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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Missing Reference: 'Qualifiers' is mentioned on line 579, but not defined ** Obsolete normative reference: RFC 2141 (Obsoleted by RFC 8141) ** Obsolete normative reference: RFC 2611 (Obsoleted by RFC 3406) ** Obsolete normative reference: RFC 2616 (Obsoleted by RFC 7230, RFC 7231, RFC 7232, RFC 7233, RFC 7234, RFC 7235) ** Obsolete normative reference: RFC 2822 (Obsoleted by RFC 5322) ** Obsolete normative reference: RFC 2915 (Obsoleted by RFC 3401, RFC 3402, RFC 3403, RFC 3404) Summary: 8 errors (**), 0 flaws (~~), 6 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Kunze 3 Internet-Draft California Digital Library 4 Intended status: Informational E. Bermès 5 Expires: 10 February 2022 Bibliothèque nationale de France 6 9 August 2021 8 The ARK Identifier Scheme 9 draft-kunze-ark-28 11 Abstract 13 The ARK (Archival Resource Key) naming scheme is designed to 14 facilitate the high-quality and persistent identification of 15 information objects. A founding principle of the ARK is that 16 persistence is purely a matter of service and is neither inherent in 17 an object nor conferred on it by a particular naming syntax. The 18 best that an identifier can do is to lead users to the services that 19 support robust reference. The term ARK itself refers both to the 20 scheme and to any single identifier that conforms to it. An ARK has 21 five components: 23 [https://NMA/]ark:[/]NAAN/Name[Qualifiers] 25 an optional and mutable Name Mapping Authority (usually a hostname), 26 the "ark:" label, the Name Assigning Authority Number (NAAN), the 27 assigned Name, and an optional and possibly mutable Qualifier 28 supported by the NMA. The NAAN and Name together form the immutable 29 persistent identifier for the object independent of the URL hostname. 30 An ARK is a special kind of URL that connects users to three things: 31 the named object, its metadata, and the provider's promise about its 32 persistence. When entered into the location field of a Web browser, 33 the ARK leads the user to the named object. That same ARK, inflected 34 by appending `?info', returns a metadata record that is both human- 35 and machine-readable. The returned record contains core metadata and 36 a commitment statement from the current provider. Tools exist for 37 minting, binding, and resolving ARKs. 39 Status of This Memo 41 This Internet-Draft is submitted in full conformance with the 42 provisions of BCP 78 and BCP 79. 44 Internet-Drafts are working documents of the Internet Engineering 45 Task Force (IETF). Note that other groups may also distribute 46 working documents as Internet-Drafts. The list of current Internet- 47 Drafts is at https://datatracker.ietf.org/drafts/current/. 49 Internet-Drafts are draft documents valid for a maximum of six months 50 and may be updated, replaced, or obsoleted by other documents at any 51 time. It is inappropriate to use Internet-Drafts as reference 52 material or to cite them other than as "work in progress." 54 This Internet-Draft will expire on 10 February 2022. 56 Copyright Notice 58 Copyright (c) 2021 IETF Trust and the persons identified as the 59 document authors. All rights reserved. 61 This document is subject to BCP 78 and the IETF Trust's Legal 62 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 63 license-info) in effect on the date of publication of this document. 64 Please review these documents carefully, as they describe your rights 65 and restrictions with respect to this document. 67 Table of Contents 69 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 70 1.1. Reasons to Use ARKs . . . . . . . . . . . . . . . . . . . 4 71 1.2. Three Requirements of ARKs . . . . . . . . . . . . . . . 5 72 1.3. Organizing Support for ARKs: Our Stuff vs. Their Stuff . 7 73 1.4. Definition of Identifier . . . . . . . . . . . . . . . . 8 74 2. ARK Anatomy . . . . . . . . . . . . . . . . . . . . . . . . . 9 75 2.1. The Name Mapping Authority (NMA) . . . . . . . . . . . . 11 76 2.2. The ARK Label Part (ark:) . . . . . . . . . . . . . . . . 12 77 2.3. The Name Assigning Authority Number (NAAN) . . . . . . . 13 78 2.4. The Name Part . . . . . . . . . . . . . . . . . . . . . . 15 79 2.4.1. Optional: Shoulders . . . . . . . . . . . . . . . . . 16 80 2.5. The Qualifier Part . . . . . . . . . . . . . . . . . . . 17 81 2.5.1. ARKs that Reveal Object Hierarchy . . . . . . . . . . 18 82 2.5.2. ARKs that Reveal Object Variants . . . . . . . . . . 19 83 2.6. Character Repertoires . . . . . . . . . . . . . . . . . . 21 84 2.7. Normalization and Lexical Equivalence . . . . . . . . . . 22 85 2.8. Resolver Chains . . . . . . . . . . . . . . . . . . . . . 23 86 3. Naming Considerations . . . . . . . . . . . . . . . . . . . . 24 87 3.1. ARKS and Usability . . . . . . . . . . . . . . . . . . . 24 88 3.2. Objects Should Wear Their Identifiers . . . . . . . . . . 24 89 3.3. Names are Political, not Technological . . . . . . . . . 24 90 3.4. Choosing a Hostname or NMA . . . . . . . . . . . . . . . 25 91 3.5. Assigners of ARKs . . . . . . . . . . . . . . . . . . . . 26 92 3.6. NAAN Namespace Management . . . . . . . . . . . . . . . . 27 93 3.7. Sub-Object Naming . . . . . . . . . . . . . . . . . . . . 28 94 4. Finding a Name Mapping Authority . . . . . . . . . . . . . . 29 95 4.1. Looking Up NMAs in a Globally Accessible File . . . . . . 30 96 5. Generic ARK Service Definition . . . . . . . . . . . . . . . 31 97 5.1. Generic ARK Access Service (access, location) . . . . . . 31 98 5.1.1. Generic Policy Service (permanence, naming, etc.) . . 32 99 5.1.2. Generic Description Service . . . . . . . . . . . . . 34 100 5.2. Overview of The HTTP URL Mapping Protocol (THUMP) . . . . 34 101 5.3. The Electronic Resource Citation (ERC) . . . . . . . . . 36 102 5.4. Advice to Web Clients . . . . . . . . . . . . . . . . . . 38 103 5.5. Enhancements and Related Specifications . . . . . . . . . 39 104 5.6. Security Considerations . . . . . . . . . . . . . . . . . 39 105 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 39 106 Appendix A. ARK Maintenance Agency: arks.org . . . . . . . . . . 42 107 Appendix B. Looking up NMAs Distributed via DNS . . . . . . . . 42 108 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44 110 1. Introduction 112 [ Note about this transitional draft. The ARKsInTheOpen.org 113 Technical Working Group (https://wiki.duraspace.org/display/ARKs/ 114 Technical+Working+Group) is in the process of revising the ARK spec 115 via a series of Internet-Drafts. This draft contains many minor but 116 noisy changes (lots of diffs but not much real change). While the 117 spec is in transition, new implementors should follow 118 https://datatracker.ietf.org/doc/html/draft-kunze-ark-18. ] 120 This document describes a scheme for the high-quality naming of 121 information resources. The scheme, called the Archival Resource Key 122 (ARK), is well suited to long-term access and identification of any 123 information resources that accommodate reasonably regular electronic 124 description. This includes digital documents, databases, software, 125 and websites, as well as physical objects (books, bones, statues, 126 etc.) and intangible objects (chemicals, diseases, vocabulary terms, 127 performances). Hereafter the term "object" refers to an information 128 resource. The term ARK itself refers both to the scheme and to any 129 single identifier that conforms to it. A reasonably concise and 130 accessible overview and rationale for the scheme is available at 131 [ARK]. 133 Schemes for persistent identification of network-accessible objects 134 are not new. In the early 1990's, the design of the Uniform Resource 135 Name [RFC2141] responded to the observed failure rate of URLs by 136 articulating an indirect, non-hostname-based naming scheme and the 137 need for responsible name management. Meanwhile, promoters of the 138 Digital Object Identifier [DOI] succeeded in building a community of 139 providers around a mature software system [Handle] that supports name 140 management. The Persistent Uniform Resource Locator [PURL] was 141 another scheme that had the advantage of working with unmodified web 142 browsers. ARKs represent an approach that attempts to build on the 143 strengths and to avoid the weaknesses of these schemes. 145 A founding principle of the ARK is that persistence is purely a 146 matter of service. Persistence is neither inherent in an object nor 147 conferred on it by a particular naming syntax. Nor is the technique 148 of name indirection -- upon which URNs, Handles, DOIs, and PURLs are 149 founded -- of central importance. Name indirection is an ancient and 150 well-understood practice; new mechanisms for it keep appearing and 151 distracting practitioner attention, with the Domain Name System (DNS) 152 [RFC1034] being a particularly dazzling and elegant example. What is 153 often forgotten is that maintenance of an indirection table is an 154 unavoidable cost to the organization providing persistence, and that 155 cost is equivalent across naming schemes. That indirection has 156 always been a native part of the web while being so lightly utilized 157 for the persistence of web-based objects indicates how unsuited most 158 organizations will probably be to the task of table maintenance and 159 to the much more fundamental challenge of keeping the objects 160 themselves viable. 162 Persistence is achieved through a provider's successful stewardship 163 of objects and their identifiers. The highest level of persistence 164 will be reinforced by a provider's robust contingency, redundancy, 165 and succession strategies. It is further safeguarded to the extent 166 that a provider's mission is shielded from funding and political 167 instabilities. These are by far the major challenges confronting 168 persistence providers, and no identifier scheme has any direct impact 169 on them. In fact, some schemes may actually be liabilities for 170 persistence because they create short- and long-term dependencies for 171 every object access on complex, special-purpose infrastructures, 172 parts of which are proprietary and all of which increase the carry- 173 forward burden for the preservation community. It is for this reason 174 that the ARK scheme relies only on educated name assignment and light 175 use of general-purpose infrastructures that are maintained mostly by 176 the internet community at large (the DNS, web servers, and web 177 browsers). 179 1.1. Reasons to Use ARKs 181 If no persistent identifier scheme contributes directly to 182 persistence, why not just use URLs? A particular URL may be as 183 durable an identifier as it is possible to have, but nothing 184 distinguishes it from an ordinary URL to the recipient who is 185 wondering if it is suitable for long-term reference. An ARK embedded 186 in a URL provides some of the necessary conditions for credible 187 persistence, inviting access to not one, but to three things: to the 188 object, to its metadata, and to a nuanced statement of commitment 189 from the provider in question (the NMA, described below) regarding 190 the object. Existence of the extra service can be probed 191 automatically by appending `?info' to the ARK. 193 The form of the ARK also supports the natural separation of naming 194 authorities into the original name assigning authority and the 195 diverse multiple name mapping (or servicing) authorities that in 196 succession and in parallel will take over custodial responsibilities 197 from the original assigner (assuming the assigner ever held that 198 responsibility) for the large majority of a long-term object's 199 archival lifetime. The name mapping authority, indicated by the 200 hostname part of the URL that contains the ARK, serves to launch the 201 ARK into cyberspace. Should it ever fail (and there is no reason why 202 a well-chosen hostname for a 100-year-old cultural memory institution 203 shouldn't last as long as the DNS), that host name is considered 204 disposeable and replaceable. Again, the form of the ARK helps 205 because it defines exactly how to recover the core immutable object 206 identity, and simple algorithms (one based on the URN model) or even 207 by-hand internet query can be used for for locating another mapping 208 authority. 210 There are tools to assist in generating ARKs and other identifiers, 211 such as [NOID] and "uuidgen", both of which rely for uniqueness on 212 human-maintained registries. This document also contains some 213 guidelines and considerations for managing namespaces and choosing 214 hostnames with persistence in mind. 216 1.2. Three Requirements of ARKs 218 The first requirement of an ARK is to give users a link from an 219 object to a promise of stewardship for it. That promise is a multi- 220 faceted covenant that binds the word of an identified service 221 provider to a specific set of responsibilities. It is critical for 222 the promise to come from a current provider and almost irrelevant, 223 over a long period of time, what the original assigner's intentions 224 were. No one can tell if successful stewardship will take place 225 because no one can predict the future. Reasonable conjecture, 226 however, may be based on past performance. There must be a way to 227 tie a promise of persistence to a provider's demonstrated or 228 perceived ability -- its reputation -- in that arena. Provider 229 reputations would then rise and fall as promises are observed 230 variously to be kept and broken. This is perhaps the best way we 231 have for gauging the strength of any persistence promise. 