
for there to be a succession of organizations that manage of an ARK
namespace.
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2.4. The Name Part
The part of the ARK just after the NAAN is the Name assigned by the
NAA, and it is also required. Semantic opaqueness in the Name part
is strongly encouraged in order to reduce an ARK's vulnerability to
era- and language-specific change. Identifier strings containing
linguistic fragments can create support difficulties down the road.
No matter how appropriate or even meaningless they are today, such
fragments may one day create confusion, give offense, or infringe on
a trademark as the semantic environment around us and our communities
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 a
couple at a time is a relatively safe and easy way to achieve a
denser namespace (more possible names for a given length of the name
string). Such names have a chance of aging and traveling well.
Tools exists that mint, bind, and resolve opaque identifiers, with or
without check characters [NOID]. More on naming considerations is
given in a subsequent section.
2.5. The Qualifier Part
The part of the ARK following the NAA-assigned Name is an optional
Qualifier. It is a string that extends the base ARK in order to
create a kind of service entry point into the object named by the
NAA. At the discretion of the providing NMA, such a service entry
point permits an ARK to support access to individual hierarchical
components and subcomponents of an object, and to variants (versions,
languages, formats) of components. A Qualifier may be invented by
the NAA or by any NMA servicing the object.
In form, the Qualifier is a ComponentPath, or a VariantPath, or a
ComponentPath followed by a VariantPath. A VariantPath is introduced
and subdivided by the reserved character `.', and a ComponentPath is
introduced and subdivided by the reserved character `/'. In this
example,
http://example.org/ark:/12025/654xz321/s3/f8.05v.tiff
the string "/s3/f8" is a ComponentPath and the string ".05v.tiff" is
a VariantPath. The ARK Qualifier is a formalization of some
currently mainstream URL syntax conventions. This formalization
specifically reserves meanings that permit recipients to make strong
inferences about logical sub-object containment and equivalence based
only on the form of the received identifiers; there is great
efficiency in not having to inspect metadata records to discover such
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relationships. NMAs are free not to disclose any of these
relationships merely by avoiding the reserved characters above.
Hierarchical components and variants are discussed further in the
next two sections.
The Qualifier, if present, differs from the Name in several important
respects. First, a Qualifier may have been assigned either by the
NAA or later by the NMA. The assignment of a Qualifier by an NMA
effectively amounts to an act of publishing a service entry point
within the conceptual object originally named by the NAA. For our
purposes, an ARK extended with a Qualifier assigned by an NMA will be
called an NMA-qualified ARK.
Second, a Qualifier assignment on the part of an NMA is made in
fulfillment of its service obligations and may reflect changing
service expectations and technology requirements. NMA-qualified ARKs
could therefore be transient, even if the base, unqualified ARK is
persistent. For example, it would be reasonable for an NMA to
support access to an image object through an actionable ARK that is
considered persistent even if the experience of that access changes
as linking, labeling, and presentation conventions evolve and as
format and security standards are updated. For an image "thumbnail",
that NMA could also support an NMA-qualified ARK that is considered
impersistent because the thumbnail will be replaced with higher
resolution images as network bandwidth and CPU speeds increase. At
the same time, for an originally scanned, high-resolution master, the
NMA could publish an NMA-qualfied ARK that is itself considered
persistent. Of course, the NMA must be able to return its separate
commitments to unqualified, NAA-assigned ARKs, to NMA-qualified ARKs,
and to any NAA-qualified ARKs that it supports.
A third difference between a Qualifier and a Name concerns the
semantic opaqueness constraint. When an NMA-qualified ARK is to be
used as a transient service entry point into a persistent object, the
priority given to semantic opaqueness observed by the NAA in the Name
part may be relaxed by the NMA in the Qualifier part. If service
priorities in the Qualifier take precedence over persistence, short-
term usability considerations may recommend somewhat semantically
laden Qualifier strings.
Finally, not only is the set of Qualifiers supported by an NMA
mutable, but different NMAs may support different Qualifier sets for
the same NAA-identified object. In this regard the NMAs act
independently of each other and of the NAA.
The next two sections describe how ARK syntax may be used to declare,
or to avoid declaring, certain kinds of relatedness among qualified
ARKs.
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2.5.1. ARKs that Reveal Object Hierarchy
An NAA or NMA may choose to reveal the presence of a hierarchical
relationship between objects using the `/' (slash) character after
the Name part of an ARK. Some authorities will choose not to
disclose this information, while others will go ahead and disclose so
that manipulators of large sets of ARKs can infer object
relationships by simple identifier inspection; for example, this
makes it possible for a system to present a collapsed view of a large
search result set.
If the ARK contains an internal slash after the NAAN, the piece to
its left indicates a containing object. For example, publishing an
ARK of the form,
ark:/12025/654/xz/321
is equivalent to publishing three ARKs,
ark:/12025/654/xz/321
ark:/12025/654/xz
ark:/12025/654
together with a declaration that the first object is contained in the
second object, and that the second object is contained in the third.
Revealing the presence of hierarchy is completely up to the assigner
(NMA or NAA). It is hard enough to commit to one object's name, let
alone to three objects' names and to a specific, ongoing relatedness
among them. Thus, regardless of whether hierarchy was present
initially, the assigner, by not using slashes, reveals no shared
inferences about hierarchical or other inter-relatedness in the
following ARKs:
ark:/12025/654_xz_321
ark:/12025/654_xz
ark:/12025/654xz321
ark:/12025/654xz
ark:/12025/654
Note that slashes around the ARK's NAAN (/12025/ in these examples)
are not part of the ARK's Name and therefore do not indicate the
existence of some sort of NAAN super object containing all objects in
its namespace. A slash must have at least one non-structural
character (one that is neither a slash nor a period) on both sides in
order for it to separate recognizable structural components. So
initial or final slashes may be removed, and double slashes may be
converted into single slashes.
