Internet-Draft: draft-kunze-ark-01.txt                          J. Kunze
ARK Identifier Scheme                    University of California (UCSF)
Expires 8 September 2001                                R. P. C. Rodgers
                                         US National Library of Medicine
                                                            8 March 2001

                  The ARK Persistent Identifier Scheme


Status of this Document

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   all provisions of Section 10 of RFC2026.

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   Copyright (C) The Internet Society (2001).  All Rights Reserved.


   The ARK (Archival Resource Key) is a scheme intended to facilitate
   the persistent naming and retrieval of information objects.  It
   comprises an identifier syntax and three services.  An ARK has four


   the prefix "ark:", the (optional and mutable) Name Mapping Authority

J. Kunze                                                        [Page 1]

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   Hostport (NMAH, where "hostport" is a hostname followed optionally by
   a colon and port number), the Name Assigning Authority Number (NAAN),
   and the assigned Name.  The NAAN and Name together form the immutable
   persistent identifier for the object.

   An ARK request is an ARK to which is appended a service request
   beginning with a question mark.  Use of an ARK request proceeds in
   two steps.  First, the NMAH, if not specified, is discovered based on
   the NAAN.  Two methods for discovery are proposed:  one is file
   based, the other based on the DNS NAPTR record.  Second, the ARK
   request is submitted to the NMAH.  Three ARK services are defined,
   gaining access to:  (1) the object (or a sensible substitute), (2) a
   description of the object (metadata), and (3) a description of the
   commitment made by the NMA regarding the persistence of the object
   (policy).  These services are defined initially to use the HTTP
   protocol, given the World Wide Web's pre-eminence among Internet
   information retrieval systems.  When the NMAH is specified, the
   "ark:" prefix may be replaced with "http://", to produce a valid URL
   that can gain access to ARK services using an unmodified Web client.

J. Kunze                                                        [Page 2]

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1.  Introduction

   This document describes a scheme for the high-quality naming of
   information resources.  The scheme, called the Archival Resource Key
   (ARK), is well suited to long-term access and identification for any
   information resources that accommodate reasonably regular electronic
   description.  This includes digital documents, databases, software,
   and websites, as well as physical objects (such as books, bones, and
   statues) 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.

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

   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.  Rather, persistence
   is achieved through a provider's successful stewardship of objects
   and their identifiers.  The highest level of persistence will be
   reinforced by a provider's robust contingency, redundancy, and
   succession strategies.  It is further safeguarded to the extent that
   a provider's mission is shielded from marketplace and political

1.1.  Three Reasons to Use 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.  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.

J. Kunze                1.1. Reasons to Use ARKs                [Page 3]

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   The second requirement of an ARK is to give users a link from an
   object to a description of it.  The problem with a naked identifier
   is that without a description real identification is incomplete.
   Identifiers common today are relatively opaque, though some contain
   ad hoc clues that reflect fleeting life cycle events such as the
   address of a short stay in a filesystem hierarchy.  Possession of
   both an identifier and an object is some improvement, but positive
   identification may still be elusive since the object itself need not
   include a matching identifier or be transparent 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.  Among the recorded
   characteristics, a checksum (e.g., [MD5]) recorded at the time of
   last handling may assist automated identification of digital objects
   (although checksums will require recomputation periodically if
   extremely persistent objects' bitstreams change as predicted due to
   inevitable media migration).

   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, with persistent identification playing a
   vital but supporting role.  Object access may not be feasible for
   various reasons, such as catastrophic loss of the object, a licensing
   agreement that keeps an archive "dark" for a period of years, or when
   an object's own lack of tangible existence precludes normal concepts
   of access (e.g., a vocabulary term might be accessed through its
   definition).  In such cases the ARK's identification role assumes a
   much higher profile.  But attempts to simplify the persistence
   problem by decoupling access from identification and concentrating
   exclusively on the latter are of questionable utility.  A perfect
   system for assigning forever unique identifiers might be created, but
   if it did so without reducing access failure rates, no one would be
   interested.  The central issue -- which may be summed up as the "HTTP
   404 Not Found" problem -- would not have been addressed.