233 The second requirement of an ARK is to give users a link from an 234 object to a description of it. The problem with a naked identifier 235 is that without a description real identification is incomplete. 236 Identifiers common today are relatively opaque, though some contain 237 ad hoc clues reflecting assertions that were briefly true, such as 238 where in a filesystem hierarchy an object lived during a short stay. 239 Possession of both an identifier and an object is some improvement, 240 but positive identification may still be uncertain since the object 241 itself might not include a matching identifier or might not carry 242 evidence obvious enough to reveal its identity without significant 243 research. In either case, what is called for is a record bearing 244 witness to the identifier's association with the object, as supported 245 by a recorded set of object characteristics. This descriptive record 246 is partly an identification "receipt" with which users and archivists 247 can verify an object's identity after brief inspection and a 248 plausible match with recorded characteristics such as title and size. 250 The final requirement of an ARK is to give users a link to the object 251 itself (or to a copy) if at all possible. Persistent identification 252 plays a vital supporting role but, strictly speaking, it can be 253 construed as no more than a record attesting to the original 254 assignment of a never-reassigned identifier. Object access may not 255 be feasible for various reasons, such as a transient service outage, 256 a catastrophic loss, a licensing agreement that keeps an archive 257 "dark" for a period of years, or when an object's own lack of 258 tangible existence confuses normal concepts of access (e.g., a 259 vocabulary term might be "accessed" through its definition). In such 260 cases the ARK's identification role assumes a much higher profile. 261 But attempts to simplify the persistence problem by decoupling access 262 from identification and concentrating exclusively on the latter are 263 of questionable utility. A perfect system for assigning forever 264 unique identifiers might be created, but if it did so without 265 reducing access failure rates, no one would be interested. The 266 central issue -- which may be crudely summed up as the "HTTP 404 Not 267 Found" problem -- would not have been addressed. 269 The central duty of an ARK is a high-quality experience of access and 270 identification. This means supporting reliable access during the 271 period described in its stewardship promise and, failing that, 272 supporting reliable access to a record describing the thing the ARK 273 is associated with. 275 ARK resolvers must support the `?info' inflection for requesting 276 metadata. Older versions of this specification distinguished between 277 two minimal inflections: `?' (brief metadata) and `??' (more 278 metadata). While these older inflections are still reserved, because 279 they have proven hard to recognize in some environments, supporting 280 them is optional. 282 1.3. Organizing Support for ARKs: Our Stuff vs. Their Stuff 284 An organization and the user community it serves can often be seen to 285 struggle with two different areas of persistent identification: the 286 Our Stuff problem and the Their Stuff problem. In the Our Stuff 287 problem, we in the organization want our own objects to acquire 288 persistent names. Since we possess or control these objects, our 289 organization tackles the Our Stuff problem directly. Whether or not 290 the objects are named by ARKs, our organization is the responsible 291 party, so it can plan for, maintain, and make commitments about the 292 objects. 294 In the Their Stuff problem, we in the organization want others' 295 objects to acquire persistent names. These are objects that we do 296 not own or control, but some of which are critically important to us. 297 But because they are beyond our influence as far as support is 298 concerned, creating and maintaining persistent identifiers for Their 299 Stuff is not especially purposeful or feasible for us to engage in. 300 There is little that we can do about someone else's stuff except 301 encourage their uptake or adoption of persistence services. 303 Co-location of persistent access and identification services is 304 natural. Any organization that undertakes ongoing support of true 305 persistent identification (which includes description) is well-served 306 if it controls, owns, or otherwise has clear internal access to the 307 identified objects, and this gives it an advantage if it wishes also 308 to support persistent access to outsiders. Conversely, persistent 309 access to outsiders requires orderly internal collection management 310 procedures that include monitoring, acquisition, verification, and 311 change control over objects, which in turn requires object 312 identifiers persistent enough to support auditable record keeping 313 practices. 315 Although organizing ARK support under one roof thus tends to make 316 sense, object hosting can successfully be separated from name 317 mapping. An example is when a name mapping authority centrally 318 provides uniform resolution services via a protocol gateway on behalf 319 of organizations that host objects behind a variety of access 320 protocols. It is also reasonable to build value-added description 321 services that rely on the underlying services of a set of mapping 322 authorities. 324 Supporting ARKs is not for every organization. By requiring 325 specific, revealed commitments to preservation, to object access, and 326 to description, the bar for providing ARK services is higher than for 327 some other identifier schemes. On the other hand, it would be hard 328 to grant credence to a persistence promise from an organization that 329 could not muster the minimum ARK services. Not that there isn't a 330 business model for an ARK-like, description-only service built on top 331 of another organization's full complement of ARK services. For 332 example, there might be competition at the description level for 333 abstracting and indexing a body of scientific literature archived in 334 a combination of open and fee-based repositories. The description- 335 only service would have no direct commitment to the objects, but 336 would act as an intermediary, forwarding commitment statements from 337 object hosting services to requestors. 339 1.4. Definition of Identifier 341 An identifier is not a string of character data -- an identifier is 342 an association between a string of data and an object. This 343 abstraction is necessary because without it a string is just data. 344 It's nonsense to talk about a string's breaking, or about its being 345 strong, maintained, and authentic. But as a representative of an 346 association, a string can do, metaphorically, the things that we 347 expect of it. 349 Without regard to whether an object is physical, digital, or 350 conceptual, to identify it is to claim an association between it and 351 a representative string, such as "Jane" or "ISBN 0596000278". What 352 gives a claim credibility is a set of verifiable assertions, or 353 metadata, about the object, such as age, height, title, or number of 354 pages. In other words, the association is made manifest by a record 355 (e.g., a cataloging or other metadata record) that vouches for it. 357 In the complete absence of any testimony (metadata) regarding an 358 association, a would-be identifier string is a meaningless sequence 359 of characters. To keep an externally visible but otherwise internal 360 string from being perceived as an identifier by outsiders, for 361 example, it suffices for an organization not to disclose the nature 362 of its association. For our immediate purpose, actual existence of 363 an association record is more important than its authenticity or 364 verifiability, which are outside the scope of this specification. 366 It is a gift to the identification process if an object carries its 367 own name as an inseparable part of itself, such as an identifier 368 imprinted on the first page of a document or embedded in a data 369 structure element of a digital document header. In cases where the 370 object is large, unwieldy, or unavailable (such as when licensing 371 restrictions are in effect), a metadata record that includes the 372 identifier string will usually suffice. That record becomes a 373 conveniently manipulable object surrogate, acting as both an 374 association "receipt" and "declaration". 376 Note that our definition of identifier extends the one in use for 377 Uniform Resource Identifiers [RFC3986]. The present document still 378 sometimes (ab)uses the terms "ARK" and "identifier" as shorthand for 379 the string part of an identifier, but the context should make the 380 meaning clear. 382 2. ARK Anatomy 384 An ARK is represented by a sequence of characters (a string) that 385 contains the Label, "ark:", optionally preceded by the beginning part 386 of a URL. Here is a diagrammed example. 388 ANATOMY OVERIEW 389 =============== 391 Resolver Service Compact ARK 392 __________________ ______________________________ 393 / \/ \ 394 https://example.org/ark:12345/x6np1wh8k/c3/s5.v7.xsl 395 .............................\________/............. 396 prefixes Base Name suffixes 397 \__________________________________________________/ 398 Mapping ARK 400 When embedded in a URL, an ARK consists of a Compact ARK preceded by 401 a Resolver Service. The larger URL-based ARK is known as a Mapping 402 ARK because it is ready to be mapped (resolved) to an information 403 response (eg, a PDF or metadata). A Mapping ARK is also know as a 404 "fully qualified ARK". The Resolver Service, which need not be 405 limited to URLs in the future, maps the URL according to rules and 406 abilities of an NMA (Name Mapping Authority). The same URL string 407 minus the Resolver Service component is known as a Compact ARK. The 408 Compact ARK is globally unique and may be resolvable via different 409 Resolver Services over time (eg, when one archive succeeds another) 410 or at the same time (eg, when one archive backs up another). 412 At a high level, after the Label comes the NAAN (Name Assigning 413 Authority Number) followed by the Name that it assigns to the 414 identified thing. The Name is a Base (or Base Name) that has 415 Prefixes (NAAN, Label, possibly a Resolver Service) and optional 416 Suffixes to identify Parts and Variant forms. During resolution, a 417 Resolver Service such as n2t.net may be able to deal with inflections 418 query strings, and content negotiation. 420 ANATOMY DETAILS 421 =============== 422 Base Compact Name Qualifiers 423 _________________ ___________ 424 / \/ \ 425 https://example.org/ark:12345/x6np1wh8k/c3/s5.v7.xsl 426 \_________/ \__/\___/\_/\_____/\____/\_____/ 427 NMA Label NAAN | Blade Parts Variants 428 Shoulder 429 \_____________/ 430 Check Zone 432 In a closer view, the Compact ARK consists of a Base Compact Name 433 followed potentially by Qualifiers. The Name (Base) often, but not 434 necessarily, consists of a Shoulder (for subdividing a NAAN 435 namespace) followed by a Blade. If a check character is present in 436 an ARK, by convention it is the right-most character of the Base 437 Name, and will have been computed over the string of characters 438 preceding it back to the beginning of the NAAN. This string, 439 including the check character itself, is the Check Zone. 441 x 443 Like the ARK itself, the NAAN "12345" and Shoulder "x6" have compact 444 and fully qualified forms. 