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2.5.2. ARKs that Reveal Object Variants
An NAA or NMA may choose to reveal the possible presence of variant
objects or object components using the `.' (period) character after
the Name part of an ARK. Some authorities will choose not to
disclose this information, while others will go ahead and disclose so
that manipulators of large sets of ARKs can infer object
relationships by simple identifier inspection; for example, this
makes it possible for a system to present a collapsed view of a large
search result set.
If the ARK contains an internal period after Name, the piece to its
left is a base name and the piece to its right, and up to the end of
the ARK or to the next period is a suffix. A Name may have more than
one suffix, for example,
ark:/12025/654.24
ark:/12025/xz4/654.24
ark:/12025/654.20v.78g.f55
ark:/12025/654.24
ark:/12025/xz4/654.24
ark:/12025/654.20v.78g.f55
There are two main rules. First, if two ARKs share the same base
name but have different suffixes, the corresponding objects were
considered variants of each other (different formats, languages,
versions, etc.) by the assigner (NMA or NAA). Thus, the following
ARKs are variants of each other:
ark:/12025/654.20v.78g.f55
ark:/12025/654.321xz
ark:/12025/654.44
Second, publishing an ARK with a suffix implies the existence of at
least one variant identified by the ARK without its suffix. The ARK
otherwise permits no further assumptions about what variants might
exist. So publishing the ARK,
ark:/12025/654.20v.78g.f55
is equivalent to publishing the four ARKs,
ark:/12025/654.20v.78g.f55
ark:/12025/654.20v.78g
ark:/12025/654.20v
ark:/12025/654
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Revealing the possibility of variants is completely up to the
assigner. It is hard enough to commit to one object's name, let
alone to multiple variants' names and to a specific, ongoing
relatedness among them. The assigner is the sole arbiter of what
constitutes a variant within its namespace, and whether to reveal
that kind of relatedness by using periods within its names.
A period must have at least one non-structural character (one that is
neither a slash nor a period) on both sides in order for it to
separate recognizable structural components. So initial or final
periods may be removed, and adjacent periods may be converted into a
single period. Multiple suffixes should be arranged in sorted order
(pure ASCII collating sequence) at the end of an ARK.
2.6. Character Repertoires
The Name and Qualifier parts are strings of visible ASCII characters
and should be less than 128 bytes in length. The length restriction
keeps the ARK short enough to append ordinary ARK request strings
without running into transport restrictions (e.g., within HTTP GET
requests). Characters may be letters, digits, or any of these six
characters:
= # * + @ _ $
The following characters may also be used, but their meanings are
reserved:
% - . /
The characters `/' and `.' are ignored if either appears as the last
character of an ARK. If used internally, they allow a name assigner
to reveal object hierarchy and object variants as previously
described.
Hyphens are considered to be insignificant and are always ignored in
ARKs. A `-' (hyphen) may appear in an ARK for readability, or it may
have crept in during the formatting and wrapping of text, but it must
be ignored in lexical comparisons. As in a telephone number, hyphens
have no meaning in an ARK. It is always safe for an NMA that
receives an ARK to remove any hyphens found in it. As a result, like
the NMAH, hyphens are "identity inert" in comparing ARKs for
equivalence. For example, the following ARKs are equivalent for
purposes of comparison and ARK service access:
ark:/12025/65-4-xz-321
http://sneezy.dopey.com/ark:/12025/654--xz32-1
ark:/12025/654xz321
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The `%' character is reserved for %-encoding all other octets that
would appear in the ARK string, in the same manner as for URIs
[RFC3986]. A %-encoded octet consists of a `%' followed by two hex
digits; for example, "%7d" stands in for `}'. Lower case hex digits
are preferred to reduce the chances of false acronym recognition;
thus it is better to use "%acT" instead of "%ACT". The character `%'
itself must be represented using "%25". As with URNs, %-encoding
permits ARKs to support legacy namespaces (e.g., ISBN, ISSN, SICI)
that have less restricted character repertoires [RFC2288].
2.7. Normalization and Lexical Equivalence
To determine if two or more ARKs identify the same object, the ARKs
are compared for lexical equivalence after first being normalized.
Since ARK strings may appear in various forms (e.g., having different
NMAHs), normalizing them minimizes the chances that comparing two ARK
strings for equality will fail unless they actually identify
different objects. In a specified-host ARK (one having an NMAH), the
NMAH never participates in such comparisons.
Normalization of an ARK for the purpose of octet-by-octet equality
comparison with another ARK consists of four steps. First, any upper
case letters in the "ark:" label and the two characters following a
`%' are converted to lower case. The case of all other letters in
the ARK string must be preserved. Second, any NMAH part is removed
(everything from an initial "http://" up to the next slash) and all
hyphens are removed.
Third, structural characters (slash and period) are normalized.
Initial and final occurrences are removed, and two structural
characters in a row (e.g., // or ./) are replaced by the first
character, iterating until each occurrence has at least one non-
structural character on either side. Finally, if there are any
components with a period on the left and a slash on the right, either
the component and the preceding period must be moved to the end of
the Name part or the ARK must be thrown out as malformed.
The fourth and final step is to arrange the suffixes in ASCII
collating sequence (that is, to sort them) and to remove duplicate
suffixes, if any. It is also permissible to throw out ARKs for which
the suffixes are not sorted.
The resulting ARK string is now normalized. Comparisons between
normalized ARKs are case-sensitive, meaning that upper case letters
are considered different from their lower case counterparts.
To keep ARK string variation to a minimum, no reserved ARK characters
should be %-encoded unless it is deliberately to conceal their
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reserved meanings. No non-reserved ARK characters should ever be
%-encoded. Finally, no %-encoded character should ever appear in an
ARK in its decoded form.
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3. Naming Considerations
The most important threats faced by persistence providers include
such things as funding loss, natural disaster, political and social
upheaval, processing faults, and errors in human oversight. There is
nothing that an identifer scheme can do about such things. Still, a
few observed identifier failures and inconveniences can be traced
back to naming practices that we now know to be less than optimal for
persistence.