1.2.  Organizing Support for ARKs

   Co-location of persistent access and identification services is
   natural.  Any organization undertaking persistent identification and
   description is in an advantaged position to undertake persistent
   access, and vice versa.  The former task becomes all the easier if
   the organization controls, owns, or otherwise has clear access to the
   objects.  Similarly, the latter cannot be managed without at least
   internal support for the former, since collection management

J. Kunze                 1.2. Support for ARKs                  [Page 4]

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   activities such as monitoring, acquisition, verification, and change
   control all require record keeping and accountability.  Organizing
   ARK services under one roof tends to make sense.

   ARK support is not for everybody.  By requiring specific, revealed
   commitments to preservation, object access, and description, the bar
   for providing ARK services is high.  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.  Such a business
   would benefit more from persistence than it would directly support

1.3.  A Definition of Identifier

   Heretofore, persistence discussion has been hampered by a borrowed
   meaning for "identifier" that emerged as a side effect of defining
   the Uniform Resource Identifier in [URI]:

        (formerly)  An identifier is a sequence of characters with a
        restricted syntax ... that can act as a reference to something
        that has identity.

   The term works in context, but falters when employed for persistence.
   Troubling phrases arise, such as,

        "The goal is to create an identifier that does not break."

   As defined this kind of identifier "breaks" when it sustains damage
   to its character sequence, but really what breaks has to do with the
   identifier's reference role.  The following definition is proposed.

        (new definition)  An identifier is an association between a
        string (a sequence of characters) and an information resource.
        That association is made manifest by a record (e.g., a
        cataloging or other metadata record) that binds the identifier
        string to a set of identifying resource characteristics.

   The identifier (the association) must be vouched for by some sort of
   record.  In the complete absence of any testimony (e.g., metadata)
   regarding an association, a would-be identifier string is a
   meaningless sequence of characters.  To keep an externally visible
   but otherwise internal identifier string opaque to outsiders, for
   example, it suffices for an organization not to disclose the nature

J. Kunze            1.3. A Definition of Identifier             [Page 5]

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   of its association.  For our immediate purpose, actual existence of
   an association record is more important than its authenticity.  If
   one is lucky an object carries its own identifier as part of itself
   (e.g., imprinted on the first page), but in processes such as
   resource discovery and retrieval the typical object is often unwieldy
   or unavailable (such as when licensing restrictions are in effect).
   A metadata record that includes the identifier string is the next
   best thing -- a conveniently manipulable surrogate that can act as
   both an association "receipt" and "declaration".

   It now makes sense to speak of preventing an identifier, as an
   association, from breaking.  Having said that, this document still
   (ab)uses the terms "ARK" and "identifier" as shorthands to refer to
   identifier strings, in other words, to sequences of characters.  Thus
   a discussion of ARK syntax refers to a string format, not an
   association format.  The context should make the meaning clear.

2.  ARK Anatomy

   An ARK is represented by a sequence of characters (a string) that
   begins with the prefix "ark:".  Here is a diagrammed example.

                  \__/\____________/ \___/ \______/
                   |    (optional)     |      |
           ARK Prefix       |          |    Name (assigned by the NAA)
                            |          |
         Name Mapping Authority       Name Assigning Authority
                Hostport (NMAH)        Number (NAAN)

   The ARK syntax can be summarized,


   where the NMAH is in brackets to indicate that it is optional.

2.1.  The Name Mapping Authority Hostport (NMAH)

   After the prefix may appear an optional Name Mapping Authority
   Hostport (NMAH) that is a temporary address where ARK service
   requests may be sent.  It consists of 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

J. Kunze                   2.1. ARK NMAH Part                   [Page 6]

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   optional NMAH part identify the same object.  Thus, for example, the
   following three ARKs are synonyms for but one information resource:


   The NMAH makes it easy to derive an identifier that is actionable in
   today's web browsers (i.e., a URL).  This amounts to substituting
   "http://" for the "ark:" prefix (although in most browsers simply
   deleting "ark:" works).  The first example ARK above thus becomes


   The NMAH part is temporary, disposable, and replaceable.  Over time
   the NMAH will likely stop working and have to be replaced with a
   currently active service provider.  This relies on a mapping
   authority discovery process, of which two alternate methods are
   outlined in a later section.  Meanwhile, a carefully chosen NMAH can
   be as durable as any Internet domain name, and so may last for a
   decade or longer.  Users should be prepared, however, to refresh the
   NMAH because the one found in an ARK may have stopped working.

