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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. Bermes 5 Expires: December 23, 2020 Bibliotheque nationale de France 6 June 21, 2020 8 The ARK Identifier Scheme 9 draft-kunze-ark-24 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[Qualifier] 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 December 23, 2020. 56 Copyright Notice 58 Copyright (c) 2020 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 63 (https://trustee.ietf.org/license-info) in effect on the date of 64 publication of this document. Please review these documents 65 carefully, as they describe your rights and restrictions with respect 66 to this document. 68 Table of Contents 70 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 71 1.1. Reasons to Use ARKs . . . . . . . . . . . . . . . . . . . 4 72 1.2. Three Requirements of ARKs . . . . . . . . . . . . . . . 5 73 1.3. Organizing Support for ARKs: Our Stuff vs. Their Stuff . 6 74 1.4. Definition of Identifier . . . . . . . . . . . . . . . . 7 75 2. ARK Anatomy . . . . . . . . . . . . . . . . . . . . . . . . . 8 76 2.1. The Name Mapping Authority (NMA) . . . . . . . . . . . . 9 77 2.2. The ARK Label Part (ark:) . . . . . . . . . . . . . . . . 11 78 2.3. The Name Assigning Authority Number (NAAN) . . . . . . . 11 79 2.4. The Name Part . . . . . . . . . . . . . . . . . . . . . . 12 80 2.5. The Qualifier Part . . . . . . . . . . . . . . . . . . . 13 81 2.5.1. ARKs that Reveal Object Hierarchy . . . . . . . . . . 14 82 2.5.2. ARKs that Reveal Object Variants . . . . . . . . . . 15 83 2.6. Character Repertoires . . . . . . . . . . . . . . . . . . 16 84 2.7. Normalization and Lexical Equivalence . . . . . . . . . . 17 85 3. Naming Considerations . . . . . . . . . . . . . . . . . . . . 19 86 3.1. ARKS Embedded in Language . . . . . . . . . . . . . . . . 19 87 3.2. Objects Should Wear Their Identifiers . . . . . . . . . . 19 88 3.3. Names are Political, not Technological . . . . . . . . . 20 89 3.4. Choosing a Hostname or NMA . . . . . . . . . . . . . . . 20 90 3.5. Assigners of ARKs . . . . . . . . . . . . . . . . . . . . 22 91 3.6. NAAN Namespace Management . . . . . . . . . . . . . . . . 22 92 3.7. Sub-Object Naming . . . . . . . . . . . . . . . . . . . . 24 93 4. Finding a Name Mapping Authority . . . . . . . . . . . . . . 24 94 4.1. Looking Up NMAs in a Globally Accessible File . . . . . . 25 95 5. Generic ARK Service Definition . . . . . . . . . . . . . . . 26 96 5.1. Generic ARK Access Service (access, location) . . . . . . 26 97 5.1.1. Generic Policy Service (permanence, naming, etc.) . . 27 98 5.1.2. Generic Description Service . . . . . . . . . . . . . 29 99 5.2. Overview of The HTTP URL Mapping Protocol (THUMP) . . . . 29 100 5.3. The Electronic Resource Citation (ERC) . . . . . . . . . 31 101 5.4. Advice to Web Clients . . . . . . . . . . . . . . . . . . 33 102 5.5. Security Considerations . . . . . . . . . . . . . . . . . 33 103 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 104 Appendix A. ARK Maintenance Agency: arks.org . . . . . . . . . . 36 105 Appendix B. Looking up NMAs Distributed via DNS . . . . . . . . 36 106 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39 108 1. Introduction 110 [ Note about this transitional draft. The ARKsInTheOpen.org 111 Technical Working Group (https://wiki.duraspace.org/display/ARKs/ 112 Technical+Working+Group) is in the process of revising the ARK spec 113 via a series of Internet-Drafts. This draft contains some minor but 114 "noisy" changes (lots of diffs). ] 116 This document describes a scheme for the high-quality naming of 117 information resources. The scheme, called the Archival Resource Key 118 (ARK), is well suited to long-term access and identification of any 119 information resources that accommodate reasonably regular electronic 120 description. This includes digital documents, databases, software, 121 and websites, as well as physical objects (books, bones, statues, 122 etc.) and intangible objects (chemicals, diseases, vocabulary terms, 123 performances). Hereafter the term "object" refers to an information 124 resource. The term ARK itself refers both to the scheme and to any 125 single identifier that conforms to it. A reasonably concise and 126 accessible overview and rationale for the scheme is available at 127 [ARK]. 129 Schemes for persistent identification of network-accessible objects 130 are not new. In the early 1990's, the design of the Uniform Resource 131 Name [RFC2141] responded to the observed failure rate of URLs by 132 articulating an indirect, non-hostname-based naming scheme and the 133 need for responsible name management. Meanwhile, promoters of the 134 Digital Object Identifier [DOI] succeeded in building a community of 135 providers around a mature software system [Handle] that supports name 136 management. The Persistent Uniform Resource Locator [PURL] was 137 another scheme that had the advantage of working with unmodified web 138 browsers. ARKs represent an approach that attempts to build on the 139 strengths and to avoid the weaknesses of these schemes. 141 A founding principle of the ARK is that persistence is purely a 142 matter of service. Persistence is neither inherent in an object nor 143 conferred on it by a particular naming syntax. Nor is the technique 144 of name indirection -- upon which URNs, Handles, DOIs, and PURLs are 145 founded -- of central importance. Name indirection is an ancient and 146 well-understood practice; new mechanisms for it keep appearing and 147 distracting practitioner attention, with the Domain Name System (DNS) 148 [RFC1034] being a particularly dazzling and elegant example. What is 149 often forgotten is that maintenance of an indirection table is an 150 unavoidable cost to the organization providing persistence, and that 151 cost is equivalent across naming schemes. That indirection has 152 always been a native part of the web while being so lightly utilized 153 for the persistence of web-based objects indicates how unsuited most 154 organizations will probably be to the task of table maintenance and 155 to the much more fundamental challenge of keeping the objects 156 themselves viable. 158 Persistence is achieved through a provider's successful stewardship 159 of objects and their identifiers. The highest level of persistence 160 will be reinforced by a provider's robust contingency, redundancy, 161 and succession strategies. It is further safeguarded to the extent 162 that a provider's mission is shielded from funding and political 163 instabilities. These are by far the major challenges confronting 164 persistence providers, and no identifier scheme has any direct impact 165 on them. In fact, some schemes may actually be liabilities for 166 persistence because they create short- and long-term dependencies for 167 every object access on complex, special-purpose infrastructures, 168 parts of which are proprietary and all of which increase the carry- 169 forward burden for the preservation community. It is for this reason 170 that the ARK scheme relies only on educated name assignment and light 171 use of general-purpose infrastructures that are maintained mostly by 172 the internet community at large (the DNS, web servers, and web 173 browsers). 175 1.1. Reasons to Use ARKs 177 If no persistent identifier scheme contributes directly to 178 persistence, why not just use URLs? A particular URL may be as 179 durable an identifier as it is possible to have, but nothing 180 distinguishes it from an ordinary URL to the recipient who is 181 wondering if it is suitable for long-term reference. An ARK embedded 182 in a URL provides some of the necessary conditions for credible 183 persistence, inviting access to not one, but to three things: to the 184 object, to its metadata, and to a nuanced statement of commitment 185 from the provider in question (the NMA, described below) regarding 186 the object. Existence of the extra service can be probed 187 automatically by appending `?info' to the ARK. 189 The form of the ARK also supports the natural separation of naming 190 authorities into the original name assigning authority and the 191 diverse multiple name mapping (or servicing) authorities that in 192 succession and in parallel will take over custodial responsibilities 193 from the original assigner (assuming the assigner ever held that 194 responsibility) for the large majority of a long-term object's 195 archival lifetime. The name mapping authority, indicated by the 196 hostname part of the URL that contains the ARK, serves to launch the 197 ARK into cyberspace. Should it ever fail (and there is no reason why 198 a well-chosen hostname for a 100-year-old cultural memory institution 199 shouldn't last as long as the DNS), that host name is considered 200 disposeable and replaceable. Again, the form of the ARK helps 201 because it defines exactly how to recover the core immutable object 202 identity, and simple algorithms (one based on the URN model) or even 203 by-hand internet query can be used for for locating another mapping 204 authority. 206 There are tools to assist in generating ARKs and other identifiers, 207 such as [NOID] and "uuidgen", both of which rely for uniqueness on 208 human-maintained registries. This document also contains some 209 guidelines and considerations for managing namespaces and choosing 210 hostnames with persistence in mind. 212 1.2. Three Requirements of ARKs 214 The first requirement of an ARK is to give users a link from an 215 object to a promise of stewardship for it. That promise is a multi- 216 faceted covenant that binds the word of an identified service 217 provider to a specific set of responsibilities. It is critical for 218 the promise to come from a current provider and almost irrelevant, 219 over a long period of time, what the original assigner's intentions 220 were. No one can tell if successful stewardship will take place 221 because no one can predict the future. Reasonable conjecture, 222 however, may be based on past performance. There must be a way to 223 tie a promise of persistence to a provider's demonstrated or 224 perceived ability -- its reputation -- in that arena. Provider 225 reputations would then rise and fall as promises are observed 226 variously to be kept and broken. This is perhaps the best way we 227 have for gauging the strength of any persistence promise. 