446 +==========+=======+==============+================================+ 447 | Form | Base | Compact Form | Fully Qualified Form | 448 +==========+=======+==============+================================+ 449 | NAAN | 12345 | ark:12345 | https://example.org/ark:12345 | 450 +----------+-------+--------------+--------------------------------+ 451 | Shoulder | x6 | ark:12345/x6 | https://example.org/ark:12345/ | 452 | | | | x6 | 453 +----------+-------+--------------+--------------------------------+ 455 Table 1: Examples of base, compact, and fully qualified forms 457 x 459 The ARK syntax can be summarized, 461 [https://NMA/]ark:[/]NAAN/Name[Qualifiers] 463 where the NMA, '/', and Qualifier parts are in brackets to indicate 464 that they are optional. The Base Compact Name is the substring 465 comprising the "ark:" label, the NAAN and the assigned Name. The 466 Resolver Service is replaceable and makes the ARK actionable for a 467 period of time. Without the Resolver Service part, what remains is 468 the Core Immutable Identity (the "persistible") part of the ARK. 470 2.1. The Name Mapping Authority (NMA) 472 Before the "ark:" label may appear an optional Name Mapping Authority 473 (NMA) that is a temporary address where ARK service requests may be 474 sent. Preceded by a URI-type protocol designation such as 475 "https://", it specifies a Resolver Service. The NMA itself is an 476 Internet hostname or host/port combination having the same format and 477 semantics as the host/port part of a URL. The most important thing 478 about the NMA is that it is "identity inert" from the point of view 479 of object identification. In other words, ARKs that differ only in 480 the optional NMA part identify the same object. Thus, for example, 481 the following three ARKs are synonyms for just one information 482 object: 484 https://loc.gov/ark:12345/x54xz321 485 https://rutgers.edu/ark:12345/x54xz321 486 ark:12345/x54xz321 488 Strictly speaking, in the realm of digital objects, these ARKs may 489 lead over time to somewhat different or diverging instances of the 490 originally named object. In an ideal world, divergence of persistent 491 objects is not desirable, but it is widely believed that digital 492 preservation efforts will inevitably lead to alterations in some 493 original objects (e.g, a format migration in order to preserve the 494 ability to display a document). If any of those objects are held 495 redundantly in more than one organization (a common preservation 496 strategy), chances are small that all holding organizations will 497 perform the same precise transformations and all maintain the same 498 object metadata. More significant divergence would be expected when 499 the holding organizations serve different audiences or compete with 500 each other. 502 The NMA part makes an ARK into an actionable URL. As with many 503 internet parameters, it is helpful to approach the NMA being liberal 504 in what you accept and conservative in what you propose. From the 505 recipient's point of view, the NMA part should be treated as 506 temporary, disposable, and replaceable. From the NMA's point of 507 view, it should be chosen with the greatest concern for longevity. A 508 carefully chosen NMA should be at least as permanent as the providing 509 organization's own hostname. In the case of a national or university 510 library, for example, there is no reason why the NMA should not be 511 considerably more permanent than soft-funded proxy hostnames such as 512 hdl.handle.net, dx.doi.org, and purl.org. In general and over time, 513 however, it is not unexpected for an NMA eventually to stop working 514 and require replacement with the NMA of a currently active service 515 provider. 517 This replacement relies on a mapping authority "resolver" discovery 518 process, of which two alternate methods are outlined in a later 519 section. The ARK, URN, Handle, and DOI schemes all use a resolver 520 discovery model that sooner or later requires matching the original 521 assigning authority with a current provider servicing that 522 authority's named objects; once found, the resolver at that provider 523 performs what amounts to a redirect to a place where the object is 524 currently held. All the schemes rely on the ongoing functionality of 525 currently mainstream technologies such as the Domain Name System 526 [RFC1034] and web browsers. The Handle and DOI schemes in addition 527 require that the Handle protocol layer and global server grid be 528 available at all times. 530 The practice of prepending "https://" and an NMA to an ARK is a way 531 of creating an actionable identifier by a method that is itself 532 temporary. Assuming that infrastructure supporting [RFC2616] 533 information retrieval will no longer be available one day, ARKs will 534 then have to be converted into new kinds of actionable identifiers. 535 By that time, if ARKs see widespread use, web browsers would 536 presumably evolve to perform this (currently simple) transformation 537 automatically. 539 2.2. The ARK Label Part (ark:) 541 The label part distinguishes an ARK from an ordinary identifier. 542 There is a new form of the label, "ark:", and an old form, "ark:/", 543 both of which must be recognized in perpetuity. Implementations 544 should generate new ARKs in the new form (without the "/") and 545 resolvers must always treat received ARKs as equivalent if they 546 differ only in regard to new form versus old form labels. Thus these 547 two ARKs are equivalent: 549 ark:/12345/x54xz321 550 ark:12345/x54xz321 552 In a URL found in the wild, the label indicates that the URL stands a 553 reasonable chance of being an ARK. If the context warrants, 554 verification that it actually is an ARK can be done by testing it for 555 existence of the three ARK services. 557 Since nothing about an identifier syntax directly affects 558 persistence, the "ark:" label (like "urn:", "doi:", and "hdl:") 559 cannot tell you whether the identifier is persistent or whether the 560 object is available. It does tell you that the original Name 561 Assigning Authority (NAA) had some sort of hopes for it, but it 562 doesn't tell you whether that NAA is still in existence, or whether a 563 decade ago it ceased to have any responsibility for providing 564 persistence, or whether it ever had any responsibility beyond naming. 566 Only a current provider can say for certain what sort of commitment 567 it intends, and the ARK label suggests that you can query the NMA 568 directly to find out exactly what kind of persistence is promised. 569 Even if what is promised is impersistence (i.e., a short-term 570 identifier), saying so is valuable information to the recipient. 571 Thus an ARK is a high-functioning identifier in the sense that it 572 provides access to the object, the metadata, and a commitment 573 statement, even if the commitment is explicitly very weak. 575 2.3. The Name Assigning Authority Number (NAAN) 577 Recalling that the general form of the ARK is, 579 [https://NMA/]ark:[/]NAAN/Name[Qualifiers] 581 the part of the ARK directly following the "ark:" (or older "ark:/") 582 label is the Name Assigning Authority Number (NAAN), up to but not 583 including the next `/' (slash) character. This part is always 584 required, as it identifies the organization that originally assigned 585 the Name of the object. Typically the organization is an 586 institution, a department, a laboratory, or any group that conducts a 587 stable, policy-driven name assigning effort. An organization may 588 request a NAAN from the ARK Maintenance Agency [ARKagency] (described 589 in Appendix A) by filling out the form [NAANrequest]. 591 For received ARKs, implementations must support a minimum NAAN length 592 of 16 octets. NAANs are opaque strings of one or more "betanumeric" 593 characters, specifically, 595 0123456789bcdfghjkmnpqrstvwxz 597 which consists of digits and consonants, minus the letter 'l'. 598 Restricting NAANs to betanumerics (alphanumerics without vowels or 599 'l') serves two goals. It reduces the chances that words -- past, 600 present, and future -- will appear in NAANs and carry unintended 601 semantics. It also helps usability by not mixing commonly confused 602 characters ('0' and 'O', '1' and 'l') and by being compatible with 603 strong transcription error detection (eg, the [NOID] check digit 604 algorithm). Since 2001, every assigned NAAN has consisted of exactly 605 five digits. 607 The NAAN designates a top-level ARK namespace. Once registered for a 608 namespace, a NAAN is never re-registered. It is possible, however, 609 for there to be a succession of organizations that manage an ARK 610 namespace. 612 There are currently four NAANs available to all organizations. An 613 ARK bearing one of these NAANs carries a specific, immutable meaning 614 that recipients can rely on for long term pragmatic benefit as 615 described below. 617 +==========+================================+==========+============+ 618 | Shared | The immutable purpose, | Expect | OK for | 619 | NAAN | meaning, or connotation of | to | long term | 620 | meaning | ARKs bearing this NAAN. | resolve? | reference? | 621 +==========+================================+==========+============+ 622 | 12345 | Example ARKs appearing in | maybe | no | 623 | examples | documentation. They might | | | 624 | | resolve, but no link checker | | | 625 | | need be concerned if they | | | 626 | | don't. They should not be | | | 627 | | considered viable for long | | | 628 | | term reference. | | | 629 +----------+--------------------------------+----------+------------+ 630 | 99152 | ARKs for controlled | yes | yes | 631 | terms | vocabulary and ontology | | | 632 | | terms, such as metadata | | | 633 | | element names and pick-list | | | 634 | | values. They should resolve | | | 635 | | to term definitions and are | | | 636 | | suitable for long term | | | 637 | | reference. | | | 638 +----------+--------------------------------+----------+------------+ 639 | 99166 | ARKs for people, groups, and | yes | yes | 640 | agents | institutions as "agents" | | | 641 | | (actors, such as creators, | | | 642 | | contributors, publishers, | | | 643 | | performers, etc). They | | | 644 | | should resolve to agent | | | 645 | | definitions and are suitable | | | 646 | | for long term reference. | | | 647 +----------+--------------------------------+----------+------------+ 648 | 99999 | ARKs for test, development, | maybe | no | 649 | test ids | or experimental purposes, | | | 650 | | often at scale. They might | | | 651 | | resolve, but no link checker | | | 652 | | need be concerned if they | | | 653 | | don't. They should not be | | | 654 | | considered viable for long | | | 655 | | term reference. | | | 656 +----------+--------------------------------+----------+------------+ 658 Table 2: Four NAANs shared across all organizations 660 To make use of a shared NAAN, an organization has several options 661 described in Section 2.4.1. 663 2.4. The Name Part 665 The part of the ARK just after the NAAN is the Name assigned by the 666 NAA, and it is also required. Semantic opaqueness in the Name part 667 is strongly encouraged in order to reduce an ARK's vulnerability to 668 era- and language-specific change. Identifier strings containing 669 linguistic fragments can create support difficulties down the road. 670 No matter how appropriate or even meaningless they are today, such 671 fragments may one day create confusion, give offense, or infringe on 672 a trademark as the semantic environment around us and our communities 673 evolves. 675 Names that look more or less like numbers avoid common problems that 676 defeat persistence and international acceptance. The use of digits 677 is highly recommended. Mixing in non-vowel alphabetic characters 678 (eg, betanumerics) a couple at a time is a relatively safe and easy 679 way to achieve a denser namespace (more possible names for a given 680 length of the name string). Such names have a chance of aging and 681 traveling well. The absence of recognizable words makes typos harder 682 to detect in opaque strings, so a common mitigation is to add a check 683 character. Tools exists that mint, bind, and resolve opaque 684 identifiers, with or without check characters [NOID]. More on naming 685 considerations is given in a subsequent section. 687 2.4.1. Optional: Shoulders 689 Just as an ARK namespace is subdivided by NAANs reserved for NAAs, it 690 is generally advantageous for an NAA to subdivide its own NAAN 691 namespace into "shoulders", where each shoulder is reserved for an 692 internal department or unit. Like the NAAN, which is a string of 693 characters that follows the "ark:" label, a shoulder is a string of 694 characters (starting with a "/") that extends the NAAN. The base 695 compact name assigned by the NAA consists of the NAAN, the shoulder, 696 a final string known as the "blade". (The shoulder plus blade 697 terminology mirrors locksmith jargon describing the information- 698 bearing parts of a key.) 700 The blade string is chosen by the NAA such that the string created by 701 concatenating the NAAN plus shoulder plus blade becomes the unique 702 base object name. Otherwise the blade may come from any source, for 703 example, it might come from a counter, a timestamp, a [NOID] minter, 704 a legacy 100-year-old accession number, etc. If there is a check 705 digit, it is expected to appear at the end of the blade and to be 706 computed over the base compact name, which is generally the most 707 important part of an ARK to make opaque. In particular, check digits 708 are not expected to cover qualifiers, which often name subobjects of 709 a persistent object that are less stable and less opaquely named than 710 the parent object (for example, ten years hence, the object's 711 thumbnail image will be of a higher resolution and the OCR text file 712 will be re-derived with improved algorithms. 714 It is important not to use any delimiter between the shoulder string 715 and blade string, especially not a "/" since it declares an object 716 boundary (see the section on ARKs that reveal object hierarchy). 