   The above method for creating an actionable identifier from an ARK
   (replacing "ark:" with "http://") is also temporary.  Assuming that
   the reign of [HTTP] in information retrieval will end one day, ARKs
   will have to be converted into new kinds of actionable identifiers.
   In any event, if ARKs see widespread use, web browsers would
   presumably evolve to perform this simple transformation

2.2.  The Name Assigning Authority Number (NAAN)

   The next part of 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 number of digits helps reduce the chances
   of finding a NAAN out of context and confusing it with nearby
   quantities such as 4-digit dates.

J. Kunze                   2.2. ARK NAAN Part                   [Page 7]

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2.3.  The Name Part

   The final part of the ARK is the Name assigned by the NAA, and it is
   also required.  The Name is a string 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 within HTTP GET requests.
   Characters may be letters, digits, or any of these seven characters:

       =   @   $   _   *   '   #

   The characters `/', `+', and `?' are reserved and must not be used at
   this time.  A `-' (hyphen) may appear in an ARK, but must be ignored
   in lexical comparisons.  The `%' character is reserved for %-encoding
   all other octets that would appear in the ARK string, in the same
   manner as for URIs [URI].  A %-encoded octet consists of a `%'
   followed by two hex digits; for example, "%7d" stands in for `}'.
   Lower case hex digits are preferred to reduce the chances of false
   acronym recognition; thus it is better to use "%acT" instead of
   "%ACT".  The character `%' itself must be represented using "%25".
   As with URNs, %-encoding permits ARKs to support legacy namespaces
   (e.g., ISBN, ISSN, SICI) that have less restricted character
   repertoires [URNBIB].

   The creation of names that include linguistically based constructs
   (having recognizable meaning from natural language) is strongly
   discouraged if long-term persistence is a naming priority.  Such
   names do not age or travel well.  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 is a relatively safe and
   easy way to achieve more compact names, although any character
   repertoire can work if potentially troublesome names will be
   discarded during a screening process.

2.4.  Lexical Equivalence

   Hyphens are always ignored in ARKs.  Hyphens may be added to an ARK's
   Name part for readability, or during the formatting and wrapping of
   text lines, but (as in phone numbers) they are treated as if they
   were not present.  Thus, 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:


J. Kunze                2.4. Lexical Equivalence                [Page 8]

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   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 ARKs for the purpose of octet-by-octet equality
   comparison consists of two steps for each ARK.  First, any upper case
   letters in the "ark:" prefix and %-encoded hex digits are converted
   to lower case.  The case of all other letters in the ARK string must
   be preserved.  Then, any NMAH is removed and all hyphens are removed.
   The resulting ARK string is now normalized.  Comparisons between
   normalized ARKs are case-sensitive, meaning that upper case letters
   are considered different from their lower case counterparts.

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

2.5.  Naming Considerations

   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 as part of the ARK.

   A valuable technique for provision of persistent objects is to try to
   have the complete identifier 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

   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

J. Kunze               2.5. Naming Considerations               [Page 9]

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   persistence strategy, and they do not discriminate based on naming
   scheme.  Whether a mistakenly re-assigned identifier 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.

   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 to outlive brands, corporations, subject
   classifications, and natural language semantics (e.g., what did the
   three letters "gay" mean forty, twenty, and two years ago?).  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 identity 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.

   Dubious persistence speculation does not make selecting naming
   strategies any easier.  For example, despite rumors to the contrary,
   there are really 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 would 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.

J. Kunze               2.5. Naming Considerations              [Page 10]

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3.  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 in
   places like the front matter of a book.

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

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

   For now, registration of ARK NAAs is in a bootstrapping phase.  To
   register, please read about the mapping authority discovery file in
   the next section and send email to

4.  Finding a Name Mapping Authority

   In order to derive an actionable identifier 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.

J. Kunze                   4. Finding an NMA                   [Page 11]

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

   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
   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.