3.1. ARKS Embedded in Language
The ARK has different goals from the URI, so it has different
character set requirements. Because linguistic constructs imperil
persistence, for ARKs non-ASCII character support is unimportant.
ARKs and URIs share goals of transcribability and transportability
within web documents, so characters are required to be visible, non-
conflicting with HTML/XML syntax, and not subject to tampering during
transmission across common transport gateways. Add the goal of
making an undelimited ARK recognizable in running prose, as in ark:/
12025/=@_22*$, and certain punctuation characters (e.g., comma,
period) end up being excluded from the ARK lest the end of a phrase
or sentence be mistaken for part of the ARK.
This consideration has more direct effect on ARK usability in a
natural language context than it has on ARK persistence. The same is
true of the rule preventing hyphens from having lexical significance.
It is fine to publish ARKs with hyphens in them (e.g., such as the
output of UUID/GUID generators), but the uniform treatment of hyphens
as insignificant reduces the possibility of users transcribing
identifiers that will have been broken through unpredictable
hyphenation by word processors. Any measure that reduces user
irritation with an identifier will increase its chances of survival.
3.2. Objects Should Wear Their Identifiers
A valuable technique for provision of persistent objects is to try to
arrange for the complete identifier to appear on, with, or near its
retrieved object. An object encountered at a moment in time when its
discovery context has long since disappeared could then easily be
traced back to its metadata, to alternate versions, to updates, etc.
This has seen reasonable success, for example, in book publishing and
software distribution. An identifier string only has meaning when
its association is known, and this a very sure, simple, and low-tech
method of reminding everyone exactly what that association is.
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3.3. Names are Political, not Technological
If persistence is the goal, a deliberate local strategy for
systematic name assignment is crucial. Names must be chosen with
great care. Poorly chosen and managed names will devastate any
persistence strategy, and they do not discriminate by identifier
scheme. Whether a mistakenly re-assigned name is a URN, DOI, PURL,
URL, or ARK, the damage -- failed access and confusion -- is not
mitigated more in one scheme than in another. Conversely, in-house
efforts to manage names responsibly will go much further towards
safeguarding persistence than any choice of naming scheme or name
resolution technology.
Branding (e.g., at the corporate or departmental level) is important
for funding and visibility, but substrings representing brands and
organizational names should be given a wide berth except when
absolutely necessary in the hostname (the identity-inert) part of the
ARK. These substrings are not only unstable because organizations
change frequently, but they are also dangerous because successor
organizations often have political or legal reasons to actively
suppress predecessor names and brands. Any measure that reduces the
chances of future political or legal pressure on an identifier will
decrease the chances that our descendants will be obliged to
deliberately break it.
3.4. Choosing a Hostname or NMA
Hostnames appearing in any identifier meant to be persistent must be
chosen with extra care. The tendency in hostname selection has
traditionally been to choose a token with recognizable attributes,
such as a corporate brand, but that tendency wreaks havoc with
persistence that is supposed to outlive brands, corporations, subject
classifications, and natural language semantics (e.g., what did the
three letters "gay" mean in 1958, 1978, and 1998?). Today's
recognized and correct attributes are tomorrow's stale or incorrect
attributes. In making hostnames (any names, actually) long-term
persistent, it helps to eliminate recognizable attributes to the
extent possible. This affects selection of any name based on URLs,
including PURLs and the explicitly disposable NMAHs.
There is no excuse for a provider that manages its internal names
impeccably not to exercise the same care in choosing what could be an
exceptionally durable hostname, especially if it would form the
prefix for all the provider's URL-based external names. Registering
an opaque hostname in the ".org" or ".net" domain would not be a bad
start. Another way is to publish your ARKs with an organizational
domain name that will be mapped by DNS to an appropriate NMA host.
This makes for shorter names with less branding vulnerability.
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It is a mistake to think that hostnames are inherently unstable. If
you require brand visibility, that may be a fact of life. But things
are easier if yours is the brand of long-lived cultural memory
institution such as a national or university library or archive.
Well-chosen hostnames from organizations that are sheltered from the
direct effects of a volatile marketplace can easily provide longer-
lived global resolvers than the domain names explicitly or implicitly
used as starting points for global resolution by indirection-based
persistent identifier schemes. For example, it is hard to imagine
circumstances under which the Library of Congress' domain name would
disappear sooner than, say, "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 consortium [N2T] of like-minded
organizations with the purpose of providing mutual preservation
support. The first goal of such a consortium would be to perpetually
rent a hostname on which to establish a web server that simply
redirects incoming member organization requests to the appropriate
member server; using ARKs, for example, a 150-member consortium could
run a very small server (24x7) that contained nothing more than 150
rewrite rules in its configuration file. Even more helpful would be
additional consortial support for a member organization that was
unable to continue providing services and needed to find a successor
archival organization. This would be a low-cost, low-tech way to
publish ARKs (or URLs) under highly persistent hostnames.
There are no obvious reasons why the organizations registering DNS
names, URN Namespaces, and DOI publisher IDs should have among them
one that is intrinsically more fallible than the next. Moreover, it
is a misconception that the demise of DNS and of HTTP need adversely
affect the persistence of URLs. At such a time, certainly URLs from
the present day might not then be actionable by our present-day
mechanisms, but resolution systems for future non-actionable URLs are
no harder to imagine than resolution systems for present-day non-
actionable URNs and DOIs. There is no more stable a namespace than
one that is dead and frozen, and that would then characterize the
space of names bearing the "http://" prefix. It is useful to
remember that just because hostnames have been carelessly chosen in
their brief history does not mean that they are unsuitable in NMAHs
(and URLs) intended for use in situations demanding the highest level
of persistence available in the Internet environment. A well-planned
name assignment strategy is everything.