718 ark:12345/x5wf6789/c2/s4.pdf # correct primordinal shoulder 719 ark:12345/x5/wf6789/c2/s4.pdf # INCORRECT 720 ^ WRONG 722 This little bit of discretion shields organizations from end users 723 making inferences about expected levels of support based on 724 recognizable shoulders. To help in-house ARK administrators reliably 725 know where the shoulder ends, it is recommended to use the "first- 726 digit convention" so that shoulders are "primordinal". A primordinal 727 shoulder is a sequence of one or more betanumeric characters ending 728 in a digit, as shown above. This means that the shoulder is all 729 consonant letters (often just one) after the NAAN and "/" up to and 730 including the first digit encountered after the NAAN. One property 731 of primordinal shoulders is that there is an infinite number of them 732 possible under any NAAN. 734 To help manage each namespace into the future, NAAs are encouraged to 735 create shoulders, even if there is only one to start with. If an 736 organization wishes to create a shoulder under one of shared NAANs 737 (99999, 12345, 99152, or 99166, described in Table 2), it should fill 738 out the Shoulder Request Form [shoulderrequest]. 740 2.5. The Qualifier Part 742 The part of the ARK following the NAA-assigned Name is an optional 743 Qualifier. It is a string that extends the Base Name in order to 744 create a kind of service entry point into the object named by the 745 NAA. At the discretion of the providing NMA, such a service entry 746 point permits an ARK to support access to individual hierarchical 747 components and subcomponents of an object, and to variants (versions, 748 languages, formats) of components. A Qualifier may be invented by 749 the NAA or by any NMA servicing the object. 751 In form, the Qualifier is a ComponentPath, or a VariantPath, or a 752 ComponentPath followed by a VariantPath. A VariantPath is introduced 753 and subdivided by the reserved character `.', and a ComponentPath is 754 introduced and subdivided by the reserved character `/'. In this 755 example, 757 https://example.org/ark:12345/x54xz321/s3/f8.v05.tiff 759 the string "/s3/f8" is a ComponentPath and the string ".v05.tiff" is 760 a VariantPath. The ARK Qualifier is a formalization of some 761 currently mainstream URL syntax conventions. This formalization 762 specifically reserves meanings that permit recipients to make strong 763 inferences about logical sub-object containment and equivalence based 764 only on the form of the received identifiers; there is great 765 efficiency in not having to inspect metadata records to discover such 766 relationships. NMAs are free not to disclose any of these 767 relationships merely by avoiding the reserved characters above. 768 Hierarchical components and variants are discussed further in the 769 next two sections. 771 The Qualifier, if present, differs from the Name in several important 772 respects. First, a Qualifier may have been assigned either by the 773 NAA or later by the NMA. The assignment of a Qualifier by an NMA 774 effectively amounts to an act of publishing a service entry point 775 within the conceptual object originally named by the NAA. For our 776 purposes, an ARK extended with a Qualifier assigned by an NMA will be 777 called an NMA-qualified ARK. 779 Second, a Qualifier assignment on the part of an NMA is made in 780 fulfillment of its service obligations and may reflect changing 781 service expectations and technology requirements. NMA-qualified ARKs 782 could therefore be transient, even if the base, unqualified ARK is 783 persistent. For example, it would be reasonable for an NMA to 784 support access to an image object through an actionable ARK that is 785 considered persistent even if the experience of that access changes 786 as linking, labeling, and presentation conventions evolve and as 787 format and security standards are updated. For an image "thumbnail", 788 that NMA could also support an NMA-qualified ARK that is considered 789 impersistent because the thumbnail will be replaced with higher 790 resolution images as network bandwidth and CPU speeds increase. At 791 the same time, for an originally scanned, high-resolution master, the 792 NMA could publish an NMA-qualfied ARK that is itself considered 793 persistent. Of course, the NMA must be able to return its separate 794 commitments to unqualified, NAA-assigned ARKs, to NMA-qualified ARKs, 795 and to any NAA-qualified ARKs that it supports. 797 A third difference between a Qualifier and a Name concerns the 798 semantic opaqueness constraint. When an NMA-qualified ARK is to be 799 used as a transient service entry point into a persistent object, the 800 priority given to semantic opaqueness observed by the NAA in the Name 801 part may be relaxed by the NMA in the Qualifier part. If service 802 priorities in the Qualifier take precedence over persistence, short- 803 term usability considerations may recommend somewhat semantically 804 laden Qualifier strings. 806 Finally, not only is the set of Qualifiers supported by an NMA 807 mutable, but different NMAs may support different Qualifier sets for 808 the same NAA-identified object. In this regard the NMAs act 809 independently of each other and of the NAA. 811 The next two sections describe how ARK syntax may be used to declare, 812 or to avoid declaring, certain kinds of relatedness among qualified 813 ARKs. 815 2.5.1. ARKs that Reveal Object Hierarchy 817 An NAA or NMA may choose to reveal the presence of a hierarchical 818 relationship between objects using the `/' (slash) character after 819 the Name part of an ARK. Some authorities will choose not to 820 disclose this information, while others will go ahead and disclose so 821 that manipulators of large sets of ARKs can infer object 822 relationships by simple identifier inspection; for example, this 823 makes it possible for a system to present a collapsed view of a large 824 search result set. 826 If the ARK contains an internal slash after the NAAN, the piece to 827 its left indicates a containing object. For example, publishing an 828 ARK of the form, 829 ark:12345/x54/xz/321 831 is equivalent to publishing three ARKs, 833 ark:12345/x54/xz/321 834 ark:12345/x54/xz 835 ark:12345/x54 837 together with a declaration that the first object is contained in the 838 second object, and that the second object is contained in the third. 840 Revealing the presence of hierarchy is completely up to the assigner 841 (NMA or NAA). It is hard enough to commit to one object's name, let 842 alone to three objects' names and to a specific, ongoing relatedness 843 among them. Thus, regardless of whether hierarchy was present 844 initially, the assigner, by not using slashes, reveals no shared 845 inferences about hierarchical or other inter-relatedness in the 846 following ARKs: 848 ark:12345/x54_xz_321 849 ark:12345/x54_xz 850 ark:12345/x54xz321 851 ark:12345/x54xz 852 ark:12345/x54 854 Note that slashes around the ARK's NAAN (/12345/ in these examples) 855 are not part of the ARK's Name and therefore do not indicate the 856 existence of some sort of NAAN super object containing all objects in 857 its namespace. A slash must have at least one non-structural 858 character (one that is neither a slash nor a period) on both sides in 859 order for it to separate recognizable structural components. So 860 initial or final slashes may be removed, and double slashes may be 861 converted into single slashes. 863 2.5.2. ARKs that Reveal Object Variants 865 An NAA or NMA may choose to reveal the possible presence of variant 866 objects or object components using the `.' (period) character after 867 the Name part of an ARK. Some authorities will choose not to 868 disclose this information, while others will go ahead and disclose so 869 that manipulators of large sets of ARKs can infer object 870 relationships by simple identifier inspection. This makes it 871 possible for a system to present a collapsed view of a large number 872 of search result items without having to issue database queries in 873 order to retrieve and analyze the inter-relatedness among all of 874 those items. 876 If the ARK contains an internal period after the Name, the piece to 877 the left of the first such period is a root name and the piece to its 878 right, and up to the end of the ARK or to the next period is a 879 suffix. A Name may have more than one suffix, for example, 881 ark:12345/x54.24 882 ark:12345/x4z/x54.24 883 ark:12345/x54.v18.fr.odf 885 There are two main rules. First, if two ARKs share the same root 886 name but have different suffixes, the corresponding objects were 887 considered variants of each other (different formats, languages, 888 versions, etc.) by the assigner (NMA or NAA). Thus, the following 889 ARKs are variants of each other: 891 ark:12345/x54.v18.fr.odf 892 ark:12345/x54.321xz 893 ark:12345/x54.44 895 Second, publishing an ARK with a suffix implies the existence of at 896 least one variant identified by the ARK without its suffix. The ARK 897 is otherwise silent about what additional variants might exist. So 898 publishing the ARK, 900 ark:12345/x54.v18.fr.odf 902 is equivalent to publishing the four ARKs, 904 ark:12345/x54.v18.fr.odf 905 ark:12345/x54.v18.fr 906 ark:12345/x54.v18 907 ark:12345/x54 909 Revealing the possibility of variants is completely up to the 910 assigner. It is hard enough to commit to one object's name, let 911 alone to multiple variants' names and to a specific, ongoing 912 relatedness among them. The assigner is the sole arbiter of what 913 constitutes a variant within its namespace, and whether to reveal 914 that kind of relatedness by using periods within its names. 916 A period must have at least one non-structural character (one that is 917 neither a slash nor a period) on both sides in order for it to 918 separate recognizable structural components. So initial or final 919 periods may be removed, and adjacent periods may be converted into a 920 single period. 922 2.6. Character Repertoires 924 The Name and Qualifier parts are strings of visible ASCII characters. 925 For received ARKs, implementations must support a minimum length of 926 255 octets for the string composed of the Base Name plus Qualifier. 927 Implementations generating strings exceeding this length should 928 understand that receiving implementations may not be able to index 929 such ARKs properly. Characters may be letters, digits, or any of 930 these seven characters: 932 = ~ * + @ _ $ 934 The following characters may also be used, but their meanings are 935 reserved: 937 % - . / 939 The characters `/' and `.' are ignored if either appears as the last 940 character of an ARK. If used internally, they allow a name assigner 941 to reveal object hierarchy and object variants as previously 942 described. 944 Hyphens are considered to be insignificant and are always ignored in 945 ARKs. A `-' (hyphen) may appear in an ARK for readability, or it may 946 have crept in during the formatting and wrapping of text, but it must 947 be ignored in lexical comparisons. As in a telephone number, hyphens 948 have no meaning in an ARK. It is always safe for an NMA that 949 receives an ARK to remove any hyphens found in it. As a result, like 950 the NMA, hyphens are "identity inert" in comparing ARKs for 951 equivalence. For example, the following ARKs are equivalent for 952 purposes of comparison and ARK service access: 954 ark:12345/x5-4-xz-321 955 https://sneezy.dopey.com/ark:12345/x54--xz32-1 956 ark:12345/x54xz321 958 The `%' character is reserved for %-encoding all other octets that 959 would appear in the ARK string, in the same manner as for URIs 960 [RFC3986]. A %-encoded octet consists of a `%' followed by two 961 uppercase hex digits; for example, "%7D" stands in for `}'. 962 Uppercase hex digits are preferred for compatibility with URI 963 encoding conventions, especially useful when URL-based ARKs are 964 compared for equivalence by ARK-unaware software systems; thus use 965 "%ACT" instead of "%acT". The character `%' itself must be 966 represented using "%25". As with URNs, %-encoding permits ARKs to 967 support legacy namespaces (e.g., ISBN, ISSN, SICI) that have less 968 restricted character repertoires [RFC2288]. 970 Implementors should be prepared to normalize some common invalid 971 characters that may be found in ARKs copy pasted from processed text. 972 For example, when pasting an ARK that was broken during line 973 wrapping, a user may inadvertently introduce newlines and spaces that 974 should be removed, or a variety of dash-like (eg, en dash) Unicode 975 characters that should be removed or converted to hyphens. 977 2.7. Normalization and Lexical Equivalence 979 To determine if two or more ARKs identify the same object, the ARKs 980 are compared for lexical equivalence after first being normalized. 981 Since ARK strings may appear in various forms (e.g., having different 982 NMAs), normalizing them minimizes the chances that comparing two ARK 983 strings for equality will fail unless they actually identify 984 different objects. In a specified-host ARK (one having an NMA), the 985 NMA never participates in such comparisons. Normalization described 986 here serves to define lexical equivalence but does not restrict how 987 implementors normalize ARKs locally for storage. 989 Normalization of a received ARK for the purpose of octet-by-octet 990 equality comparison with another ARK consists of the following steps. 