   There is never a requirement that an NAA also run an NMA service,
   although it seems not an unlikely scenario.  Over time NAAs and NMAs
   would come and go.  One NMA would 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.

J. Kunze                   4. Finding an NMA                   [Page 12]

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4.1.  Looking Up NMAHs in a Globally Accessible File

   This subsection describes a way to look up NMAHs using a simple 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 the Domain Name System [DNS].

   A copy of the current file (at the time of writing) appears in an
   appendix and is available on the web.  A minimal version of the file
   appears below.  Comment lines (lines that begin with `#') explain the
   format and give the file's modification time, reloading address, and
   NAA registration instructions.  There is even a Perl script that
   processes the file embedded in the file's comments.  Because this is
   still a proposed file, none of the values in it are real.

J. Kunze            4.1. NMAH Discovery (/etc/natab)           [Page 13]

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       # Name Assigning Authority / Name Mapping Authority Lookup Table
       #     Last change:   22 February 2001
       #     Reload from:
       #     Mirrored at:
       #                    http://....../etc/natab
       #     To register:
       #     Process with:  Perl script at end of this file (optional)
       # Each NAA appears at the beginning of a line with the NAA Number
       # first, a colon, and an ARK or URL to a statement of naming policy
       # (see for an example).
       # All the NMA hostports that service an NAA are listed, one per
       # line, indented, after the corresponding NAA line.
       #   US Library of Congress
       #   US National Library of Medicine
       #   US National Agriculture Library
       #--- end of data ---
       # The enclosed Perl script takes an NAA as argument and outputs
       # the NMAs in this file listed under any matching NAA.
       # my $naa = shift;
       # while (<>) {
       #     next if (! /^$naa:/);
       #     while (<>) {
       #         last if (! /^[#\s]./);
       #         print "$1\n" if (/^\s+(\S+)/);
       #     }
       # }
       # end of file

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4.2.  Looking up NMAHs Distributed via DNS

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

   In its full generality the [NAPTR] algorithm ambitiously accommodates
   a complex set of preferences, orderings, protocols, mapping services,
   regular expression rewriting rules, and DNS record types.  This
   subsection proposes a drastic simplification of it for the special
   case of ARK mapping authority discovery.  The simplified algorithm is
   called Maptr.  It uses only one DNS record type (NAPTR) and restricts
   most of its field values to constants.  The following hypothetical
   excerpt from a DNS data file for the NAAN known as 12026 shows three
   example NAPTR records ready to use with the Maptr algorithm.
     ;; US Library of Congress
     ;;       order pref flags service regexp replacement
      IN NAPTR  0     0   "h"  "ark"     ""
      IN NAPTR  0     0   "h"  "ark"     ""
      IN NAPTR  0     0   "h"  "ark"     ""

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

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

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

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

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

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

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

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

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

   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.

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.  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

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5.1.  Generic ARK Access Service (access, location)

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

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

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

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

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

   The permanence declaration for an object is a rating defined with
   respect to an identified permanence provider (guarantor), and may
   include the following aspects.  One permanence rating framework is
   given in [NLMPerm].

        (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" -- documentation, whether abbreviated or
        detailed, of any or all corrections, migrations, revisions, etc.

   Naming policy for an object includes an historical description of the
   NAA's (and its successor NAA's) policies regarding differentiation of
   objects.  It may include the following aspects.

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        (e) "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

        (f) "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.

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

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

5.3.  Generic Description Service

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

6.  Overview of the HTTP Key Mapping Protocol (HKMP)

   The HTTP Key Mapping Protocol (HKMP) is a way of taking a key (a kind
   of identifier) and asking such questions as, what information does
   this identify and how permanent is it?  [HKMP] 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.  HKMP is designed so that
   a person can enter ARK requests directly into the location field of
   current browser interfaces.  Because it runs over HTTP, HKMP can be
   simulated and tested within keyboard-based [TELNET] sessions.

   The asker (a person or client program) starts with an identifier,

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   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 an HKMP request (contained within an
   HTTP request), receives an answer via an HKMP response (contained
   within an HTTP response), and closes the session.  That concludes the
   connected portion of the protocol.