229 The second requirement of an ARK is to give users a link from an 230 object to a description of it. The problem with a naked identifier 231 is that without a description real identification is incomplete. 232 Identifiers common today are relatively opaque, though some contain 233 ad hoc clues reflecting assertions that were briefly true, such as 234 where in a filesystem hierarchy an object lived during a short stay. 235 Possession of both an identifier and an object is some improvement, 236 but positive identification may still be uncertain since the object 237 itself might not include a matching identifier or might not carry 238 evidence obvious enough to reveal its identity without significant 239 research. In either case, what is called for is a record bearing 240 witness to the identifier's association with the object, as supported 241 by a recorded set of object characteristics. This descriptive record 242 is partly an identification "receipt" with which users and archivists 243 can verify an object's identity after brief inspection and a 244 plausible match with recorded characteristics such as title and size. 246 The final requirement of an ARK is to give users a link to the object 247 itself (or to a copy) if at all possible. High quality access is the 248 central duty of an ARK. Persistent identification plays a vital 249 supporting role but, strictly speaking, it can be construed as no 250 more than a record attesting to the original assignment of a never- 251 reassigned identifier. Object access may not be feasible for various 252 reasons, such as a transient service outage, a catastrophic loss, a 253 licensing agreement that keeps an archive "dark" for a period of 254 years, or when an object's own lack of tangible existence confuses 255 normal concepts of access (e.g., a vocabulary term might be 256 "accessed" through its definition). In such cases the ARK's 257 identification role assumes a much higher profile. But attempts to 258 simplify the persistence problem by decoupling access from 259 identification and concentrating exclusively on the latter are of 260 questionable utility. A perfect system for assigning forever unique 261 identifiers might be created, but if it did so without reducing 262 access failure rates, no one would be interested. The central issue 263 -- which may be summed up as the "HTTP 404 Not Found" problem -- 264 would not have been addressed. 266 ARK resolvers must support the `?info' inflection for requesting 267 metadata. Older versions of this specification distinguished between 268 two minimal inflections: `?' (brief metadata) and `??' (more 269 metadata). While these older inflections are still reserved, because 270 they have proven hard to recognize in some environments, supporting 271 them is optional. 273 1.3. Organizing Support for ARKs: Our Stuff vs. Their Stuff 275 An organization and the user community it serves can often be seen to 276 struggle with two different areas of persistent identification: the 277 Our Stuff problem and the Their Stuff problem. In the Our Stuff 278 problem, we in the organization want our own objects to acquire 279 persistent names. Since we possess or control these objects, our 280 organization tackles the Our Stuff problem directly. Whether or not 281 the objects are named by ARKs, our organization is the responsible 282 party, so it can plan for, maintain, and make commitments about the 283 objects. 285 In the Their Stuff problem, we in the organization want others' 286 objects to acquire persistent names. These are objects that we do 287 not own or control, but some of which are critically important to us. 288 But because they are beyond our influence as far as support is 289 concerned, creating and maintaining persistent identifiers for Their 290 Stuff is not especially purposeful or feasible for us to engage in. 291 There is little that we can do about someone else's stuff except 292 encourage their uptake or adoption of persistence services. 294 Co-location of persistent access and identification services is 295 natural. Any organization that undertakes ongoing support of true 296 persistent identification (which includes description) is well-served 297 if it controls, owns, or otherwise has clear internal access to the 298 identified objects, and this gives it an advantage if it wishes also 299 to support persistent access to outsiders. Conversely, persistent 300 access to outsiders requires orderly internal collection management 301 procedures that include monitoring, acquisition, verification, and 302 change control over objects, which in turn requires object 303 identifiers persistent enough to support auditable record keeping 304 practices. 306 Although organizing ARK support under one roof thus tends to make 307 sense, object hosting can successfully be separated from name 308 mapping. An example is when a name mapping authority centrally 309 provides uniform resolution services via a protocol gateway on behalf 310 of organizations that host objects behind a variety of access 311 protocols. It is also reasonable to build value-added description 312 services that rely on the underlying services of a set of mapping 313 authorities. 315 Supporting ARKs is not for every organization. By requiring 316 specific, revealed commitments to preservation, to object access, and 317 to description, the bar for providing ARK services is higher than for 318 some other identifier schemes. On the other hand, it would be hard 319 to grant credence to a persistence promise from an organization that 320 could not muster the minimum ARK services. Not that there isn't a 321 business model for an ARK-like, description-only service built on top 322 of another organization's full complement of ARK services. For 323 example, there might be competition at the description level for 324 abstracting and indexing a body of scientific literature archived in 325 a combination of open and fee-based repositories. The description- 326 only service would have no direct commitment to the objects, but 327 would act as an intermediary, forwarding commitment statements from 328 object hosting services to requestors. 330 1.4. Definition of Identifier 332 An identifier is not a string of character data -- an identifier is 333 an association between a string of data and an object. This 334 abstraction is necessary because without it a string is just data. 335 It's nonsense to talk about a string's breaking, or about its being 336 strong, maintained, and authentic. But as a representative of an 337 association, a string can do, metaphorically, the things that we 338 expect of it. 340 Without regard to whether an object is physical, digital, or 341 conceptual, to identify it is to claim an association between it and 342 a representative string, such as "Jane" or "ISBN 0596000278". What 343 gives a claim credibility is a set of verifiable assertions, or 344 metadata, about the object, such as age, height, title, or number of 345 pages. In other words, the association is made manifest by a record 346 (e.g., a cataloging or other metadata record) that vouches for it. 348 In the complete absence of any testimony (metadata) regarding an 349 association, a would-be identifier string is a meaningless sequence 350 of characters. To keep an externally visible but otherwise internal 351 string from being perceived as an identifier by outsiders, for 352 example, it suffices for an organization not to disclose the nature 353 of its association. For our immediate purpose, actual existence of 354 an association record is more important than its authenticity or 355 verifiability, which are outside the scope of this specification. 357 It is a gift to the identification process if an object carries its 358 own name as an inseparable part of itself, such as an identifier 359 imprinted on the first page of a document or embedded in a data 360 structure element of a digital document header. In cases where the 361 object is large, unwieldy, or unavailable (such as when licensing 362 restrictions are in effect), a metadata record that includes the 363 identifier string will usually suffice. That record becomes a 364 conveniently manipulable object surrogate, acting as both an 365 association "receipt" and "declaration". 367 Note that our definition of identifier extends the one in use for 368 Uniform Resource Identifiers [RFC3986]. The present document still 369 sometimes (ab)uses the terms "ARK" and "identifier" as shorthand for 370 the string part of an identifier, but the context should make the 371 meaning clear. 373 2. ARK Anatomy 375 An ARK is represented by a sequence of characters (a string) that 376 contains the label, "ark:", optionally preceded by the beginning part 377 of a URL. Here is a diagrammed example. 379 ARK ANATOMY 380 =========== 382 Resolver Service Base Object Name Qualifiers 383 __________________ ________________ _____________ 384 / \/ ... \/ \ 385 https://example.org/ark:12345/x54xz321/s3/f8.05v.tiff 386 \_________/ \__/\___/ \______/\____/\_______/ 387 | | | ... | | | 388 | Label | | | Sub-parts Variants 389 | | | | 390 Name Mapping Authority (NMA) | | Assigned Name ... 