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3.5. Assigners of ARKs
A Name Assigning Authority (NAA) is an organization that creates (or
delegates creation of) long-term associations between identifiers and
information objects. Examples of NAAs include national libraries,
national archives, and publishers. An NAA may arrange with an
external organization for identifier assignment. The US Library of
Congress, for example, allows OCLC (the Online Computer Library
Center, a major world cataloger of books) to create associations
between Library of Congress call numbers (LCCNs) and the books that
OCLC processes. A cataloging record is generated that testifies to
each association, and the identifier is included by the publisher,
for example, in the front matter of a book.
An NAA does not so much create an identifier as create an
association. The NAA first draws an unused identifier string from
its namespace, which is the set of all identifiers under its control.
It then records the assignment of the identifier to an information
object having sundry witnessed characteristics, such as a particular
author and modification date. A namespace is usually reserved for an
NAA by agreement with recognized community organizations (such as
IANA and ISO) that all names containing a particular string be under
its control. In the ARK an NAA is represented by the Name Assigning
Authority Number (NAAN).
The ARK namespace reserved for an NAA is the set of names bearing its
particular NAAN. For example, all strings beginning with "ark:/
12025/" are under control of the NAA registered under 12025, which
might be the National Library of Finland. Because each NAA has a
different NAAN, names from one namespace cannot conflict with those
from another. Each NAA is free to assign names from its namespace
(or delegate assignment) according to its own policies. These
policies must be documented in a manner similar to the declarations
required for URN Namespace registration [RFC2611].
To register for a NAAN, please read about the mapping authority
discovery file in the next section and send email to ark@cdlib.org.
3.6. NAAN Namespace Management
Every NAA must have a namespace management strategy. A time-honored
technique is to hierarchically partition a namespace into
subnamespaces using prefixes that guarantee non-collision of names in
different partition. This practice is strongly encouraged for all
NAAs, especially when subnamespace management will be delegated to
other departments, units, or projects within an organization. For
example, with a NAAN that is assigned to a university and managed by
its main library, care should be taken to reserve semantically opaque
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prefixes that will set aside large parts of the unused namespace for
future assignments. Prefix-based partition management is an
important responsibility of the NAA.
This sort of delegation by prefix is well-used in the formation of
DNS names and ISBN identifiers. An important difference is that in
the former, the hierarchy is deliberately exposed and in the latter
it is hidden. Rather than using lexical boundary markers such as the
period (`.') found in domain names, the ISBN uses a publisher prefix
but doesn't disclose where the prefix ends and the publisher's
assigned name begins. This practice of non-disclosure, borrowed from
the ISBN and ISSN schemes, is encouraged in assigning ARKs, because
it reduces the visibility of an assertion that is probably not
important now and may become a vulnerability later.
Reasonable prefixes for assigned names usually consist of consonants
and digits and are 1-5 characters in length. For example, the
constant prefix "x9t" might be delegated to a book digitization
project that creates identifiers such as
http://444.berkeley.edu/ark:/28722/x9t38rk45c
If longevity is the goal, it is important to keep the prefixes free
of recognizable semantics; for example, using an acronym representing
a project or a department is discouraged. At the same time, you may
wish to set aside a subnamespace for testing purposes under a prefix
such as "fk..." that can serve as a visual clue and reminder to
maintenance staff that this "fake" identifier was never published.
There are other measures one can take to avoid user confusion,
transcription errors, and the appearance of accidental semantics when
creating identifiers. If you are generating identifiers
automatically, pure numeric identifiers are likeley to be
semantically opaque enough, but it's probably useful to avoid leading
zeroes because some users mistakenly treat them as optional, thinking
(arithmetically) that they don't contribute to the "value" of the
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.
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3.7. Sub-Object Naming
As mentioned previously, semantically opaque identifiers are very
useful for long-term naming of abstract objects, however, it may be
appropriate to extend these names with less opaque extensions that
reference contemporary service entry points (sub-objects) in support
of the object. Sub-object extensions beginning with a digit or
underscore (`_') are reserved for the possibilty of developing a
future registry of canonical service points (e.g., numeric references
to versions, formats, languages, etc).
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4. Finding a Name Mapping Authority
In order to derive an actionable identifier (these days, a URL) from
an ARK, a hostport (hostname or hostname plus port combination) for a
working Name Mapping Authority (NMA) must be found. An NMA is a
service that is able to respond to the three basic ARK service
requests. Relying on registration and client-side discovery, NMAs
make known which NAAs' identifiers they are willing to service.
Upon encountering an ARK, a user (or client software) looks inside it
for the optional NMAH part (the hostport of the NMA's ARK service).
If it contains an NMAH that is working, this NMAH discovery step may
be skipped; the NMAH effectively uses the beginning of an ARK to
cache the results of a prior mapping authority discovery process. If
a new NMAH needs to found, the client looks inside the ARK again for
the NAAN (Name Assigning Authority Number). Querying a global
database, it then uses the NAAN to look up all current NMAHs that
service ARKs issued by the identified NAA.
The global database is key, and ideally the lookup would be automatic
and transparent to the user. For this, the most promising method is
probably the Name-to-Thing (N2T) Resolver [N2T] at n2t.info. It is a
proposed low-cost, highly reliable, consortially maintained NMAH that
simply exists to support actionable HTTP-based URLs for as long as
HTTP is used. One of its big advantages over the other two methods
and the URN, Handle, DOI, and PURL methods, is that N2T addresses the
namespace splitting problem. When objects maintained by one NMA are
inherited by more than one successor NMA, until now one of those
successors would be required to maintain forwarding tables on behalf
of the other successors.
There are two other ways to discover an NMAH, one of them described
in a subsection below. Another way, described in an appendix, is
based on a simplification of the URN resolver discovery method,
itself very similar in principle to the resolver discovery method
used by Handles and DOIs. None of these methods does more than what
can be done with a very small, consortially maintained web server
such as [N2T].
In the interests of long-term persistence, however, ARK mechanisms
are first defined in high-level, protocol-independent terms so that
mechanisms may evolve and be replaced over time without compromising
fundamental service objectives. Either or both specific methods
given here may eventually be supplanted by better methods since, by
design, the ARK scheme does not depend on a particular method, but
only on having some method to locate an active NMAH.