992 1. The NMA part (eg, everything from an initial "https://" up to 993 the next slash), if present is removed. 995 2. Any URI query string is removed (everything from the first 996 literal '?' to the end of the string). 998 3. The first case-insensitive match on "ark:/" or "ark:" is 999 converted to "ark:" (replacing any uppercase letters and 1000 removing any terminal '/'). 1002 4. Any uppercase letters in the NAAN are converted to lowercase. 1004 5. In the string that remains, the two characters following every 1005 occurrence of `%' are converted to lowercase. The case of all 1006 other letters in the ARK string must be preserved. 1008 6. All hyphens and common dash-like characters (eg, U+2010 to 1009 U+2015) are removed. 1011 7. If normalization is being done as part of a resolution step, and 1012 if the end of the remaining string matches a known inflection, 1013 the inflection is noted and removed. 1015 8. Structural characters (slash and period) are normalized: initial 1016 and final occurrences are removed, and two structural characters 1017 in a row (e.g., // or ./) are replaced by the first character, 1018 iterating until each occurrence has at least one non-structural 1019 character on either side. 1021 9. If there are any components with a period on the left and a 1022 slash on the right, either the component and the preceding 1023 period must be moved to the end of the Name part or the ARK must 1024 be thrown out as malformed. 1026 The resulting ARK string is now normalized. Comparisons between 1027 normalized ARKs are case-sensitive, meaning that uppercase letters 1028 are considered different from their lowercase counterparts. 1030 To keep ARK string variation to a minimum, no reserved ARK characters 1031 should be %-encoded unless it is deliberately to conceal their 1032 reserved meanings. No non-reserved ARK characters should ever be 1033 %-encoded. Finally, no %-encoded character should ever appear in an 1034 ARK in its decoded form. 1036 2.8. Resolver Chains 1038 Resolution is a computation, often multi-stage, that maps a client 1039 identifier to a response. The response may be any "thing", such as a 1040 spreadsheet, a landing page, a metadata record, or a 404 Not Found. 1041 A single-stage retrieval of a web page is a resolution. More 1042 interesting kinds of resolution involve forwarding (indirection) and/ 1043 or proxying. 1045 On the web, forwarding is done with HTTP redirects. In general ARK 1046 resolution on the web involves a chain of one or more redirects that 1047 ends with the web server, known as the Responder, that responds 1048 without redirecting. The Responder might be a proxy and itself 1049 intiate a sub-resolution request chain unbeknownst to the original 1050 client, but that is out of scope here. An ARK might have a Resource 1051 Responder that is a different host from the Metadata Responder. The 1052 client starts resolution by contacting the NMA (server host) found in 1053 the original Mapping ARK URL. This is known as the First Resolver. 1055 3. Naming Considerations 1057 The most important threats faced by persistence providers include 1058 such things as funding loss, natural disaster, political and social 1059 upheaval, processing faults, and errors in human oversight. There is 1060 nothing that an identifer scheme can do about such things. Still, a 1061 few observed identifier failures and inconveniences can be traced 1062 back to naming practices that we now know to be less than optimal for 1063 persistence. 1065 3.1. ARKS and Usability 1067 Because linguistic constructs imperil persistence, for ARKs non-ASCII 1068 character support is unimportant. ARKs and URIs share goals of 1069 transcribability and transportability within web documents, so 1070 characters are required to be visible, non-conflicting with HTML/XML 1071 syntax, and not subject to tampering during transmission across 1072 common transport gateways. 1074 Any measure that reduces user irritation with an identifier will 1075 increase its chances of survival. This explains the rule preventing 1076 hyphens from having lexical significance. It is fine to publish ARKs 1077 with hyphens in them (e.g., such as the output of UUID/GUID 1078 generators), but the uniform treatment of hyphens (and their Unicode 1079 equivalents) as insignificant reduces the possibility of users 1080 transcribing identifiers that will have been broken through 1081 unpredictable hyphenation by word processors. 1083 3.2. Objects Should Wear Their Identifiers 1085 A valuable technique for provision of persistent objects is to try to 1086 arrange for the complete identifier to appear on, with, or near its 1087 retrieved object. An object encountered at a moment in time when its 1088 discovery context has long since disappeared could then easily be 1089 traced back to its metadata, to alternate versions, to updates, etc. 1090 This has seen reasonable success, for example, in book publishing and 1091 software distribution. An identifier string only has meaning when 1092 its association is known, and this a very sure, simple, and low-tech 1093 method of reminding everyone exactly what that association is. 1095 3.3. Names are Political, not Technological 1097 If persistence is the goal, a deliberate local strategy for 1098 systematic name assignment is crucial. Names must be chosen with 1099 great care. Poorly chosen and managed names will devastate any 1100 persistence strategy, and they do not discriminate by identifier 1101 scheme. Whether a mistakenly re-assigned name is a URN, DOI, PURL, 1102 URL, or ARK, the damage -- failed access and confusion -- is not 1103 mitigated more in one scheme than in another. Conversely, in-house 1104 efforts to manage names responsibly will go much further towards 1105 safeguarding persistence than any choice of naming scheme or name 1106 resolution technology. 1108 Branding (e.g., at the corporate or departmental level) is important 1109 for funding and visibility, but substrings representing brands and 1110 organizational names should be given a wide berth except when 1111 absolutely necessary in the hostname (the identity-inert) part of the 1112 ARK. These substrings are not only unstable because organizations 1113 change frequently, but they are also dangerous because successor 1114 organizations often have political or legal reasons to actively 1115 suppress predecessor names and brands. Any measure that reduces the 1116 chances of future political or legal pressure on an identifier will 1117 decrease the chances that our descendants will be obliged to 1118 deliberately break it. 1120 3.4. Choosing a Hostname or NMA 1122 Hostnames appearing in any identifier meant to be persistent must be 1123 chosen with extra care. The tendency in hostname selection has 1124 traditionally been to choose a token with recognizable attributes, 1125 such as a corporate brand, but that tendency wreaks havoc with 1126 persistence that is supposed to outlive brands, corporations, subject 1127 classifications, and natural language semantics (e.g., what did the 1128 three letters "gay" mean in 1958, 1978, and 1998?). Today's 1129 recognized and correct attributes are tomorrow's stale or incorrect 1130 attributes. In making hostnames (any names, actually) long-term 1131 persistent, it helps to eliminate recognizable attributes to the 1132 extent possible. This affects selection of any name based on URLs, 1133 including PURLs and the explicitly disposable NMAs. 1135 There is no excuse for a provider that manages its internal names 1136 impeccably not to exercise the same care in choosing what could be an 1137 exceptionally durable hostname, especially if it would form the 1138 prefix for all the provider's URL-based external names. Registering 1139 an opaque hostname in the ".org" or ".net" domain would not be a bad 1140 start. Another way is to publish your ARKs with an organizational 1141 domain name that will be mapped by DNS to an appropriate NMA host. 1142 This makes for shorter names with less branding vulnerability. 1144 It is a mistake to think that hostnames are inherently unstable. If 1145 you require brand visibility, that may be a fact of life. But things 1146 are easier if yours is the brand of long-lived cultural memory 1147 institution such as a national or university library or archive. 1148 Well-chosen hostnames from organizations that are sheltered from the 1149 direct effects of a volatile marketplace can easily provide longer- 1150 lived global resolvers than the domain names explicitly or implicitly 1151 used as starting points for global resolution by indirection-based 1152 persistent identifier schemes. For example, it is hard to imagine 1153 circumstances under which the Library of Congress' domain name would 1154 disappear sooner than, say, "handle.net". 1156 For smaller libraries, archives, and preservation organizations, 1157 there is a natural concern about whether they will be able to keep 1158 their web servers and domain names in the face of uncertain funding. 1159 One option is to form or join a group of like-minded organizations 1160 with the purpose of providing mutual preservation support. The first 1161 goal of such a group would be to perpetually rent a hostname on which 1162 to establish a web server that simply redirects incoming member 1163 organization requests to the appropriate member server; using ARKs, 1164 for example, a 150-member group could run a very small server (24x7) 1165 that contained nothing more than 150 rewrite rules in its 1166 configuration file. Even more helpful would be additional consortial 1167 support for a member organization that was unable to continue 1168 providing services and needed to find a successor archival 1169 organization. This would be a low-cost, low-tech way to publish ARKs 1170 (or URLs) under highly persistent hostnames. 1172 There are no obvious reasons why the organizations registering DNS 1173 names, URN Namespaces, and DOI publisher IDs should have among them 1174 one that is intrinsically more fallible than the next. Moreover, it 1175 is a misconception that the demise of DNS and of HTTP need adversely 1176 affect the persistence of URLs. At such a time, certainly URLs from 1177 the present day might not then be actionable by our present-day 1178 mechanisms, but resolution systems for future non-actionable URLs are 1179 no harder to imagine than resolution systems for present-day non- 1180 actionable URNs and DOIs. There is no more stable a namespace than 1181 one that is dead and frozen, and that would then characterize the 1182 space of names bearing the "http://" or "https://" prefix. It is 1183 useful to remember that just because hostnames have been carelessly 1184 chosen in their brief history does not mean that they are unsuitable 1185 in NMAs (and URLs) intended for use in situations demanding the 1186 highest level of persistence available in the Internet environment. 1187 A well-planned name assignment strategy is everything. 1189 3.5. Assigners of ARKs 1191 A Name Assigning Authority (NAA) is an organization that creates (or 1192 delegates creation of) long-term associations between identifiers and 1193 information objects. Examples of NAAs include national libraries, 1194 national archives, and publishers. An NAA may arrange with an 1195 external organization for identifier assignment. The US Library of 1196 Congress, for example, allows OCLC (the Online Computer Library 1197 Center, a major world cataloger of books) to create associations 1198 between Library of Congress call numbers (LCCNs) and the books that 1199 OCLC processes. A cataloging record is generated that testifies to 1200 each association, and the identifier is included by the publisher, 1201 for example, in the front matter of a book. 1203 An NAA does not so much create an identifier as create an 1204 association. The NAA first draws an unused identifier string from 1205 its namespace, which is the set of all identifiers under its control. 1206 It then records the assignment of the identifier to an information 1207 object having sundry witnessed characteristics, such as a particular 1208 author and modification date. A namespace is usually reserved for an 1209 NAA by agreement with recognized community organizations (such as 1210 IANA and ISO) that all names containing a particular string be under 1211 its control. In the ARK an NAA is represented by the Name Assigning 1212 Authority Number (NAAN). 1214 The ARK namespace reserved for an NAA is the set of names bearing its 1215 particular NAAN. For example, all strings beginning with 1216 "ark:12345/" are under control of the NAA registered under 12345, 1217 which might be the National Library of Finland. Because each NAA has 1218 a different NAAN, names from one namespace cannot conflict with those 1219 from another. Each NAA is free to assign names from its namespace 1220 (or delegate assignment) according to its own policies. These 1221 policies must be documented in a manner similar to the declarations 1222 required for URN Namespace registration [RFC2611]. 1224 Organizations can request or update a NAAN by filling out the NAAN 1225 Request Form [NAANrequest]. 