391 | +---------- Shoulder: /x5 392 Name Assigning Authority Number (NAAN) 394 The ARK syntax can be summarized, 396 [https://NMA/]ark:[/]NAAN/Name[Qualifier] 398 where the NMA, '/', and Qualifier parts are in brackets to indicate 399 that they are optional. The Base Object Name is the substring 400 comprising the "ark:" label, the NAAN and the assigned Name. The 401 Resolver Service is replaceable and makes the ARK actionable for a 402 period of time. Without the Resolver Service part, what remains is 403 the Core Immutable Identity (the "persistible") part of the ARK. 405 2.1. The Name Mapping Authority (NMA) 407 Before the "ark:" label may appear an optional Name Mapping Authority 408 (NMA) that is a temporary address where ARK service requests may be 409 sent. Preceded by a URI-type protocol designation such as 410 "https://", it specifies a Resolver Service. The NMA itself is an 411 Internet hostname or host/port combination having the same format and 412 semantics as the host/port part of a URL. The most important thing 413 about the NMA is that it is "identity inert" from the point of view 414 of object identification. In other words, ARKs that differ only in 415 the optional NMA part identify the same object. Thus, for example, 416 the following three ARKs are synonyms for just one information 417 object: 419 https://loc.gov/ark:12345/x54xz321 420 https://rutgers.edu/ark:12345/x54xz321 421 ark:12345/x54xz321 423 Strictly speaking, in the realm of digital objects, these ARKs may 424 lead over time to somewhat different or diverging instances of the 425 originally named object. In an ideal world, divergence of persistent 426 objects is not desirable, but it is widely believed that digital 427 preservation efforts will inevitably lead to alterations in some 428 original objects (e.g, a format migration in order to preserve the 429 ability to display a document). If any of those objects are held 430 redundantly in more than one organization (a common preservation 431 strategy), chances are small that all holding organizations will 432 perform the same precise transformations and all maintain the same 433 object metadata. More significant divergence would be expected when 434 the holding organizations serve different audiences or compete with 435 each other. 437 The NMA part makes an ARK into an actionable URL. As with many 438 internet parameters, it is helpful to approach the NMA being liberal 439 in what you accept and conservative in what you propose. From the 440 recipient's point of view, the NMA part should be treated as 441 temporary, disposable, and replaceable. From the NMA's point of 442 view, it should be chosen with the greatest concern for longevity. A 443 carefully chosen NMA should be at least as permanent as the providing 444 organization's own hostname. In the case of a national or university 445 library, for example, there is no reason why the NMA should not be 446 considerably more permanent than soft-funded proxy hostnames such as 447 hdl.handle.net, dx.doi.org, and purl.org. In general and over time, 448 however, it is not unexpected for an NMA eventually to stop working 449 and require replacement with the NMA of a currently active service 450 provider. 452 This replacement relies on a mapping authority "resolver" discovery 453 process, of which two alternate methods are outlined in a later 454 section. The ARK, URN, Handle, and DOI schemes all use a resolver 455 discovery model that sooner or later requires matching the original 456 assigning authority with a current provider servicing that 457 authority's named objects; once found, the resolver at that provider 458 performs what amounts to a redirect to a place where the object is 459 currently held. All the schemes rely on the ongoing functionality of 460 currently mainstream technologies such as the Domain Name System 461 [RFC1034] and web browsers. The Handle and DOI schemes in addition 462 require that the Handle protocol layer and global server grid be 463 available at all times. 465 The practice of prepending "https://" and an NMA to an ARK is a way 466 of creating an actionable identifier by a method that is itself 467 temporary. Assuming that infrastructure supporting [RFC2616] 468 information retrieval will no longer be available one day, ARKs will 469 then have to be converted into new kinds of actionable identifiers. 470 By that time, if ARKs see widespread use, web browsers would 471 presumably evolve to perform this (currently simple) transformation 472 automatically. 474 2.2. The ARK Label Part (ark:) 476 The label part distinguishes an ARK from an ordinary identifier. 477 There is a new form of the label, "ark:", and an old form, "ark:/", 478 both of which must be recognized in perpetuity. Implementations 479 should generate new ARKs in the new form (without the "/") and 480 resolvers must always treat received ARKs as equivalent if they 481 differ only in regard to new form versus old form labels. Thus these 482 two ARKs are equivalent: 484 ark:/12345/x54xz321 485 ark:12345/x54xz321 487 In a URL found in the wild, the label indicates that the URL stands a 488 reasonable chance of being an ARK. If the context warrants, 489 verification that it actually is an ARK can be done by testing it for 490 existence of the three ARK services. 492 Since nothing about an identifier syntax directly affects 493 persistence, the "ark:" label (like "urn:", "doi:", and "hdl:") 494 cannot tell you whether the identifier is persistent or whether the 495 object is available. It does tell you that the original Name 496 Assigning Authority (NAA) had some sort of hopes for it, but it 497 doesn't tell you whether that NAA is still in existence, or whether a 498 decade ago it ceased to have any responsibility for providing 499 persistence, or whether it ever had any responsibility beyond naming. 501 Only a current provider can say for certain what sort of commitment 502 it intends, and the ARK label suggests that you can query the NMA 503 directly to find out exactly what kind of persistence is promised. 504 Even if what is promised is impersistence (i.e., a short-term 505 identifier), saying so is valuable information to the recipient. 506 Thus an ARK is a high-functioning identifier in the sense that it 507 provides access to the object, the metadata, and a commitment 508 statement, even if the commitment is explicitly very weak. 510 2.3. The Name Assigning Authority Number (NAAN) 512 Recalling that the general form of the ARK is, 514 [https://NMA/]ark:[/]NAAN/Name[Qualifier] 516 the part of the ARK directly following the "ark:" (or older "ark:/") 517 label is the Name Assigning Authority Number (NAAN), up to but not 518 including the next `/' (slash) character. This part is always 519 required, as it identifies a hostname of the organization that 520 originally assigned the Name of the object. Typically the 521 organization is an institution, a department, a laboratory, or any 522 group that conducts an stable, policy-driven name assigning effort. 523 It is used to discover a currently valid NMA and to provide top-level 524 partitioning of the space of all ARKs. 526 An organization may request a NAAN from the ARK Maintenance Agency 527 [ARKagency] (described in Appendix A) by filling out the form at 528 [NAANrequest]. NAANs are opaque strings of one or more characters 529 drawn from this set, 531 0123456789bcdfghjkmnpqrstvwxz 533 which consists of digits and consonants, minus the letter 'l'. 534 Restricting NAANs to this set serves two goals. It reduces the 535 chances that words -- past, present, and future -- will appear in 536 NAANs and carry unintended semantics. It also helps usability by not 537 mixing commonly confused characters ('0' and 'O', '1' and 'l') and by 538 being compatible with strong transcription error detection (eg, the 539 [NOID] check digit algorithm). Since 2001, every assigned NAAN has 540 consisted of exactly five digits, and no immediate change in that 541 practice is foreseen. 543 The NAAN designates a top-level ARK namespace. Once registered for a 544 namespace, a NAAN is never re-registered. It is possible, however, 545 for there to be a succession of organizations that manage an ARK 546 namespace. 548 2.4. The Name Part 550 The part of the ARK just after the NAAN is the Name assigned by the 551 NAA, and it is also required. Semantic opaqueness in the Name part 552 is strongly encouraged in order to reduce an ARK's vulnerability to 553 era- and language-specific change. Identifier strings containing 554 linguistic fragments can create support difficulties down the road. 555 No matter how appropriate or even meaningless they are today, such 556 fragments may one day create confusion, give offense, or infringe on 557 a trademark as the semantic environment around us and our communities 558 evolves. 560 Names that look more or less like numbers avoid common problems that 561 defeat persistence and international acceptance. The use of digits 562 is highly recommended. Mixing in non-vowel alphabetic characters a 563 couple at a time is a relatively safe and easy way to achieve a 564 denser namespace (more possible names for a given length of the name 565 string). Such names have a chance of aging and traveling well. 566 Tools exists that mint, bind, and resolve opaque identifiers, with or 567 without check characters [NOID]. More on naming considerations is 568 given in a subsequent section. 570 2.5. The Qualifier Part 572 The part of the ARK following the NAA-assigned Name is an optional 573 Qualifier. It is a string that extends the base ARK in order to 574 create a kind of service entry point into the object named by the 575 NAA. At the discretion of the providing NMA, such a service entry 576 point permits an ARK to support access to individual hierarchical 577 components and subcomponents of an object, and to variants (versions, 578 languages, formats) of components. A Qualifier may be invented by 579 the NAA or by any NMA servicing the object. 