At the time of issuance, at least one NMAH for an ARK should be
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prepared to service it. That NMA may or may not be administered by
the Name Assigning Authority (NAA) that created it. Consider the
following hypothetical example of providing long-term access to a
cancer research journal. The publisher wishes to turn a profit and
the National Library of Medicine wishes to preserve the scholarly
record. An agreement might be struck whereby the publisher would act
as the NAA and the national library would archive the journal issue
when it appears, but without providing direct access for the first
six months. During the first six months of peak commercial
viability, the publisher would retain exclusive delivery rights and
would charge access fees. Again, by agreement, both the library and
the publisher would act as NMAs, but during that initial period the
library would redirect requests for issues less than six months old
to the publisher. At the end of the waiting period, the library
would then begin servicing requests for issues older than six months
by tapping directly into its own archives. Meanwhile, the publisher
might routinely redirect incoming requests for older issues to the
library. Long-term access is thereby preserved, and so is the
commercial incentive to publish content.
Although it will be common for an NAA also to run an NMA service, it
is never a requirement. Over time NAAs and NMAs will come and go.
One NMA will succeed another, and there might be many NMAs serving
the same ARKs simultaneously (e.g., as mirrors or as competitors).
There might also be asymmetric but coordinated NMAs as in the
library-publisher example above.
4.1. Looking Up NMAHs in a Globally Accessible File
This subsection describes a way to look up NMAHs using a simple name
authority table represented as a plain text file. For efficient
access the file may be stored in a local filesystem, but it needs to
be reloaded periodically to incorporate updates. It is not expected
that the size of the file or frequency of update should impose an
undue maintenance or searching burden any time soon, for even
primitive linear search of a file with ten-thousand NAAs is a
subsecond operation on modern server machines. The proposed file
strategy is similar to the /etc/hosts file strategy that supported
Internet host address lookup for a period of years before the advent
of DNS.
The name authority table file is updated on an ongoing basis and is
available for copying over the internet from the California Digital
Library at http://www.cdlib.org/inside/diglib/ark/natab and from a
number of mirror sites. The file contains comment lines (lines that
begin with `#') explaining the format and giving the file's
modification time, reloading address, and NAA registration
instructions. There is even a Perl script that processes the file
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embedded in the file's comments. The currently registered Name
Assigning Authorities are:
12025 National Library of Medicine
12026 Library of Congress
12027 National Agriculture Library
13030 California Digital Library
13038 World Intellectual Property Organization
20775 University of California San Diego
29114 University of California San Francisco
28722 University of California Berkeley
21198 University of California Los Angeles
15230 Rutgers University
13960 Internet Archive
64269 Digital Curation Centre
62624 New York University
67531 University of North Texas
27927 Ithaka Electronic-Archiving Initiative
12148 Bibliotheque nationale de France
/ National Library of France
78319 Google
88435 Princeton University
78428 University of Washington
89901 Archives of the Region of Vaestra Goetaland
and City of Gothenburg, Sweden
80444 Northwest Digital Archives
25593 Emory University
25031 University of Kansas
17101 Centre for Ecology & Hydrology, UK
65323 University of Calgary
61001 University of Chicago
52327 Bibliotheque et Archives Nationales du Quebec
/ National Libary and Archives of Quebec
39331 National Szechenyi Library / National Library of Hungary
26677 Library and Archives Canada / Bibliotheque et Archives Canada
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5. Generic ARK Service Definition
An ARK request's output is delivered information; examples include
the object itself, a policy declaration (e.g., a promise of support),
a descriptive metadata record, or an error message. The experience
of object delivery is expected to be an evolving mix of information
that reflects changing service expectations and technology
requirements; contemporary examples include such things as an object
summary and component links formatted for human consumption. ARK
services must be couched in high-level, protocol-independent terms if
persistence is to outlive today's networking infrastructural
assumptions. The high-level ARK service definitions listed below are
followed in the next section by a concrete method (one of many
possible methods) for delivering these services with today's
technology.
5.1. Generic ARK Access Service (access, location)
Returns (a copy of) the object or a redirect to the same, although a
sensible object proxy may be substituted. Examples of sensible
substitutes include,
o a table of contents instead of a large complex document,
o a home page instead of an entire web site hierarchy,
o a rights clearance challenge before accessing protected data,
o directions for access to an offline object (e.g., a book),
o a description of an intangible object (a disease, an event), or
o an applet acting as "player" for a large multimedia object.
May also return a discriminated list of alternate object locators.
If access is denied, returns an explanation of the object's current
(perhaps permanent) inaccessibility.
5.1.1. Generic Policy Service (permanence, naming, etc.)
Returns declarations of policy and support commitments for given
ARKs. Declarations are returned in either a structured metadata
format or a human readable text format; sometimes one format may
serve both purposes. Policy subareas may be addressed in separate
requests, but the following areas should should be covered: object
permanence, object naming, object fragment addressing, and
operational service support.
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The permanence declaration for an object is a rating defined with
respect to an identified permanence provider (guarantor), which will
be the NMA. It may include the following aspects.
(a) "object availability" -- whether and how access to the object
is supported (e.g., online 24x7, or offline only),
(b) "identifier validity" -- under what conditions the identifier
will be or has been re-assigned,
(c) "content invariance" -- under what conditions the content of
the object is subject to change, and
(d) "change history" -- access to corrections, migrations, and
revisions, whether through links to the changed objects themselves
or through a document summarizing the change history
One approach to a permanence rating framework, conceived
independently from ARKs, is given in [NLMPerm]. Under ongoing
development and limited deployment at the US National Library of
Medicine, it identifies the following "permanence levels":
Not Guaranteed: No commitment has been made to retain this
resource. It could become unavailable at any time. Its
identifier could be changed.
Permanent: Dynamic Content: A commitment has been made to keep
this resource permanently available. Its identifier will always
provide access to the resource. Its content could be revised or
replaced.