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

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


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

   In the following example HKMP 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 three pieces
   separated from each other by blank lines:  the client's piece (lines
   1-3), the server's HTTP/HKMP response headers (4-7), and the body of
   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.

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    1  C: ...  [opens session]
       C: GET HTTP/1.1
       S: HTTP/1.1 200 OK
    5  S: Content-Type: text/plain
       S: HKMP-Status: 0.1 200 OK
       S: erc:
       S: who:    Lederberg, Joshua
   10  S: what:   Studies of Human Families for Genetic Linkage
       S: when:   1974
       S: where:
       S: ...  [closes session]

   The first two server response lines (4-5) above are typical of HTTP.
   The next line (6) is peculiar to HKMP, and indicates the HKMP version
   and a normal return status.  The balance of the response (8-11) is
   the single metadata record that comprises the ARK description service
   response.  The record is in the format of an Electronic Resource
   Citation [ERC], which is discussed in more detail in the next
   section.  For now, note that it contains four elements that answer
   the top priority questions regarding an expression of the object:
   who played a major role in expressing it, what the expression was
   called, when is was created, and where the expression may be found.
   This quartet of elements comes up again and again in ERCs.

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

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    1  C: ...  [opens session]
       C: GET HTTP/1.1
       S: HTTP/1.1 200 OK
    5  S: Content-Type: text/plain
       S: HKMP-Status: 0.1 200 OK
       S: erc:
       S: who:    Lederberg, Joshua
   10  S: what:   Studies of Human Families for Genetic Linkage
       S: when:   1974
       S: where:
       S: erc-support:
       S: who:    NIH/NLM/LHNCBC
   15  S: what:   Permanent, Unchanging Content
       S: when:   2001 04 21
       S: where:
       S: ...  [closes session]

   Again, a single metadata record (lines 8-17) is returned, but it
   consists of two segments.  The first segment (8-12) gives the same
   basic citation information as in the previous example.  It is
   returned in order to establish context for the persistence
   declaration in the second segment (13-17).

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

7.  Overview of Electronic Resource Citations (ERCs)

   An Electronic Resource Citation (or ERC, pronounced e-r-c) [ERC] is a
   simple, compact, and printable record designed to hold data
   associated with an information resource.  By design, the ERC is a
   metadata format that balances the needs for expressive power, very
   simple machine processing, and direct human manipulation.

   A founding principle of the ERC 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 ERC format must only minimally strain
   people's ability to read, understand, change, and transmit ERCs

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   without their relying on intermediation with specialized software
   tools.  The basic ERC needs to be succinct, transparent, and
   trivially parseable by software.

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

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

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

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   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.

7.1.  ERC Syntax

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

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

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

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

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

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

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

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

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

7.2.  ERC Stories

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

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

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

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   The four questions are answered by using corresponding element
   labels.  The four element labels can be re-used in each story
   segment, but their meaning changes depending on the segment (the
   story type) in which they appear.  In the example above, "who" is
   first used to name a document's author and subsequently used to name
   the permanence guarantor (provider).  Similarly, "when" first lists
   the date of object creation and in the next segment lists the date of
   a commitment decision.  Four labels appearing across three segments
   effectively map to twelve semantically distinct elements.  Distinct
   element meanings are mapped to Dublin Core elements in a later

7.3.  The ERC Anchoring Story

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

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

   The anchoring story has special rules designed to keep ERC processing
   simple and predictable.  Each of the four basic elements (who, what,
   when, and where) must be present, unless a best effort to supply it
   fails.  In the event of failure, the element still appears but a

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   special value (described later) is used to explain the missing value.
   While the requirement that each of the four elements be present only
   applies to the anchoring story segment, as usual these elements
   appear at the beginning of the segment and may only be used in the
   prescribed order.  A minimal ERC would normally consist of just an
   anchoring story and the element quartet, as illustrated in the next

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

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

       erc: National Research Council | The Digital Dilemma | 2000

   Although machine readable, this abbreviated ERC format is neither
   required nor permitted at this time in HKMP responses.