581 In form, the Qualifier is a ComponentPath, or a VariantPath, or a 582 ComponentPath followed by a VariantPath. A VariantPath is introduced 583 and subdivided by the reserved character `.', and a ComponentPath is 584 introduced and subdivided by the reserved character `/'. In this 585 example, 587 https://example.org/ark:12345/x54xz321/s3/f8.05v.tiff 589 the string "/s3/f8" is a ComponentPath and the string ".05v.tiff" is 590 a VariantPath. The ARK Qualifier is a formalization of some 591 currently mainstream URL syntax conventions. This formalization 592 specifically reserves meanings that permit recipients to make strong 593 inferences about logical sub-object containment and equivalence based 594 only on the form of the received identifiers; there is great 595 efficiency in not having to inspect metadata records to discover such 596 relationships. NMAs are free not to disclose any of these 597 relationships merely by avoiding the reserved characters above. 598 Hierarchical components and variants are discussed further in the 599 next two sections. 601 The Qualifier, if present, differs from the Name in several important 602 respects. First, a Qualifier may have been assigned either by the 603 NAA or later by the NMA. The assignment of a Qualifier by an NMA 604 effectively amounts to an act of publishing a service entry point 605 within the conceptual object originally named by the NAA. For our 606 purposes, an ARK extended with a Qualifier assigned by an NMA will be 607 called an NMA-qualified ARK. 609 Second, a Qualifier assignment on the part of an NMA is made in 610 fulfillment of its service obligations and may reflect changing 611 service expectations and technology requirements. NMA-qualified ARKs 612 could therefore be transient, even if the base, unqualified ARK is 613 persistent. For example, it would be reasonable for an NMA to 614 support access to an image object through an actionable ARK that is 615 considered persistent even if the experience of that access changes 616 as linking, labeling, and presentation conventions evolve and as 617 format and security standards are updated. For an image "thumbnail", 618 that NMA could also support an NMA-qualified ARK that is considered 619 impersistent because the thumbnail will be replaced with higher 620 resolution images as network bandwidth and CPU speeds increase. At 621 the same time, for an originally scanned, high-resolution master, the 622 NMA could publish an NMA-qualfied ARK that is itself considered 623 persistent. Of course, the NMA must be able to return its separate 624 commitments to unqualified, NAA-assigned ARKs, to NMA-qualified ARKs, 625 and to any NAA-qualified ARKs that it supports. 627 A third difference between a Qualifier and a Name concerns the 628 semantic opaqueness constraint. When an NMA-qualified ARK is to be 629 used as a transient service entry point into a persistent object, the 630 priority given to semantic opaqueness observed by the NAA in the Name 631 part may be relaxed by the NMA in the Qualifier part. If service 632 priorities in the Qualifier take precedence over persistence, short- 633 term usability considerations may recommend somewhat semantically 634 laden Qualifier strings. 636 Finally, not only is the set of Qualifiers supported by an NMA 637 mutable, but different NMAs may support different Qualifier sets for 638 the same NAA-identified object. In this regard the NMAs act 639 independently of each other and of the NAA. 641 The next two sections describe how ARK syntax may be used to declare, 642 or to avoid declaring, certain kinds of relatedness among qualified 643 ARKs. 645 2.5.1. ARKs that Reveal Object Hierarchy 647 An NAA or NMA may choose to reveal the presence of a hierarchical 648 relationship between objects using the `/' (slash) character after 649 the Name part of an ARK. Some authorities will choose not to 650 disclose this information, while others will go ahead and disclose so 651 that manipulators of large sets of ARKs can infer object 652 relationships by simple identifier inspection; for example, this 653 makes it possible for a system to present a collapsed view of a large 654 search result set. 656 If the ARK contains an internal slash after the NAAN, the piece to 657 its left indicates a containing object. For example, publishing an 658 ARK of the form, 660 ark:12345/x54/xz/321 662 is equivalent to publishing three ARKs, 663 ark:12345/x54/xz/321 664 ark:12345/x54/xz 665 ark:12345/x54 667 together with a declaration that the first object is contained in the 668 second object, and that the second object is contained in the third. 670 Revealing the presence of hierarchy is completely up to the assigner 671 (NMA or NAA). It is hard enough to commit to one object's name, let 672 alone to three objects' names and to a specific, ongoing relatedness 673 among them. Thus, regardless of whether hierarchy was present 674 initially, the assigner, by not using slashes, reveals no shared 675 inferences about hierarchical or other inter-relatedness in the 676 following ARKs: 678 ark:12345/x54_xz_321 679 ark:12345/x54_xz 680 ark:12345/x54xz321 681 ark:12345/x54xz 682 ark:12345/x54 684 Note that slashes around the ARK's NAAN (/12345/ in these examples) 685 are not part of the ARK's Name and therefore do not indicate the 686 existence of some sort of NAAN super object containing all objects in 687 its namespace. A slash must have at least one non-structural 688 character (one that is neither a slash nor a period) on both sides in 689 order for it to separate recognizable structural components. So 690 initial or final slashes may be removed, and double slashes may be 691 converted into single slashes. 693 2.5.2. ARKs that Reveal Object Variants 695 An NAA or NMA may choose to reveal the possible presence of variant 696 objects or object components using the `.' (period) character after 697 the Name part of an ARK. Some authorities will choose not to 698 disclose this information, while others will go ahead and disclose so 699 that manipulators of large sets of ARKs can infer object 700 relationships by simple identifier inspection; for example, this 701 makes it possible for a system to present a collapsed view of a large 702 search result set. 704 If the ARK contains an internal period after Name, the piece to its 705 left is a root name and the piece to its right, and up to the end of 706 the ARK or to the next period is a suffix. A Name may have more than 707 one suffix, for example, 708 ark:12345/x54.24 709 ark:12345/x4z/x54.24 710 ark:12345/x54.20v.78g.f55 712 There are two main rules. First, if two ARKs share the same root 713 name but have different suffixes, the corresponding objects were 714 considered variants of each other (different formats, languages, 715 versions, etc.) by the assigner (NMA or NAA). Thus, the following 716 ARKs are variants of each other: 718 ark:12345/x54.20v.78g.f55 719 ark:12345/x54.321xz 720 ark:12345/x54.44 722 Second, publishing an ARK with a suffix implies the existence of at 723 least one variant identified by the ARK without its suffix. The ARK 724 otherwise permits no further assumptions about what variants might 725 exist. So publishing the ARK, 727 ark:12345/x54.20v.78g.f55 729 is equivalent to publishing the four ARKs, 731 ark:12345/x54.20v.78g.f55 732 ark:12345/x54.20v.78g 733 ark:12345/x54.20v 734 ark:12345/x54 736 Revealing the possibility of variants is completely up to the 737 assigner. It is hard enough to commit to one object's name, let 738 alone to multiple variants' names and to a specific, ongoing 739 relatedness among them. The assigner is the sole arbiter of what 740 constitutes a variant within its namespace, and whether to reveal 741 that kind of relatedness by using periods within its names. 743 A period must have at least one non-structural character (one that is 744 neither a slash nor a period) on both sides in order for it to 745 separate recognizable structural components. So initial or final 746 periods may be removed, and adjacent periods may be converted into a 747 single period. Multiple suffixes should be arranged in sorted order 748 (pure ASCII collating sequence) at the end of an ARK. 750 2.6. Character Repertoires 752 The Name and Qualifier parts are strings of visible ASCII characters. 753 For received ARKs, implementations must support a minimum length of 754 255 octets for the string composed of the Base ARK plus Qualifier. 755 Implementations generating strings exceeding this length should 756 understand that receiving implementations may not be able to index 757 such ARKs properly. Characters may be letters, digits, or any of 758 these seven characters: 760 = ~ * + @ _ $ 762 The following characters may also be used, but their meanings are 763 reserved: 765 % - . / 767 The characters `/' and `.' are ignored if either appears as the last 768 character of an ARK. If used internally, they allow a name assigner 769 to reveal object hierarchy and object variants as previously 770 described. 772 Hyphens are considered to be insignificant and are always ignored in 773 ARKs. A `-' (hyphen) may appear in an ARK for readability, or it may 774 have crept in during the formatting and wrapping of text, but it must 775 be ignored in lexical comparisons. As in a telephone number, hyphens 776 have no meaning in an ARK. It is always safe for an NMA that 777 receives an ARK to remove any hyphens found in it. As a result, like 778 the NMA, hyphens are "identity inert" in comparing ARKs for 779 equivalence. For example, the following ARKs are equivalent for 780 purposes of comparison and ARK service access: 782 ark:12345/x5-4-xz-321 783 https://sneezy.dopey.com/ark:12345/x54--xz32-1 784 ark:12345/x54xz321 786 The `%' character is reserved for %-encoding all other octets that 787 would appear in the ARK string, in the same manner as for URIs 788 [RFC3986]. A %-encoded octet consists of a `%' followed by two hex 789 digits; for example, "%7d" stands in for `}'. Lower case hex digits 790 are preferred to reduce the chances of false acronym recognition; 791 thus it is better to use "%acT" instead of "%ACT". The character `%' 792 itself must be represented using "%25". As with URNs, %-encoding 793 permits ARKs to support legacy namespaces (e.g., ISBN, ISSN, SICI) 794 that have less restricted character repertoires [RFC2288]. 