Permanent: Stable Content: A commitment has been made to keep this
resource permanently available. Its identifier will always
provide access to the resource. Its content is subject only to
minor corrections or additions.
Permanent: Unchanging Content: A commitment has been made to keep
this resource permanently available. Its identifier will always
provide access to the resource. Its content will not change.
Naming policy for an object includes an historical description of the
NAA's (and its successor NAA's) policies regarding differentiation of
objects. Since it the NMA who responds to requests for policy
statements, it is useful for the NMA to be able to produce or
summarize these historical NAA documents. Naming policy may include
the following aspects.
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(i) "similarity" -- (or "unity") the limit, defined by the NAA, to
the level of dissimilarity beyond which two similar objects
warrant separate identifiers but before which they share one
single identifier, and
(ii) "granularity" -- the limit, defined by the NAA, to the level
of object subdivision beyond which sub-objects do not warrant
separately assigned identifiers but before which sub-objects are
assigned separate identifiers.
Subnaming policy for an object describes the qualifiers that the NMA,
in fulfilling its ongoing and evolving service obligations, allows as
extensions to an NAA-assigned ARK. To the conceptual object that the
NAA named with an ARK, the NMA may add component access points and
derivatives (e.g., format migrations in aid of preservation) in order
to provide both basic and value-added services.
Addressing policy for an object includes a description of how, during
access, object components (e.g., paragraphs, sections) or views
(e.g., image conversions) may or may not be "addressed", in other
words, how the NMA permits arguments or parameters to modify the
object delivered as the result of an ARK request. If supported,
these sorts of operations would provide things like byte-ranged
fragment delivery and open-ended format conversions, or any set of
possible transformations that would be too numerous to list or to
identify with separately assigned ARKs.
Operational service support policy includes a description of general
operational aspects of the NMA service, such as after-hours staffing
and trouble reporting procedures.
5.1.2. Generic Description Service
Returns a description of the object. Descriptions are returned in
either a structured metadata format or a human readable text format;
sometimes one format may serve both purposes. A description must at
a minimum answer the who, what, when, and where questions concerning
an expression of the object. Standalone descriptions should be
accompanied by the modification date and source of the description
itself. May also return discriminated lists of ARKs that are related
to the given ARK.
5.2. Overview of The HTTP URL Mapping Protocol (THUMP)
The HTTP URL Mapping Protocol (THUMP) is a way of taking a key (any
identifier) and asking such questions as, what information does this
identify and how permanent is it? [THUMP] is in fact one specific
method under development for delivering ARK services. The protocol
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runs over HTTP to exploit the web browser's current pre-eminence as
user interface to the Internet. THUMP is designed so that a person
can enter ARK requests directly into the location field of current
browser interfaces. Because it runs over HTTP, THUMP can be
simulated and tested via keyboard-based interactions [RFC0854].
The asker (a person or client program) starts with an identifier,
such as an ARK or a URL. The identifier reveals to the asker (or
allows the asker to infer) the Internet host name and port number of
a server system that responds to questions. Here, this is just the
NMAH that is obtained by inspection and possibly lookup based on the
ARK's NAAN. The asker then sets up an HTTP session with the server
system, sends a question via a THUMP request (contained within an
HTTP request), receives an answer via a THUMP response (contained
within an HTTP response), and closes the session. That concludes the
connected portion of the protocol.
A THUMP request is a string of characters beginning with a `?'
(question mark) that is appended to the identifier string. The
resulting string is sent as an argument to HTTP's GET command.
Request strings too long for GET may be sent using HTTP's POST
command. The three most common requests correspond to three
degenerate special cases that keep the user's learning and typing
burden low. First, a simple key with no request at all is the same
as an ordinary access request. Thus a plain ARK entered into a
browser's location field behaves much like a plain URL, and returns
access to the primary identified object, for instance, an HTML
document.
The second special case is a minimal ARK description request string
consisting of just "?". For example, entering the string,
ark.nlm.nih.gov/12025/psbbantu?
into the browser's location field directly precipitates a request for
a metadata record describing the object identified by ark:/12025/
psbbantu. The browser, unaware of THUMP, prepares and sends an HTTP
GET request in the same manner as for a URL. THUMP is designed so
that the response (indicated by the returned HTTP content type) is
normally displayed, whether the output is structured for machine
processing (text/plain) or formatted for human consumption (text/
html).
In the following example THUMP session, each line has been annotated
to include a line number and whether it was the client or server that
sent it. Without going into much depth, the session has four pieces
separated from each other by blank lines: the client's piece (lines
1-3), the server's HTTP/THUMP response headers (4-7), and the body of
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the server's response (8-13). The first and last lines (1 and 13)
correspond to the client's steps to start the TCP session and the
server's steps to end it, respectively.
1 C: [opens session]
C: GET http://ark.nlm.nih.gov/ark:/12025/psbbantu? HTTP/1.1
C:
S: HTTP/1.1 200 OK
5 S: Content-Type: text/plain
S: THUMP-Status: 0.6 200 OK
S:
S: erc:
S: who: Lederberg, Joshua
10 S: what: Studies of Human Families for Genetic Linkage
S: when: 1974
S: where: http://profiles.nlm.nih.gov/BB/A/N/T/U/_/bbantu.pdf
S: [closes session]
The first two server response lines (4-5) above are typical of HTTP.
The next line (6) is peculiar to THUMP, and indicates the THUMP
version and a normal return status.
The balance of the response consists of a single metadata record
(8-12) that comprises the ARK description service response. The
returned record is in the format of an Electronic Resource Citation
[ERC], which is discussed in overview in the next section. For now,
note that it contains four elements that answer the top priority
questions regarding an expression of the object: who played a major
role in expressing it, what the expression was called, when is was
created, and where the expression may be found. This quartet of
elements comes up again and again in ERCs.
The third degenerate special case of an ARK request (and no other
cases will be described in this document) is the string "??",
corresponding to a minimal permanence policy request. It can be seen
in use appended to an ARK (on line 2) in the example session that
follows.