1227 3.6. NAAN Namespace Management 1229 Every NAA should have a namespace management strategy. A classic 1230 hierarchical approach is to partition a NAAN namespace into 1231 subnamespaces known as "shoulders". As explained in Section 2.4.1, 1232 each shoulder is a unique prefix that guarantees non-collision of 1233 names in different partitions. This practice is strongly encouraged 1234 for all NAAs, especially when subnamespace management and assignment 1235 streams will be delegated to departments, units, or projects within 1236 an organization. For example, with a NAAN that is assigned to a 1237 university and managed by its main library, the library should take 1238 care to reserve shoulders (semantically opaque shoulders being 1239 preferred) for distinct assignment streams. Prefix-based partition 1240 management is typically an important responsibility of the NAA. 1242 This shoulder delegation approach plays out differently in two real- 1243 world examples: DNS names and ISBN identifiers. In the former, the 1244 hierarchy is deliberately exposed and in the latter it is hidden. 1245 Rather than using lexical boundary markers such as the period (`.') 1246 found in domain names, the ISBN uses a publisher prefix but doesn't 1247 disclose where the prefix ends and the publisher's assigned name 1248 begins. This practice of non-disclosure, found in the ISBN and ISSN 1249 schemes, is encouraged in assigning ARKs because it reduces the 1250 visibility of an assertion that is probably not important now and may 1251 become a vulnerability later. 1253 If longevity is the goal, it is important to keep the prefixes free 1254 of recognizable semantics; for example, using an acronym representing 1255 a project or a department is discouraged. At the same time, you may 1256 wish to set aside a subnamespace for testing purposes under a 1257 shoulder such as "fk9..." that can serve as a visual clue and 1258 reminder to maintenance staff that this "fake" identifier was never 1259 published. 1261 There are other measures one can take to avoid user confusion, 1262 transcription errors, and the appearance of accidental semantics when 1263 creating identifiers. If you are generating identifiers 1264 automatically, pure numeric identifiers are likeley to be 1265 semantically opaque enough, but it's probably useful to avoid leading 1266 zeroes because some users mistakenly treat them as optional, thinking 1267 (arithmetically) that they don't contribute to the "value" of the 1268 identifier. 1270 If you need lots of identifiers and you don't want them to get too 1271 long, you can mix digits with consonants (but avoid vowels since they 1272 might accidentally spell words) to get more identifiers without 1273 increasing the string length. In this case you may not want more 1274 than a two letters in a row because it reduces the chance of 1275 generating acronyms. Generator tools such as [NOID] provide support 1276 for these sorts of identifiers, and can also add a computed check 1277 character as a guarantee against the most common transcription 1278 errors. If used, it is recommended that the check character be 1279 appended to the original Base Compact Name string (ie, minus the 1280 check character), that original string having been the basis for 1281 computing the check character. 1283 3.7. Sub-Object Naming 1285 As mentioned previously, semantically opaque identifiers are very 1286 useful for long-term naming of abstract objects, however, it may be 1287 appropriate to extend these names with less opaque extensions that 1288 reference contemporary service entry points (sub-objects) in support 1289 of the object. Sub-object extensions beginning with a digit or 1290 underscore (`_') are reserved for the possibilty of developing a 1291 future registry of canonical service points (e.g., numeric references 1292 to versions, formats, languages, etc). 1294 4. Finding a Name Mapping Authority 1296 In order to derive an actionable identifier (these days, a URL) from 1297 an ARK, a hostname (or hostname plus port combination) for a working 1298 Name Mapping Authority (NMA) must be found. An NMA is a service that 1299 is able to respond to basic ARK service requests. Relying on 1300 registration and client-side discovery, NMAs make known which NAAs' 1301 identifiers they are willing to service. 1303 Upon encountering an ARK, a user (or client software) looks inside it 1304 for the optional NMA part (the host part of the NMA's ARK service). 1305 If it contains an NMA that is working, this NMA discovery step may be 1306 skipped; the NMA effectively uses the beginning of an ARK to cache 1307 the results of a prior mapping authority discovery process. If a new 1308 NMA needs to found, the client looks inside the ARK again for the 1309 NAAN (Name Assigning Authority Number). Querying a global database, 1310 it then uses the NAAN to look up all current NMAs that service ARKs 1311 issued by the identified NAA. 1313 The global database is key, and ideally the lookup would be automatic 1314 and transparent to the user. For this, the current most promising 1315 method is the Name-to-Thing (N2T) Resolver [N2T] at n2t.net. It is a 1316 reliable, low-cost NMA supported by the ARK Alliance that primarily 1317 exists to support actionable HTTP-based URLs for as long as HTTP is 1318 used. One of its big advantages over the other two methods and the 1319 URN, Handle, DOI, and PURL methods, is that N2T addresses the 1320 namespace splitting problem. When objects maintained by one NMA are 1321 inherited by more than one successor NMA, until now one of those 1322 successors would be required to maintain forwarding tables on behalf 1323 of the other successors. 1325 There are two other ways to discover an NMA, one of them described in 1326 a subsection below. Another way, described in an appendix, is based 1327 on a simplification of the URN resolver discovery method, itself very 1328 similar in principle to the resolver discovery method used by Handles 1329 and DOIs. None of these methods does more than what can be done with 1330 a very small, consortially maintained web server such as [N2T]. 1332 In the interests of long-term persistence, however, ARK mechanisms 1333 are first defined in high-level, protocol-independent terms so that 1334 mechanisms may evolve and be replaced over time without compromising 1335 fundamental service objectives. Either or both specific methods 1336 given here may eventually be supplanted by better methods since, by 1337 design, the ARK scheme does not depend on a particular method, but 1338 only on having some method to locate an active NMA. 1340 At the time of issuance, at least one NMA for an ARK should be 1341 prepared to service it. That NMA may or may not be administered by 1342 the Name Assigning Authority (NAA) that created it. Consider the 1343 following hypothetical example of providing long-term access to a 1344 cancer research journal. The publisher wishes to turn a profit and 1345 the National Library of Medicine wishes to preserve the scholarly 1346 record. An agreement might be struck whereby the publisher would act 1347 as the NAA and the national library would archive the journal issue 1348 when it appears, but without providing direct access for the first 1349 six months. During the first six months of peak commercial 1350 viability, the publisher would retain exclusive delivery rights and 1351 would charge access fees. Again, by agreement, both the library and 1352 the publisher would act as NMAs, but during that initial period the 1353 library would redirect requests for issues less than six months old 1354 to the publisher. At the end of the waiting period, the library 1355 would then begin servicing requests for issues older than six months 1356 by tapping directly into its own archives. Meanwhile, the publisher 1357 might routinely redirect incoming requests for older issues to the 1358 library. Long-term access is thereby preserved, and so is the 1359 commercial incentive to publish content. 1361 Although it will be common for an NAA also to run an NMA service, it 1362 is never a requirement. Over time NAAs and NMAs will come and go. 1363 One NMA will succeed another, and there might be many NMAs serving 1364 the same ARKs simultaneously (e.g., as mirrors or as competitors). 1365 There might also be asymmetric but coordinated NMAs as in the 1366 library-publisher example above. 1368 4.1. Looking Up NMAs in a Globally Accessible File 1370 This subsection describes a way to look up NMAs using a simple name 1371 authority table represented as a plain text file. For efficient 1372 access the file may be stored in a local filesystem, but it needs to 1373 be reloaded periodically to incorporate updates. It is not expected 1374 that the size of the file or frequency of update should impose an 1375 undue maintenance or searching burden any time soon, for even 1376 primitive linear search of a file with ten-thousand NAAs is a 1377 subsecond operation on modern server machines. The proposed file 1378 strategy is similar to the /etc/hosts file strategy that supported 1379 Internet host address lookup for a period of years before the advent 1380 of DNS. 1382 The name authority table file is updated on an ongoing basis and is 1383 available for copying over the internet from a number of mirror sites 1384 [NAANregistry]. The file contains comment lines (lines that begin 1385 with `#') explaining the format and giving the file's modification 1386 time, reloading address, and NAA registration instructions. 1388 5. Generic ARK Service Definition 1390 An ARK request's output is delivered information; examples include 1391 the object itself, a policy declaration (e.g., a promise of support), 1392 a descriptive metadata record, or an error message. The experience 1393 of object delivery is expected to be an evolving mix of information 1394 that reflects changing service expectations and technology 1395 requirements; contemporary examples include such things as an object 1396 summary and component links formatted for human consumption. ARK 1397 services must be couched in high-level, protocol-independent terms if 1398 persistence is to outlive today's networking infrastructural 1399 assumptions. The high-level ARK service definitions listed below are 1400 followed in the next section by a concrete method (one of many 1401 possible methods) for delivering these services with today's 1402 technology. Note that some services may be invoked in one operation, 1403 such as when an '?info' inflection returns both a description and a 1404 permanence declaration for an object. 1406 5.1. Generic ARK Access Service (access, location) 1408 Returns (a copy of) the object or a redirect to the same, although a 1409 sensible object proxy may be substituted. Examples of sensible 1410 substitutes include, 1412 * a table of contents instead of a large complex document, 1414 * a home page instead of an entire web site hierarchy, 1416 * a rights clearance challenge before accessing protected data, 1418 * directions for access to an offline object (e.g., a book), 1420 * a description of an intangible object (a disease, an event), or 1422 * an applet acting as "player" for a large multimedia object. 1424 May also return a discriminated list of alternate object locators. 1425 If access is denied, returns an explanation of the object's current 1426 (perhaps permanent) inaccessibility. 1428 5.1.1. Generic Policy Service (permanence, naming, etc.) 1430 Returns declarations of policy and support commitments for given 1431 ARKs. Declarations are returned in either a structured metadata 1432 format or a human readable text format; sometimes one format may 1433 serve both purposes. Policy subareas may be addressed in separate 1434 requests, but the following areas should be covered: object 1435 permanence, object naming, object fragment addressing, and 1436 operational service support. 1438 The permanence declaration for an object is a rating defined with 1439 respect to an identified permanence provider (guarantor), which will 1440 be the NMA. It may include the following aspects. 1442 (a) "object availability" -- whether and how access to the object 1443 is supported (e.g., online 24x7, or offline only), 1445 (b) "identifier validity" -- under what conditions the identifier 1446 will be or has been re-assigned, 1448 (c) "content invariance" -- under what conditions the content of 1449 the object is subject to change, and 1451 (d) "change history" -- access to corrections, migrations, and 1452 revisions, whether through links to the changed objects themselves 1453 or through a document summarizing the change history 1455 One approach to persistence statements, conceived independently from 1456 ARKs, can be found at [PStatements], with ongoing work available at 1457 [ARKspecs]. An older approach to a permanence rating framework is 1458 given in [NLMPerm], which identified the following "permanence 1459 levels": 1461 Not Guaranteed: No commitment has been made to retain this 1462 resource. It could become unavailable at any time. Its 1463 identifier could be changed. 