796 2.7. Normalization and Lexical Equivalence 798 To determine if two or more ARKs identify the same object, the ARKs 799 are compared for lexical equivalence after first being normalized. 800 Since ARK strings may appear in various forms (e.g., having different 801 NMAs), normalizing them minimizes the chances that comparing two ARK 802 strings for equality will fail unless they actually identify 803 different objects. In a specified-host ARK (one having an NMA), the 804 NMA never participates in such comparisons. Normalization described 805 here serves to define lexical equivalence but does not restrict how 806 implementors normalize ARKs locally for storage. 808 Normalization of a received ARK for the purpose of octet-by-octet 809 equality comparison with another ARK consists of the following steps. 811 1. The NMA part (eg, everything from an initial "https://" up to the 812 next slash), if present is removed. 814 2. Any URI query string is removed (everything from the first 815 literal '?' to the end of the string). 817 3. The first case-insensitive match on "ark:/" or "ark:" is 818 converted to "ark:" (replacing any upper case letters and 819 removing any terminal '/'). 821 4. In the string that remains, the two characters following every 822 occurrence of `%' are converted to lower case. The case of all 823 other letters in the ARK string must be preserved. 825 5. All hyphens are removed. 827 6. If normalization is being done as part of a resolution step, and 828 if the end of the remaining string matches a known inflection, 829 the inflection is noted and removed. 831 7. Structural characters (slash and period) are normalized: initial 832 and final occurrences are removed, and two structural characters 833 in a row (e.g., // or ./) are replaced by the first character, 834 iterating until each occurrence has at least one non-structural 835 character on either side. 837 8. If there are any components with a period on the left and a slash 838 on the right, either the component and the preceding period must 839 be moved to the end of the Name part or the ARK must be thrown 840 out as malformed. 842 9. The final step is to arrange the suffixes in ASCII collating 843 sequence (that is, to sort them) and to remove duplicate 844 suffixes, if any. It is also permissible to throw out ARKs for 845 which the suffixes are not sorted. 847 The resulting ARK string is now normalized. Comparisons between 848 normalized ARKs are case-sensitive, meaning that upper case letters 849 are considered different from their lower case counterparts. 851 To keep ARK string variation to a minimum, no reserved ARK characters 852 should be %-encoded unless it is deliberately to conceal their 853 reserved meanings. No non-reserved ARK characters should ever be 854 %-encoded. Finally, no %-encoded character should ever appear in an 855 ARK in its decoded form. 857 3. Naming Considerations 859 The most important threats faced by persistence providers include 860 such things as funding loss, natural disaster, political and social 861 upheaval, processing faults, and errors in human oversight. There is 862 nothing that an identifer scheme can do about such things. Still, a 863 few observed identifier failures and inconveniences can be traced 864 back to naming practices that we now know to be less than optimal for 865 persistence. 867 3.1. ARKS Embedded in Language 869 The ARK has different goals from the URI, so it has different 870 character set requirements. Because linguistic constructs imperil 871 persistence, for ARKs non-ASCII character support is unimportant. 872 ARKs and URIs share goals of transcribability and transportability 873 within web documents, so characters are required to be visible, non- 874 conflicting with HTML/XML syntax, and not subject to tampering during 875 transmission across common transport gateways. Add the goal of 876 making an undelimited ARK recognizable in running prose, as in 877 ark:12345/=@_22*$, and certain punctuation characters (e.g., comma, 878 period) end up being excluded from the ARK lest the end of a phrase 879 or sentence be mistaken for part of the ARK. 881 This consideration has more direct effect on ARK usability in a 882 natural language context than it has on ARK persistence. The same is 883 true of the rule preventing hyphens from having lexical significance. 884 It is fine to publish ARKs with hyphens in them (e.g., such as the 885 output of UUID/GUID generators), but the uniform treatment of hyphens 886 as insignificant reduces the possibility of users transcribing 887 identifiers that will have been broken through unpredictable 888 hyphenation by word processors. Any measure that reduces user 889 irritation with an identifier will increase its chances of survival. 891 3.2. Objects Should Wear Their Identifiers 893 A valuable technique for provision of persistent objects is to try to 894 arrange for the complete identifier to appear on, with, or near its 895 retrieved object. An object encountered at a moment in time when its 896 discovery context has long since disappeared could then easily be 897 traced back to its metadata, to alternate versions, to updates, etc. 898 This has seen reasonable success, for example, in book publishing and 899 software distribution. An identifier string only has meaning when 900 its association is known, and this a very sure, simple, and low-tech 901 method of reminding everyone exactly what that association is. 903 3.3. Names are Political, not Technological 905 If persistence is the goal, a deliberate local strategy for 906 systematic name assignment is crucial. Names must be chosen with 907 great care. Poorly chosen and managed names will devastate any 908 persistence strategy, and they do not discriminate by identifier 909 scheme. Whether a mistakenly re-assigned name is a URN, DOI, PURL, 910 URL, or ARK, the damage -- failed access and confusion -- is not 911 mitigated more in one scheme than in another. Conversely, in-house 912 efforts to manage names responsibly will go much further towards 913 safeguarding persistence than any choice of naming scheme or name 914 resolution technology. 916 Branding (e.g., at the corporate or departmental level) is important 917 for funding and visibility, but substrings representing brands and 918 organizational names should be given a wide berth except when 919 absolutely necessary in the hostname (the identity-inert) part of the 920 ARK. These substrings are not only unstable because organizations 921 change frequently, but they are also dangerous because successor 922 organizations often have political or legal reasons to actively 923 suppress predecessor names and brands. Any measure that reduces the 924 chances of future political or legal pressure on an identifier will 925 decrease the chances that our descendants will be obliged to 926 deliberately break it. 928 3.4. Choosing a Hostname or NMA 930 Hostnames appearing in any identifier meant to be persistent must be 931 chosen with extra care. The tendency in hostname selection has 932 traditionally been to choose a token with recognizable attributes, 933 such as a corporate brand, but that tendency wreaks havoc with 934 persistence that is supposed to outlive brands, corporations, subject 935 classifications, and natural language semantics (e.g., what did the 936 three letters "gay" mean in 1958, 1978, and 1998?). Today's 937 recognized and correct attributes are tomorrow's stale or incorrect 938 attributes. In making hostnames (any names, actually) long-term 939 persistent, it helps to eliminate recognizable attributes to the 940 extent possible. This affects selection of any name based on URLs, 941 including PURLs and the explicitly disposable NMAs. 943 There is no excuse for a provider that manages its internal names 944 impeccably not to exercise the same care in choosing what could be an 945 exceptionally durable hostname, especially if it would form the 946 prefix for all the provider's URL-based external names. Registering 947 an opaque hostname in the ".org" or ".net" domain would not be a bad 948 start. Another way is to publish your ARKs with an organizational 949 domain name that will be mapped by DNS to an appropriate NMA host. 950 This makes for shorter names with less branding vulnerability. 952 It is a mistake to think that hostnames are inherently unstable. If 953 you require brand visibility, that may be a fact of life. But things 954 are easier if yours is the brand of long-lived cultural memory 955 institution such as a national or university library or archive. 956 Well-chosen hostnames from organizations that are sheltered from the 957 direct effects of a volatile marketplace can easily provide longer- 958 lived global resolvers than the domain names explicitly or implicitly 959 used as starting points for global resolution by indirection-based 960 persistent identifier schemes. For example, it is hard to imagine 961 circumstances under which the Library of Congress' domain name would 962 disappear sooner than, say, "handle.net". 964 For smaller libraries, archives, and preservation organizations, 965 there is a natural concern about whether they will be able to keep 966 their web servers and domain names in the face of uncertain funding. 967 One option is to form or join a consortium [N2T] of like-minded 968 organizations with the purpose of providing mutual preservation 969 support. The first goal of such a consortium would be to perpetually 970 rent a hostname on which to establish a web server that simply 971 redirects incoming member organization requests to the appropriate 972 member server; using ARKs, for example, a 150-member consortium could 973 run a very small server (24x7) that contained nothing more than 150 974 rewrite rules in its configuration file. Even more helpful would be 975 additional consortial support for a member organization that was 976 unable to continue providing services and needed to find a successor 977 archival organization. This would be a low-cost, low-tech way to 978 publish ARKs (or URLs) under highly persistent hostnames. 