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1 C: [opens session]
C: GET http://ark.nlm.nih.gov/ark:/12025/psbbantu?? HTTP/1.1
C:
S: HTTP/1.1 200 OK
5 S: Content-Type: text/plain
S: THUMP-Status: 0.6 200 OK
S:
S: erc:
S: who: Lederberg, Joshua
10 S: what: Studies of Human Families for Genetic Linkage
S: when: 1974
S: where: http://profiles.nlm.nih.gov/BB/A/N/T/U/_/bbantu.pdf
S: erc-support:
S: who: USNLM
15 S: what: Permanent, Unchanging Content
S: when: 20010421
S: where: http://ark.nlm.nih.gov/yy22948
S: [closes session]
Each segment in an ERC tells a different story relating to the
object, so although the same four questions (elements) appear in
each, the answers depend on the segment's story type. While the
first segment tells the story of an expression of the object, the
second segment tells the story of the support commitment made to it:
who made the commitment, what the nature of the commitment was, when
it was made, and where a fuller explanation of the commitment may be
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 previous section shows two limited examples of what is fully
described elsewhere [ERC]. The rest of this short section provides
some of the background and rationale for this record format.
A founding principle of Kernel metadata is that direct human contact
with metadata will be a necessary and sufficient condition for the
near term rapid development of metadata standards, systems, and
services. Thus the machine-processable Kernel elements must only
minimally strain people's ability to read, understand, change, and
transmit ERCs without their relying on intermediation with
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specialized software tools. The basic ERC needs to be succinct,
transparent, and trivially parseable by software.
In the current Internet, it is natural seriously to consider using
XML as an exchange format because of predictions that it will obviate
many ad hoc formats and programs, and unify much of the world's
information under one reliable data structuring discipline that is
easy to generate, verify, parse, and render. It appears, however,
that XML is still only catching on after years of standards work and
implementation experience. The reasons for it are unclear, but for
now very simple XML interpretation is still out of reach. Another
important caution is that XML structures are hard on the eyeballs,
taking up an amount of display (and page) space that significantly
exceeds that of traditional formats. Until these conflicts with ERC
principle are resolved, XML is not a first choice for representing
ERCs. Borrowing instead from the data structuring format that
underlies the successful spread of email and web services, the first
ERC format uses [ANVL], which is based on email and HTTP headers
[RFC2822]. There is a naturalness to ANVL's label-colon-value format
(seen in the previous section) that barely needs explanation to a
person beginning to enter ERC metadata.
Besides simplicity of ERC system implementation and data entry
mechanics, ERC semantics (what the record and its constituent parts
mean) must also be easy to explain. ERC semantics are based on a
reformulation and extension of the Dublin Core [RFC5013] hypothesis,
which suggests that the fifteen Dublin Core metadata elements have a
key role to play in cross-domain resource description. The ERC
design recognizes that the Dublin Core's primary contribution is the
international, interdisciplinary consensus that identified fifteen
semantic buckets (element categories), regardless of how they are
labeled. The ERC then adds a definition for a record and some
minimal compliance rules. In pursuing the limits of simplicity, the
ERC design combines and relabels some Dublin Core buckets to isolate
a tiny kernel (subset) of four elements for basic cross-domain
resource description.
For the cross-domain kernel, the ERC uses the four basic elements --
who, what, when, and where -- to pretend that every object in the
universe can have a uniform minimal description. Each has a name or
other identifier, a location, some responsible person or party, and a
date. It doesn't matter what type of object it is, or whether one
plans to read it, interact with it, smoke it, wear it, or navigate
it. Of course, this approach is flawed because uniformity of
description for some object types requires more semantic contortion
and sacrifice than for others. That is why at the beginning of this
document, the ARK was said to be suited to objects that accommodate
reasonably regular electronic description.
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While insisting on uniformity at the most basic level provides
powerful cross-domain leverage, the semantic sacrifice is great for
many applications. So the ERC also permits a semantically rich and
nuanced description to co-exist in a record along with a basic
description. In that way both sophisticated and naive recipients of
the record can extract the level of meaning from it that best suits
their needs and abilities. Key to unlocking the richer description
is a controlled vocabulary of ERC record types (not explained in this
document) that permit knowledgeable recipients to apply defined sets
of additional assumptions to the record.
5.4. Advice to Web Clients
ARKs are envisaged to appear wherever durable object references are
planned. Library cataloging records, literature citations, and
bibliographies are important examples. In many of these places URLs
(Uniform Resource Locators) are currently used, and inside some of
those URLs are embedded URNs, Handles, and DOIs. Unfortunately,
there's no suggestion of a way to probe for extra services that would
build confidence in those identifiers; in other words, there's no way
to tell whether any of those identifiers is any better managed than
the average URL.
ARKs are also envisaged to appear in hypertext links (where they are
not normally shown to users) and in rendered text (displayed or
printed). A normal HTML link for which the URL is not displayed
looks like this.
Click Here
A URL with an embedded ARK invites access (via `?' and `??') to extra
services:
Click Here
Using the [N2T] resolver to provide identifier-scheme-agnostic
protection against hostname instability, this ARK could be published
as:
Click Here
An NAA will typically make known the associations it creates by
publishing them in catalogs, actively advertizing them, or simply
leaving them on web sites for visitors (e.g., users, indexing
spiders) to stumble across in browsing.
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5.5. Security Considerations
The ARK naming scheme poses no direct risk to computers and networks.
Implementors of ARK services need to be aware of security issues when
querying networks and filesystems for Name Mapping Authority
services, and the concomitant risks from spoofing and obtaining
incorrect information. These risks are no greater for ARK mapping
authority discovery than for other kinds of service discovery. For
example, recipients of ARKs with a specified hostport (NMAH) should
treat it like a URL and be aware that the identified ARK service may
no longer be operational.
Apart from mapping authority discovery, ARK clients and servers
subject themselves to all the risks that accompany normal operation
of the protocols underlying mapping services (e.g., HTTP, Z39.50).