1465 Permanent: Dynamic Content: A commitment has been made to keep 1466 this resource permanently available. Its identifier will always 1467 provide access to the resource. Its content could be revised or 1468 replaced. 1470 Permanent: Stable Content: A commitment has been made to keep this 1471 resource permanently available. Its identifier will always 1472 provide access to the resource. Its content is subject only to 1473 minor corrections or additions. 1475 Permanent: Unchanging Content: A commitment has been made to keep 1476 this resource permanently available. Its identifier will always 1477 provide access to the resource. Its content will not change. 1479 Naming policy for an object includes an historical description of the 1480 NAA's (and its successor NAA's) policies regarding differentiation of 1481 objects. Since it is the NMA that responds to requests for policy 1482 statements, it is useful for the NMA to be able to produce or 1483 summarize these historical NAA documents. Naming policy may include 1484 the following aspects. 1486 (i) "similarity" -- (or "unity") the limit, defined by the NAA, to 1487 the level of dissimilarity beyond which two similar objects 1488 warrant separate identifiers but before which they share one 1489 single identifier, and 1491 (ii) "granularity" -- the limit, defined by the NAA, to the level 1492 of object subdivision beyond which sub-objects do not warrant 1493 separately assigned identifiers but before which sub-objects are 1494 assigned separate identifiers. 1496 Subnaming policy for an object describes the qualifiers that the NMA, 1497 in fulfilling its ongoing and evolving service obligations, allows as 1498 extensions to an NAA-assigned ARK. To the conceptual object that the 1499 NAA named with an ARK, the NMA may add component access points and 1500 derivatives (e.g., format migrations in aid of preservation) in order 1501 to provide both basic and value-added services. 1503 Addressing policy for an object includes a description of how, during 1504 access, object components (e.g., paragraphs, sections) or views 1505 (e.g., image conversions) may or may not be "addressed", in other 1506 words, how the NMA permits arguments or parameters to modify the 1507 object delivered as the result of an ARK request. If supported, 1508 these sorts of operations would provide things like byte-ranged 1509 fragment delivery and open-ended format conversions, or any set of 1510 possible transformations that would be too numerous to list or to 1511 identify with separately assigned ARKs. 1513 Operational service support policy includes a description of general 1514 operational aspects of the NMA service, such as after-hours staffing 1515 and trouble reporting procedures. 1517 5.1.2. Generic Description Service 1519 Returns a description of the object. Descriptions are returned in a 1520 structured metadata format, a human-readable text format, or in one 1521 format that serves both purposes (such as human-readable HTML with 1522 embedded machine-readable metadata, or perhaps YAML). A description 1523 must at a minimum answer the who, what, when, and where questions 1524 ("where" being the long-term identifier as opposed to a transient 1525 redirect target) concerning an expression of the object. Standalone 1526 descriptions should be accompanied by the modification date and 1527 source of the description itself. May also return discriminated 1528 lists of ARKs that are related to the given ARK. 1530 5.2. Overview of The HTTP URL Mapping Protocol (THUMP) 1532 The HTTP URL Mapping Protocol (THUMP) is a way of taking a key (any 1533 identifier) and asking such questions as, what information does this 1534 identify and how permanent is it? [THUMP] is in fact one specific 1535 method under development for delivering ARK services. The protocol 1536 runs over HTTP to exploit the web browser's current pre-eminence as 1537 user interface to the Internet. THUMP is designed so that a person 1538 can enter ARK requests directly into the location field of current 1539 browser interfaces. Because it runs over HTTP, THUMP can be 1540 simulated and tested via keyboard-based interactions [RFC0854]. 1542 The asker (a person or client program) starts with an identifier, 1543 such as an ARK or a URL. The identifier reveals to the asker (or 1544 allows the asker to infer) the Internet host name and port number of 1545 a server system that responds to questions. Here, this is just the 1546 NMA that is obtained by inspection and possibly lookup based on the 1547 ARK's NAAN. The asker then sets up an HTTP session with the server 1548 system, sends a question via a THUMP request (contained within an 1549 HTTP request), receives an answer via a THUMP response (contained 1550 within an HTTP response), and closes the session. That concludes the 1551 connected portion of the protocol. 1553 A THUMP request is a string of characters beginning with a `?' 1554 (question mark) that is appended to the identifier string. The 1555 resulting string is sent as an argument to HTTP's GET command. 1556 Request strings too long for GET may be sent using HTTP's POST 1557 command. The two most common requests correspond to two degenerate 1558 special cases. First, a simple key with no request at all is the 1559 same as an ordinary access request. Thus a plain ARK entered into a 1560 browser's location field behaves much like a plain URL, and returns 1561 access to the primary identified object, for instance, an HTML 1562 document. 1564 The second special case is a minimal ARK description request string 1565 consisting of just "?info". For example, entering the string, 1567 n2t.net/ark:67531/metadc107835?info 1569 into the browser's location field directly precipitates a request for 1570 a metadata record describing the object identified by ark:67531/ 1571 metadc107835. The browser, unaware of THUMP, prepares and sends an 1572 HTTP GET request in the same manner as for a URL. THUMP is designed 1573 so that the response (indicated by the returned HTTP content type) is 1574 normally displayed, whether the output is structured for machine 1575 processing (text/plain) or formatted for human consumption (text/ 1576 html). In addition to '?info', this specification reserves both '?' 1577 and '??' (originally older forms) for future use. 1579 The following example THUMP session assumes metadata being returned 1580 by a resolver (as server) to a browser client. Each line has been 1581 annotated to include a line number and whether it was the client or 1582 server that sent it. Without going into much depth, the session has 1583 four pieces separated from each other by blank lines: the client's 1584 piece (lines 1-3), the server's HTTP/THUMP response headers (4-8), 1585 and the body of the server's response (9-18). The first and last 1586 lines (1 and 19) correspond to the client's steps to start the TCP 1587 session and the server's steps to end it, respectively. 1589 1 C: [opens session] 1590 C: GET https://n2t.net/ark:67531/metadc107835?info HTTP/1.1 1591 C: 1592 S: HTTP/1.1 200 OK 1593 5 S: Content-Type: text/plain 1594 S: THUMP-Status: 0.6 200 OK 1595 S: Link: rel="describes"; 1596 S: 1597 S: erc: 1598 10 S: who: Austin, Larry 1599 S: what: A Study of Rhythm in Bach's Orgelbüchlein 1600 S: when: 1952 1601 S: where: https://digital.library.unt.edu/ark:/67531/metadc107835 1602 S: erc-support: 1603 15 S: who: University of North Texas Libraries 1604 S: what: Permanent: Stable Content: 1605 S: when: 20081203 1606 S: where: https://digital.library.unt.edu/ark:/67531/ 1607 S: [closes session] 1609 The first two server response lines (4-5) above are typical of HTTP. 1610 The next line (6) is peculiar to THUMP, and indicates the THUMP 1611 version and a normal return status. The final header line (7) 1612 asserts, for the benefit of recipients unfamiliar with ARK 1613 inflections, that the response describes the uninflected ARK. 1615 The balance of the response consists of a single metadata record 1616 (9-18) that comprises the ARK description service response. The 1617 returned record is in the format of an Electronic Resource Citation 1618 [ERC], which is discussed in overview in the next section. For now, 1619 note that it contains four elements that answer the top priority 1620 questions regarding an expression of the object: who played a major 1621 role in expressing it, what the expression was called, when it was 1622 created, and where the expression may be found (note that "where" is 1623 preferably a persistent, citable identifier rather than an unstable 1624 URL sometimes mistakenly referred to as a "location"). This quartet 1625 of elements comes up again and again in ERCs. Lines 13-17 contain a 1626 minimal persistence statement. 1628 Each segment in an ERC tells a different story relating to the 1629 object, so although the same four questions (elements) appear in 1630 each, the answers depend on the segment's story type. While the 1631 first segment tells the story of an expression of the object, the 1632 second segment tells the story of the support commitment made to it: 1633 who made the commitment, what the nature of the commitment was, when 1634 it was made, and where a fuller explanation of the commitment may be 1635 found. 1637 5.3. The Electronic Resource Citation (ERC) 1639 An Electronic Resource Citation (or ERC, pronounced e-r-c) [ERC] is a 1640 kind of object description that uses Dublin Core Kernel metadata 1641 elements [DCKernel]. The ERC with Kernel elements provides a simple, 1642 compact, and printable record for holding data associated with an 1643 information resource. As originally designed [Kernel], Kernel 1644 metadata balances the needs for expressive power, very simple machine 1645 processing, and direct human manipulation. The ERC sense of 1646 "citation" is not limited to the traditional referencing of a result 1647 or information fixed in time on a printed page, but to a more general 1648 kind of reference, both backward, to digital material that cannot be 1649 known to be fixed in time (true of virtually all online information), 1650 and forward, to material that is all the more valuable for improving 1651 or evolving over time. 1653 The previous section shows two limited examples of what is fully 1654 described elsewhere [ERC]. The rest of this short section provides 1655 some of the background and rationale for this record format. 1657 A founding principle of Kernel metadata is that direct human contact 1658 with metadata will be a necessary and sufficient condition for the 1659 near term rapid development of metadata standards, systems, and 1660 services. Thus the machine-processable Kernel elements must only 1661 minimally strain people's ability to read, understand, change, and 1662 transmit ERCs without their relying on intermediation with 1663 specialized software tools. The basic ERC needs to be succinct, 1664 transparent, and trivially parseable by software. 1666 Borrowing from the data structuring format that underlies the 1667 successful spread of email and web services, the ERC format uses 1668 [ANVL], which is based on email and HTTP headers [RFC2822]. There is 1669 a naturalness to ANVL's label-colon-value format (seen in the 1670 previous section) that barely needs explanation to a person beginning 1671 to enter ERC metadata. 1673 While ANVL elements are expected at the top level and don't 1674 themselves support hierarchy, the value of an ANVL element may be an 1675 arbitrary encoded hierarchy of JSON or XML. Typically, the name of 1676 such an ANVL element ends in "json" or "xml", for example, "json" or 1677 "geojson". Care should be taken to escape structural characters that 1678 appear in element names and values, specifically, line terminators 1679 (both newlines ("\n") and carriage returns ("\r")) and, in element 1680 names, colons (":"). 1682 Besides simplicity of ERC system implementation and data entry 1683 mechanics, ERC semantics (what the record and its constituent parts 1684 mean) must also be easy to explain. ERC semantics are based on a 1685 reformulation and extension of the Dublin Core [RFC5013] hypothesis, 1686 which suggests that the fifteen Dublin Core metadata elements have a 1687 key role to play in cross-domain resource description. The ERC 1688 design recognizes that the Dublin Core's primary contribution is the 1689 international, interdisciplinary consensus that identified fifteen 1690 semantic buckets (element categories), regardless of how they are 1691 labeled. The ERC then adds a definition for a record and some 1692 minimal compliance rules. In pursuing the limits of simplicity, the 1693 ERC design combines and relabels some Dublin Core buckets to isolate 1694 a tiny kernel (subset) of four elements for basic cross-domain 1695 resource description. 1697 For the cross-domain kernel, the ERC uses the four basic elements -- 1698 who, what, when, and where -- to pretend that every object in the 1699 universe can have a uniform minimal description. Each has a name or 1700 other identifier, a locator (a means to access it), some responsible 1701 person or party, and a date. It doesn't matter what type of object 1702 it is, or whether one plans to read it, interact with it, smoke it, 1703 wear it, or navigate it. Of course, this approach is flawed because 1704 uniformity of description for some object types requires more 1705 semantic contortion and sacrifice than for others. That is why at 1706 the beginning of this document, the ARK was said to be suited to 1707 objects that accommodate reasonably regular electronic description. 