980 There are no obvious reasons why the organizations registering DNS 981 names, URN Namespaces, and DOI publisher IDs should have among them 982 one that is intrinsically more fallible than the next. Moreover, it 983 is a misconception that the demise of DNS and of HTTP need adversely 984 affect the persistence of URLs. At such a time, certainly URLs from 985 the present day might not then be actionable by our present-day 986 mechanisms, but resolution systems for future non-actionable URLs are 987 no harder to imagine than resolution systems for present-day non- 988 actionable URNs and DOIs. There is no more stable a namespace than 989 one that is dead and frozen, and that would then characterize the 990 space of names bearing the "http://" or "https://" prefix. It is 991 useful to remember that just because hostnames have been carelessly 992 chosen in their brief history does not mean that they are unsuitable 993 in NMAs (and URLs) intended for use in situations demanding the 994 highest level of persistence available in the Internet environment. 995 A well-planned name assignment strategy is everything. 997 3.5. Assigners of ARKs 999 A Name Assigning Authority (NAA) is an organization that creates (or 1000 delegates creation of) long-term associations between identifiers and 1001 information objects. Examples of NAAs include national libraries, 1002 national archives, and publishers. An NAA may arrange with an 1003 external organization for identifier assignment. The US Library of 1004 Congress, for example, allows OCLC (the Online Computer Library 1005 Center, a major world cataloger of books) to create associations 1006 between Library of Congress call numbers (LCCNs) and the books that 1007 OCLC processes. A cataloging record is generated that testifies to 1008 each association, and the identifier is included by the publisher, 1009 for example, in the front matter of a book. 1011 An NAA does not so much create an identifier as create an 1012 association. The NAA first draws an unused identifier string from 1013 its namespace, which is the set of all identifiers under its control. 1014 It then records the assignment of the identifier to an information 1015 object having sundry witnessed characteristics, such as a particular 1016 author and modification date. A namespace is usually reserved for an 1017 NAA by agreement with recognized community organizations (such as 1018 IANA and ISO) that all names containing a particular string be under 1019 its control. In the ARK an NAA is represented by the Name Assigning 1020 Authority Number (NAAN). 1022 The ARK namespace reserved for an NAA is the set of names bearing its 1023 particular NAAN. For example, all strings beginning with 1024 "ark:12345/" are under control of the NAA registered under 12345, 1025 which might be the National Library of Finland. Because each NAA has 1026 a different NAAN, names from one namespace cannot conflict with those 1027 from another. Each NAA is free to assign names from its namespace 1028 (or delegate assignment) according to its own policies. These 1029 policies must be documented in a manner similar to the declarations 1030 required for URN Namespace registration [RFC2611]. 1032 Organizations can request or update a NAAN by filling out a form 1033 [NAANrequest]. 1035 3.6. NAAN Namespace Management 1037 Every NAA must have a namespace management strategy. A time-honored 1038 technique is to hierarchically partition a namespace into 1039 subnamespaces using prefixes that guarantee non-collision of names in 1040 different partition. This practice is strongly encouraged for all 1041 NAAs, especially when subnamespace management will be delegated to 1042 other departments, units, or projects within an organization. For 1043 example, with a NAAN that is assigned to a university and managed by 1044 its main library, care should be taken to reserve semantically opaque 1045 prefixes that will set aside large parts of the unused namespace for 1046 future assignments. Prefix-based partition management is an 1047 important responsibility of the NAA. 1049 This sort of delegation by prefix is well-used in the formation of 1050 DNS names and ISBN identifiers. An important difference is that in 1051 the former, the hierarchy is deliberately exposed and in the latter 1052 it is hidden. Rather than using lexical boundary markers such as the 1053 period (`.') found in domain names, the ISBN uses a publisher prefix 1054 but doesn't disclose where the prefix ends and the publisher's 1055 assigned name begins. This practice of non-disclosure, borrowed from 1056 the ISBN and ISSN schemes, is encouraged in assigning ARKs, because 1057 it reduces the visibility of an assertion that is probably not 1058 important now and may become a vulnerability later. 1060 Reasonable prefixes for assigned names usually consist of consonants 1061 and digits and are 1-5 characters in length. For example, the 1062 constant prefix "x9t" might be delegated to a book digitization 1063 project that creates identifiers such as 1065 https://444.berkeley.edu/ark:28722/x9t38rk45c 1067 If longevity is the goal, it is important to keep the prefixes free 1068 of recognizable semantics; for example, using an acronym representing 1069 a project or a department is discouraged. At the same time, you may 1070 wish to set aside a subnamespace for testing purposes under a prefix 1071 such as "fk..." that can serve as a visual clue and reminder to 1072 maintenance staff that this "fake" identifier was never published. 1074 There are other measures one can take to avoid user confusion, 1075 transcription errors, and the appearance of accidental semantics when 1076 creating identifiers. If you are generating identifiers 1077 automatically, pure numeric identifiers are likeley to be 1078 semantically opaque enough, but it's probably useful to avoid leading 1079 zeroes because some users mistakenly treat them as optional, thinking 1080 (arithmetically) that they don't contribute to the "value" of the 1081 identifier. 1083 If you need lots of identifiers and you don't want them to get too 1084 long, you can mix digits with consonants (but avoid vowels since they 1085 might accidentally spell words) to get more identifiers without 1086 increasing the string length. In this case you may not want more 1087 than a two letters in a row because it reduces the chance of 1088 generating acronyms. Generator tools such as [NOID] provide support 1089 for these sorts of identifiers, and can also add a computed check 1090 character as a guarantee against the most common transcription 1091 errors. 1093 3.7. Sub-Object Naming 1095 As mentioned previously, semantically opaque identifiers are very 1096 useful for long-term naming of abstract objects, however, it may be 1097 appropriate to extend these names with less opaque extensions that 1098 reference contemporary service entry points (sub-objects) in support 1099 of the object. Sub-object extensions beginning with a digit or 1100 underscore (`_') are reserved for the possibilty of developing a 1101 future registry of canonical service points (e.g., numeric references 1102 to versions, formats, languages, etc). 1104 4. Finding a Name Mapping Authority 1106 In order to derive an actionable identifier (these days, a URL) from 1107 an ARK, a hostname (or hostname plus port combination) for a working 1108 Name Mapping Authority (NMA) must be found. An NMA is a service that 1109 is able to respond to basic ARK service requests. Relying on 1110 registration and client-side discovery, NMAs make known which NAAs' 1111 identifiers they are willing to service. 1113 Upon encountering an ARK, a user (or client software) looks inside it 1114 for the optional NMA part (the host part of the NMA's ARK service). 1115 If it contains an NMA that is working, this NMA discovery step may be 1116 skipped; the NMA effectively uses the beginning of an ARK to cache 1117 the results of a prior mapping authority discovery process. If a new 1118 NMA needs to found, the client looks inside the ARK again for the 1119 NAAN (Name Assigning Authority Number). Querying a global database, 1120 it then uses the NAAN to look up all current NMAs that service ARKs 1121 issued by the identified NAA. 1123 The global database is key, and ideally the lookup would be automatic 1124 and transparent to the user. For this, the most promising method is 1125 probably the Name-to-Thing (N2T) Resolver [N2T] at n2t.net. It is a 1126 proposed low-cost, highly reliable, consortially maintained NMA that 1127 simply exists to support actionable HTTP-based URLs for as long as 1128 HTTP is used. One of its big advantages over the other two methods 1129 and the URN, Handle, DOI, and PURL methods, is that N2T addresses the 1130 namespace splitting problem. When objects maintained by one NMA are 1131 inherited by more than one successor NMA, until now one of those 1132 successors would be required to maintain forwarding tables on behalf 1133 of the other successors. 1135 There are two other ways to discover an NMA, one of them described in 1136 a subsection below. Another way, described in an appendix, is based 1137 on a simplification of the URN resolver discovery method, itself very 1138 similar in principle to the resolver discovery method used by Handles 1139 and DOIs. None of these methods does more than what can be done with 1140 a very small, consortially maintained web server such as [N2T]. 