7.4.  ERC Elements

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

   An ERC segment allows an unlimited number of elements with the same
   label.  Multiple such elements are treated as if they were combined
   into one element with multiple values.  For example, the three

       who: Bullock, T.H.
       who: Achimowicz, J.Z. | Duckrow, R.B. | Spencer, S.S.
       who: Iragui-Madoz, V.J.

   are treated as if all the values were in one element with five-

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       who: Bullock, T.H.  | Achimowicz, J.Z. | Duckrow, R.B.
               | Spencer, S.S.  | Iragui-Madoz, V.J.

   Required ordering is preserved as long as all who's precede all
   what's, all what's precede all when's, and all when's precede all

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

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

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

       who/created:    ...
       who/published:  ...
       who/modified:   ...
       when/valid:     ...
       when/modified:  ...
       when/published: ...
       when/created:   ...

   Using past tense verbs for qualifiers also reminds providers and
   recipients that element values contain transient assertions that may
   have been true once, but that tend to become less true over time.
   Recipients that don't understand the meaning of a qualifier can fall
   back onto the semantic category (bucket) designated by the
   unqualified element label.  Inevitably recipients (people and
   software) will have diverse abilities in understanding elements and

   Any number of other elements and qualifiers may be used in
   conjunction with the quartet of basic segment questions.  The only

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

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

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

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

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

7.5.  ERC Element Values

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

       VALUE =    [markup_flags]    (:ccode)    ,    DATA

   In processing, the first non-whitespace character of an ERC element

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   value is examined.  An initial `[' is reserved to introduce a
   bracketed set of markup flags (not described in this document) that
   ends with `]'.  If ERC data is machine-generated, each value string
   may be preceded by "[]" to prevent any of its data from being
   mistaken for markup flags.  Once past the optional markup, the
   remaining value may optionally begin with a controlled code.  A
   controlled code always has the form "(:ccode)", for example,

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

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

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

       (:unkn)   unknown (e.g., Anonymous)
       (:unav)   value unavailable indefinitely
       (:none)   never had a value, never will
       (:unac)   temporarily inaccessible
       (:unap)   not applicable (makes no sense)
       (:null)   explicitly empty
       (:igno)   element here only to satisfy syntax
       (:unas)   value unassigned (e.g., untitled painting)
       (:elwr)   value appears elsewhere in record
       (:remo)   withheld, suppressed intentionally

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

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

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

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

7.6.  ERC Element Encoding and Dates

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

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

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

   One particularly useful construct in ERC element values is the pair

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   of special encoding markers ("%{" and "%}") that indicates a
   "whitespace-squeezed" block.  Whatever string of characters they
   enclose will be treated as if none of the contained whitespace
   (SPACEs, TABs, Newlines) were present.  This comes in handy for
   writing long, multi-part URLs in a readable way.  For example, the
   value in

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

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


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

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

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

       2000 12 29 235955           (recommended for readability)
       2000 12 29 23 59 55
       20   001 229 235  9 5 5
       20001229235955              (normalized date and time)

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

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

J. Kunze          7.6. ERC Element Encoding and Dates          [Page 32]

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7.7.  ERC Stub Records and Internal Support

   The ERC design introduces the concept of a "stub" record, which is an
   incomplete ERC record intended to be supplemented with additional
   elements before being released as a standalone ERC record.  Stub ERC
   records have no minimum required elements.  They may be useful in
   supporting internal procedures using the ERC syntax.  Often they rely
   on the convenience and accuracy of automatically supplied elements,
   even the basic ones.  To be ready for external use, however, an ERC
   stub must be transformed into a complete ERC record having the usual
   required elements.  An ERC stub record can be convenient for metadata
   embedded in a document, where elements such as location, modification
   date, and size -- which one would not omit from an externalized
   record -- are omitted simply because they are much better supplied by
   a computation.  A separate local administrative procedure, not
   defined for ERC's in general, would effect the promotion of stubs
   into complete records.