1709 While insisting on uniformity at the most basic level provides 1710 powerful cross-domain leverage, the semantic sacrifice is great for 1711 many applications. So the ERC also permits a semantically rich and 1712 nuanced description to co-exist in a record along with a basic 1713 description. In that way both sophisticated and naive recipients of 1714 the record can extract the level of meaning from it that best suits 1715 their needs and abilities. Key to unlocking the richer description 1716 is a controlled vocabulary of ERC record types (not explained in this 1717 document) that permit knowledgeable recipients to apply defined sets 1718 of additional assumptions to the record. 1720 5.4. Advice to Web Clients 1722 ARKs are envisaged to appear wherever durable object references are 1723 planned. Library cataloging records, literature citations, and 1724 bibliographies are important examples. In many of these places URLs 1725 (Uniform Resource Locators) are currently used, and inside some of 1726 those URLs are embedded URNs, Handles, and DOIs. Unfortunately, 1727 there's no suggestion of a way to probe for extra services that would 1728 build confidence in those identifiers; in other words, there's no way 1729 to tell whether any of those identifiers is any better managed than 1730 the average URL. 1732 ARKs are also envisaged to appear in hypertext links (where they are 1733 not normally shown to users) and in rendered text (displayed or 1734 printed). A normal HTML link for which the URL is not displayed 1735 looks like this. 1737 Click Here 1739 A URL with an embedded ARK invites access (via `?info') to extra 1740 services: 1742 Click Here 1744 Using the [N2T] resolver to provide identifier-scheme-agnostic 1745 protection against hostname instability, this ARK could be published 1746 as: 1748 Click Here 1749 An NAA will typically make known the associations it creates by 1750 publishing them in catalogs, actively advertizing them, or simply 1751 leaving them on web sites for visitors (e.g., users, indexing 1752 spiders) to stumble across in browsing. 1754 5.5. Enhancements and Related Specifications 1756 ARK services, data models, inflections, and applications continue to 1757 evolve. Follow-on developments and specifications will be made 1758 available from the ARK Maintenance Agency [ARKspecs]. 1760 5.6. Security Considerations 1762 The ARK naming scheme poses no direct risk to computers and networks. 1763 Implementors of ARK services need to be aware of security issues when 1764 querying networks and filesystems for Name Mapping Authority 1765 services, and the concomitant risks from spoofing and obtaining 1766 incorrect information. These risks are no greater for ARK mapping 1767 authority discovery than for other kinds of service discovery. For 1768 example, recipients of ARKs with a specified host (NMA) should treat 1769 it like a URL and be aware that the identified ARK service may no 1770 longer be operational. 1772 Apart from mapping authority discovery, ARK clients and servers 1773 subject themselves to all the risks that accompany normal operation 1774 of the protocols underlying mapping services (e.g., HTTP, Z39.50). 1775 As specializations of such protocols, an ARK service may limit 1776 exposure to the usual risks. Indeed, ARK services may enhance a kind 1777 of security by helping users identify long-term reliable references 1778 to information objects. 1780 6. References 1782 [ANVL] Kunze, J., Kahle, B., Masanes, J., and G. Mohr, "A Name- 1783 Value Language", 2005, 1784 . 1786 [ARK] Kunze, J., "Towards Electronic Persistence Using ARK 1787 Identifiers", IWAW/ECDL Annual Workshop Proceedings 3rd, 1788 August 2003, . 1790 [ARKagency] 1791 Alliance, A., "ARK Maintenance Agency", 2021, 1792 . 1794 [ARKspecs] Alliance, A., "ARK Maintenance Agency Specifications", 1795 2021, . 1797 [DCKernel] Initiative, D. C. M., "Kernel Metadata Working Group", 1798 2001-2008, . 1800 [DOI] Foundation, I. D., "The Digital Object Identifier (DOI) 1801 System", February 2001, . 1803 [ERC] Kunze, J. and A. Turner, "Kernel Metadata and Electronic 1804 Resource Citations", October 2007, 1805 . 1807 [Handle] Lannom, L., "Handle System Overview", ICSTI Forum No. 30, 1808 April 1999, . 1810 [Kernel] Kunze, J., "A Metadata Kernel for Electronic Permanence", 1811 Journal of Digital Information Vol 2, Issue 2, 1812 ISSN 1368-7506, January 2002, 1813 . 1815 [N2T] Alliance, A., "Name-to-Thing Resolver", August 2006, 1816 . 1818 [NAANregistry] 1819 ARKs.org, "NAAN Registry", 2019, 1820 . 1822 [NAANrequest] 1823 ARKs.org, "NAAN Request Form", 2018, 1824 . 1826 [NLMPerm] Byrnes, M., "Permanence Levels and the Archives for NLM's 1827 Permanent Web Documents", March 2005, 1828 . 1831 [NOID] Kunze, J., "Nice Opaque Identifiers", April 2006, 1832 . 1834 [PStatements] 1835 Kunze, J., "Persistence statements: describing digital 1836 stickiness", October 2016, 1837 . 1839 [PURL] Shafer, K., "Introduction to Persistent Uniform Resource 1840 Locators", 1996, 1841 . 1844 [shoulderrequest] 1845 ARKs.org, "Shoulder Request Form", 2021, 1846 . 1848 [RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol 1849 Specification", STD 8, RFC 854, DOI 10.17487/RFC0854, May 1850 1983, . 1852 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1853 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1854 . 1856 [RFC2141] Moats, R., "URN Syntax", RFC 2141, DOI 10.17487/RFC2141, 1857 May 1997, . 1859 [RFC2288] Lynch, C., Preston, C., and R. Daniel, "Using Existing 1860 Bibliographic Identifiers as Uniform Resource Names", 1861 RFC 2288, DOI 10.17487/RFC2288, February 1998, 1862 . 1864 [RFC2611] Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom, 1865 "URN Namespace Definition Mechanisms", BCP 33, RFC 2611, 1866 DOI 10.17487/RFC2611, June 1999, 1867 . 1869 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1870 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1871 Transfer Protocol -- HTTP/1.1", RFC 2616, 1872 DOI 10.17487/RFC2616, June 1999, 1873 . 1875 [RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822, 1876 DOI 10.17487/RFC2822, April 2001, 1877 . 1879 [RFC2915] Mealling, M. and R. Daniel, "The Naming Authority Pointer 1880 (NAPTR) DNS Resource Record", RFC 2915, 1881 DOI 10.17487/RFC2915, September 2000, 1882 . 1884 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 1885 Resource Identifier (URI): Generic Syntax", STD 66, 1886 RFC 3986, DOI 10.17487/RFC3986, January 2005, 1887 . 1889 [RFC5013] Kunze, J. and T. Baker, "The Dublin Core Metadata Element 1890 Set", RFC 5013, DOI 10.17487/RFC5013, August 2007, 1891 . 1893 [THUMP] Gamiel, K. and J. Kunze, "The HTTP URL Mapping Protocol", 1894 August 2007, . 1897 Appendix A. ARK Maintenance Agency: arks.org 1899 The ARK Maintenance Agency [ARKagency] at arks.org has several 1900 functions. 1902 * To manage the registry of organizations that will be assigning 1903 ARKs. Organizations can request or update a NAAN by filling out 1904 the NAAN Request Form [NAANrequest]. 1906 * To be a clearinghouse for information about ARKs, such as best 1907 practices, introductory documentation, tutorials, community 1908 forums, etc. These supplemental resources help ARK implementor in 1909 high-level applications across different sectors and disciplines, 1910 and with a variety of metadata standards. 1912 * To be a locus of discussion about future versions of the ARK 1913 specification. 1915 Appendix B. Looking up NMAs Distributed via DNS 1917 This subsection introduces an older method for looking up NMAs that 1918 is based on the method for discovering URN resolvers described in 1919 [RFC2915]. It relies on querying the DNS system already installed in 1920 the background infrastructure of most networked computers. A query 1921 is submitted to DNS asking for a list of resolvers that match a given 1922 NAAN. DNS distributes the query to the particular DNS servers that 1923 can best provide the answer, unless the answer can be found more 1924 quickly in a local DNS cache as a side-effect of a recent query. 1925 Responses come back inside Name Authority Pointer (NAPTR) records. 1926 The normal result is one or more candidate NMAs. 1928 In its full generality the [RFC2915] algorithm ambitiously 1929 accommodates a complex set of preferences, orderings, protocols, 1930 mapping services, regular expression rewriting rules, and DNS record 1931 types. This subsection proposes a drastic simplification of it for 1932 the special case of ARK mapping authority discovery. The simplified 1933 algorithm is called Maptr. It uses only one DNS record type (NAPTR) 1934 and restricts most of its field values to constants. The following 1935 hypothetical excerpt from a DNS data file for the NAAN known as 12026 1936 shows three example NAPTR records ready to use with the Maptr 1937 algorithm. 1939 12026.ark.arpa. 1940 ;; US Library of Congress 1941 ;; order pref flags service regexp replacement 1942 IN NAPTR 0 0 "h" "ark" "USLC" lhc.nlm.nih.gov:8080 1943 IN NAPTR 0 0 "h" "ark" "USLC" foobar.zaf.org 1944 IN NAPTR 0 0 "h" "ark" "USLC" sneezy.dopey.com 1946 All the fields are held constant for Maptr except for the "flags", 1947 "regexp", and "replacement" fields. The "service" field contains the 1948 constant value "ark" so that NAPTR records participating in the Maptr 1949 algorithm will not be confused with other NAPTR records. The "order" 1950 and "pref" fields are held to 0 (zero) and otherwise ignored for now; 1951 the algorithm may evolve to use these fields for ranking decisions 1952 when usage patterns and local administrative needs are better 1953 understood. 1955 When a Maptr query returns a record with a flags field of "h" (for 1956 host, a Maptr extension to the NAPTR flags), the replacement field 1957 contains the NMA (host) of an ARK service provider. When a query 1958 returns a record with a flags field of "" (the empty string), the 1959 client needs to submit a new query containing the domain name found 1960 in the replacement field. This second sort of record exploits the 1961 distributed nature of DNS by redirecting the query to another domain 1962 name. It looks like this. 1964 12345.ark.arpa. 1965 ;; Digital Library Consortium 1966 ;; order pref flags service regexp replacement 1967 IN NAPTR 0 0 "" "ark" "" dlc.spct.org. 1969 Here is the Maptr algorithm for ARK mapping authority discovery. In 1970 it replace with the NAAN from the ARK for which an NMA is 1971 sought. 1973 1. Initialize the DNS query: type=NAPTR, query=.ark.arpa. 1975 2. Submit the query to DNS and retrieve (NAPTR) records, discarding 1976 any record that does not have "ark" for the service field. 1978 3. All remaining records with a flags fields of "h" contain 1979 candidate NMAs in their replacement fields. Set them aside, if 1980 any. 1982 4. Any record with an empty flags field ("") has a replacement field 1983 containing a new domain name to which a subsequent query should 1984 be redirected. For each such record, set query= 1985 then go to step (2). When all such records have been recursively 1986 exhausted, go to step (5). 1988 5. All redirected queries have been resolved and a set of candidate 1989 NMAs has been accumulated from steps (3). If there are zero 1990 NMAs, exit -- no mapping authority was found. If there is one or 1991 more NMA, choose one using any criteria you wish, then exit. 1993 A Perl script that implements this algorithm is included here. 1995 #!/depot/bin/perl 1997 use Net::DNS; # include simple DNS package 1998 my $qtype = "NAPTR"; # initialize query type 1999 my $naa = shift; # get NAAN script argument 2000 my $mad = new Net::DNS::Resolver; # mapping authority discovery 2002 &maptr("$naa.ark.arpa"); # call maptr - that's it 2004 sub maptr { # recursive maptr algorithm 2005 my $dname = shift; # domain name as argument 2006 my ($rr, $order, $pref, $flags, $service, $regexp, 2007 $replacement); 2008 my $query = $mad->query($dname, $qtype); 2009 return # non-productive query 2010 if (! $query || ! $query->answer); 2011 foreach $rr ($query->answer) { 2012 next # skip records of wrong type 2013 if ($rr->type ne $qtype); 2014 ($order, $pref, $flags, $service, $regexp, 2015 $replacement) = split(/\s/, $rr->rdatastr); 2016 if ($flags eq "") { 2017 &maptr($replacement); # recurse 2018 } elsif ($flags eq "h") { 2019 print "$replacement\n"; # candidate NMA 2020 } 2021 } 2022 } 2024 The global database thus distributed via DNS and the Maptr algorithm 2025 can easily be seen to mirror the contents of the Name Authority 2026 Table file described in the previous section. 2028 Authors' Addresses 2030 John A. Kunze 2031 California Digital Library 2032 1111 Franklin Street 2033 Oakland, CA 94607 2034 United States of America 2035 Email: jak@ucop.edu 2037 Emmanuelle Bermès 2038 Bibliothèque nationale de France 2039 Quai François Mauriac 2040 75706 Paris 2041 France 2043 Email: emmanuelle.bermes@bnf.fr