1142 In the interests of long-term persistence, however, ARK mechanisms 1143 are first defined in high-level, protocol-independent terms so that 1144 mechanisms may evolve and be replaced over time without compromising 1145 fundamental service objectives. Either or both specific methods 1146 given here may eventually be supplanted by better methods since, by 1147 design, the ARK scheme does not depend on a particular method, but 1148 only on having some method to locate an active NMA. 1150 At the time of issuance, at least one NMA for an ARK should be 1151 prepared to service it. That NMA may or may not be administered by 1152 the Name Assigning Authority (NAA) that created it. Consider the 1153 following hypothetical example of providing long-term access to a 1154 cancer research journal. The publisher wishes to turn a profit and 1155 the National Library of Medicine wishes to preserve the scholarly 1156 record. An agreement might be struck whereby the publisher would act 1157 as the NAA and the national library would archive the journal issue 1158 when it appears, but without providing direct access for the first 1159 six months. During the first six months of peak commercial 1160 viability, the publisher would retain exclusive delivery rights and 1161 would charge access fees. Again, by agreement, both the library and 1162 the publisher would act as NMAs, but during that initial period the 1163 library would redirect requests for issues less than six months old 1164 to the publisher. At the end of the waiting period, the library 1165 would then begin servicing requests for issues older than six months 1166 by tapping directly into its own archives. Meanwhile, the publisher 1167 might routinely redirect incoming requests for older issues to the 1168 library. Long-term access is thereby preserved, and so is the 1169 commercial incentive to publish content. 1171 Although it will be common for an NAA also to run an NMA service, it 1172 is never a requirement. Over time NAAs and NMAs will come and go. 1173 One NMA will succeed another, and there might be many NMAs serving 1174 the same ARKs simultaneously (e.g., as mirrors or as competitors). 1175 There might also be asymmetric but coordinated NMAs as in the 1176 library-publisher example above. 1178 4.1. Looking Up NMAs in a Globally Accessible File 1180 This subsection describes a way to look up NMAs using a simple name 1181 authority table represented as a plain text file. For efficient 1182 access the file may be stored in a local filesystem, but it needs to 1183 be reloaded periodically to incorporate updates. It is not expected 1184 that the size of the file or frequency of update should impose an 1185 undue maintenance or searching burden any time soon, for even 1186 primitive linear search of a file with ten-thousand NAAs is a 1187 subsecond operation on modern server machines. The proposed file 1188 strategy is similar to the /etc/hosts file strategy that supported 1189 Internet host address lookup for a period of years before the advent 1190 of DNS. 1192 The name authority table file is updated on an ongoing basis and is 1193 available for copying over the internet from a number of mirror sites 1194 [NAANregistry]. The file contains comment lines (lines that begin 1195 with `#') explaining the format and giving the file's modification 1196 time, reloading address, and NAA registration instructions. 1198 5. Generic ARK Service Definition 1200 An ARK request's output is delivered information; examples include 1201 the object itself, a policy declaration (e.g., a promise of support), 1202 a descriptive metadata record, or an error message. The experience 1203 of object delivery is expected to be an evolving mix of information 1204 that reflects changing service expectations and technology 1205 requirements; contemporary examples include such things as an object 1206 summary and component links formatted for human consumption. ARK 1207 services must be couched in high-level, protocol-independent terms if 1208 persistence is to outlive today's networking infrastructural 1209 assumptions. The high-level ARK service definitions listed below are 1210 followed in the next section by a concrete method (one of many 1211 possible methods) for delivering these services with today's 1212 technology. Note that some services may be invoked in one operation, 1213 such as when an '?info' inflection returns both a description and a 1214 permanence declaration for an object. 1216 5.1. Generic ARK Access Service (access, location) 1218 Returns (a copy of) the object or a redirect to the same, although a 1219 sensible object proxy may be substituted. Examples of sensible 1220 substitutes include, 1222 o a table of contents instead of a large complex document, 1224 o a home page instead of an entire web site hierarchy, 1226 o a rights clearance challenge before accessing protected data, 1228 o directions for access to an offline object (e.g., a book), 1230 o a description of an intangible object (a disease, an event), or 1232 o an applet acting as "player" for a large multimedia object. 1234 May also return a discriminated list of alternate object locators. 1235 If access is denied, returns an explanation of the object's current 1236 (perhaps permanent) inaccessibility. 1238 5.1.1. Generic Policy Service (permanence, naming, etc.) 1240 Returns declarations of policy and support commitments for given 1241 ARKs. Declarations are returned in either a structured metadata 1242 format or a human readable text format; sometimes one format may 1243 serve both purposes. Policy subareas may be addressed in separate 1244 requests, but the following areas should be covered: object 1245 permanence, object naming, object fragment addressing, and 1246 operational service support. 1248 The permanence declaration for an object is a rating defined with 1249 respect to an identified permanence provider (guarantor), which will 1250 be the NMA. It may include the following aspects. 1252 (a) "object availability" -- whether and how access to the object 1253 is supported (e.g., online 24x7, or offline only), 1255 (b) "identifier validity" -- under what conditions the identifier 1256 will be or has been re-assigned, 1258 (c) "content invariance" -- under what conditions the content of 1259 the object is subject to change, and 1261 (d) "change history" -- access to corrections, migrations, and 1262 revisions, whether through links to the changed objects themselves 1263 or through a document summarizing the change history 1265 A recent approach to persistence statements, conceived independently 1266 from ARKs, can be found at [PStatements], with ongoing work available 1267 at [ARKagency]. An older approach to a permanence rating framework 1268 is given in [NLMPerm], which identified the following "permanence 1269 levels": 1271 Not Guaranteed: No commitment has been made to retain this 1272 resource. It could become unavailable at any time. Its 1273 identifier could be changed. 1275 Permanent: Dynamic Content: A commitment has been made to keep 1276 this resource permanently available. Its identifier will always 1277 provide access to the resource. Its content could be revised or 1278 replaced. 1280 Permanent: Stable Content: A commitment has been made to keep this 1281 resource permanently available. Its identifier will always 1282 provide access to the resource. Its content is subject only to 1283 minor corrections or additions. 1285 Permanent: Unchanging Content: A commitment has been made to keep 1286 this resource permanently available. Its identifier will always 1287 provide access to the resource. Its content will not change. 1289 Naming policy for an object includes an historical description of the 1290 NAA's (and its successor NAA's) policies regarding differentiation of 1291 objects. Since it is the NMA who responds to requests for policy 1292 statements, it is useful for the NMA to be able to produce or 1293 summarize these historical NAA documents. Naming policy may include 1294 the following aspects. 1296 (i) "similarity" -- (or "unity") the limit, defined by the NAA, to 1297 the level of dissimilarity beyond which two similar objects 1298 warrant separate identifiers but before which they share one 1299 single identifier, and 1301 (ii) "granularity" -- the limit, defined by the NAA, to the level 1302 of object subdivision beyond which sub-objects do not warrant 1303 separately assigned identifiers but before which sub-objects are 1304 assigned separate identifiers. 1306 Subnaming policy for an object describes the qualifiers that the NMA, 1307 in fulfilling its ongoing and evolving service obligations, allows as 1308 extensions to an NAA-assigned ARK. To the conceptual object that the 1309 NAA named with an ARK, the NMA may add component access points and 1310 derivatives (e.g., format migrations in aid of preservation) in order 1311 to provide both basic and value-added services. 1313 Addressing policy for an object includes a description of how, during 1314 access, object components (e.g., paragraphs, sections) or views 1315 (e.g., image conversions) may or may not be "addressed", in other 1316 words, how the NMA permits arguments or parameters to modify the 1317 object delivered as the result of an ARK request. If supported, 1318 these sorts of operations would provide things like byte-ranged 1319 fragment delivery and open-ended format conversions, or any set of 1320 possible transformations that would be too numerous to list or to 1321 identify with separately assigned ARKs. 1323 Operational service support policy includes a description of general 1324 operational aspects of the NMA service, such as after-hours staffing 1325 and trouble reporting procedures. 