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

8.  Advice to Web Clients

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

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

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

J. Kunze                8. Advice to Web Clients               [Page 33]

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

   The same link with an ARK instead of a URL:

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

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

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

   A simple expedient that works for now without browser modification is
   to use a specified-host ARK (one with an NMAH) but without the usual
   "ark:" prefix (a prefix of "http://" is normally assumed), as in,

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

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

9.  Security Considerations

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

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

10.  Authors' Addresses

   John A. Kunze
   Center for Knowledge Management

J. Kunze                 10. Authors' Addresses                [Page 34]

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   University of California, San Francisco
   530 Parnassus Ave, Box 0840
   San Francisco, CA  94143-0840, USA

   Fax:   +1 415-476-4653

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

   Fax:   +1 301-496-0673

11.  References

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

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

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

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

   [ERC]      J. Kunze, "Electronic Resource Citations", work in

   [HKMP]     J. Kunze, "HTTP Key Mapping Protocol", work in progress.

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

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

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

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

J. Kunze                     11. References                    [Page 35]

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

   [REG]      J. Kunze, "Resource Metadata Vocabulary", work in

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

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

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

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

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

12.  Appendix:  An NLM Prototype ARK Service

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

   Comments and feedback may be addressed to

13.  Appendix:  Current ARK Name Authority Table

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

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   # Name Assigning Authority / Name Mapping Authority Lookup Table
   #     Last change:   22 February 2001
   #     Reload from:
   #     Mirrored at:
   #                    http://....../etc/natab
   #     To register:
   #     Process with:  Perl script at end of this file (optional)
   # Each NAA appears at the beginning of a line with the NAA Number
   # first, a colon, and an ARK or URL to a statement of naming policy
   # (see for an example).
   # All the NMA hostports that service an NAA are listed, one per
   # line, indented, after the corresponding NAA line.
   #   US Library of Congress
   #   US National Library of Medicine
   #   US National Agriculture Library
   #--- end of data ---
   # The enclosed Perl script takes an NAA as argument and outputs
   # the NMAs in this file listed under any matching NAA.
   # my $naa = shift;
   # while (<>) {
   #     next if (! /^$naa:/);
   #     while (<>) {
   #         last if (! /^[#\s]./);
   #         print "$1\n" if (/^\s+(\S+)/);
   #     }
   # }
   # end of file

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14.  Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the  purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive

Expires 8 September 2001

J. Kunze                  14. Copyright Notice                 [Page 38]

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                           Table of Contents

Status of this Document ...........................................    1
Abstract ..........................................................    1
1.  Introduction ..................................................    3
1.1.  Three Reasons to Use ARKs ...................................    3
1.2.  Organizing Support for ARKs .................................    4
1.3.  A Definition of Identifier ..................................    5
2.  ARK Anatomy ...................................................    6
2.1.  The Name Mapping Authority Hostport (NMAH) ..................    6
2.2.  The Name Assigning Authority Number (NAAN) ..................    7
2.3.  The Name Part ...............................................    8
2.4.  Lexical Equivalence .........................................    8
2.5.  Naming Considerations .......................................    9
3.  Assigners of ARKs .............................................   11
4.  Finding a Name Mapping Authority ..............................   11
4.1.  Looking Up NMAHs in a Globally Accessible File ..............   13
4.2.  Looking up NMAHs Distributed via DNS ........................   15
5.  Generic ARK Service Definition ................................   16
5.1.  Generic ARK Access Service (access, location) ...............   17
5.2.  Generic Policy Service (permanence, naming, etc.)  ..........   17
5.3.  Generic Description Service .................................   18
6.  Overview of the HTTP Key Mapping Protocol (HKMP) ..............   18
7.  Overview of Electronic Resource Citations (ERCs) ..............   21
7.1.  ERC Syntax ..................................................   23
7.2.  ERC Stories .................................................   24
7.3.  The ERC Anchoring Story .....................................   25
7.4.  ERC Elements ................................................   26
7.5.  ERC Element Values ..........................................   29
7.6.  ERC Element Encoding and Dates ..............................   31
7.7.  ERC Stub Records and Internal Support .......................   33
8.  Advice to Web Clients .........................................   33
9.  Security Considerations .......................................   34
10.  Authors' Addresses ...........................................   34
11.  References ...................................................   35
12.  Appendix:  An NLM Prototype ARK Service ......................   36
13.  Appendix:  Current ARK Name Authority Table ..................   36
14.  Copyright Notice .............................................   38

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