As specializations of such protocols, an ARK service may limit
exposure to the usual risks. Indeed, ARK services may enhance a kind
of security by helping users identify long-term reliable references
to information objects.
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6. References
[ANVL] Kunze, J. and B. Kahle, "A Name-Value Language", 2008,
.
[ARK] Kunze, J., "Towards Electronic Persistence Using ARK
Identifiers", IWAW/ECDL Annual Workshop Proceedings 3rd,
August 2003,
.
[DCKernel]
DCMI, "Kernel Metadata Working Group", 2001-2008,
.
[DOI] IDF, "The Digital Object Identifier (DOI) System",
February 2001, .
[ERC] Kunze, J. and A. Turner, "Kernel Metadata and Electronic
Resource Citations", October 2007,
.
[Handle] Lannom, L., "Handle System Overview", ICSTI Forum No. 30,
April 1999, .
[Kernel] Kunze, J., "A Metadata Kernel for Electronic Permanence",
Journal of Digital Information Vol 2, Issue 2, ISSN 1368-
7506, January 2002,
.
[N2T] CDL, "Name-to-Thing Resolver", August 2006,
.
[NLMPerm] Byrnes, M., "Defining NLM's Commitment to the Permanence
of Electronic Information", ARL 212:8-9, October 2000,
.
[NOID] Kunze, J., "Nice Opaque Identifiers", February 2005,
.
[PURL] Shafer, K., "Introduction to Persistent Uniform Resource
Locators", 1996, .
[RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, May 1983.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
Kunze & Rodgers [Page 38]
ARK May 2008
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC2288] Lynch, C., Preston, C., and R. Jr, "Using Existing
Bibliographic Identifiers as Uniform Resource Names",
RFC 2288, February 1998.
[RFC2611] Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
"URN Namespace Definition Mechanisms", BCP 33, RFC 2611,
June 1999.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC2915] Mealling, M. and R. Daniel, "The Naming Authority Pointer
(NAPTR) DNS Resource Record", RFC 2915, September 2000.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC5013] Kunze, J. and T. Baker, "The Dublin Core Metadata Element
Set", RFC 5013, August 2007.
[THUMP] Gamiel, K. and J. Kunze, "The HTTP URL Mapping Protocol",
August 2007,
.
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Appendix A. ARK Maintenance Agency
Production settings in which ARKs are used include the University of
California, the National Library of France, the Internet Archive, and
Portico, with maintenance based at the California Digital Library
(CDL), housed at the University of California Office of the
President.
http://ark.cdlib.org/
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Appendix B. Looking up NMAHs Distributed via DNS
This subsection introduces an older method for looking up NMAHs that
is based on the method for discovering URN resolvers described in
[RFC2915]. It relies on querying the DNS system already installed in
the background infrastructure of most networked computers. A query
is submitted to DNS asking for a list of resolvers that match a given
NAAN. DNS distributes the query to the particular DNS servers that
can best provide the answer, unless the answer can be found more
quickly in a local DNS cache as a side-effect of a recent query.
Responses come back inside Name Authority Pointer (NAPTR) records.
The normal result is one or more candidate NMAHs.
In its full generality the [RFC2915] algorithm ambitiously
accommodates a complex set of preferences, orderings, protocols,
mapping services, regular expression rewriting rules, and DNS record
types. This subsection proposes a drastic simplification of it for
the special case of ARK mapping authority discovery. The simplified
algorithm is called Maptr. It uses only one DNS record type (NAPTR)
and restricts most of its field values to constants. The following
hypothetical excerpt from a DNS data file for the NAAN known as 12026
shows three example NAPTR records ready to use with the Maptr
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
hostport, a Maptr extension to the NAPTR flags), the replacement
field contains the NMAH (hostport) of an ARK service provider. When
a query returns a record with a flags field of "" (the empty string),
the client needs to submit a new query containing the domain name
found in the replacement field. This second sort of record exploits
the distributed nature of DNS by redirecting the query to another
domain name. It looks like this.
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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 with the NAAN from the ARK for which an NMAH is
sought.
1. Initialize the DNS query: type=NAPTR, query=.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 NMAHs 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=
then go to step (2). When all such records have been recursively
exhausted, go to step (5).
5. All redirected queries have been resolved and a set of candidate
NMAHs has been accumulated from steps (3). If there are zero
NMAHs, exit -- no mapping authority was found. If there is one
or more NMAH, choose one using any criteria you wish, then exit.
A Perl script that implements this algorithm is included here.
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#!/depot/bin/perl
use Net::DNS; # include simple DNS package
my $qtype = "NAPTR"; # initialize query type
my $naa = shift; # get NAAN script argument
my $mad = new Net::DNS::Resolver; # mapping authority discovery
&maptr("$naa.ark.arpa"); # call maptr - that's it
sub maptr { # recursive maptr algorithm
my $dname = shift; # domain name as argument
my ($rr, $order, $pref, $flags, $service, $regexp,
$replacement);
my $query = $mad->query($dname, $qtype);
return # non-productive query
if (! $query || ! $query->answer);
foreach $rr ($query->answer) {
next # skip records of wrong type
if ($rr->type ne $qtype);
($order, $pref, $flags, $service, $regexp,
$replacement) = split(/\s/, $rr->rdatastr);
if ($flags eq "") {
&maptr($replacement); # recurse
} elsif ($flags eq "h") {
print "$replacement\n"; # candidate NMAH
}
}
}
The global database thus distributed via DNS and the Maptr algorithm
can easily be seen to mirror the contents of the Name Authority Table
file described in the previous section.
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Authors' Addresses
John A. Kunze
California Digital Library
415 20th St, 4th Floor
Oakland, CA 94612
US
Fax: +1 510-893-5212
Email: jak@ucop.edu
R. P. C. Rodgers
US National Library of Medicine
8600 Rockville Pike, Bldg. 38A
Bethesda, MD 20894
USA
Fax: +1 301-496-0673
Email: rodgers@nlm.nih.gov
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