1327 5.1.2. Generic Description Service 1329 Returns a description of the object. Descriptions are returned in a 1330 structured metadata format, human readable text format, or in one 1331 format that serves both purposes (such as human-readable HTML with 1332 embedded machine-readable metadata). A description must at a minimum 1333 answer the who, what, when, and where questions concerning an 1334 expression of the object. Standalone descriptions should be 1335 accompanied by the modification date and source of the description 1336 itself. May also return discriminated lists of ARKs that are related 1337 to the given ARK. 1339 5.2. Overview of The HTTP URL Mapping Protocol (THUMP) 1341 The HTTP URL Mapping Protocol (THUMP) is a way of taking a key (any 1342 identifier) and asking such questions as, what information does this 1343 identify and how permanent is it? [THUMP] is in fact one specific 1344 method under development for delivering ARK services. The protocol 1345 runs over HTTP to exploit the web browser's current pre-eminence as 1346 user interface to the Internet. THUMP is designed so that a person 1347 can enter ARK requests directly into the location field of current 1348 browser interfaces. Because it runs over HTTP, THUMP can be 1349 simulated and tested via keyboard-based interactions [RFC0854]. 1351 The asker (a person or client program) starts with an identifier, 1352 such as an ARK or a URL. The identifier reveals to the asker (or 1353 allows the asker to infer) the Internet host name and port number of 1354 a server system that responds to questions. Here, this is just the 1355 NMA that is obtained by inspection and possibly lookup based on the 1356 ARK's NAAN. The asker then sets up an HTTP session with the server 1357 system, sends a question via a THUMP request (contained within an 1358 HTTP request), receives an answer via a THUMP response (contained 1359 within an HTTP response), and closes the session. That concludes the 1360 connected portion of the protocol. 1362 A THUMP request is a string of characters beginning with a `?' 1363 (question mark) that is appended to the identifier string. The 1364 resulting string is sent as an argument to HTTP's GET command. 1365 Request strings too long for GET may be sent using HTTP's POST 1366 command. The two most common requests correspond to two degenerate 1367 special cases. First, a simple key with no request at all is the 1368 same as an ordinary access request. Thus a plain ARK entered into a 1369 browser's location field behaves much like a plain URL, and returns 1370 access to the primary identified object, for instance, an HTML 1371 document. 1373 The second special case is a minimal ARK description request string 1374 consisting of just "?info". For example, entering the string, 1375 ark.nlm.nih.gov/12345/psbbantu?info 1377 into the browser's location field directly precipitates a request for 1378 a metadata record describing the object identified by ark:12345/ 1379 psbbantu. The browser, unaware of THUMP, prepares and sends an HTTP 1380 GET request in the same manner as for a URL. THUMP is designed so 1381 that the response (indicated by the returned HTTP content type) is 1382 normally displayed, whether the output is structured for machine 1383 processing (text/plain) or formatted for human consumption (text/ 1384 html). In addition to '?info', this specification reserves both '?' 1385 and '??' (originally older forms) for future use. 1387 In the following example THUMP session, each line has been annotated 1388 to include a line number and whether it was the client or server that 1389 sent it. Without going into much depth, the session has four pieces 1390 separated from each other by blank lines: the client's piece (lines 1391 1-3), the server's HTTP/THUMP response headers (4-7), and the body of 1392 the server's response (8-13). The first and last lines (1 and 13) 1393 correspond to the client's steps to start the TCP session and the 1394 server's steps to end it, respectively. 1396 1 C: [opens session] 1397 C: GET https://ark.nlm.nih.gov/ark:12345/psbbantu?info HTTP/1.1 1398 C: 1399 S: HTTP/1.1 200 OK 1400 5 S: Content-Type: text/plain 1401 S: THUMP-Status: 0.6 200 OK 1402 S: 1403 S: erc: 1404 S: who: Lederberg, Joshua 1405 10 S: what: Studies of Human Families for Genetic Linkage 1406 S: when: 1974 1407 S: where: https://profiles.nlm.nih.gov/BB/A/N/T/U/_/bbantu.pdf 1408 S: erc-support: 1409 S: who: USNLM 1410 15 S: what: Permanent, Unchanging Content 1411 S: when: 20010421 1412 S: where: https://ark.nlm.nih.gov/yy22948 1413 S: [closes session] 1415 The first two server response lines (4-5) above are typical of HTTP. 1416 The next line (6) is peculiar to THUMP, and indicates the THUMP 1417 version and a normal return status. 1419 The balance of the response consists of a single metadata record 1420 (8-17) that comprises the ARK description service response. The 1421 returned record is in the format of an Electronic Resource Citation 1422 [ERC], which is discussed in overview in the next section. For now, 1423 note that it contains four elements that answer the top priority 1424 questions regarding an expression of the object: who played a major 1425 role in expressing it, what the expression was called, when is was 1426 created, and where the expression may be found. This quartet of 1427 elements comes up again and again in ERCs. Lines 13-17 contain a 1428 minimal persistence statement. 1430 Each segment in an ERC tells a different story relating to the 1431 object, so although the same four questions (elements) appear in 1432 each, the answers depend on the segment's story type. While the 1433 first segment tells the story of an expression of the object, the 1434 second segment tells the story of the support commitment made to it: 1435 who made the commitment, what the nature of the commitment was, when 1436 it was made, and where a fuller explanation of the commitment may be 1437 found. 1439 5.3. The Electronic Resource Citation (ERC) 1441 An Electronic Resource Citation (or ERC, pronounced e-r-c) [ERC] is a 1442 kind of object description that uses Dublin Core Kernel metadata 1443 elements [DCKernel]. The ERC with Kernel elements provides a simple, 1444 compact, and printable record for holding data associated with an 1445 information resource. As originally designed [Kernel], Kernel 1446 metadata balances the needs for expressive power, very simple machine 1447 processing, and direct human manipulation. 1449 The previous section shows two limited examples of what is fully 1450 described elsewhere [ERC]. The rest of this short section provides 1451 some of the background and rationale for this record format. 1453 A founding principle of Kernel metadata is that direct human contact 1454 with metadata will be a necessary and sufficient condition for the 1455 near term rapid development of metadata standards, systems, and 1456 services. Thus the machine-processable Kernel elements must only 1457 minimally strain people's ability to read, understand, change, and 1458 transmit ERCs without their relying on intermediation with 1459 specialized software tools. The basic ERC needs to be succinct, 1460 transparent, and trivially parseable by software. 1462 Borrowing from the data structuring format that underlies the 1463 successful spread of email and web services, the ERC format uses 1464 [ANVL], which is based on email and HTTP headers [RFC2822]. There is 1465 a naturalness to ANVL's label-colon-value format (seen in the 1466 previous section) that barely needs explanation to a person beginning 1467 to enter ERC metadata. 1469 While ANVL elements are expected at the top level and don't 1470 themselves support hierarchy, the value of an ANVL element may be an 1471 arbitrary encoded hierarchy of JSON or XML. Typically, the name of 1472 such an ANVL element ends in "json" or "xml", for example, "json" or 1473 "geojson". Care should be taken to escape structural characters that 1474 appear in element names and values, specifically, line terminators 1475 (both newlines ("\n") and carriage returns ("\r")) and, in element 1476 names, colons (":"). 1478 Besides simplicity of ERC system implementation and data entry 1479 mechanics, ERC semantics (what the record and its constituent parts 1480 mean) must also be easy to explain. ERC semantics are based on a 1481 reformulation and extension of the Dublin Core [RFC5013] hypothesis, 1482 which suggests that the fifteen Dublin Core metadata elements have a 1483 key role to play in cross-domain resource description. The ERC 1484 design recognizes that the Dublin Core's primary contribution is the 1485 international, interdisciplinary consensus that identified fifteen 1486 semantic buckets (element categories), regardless of how they are 1487 labeled. The ERC then adds a definition for a record and some 1488 minimal compliance rules. In pursuing the limits of simplicity, the 1489 ERC design combines and relabels some Dublin Core buckets to isolate 1490 a tiny kernel (subset) of four elements for basic cross-domain 1491 resource description. 1493 For the cross-domain kernel, the ERC uses the four basic elements -- 1494 who, what, when, and where -- to pretend that every object in the 1495 universe can have a uniform minimal description. Each has a name or 1496 other identifier, a location, some responsible person or party, and a 1497 date. It doesn't matter what type of object it is, or whether one 1498 plans to read it, interact with it, smoke it, wear it, or navigate 1499 it. Of course, this approach is flawed because uniformity of 1500 description for some object types requires more semantic contortion 1501 and sacrifice than for others. That is why at the beginning of this 1502 document, the ARK was said to be suited to objects that accommodate 1503 reasonably regular electronic description. 1505 While insisting on uniformity at the most basic level provides 1506 powerful cross-domain leverage, the semantic sacrifice is great for 1507 many applications. So the ERC also permits a semantically rich and 1508 nuanced description to co-exist in a record along with a basic 1509 description. In that way both sophisticated and naive recipients of 1510 the record can extract the level of meaning from it that best suits 1511 their needs and abilities. Key to unlocking the richer description 1512 is a controlled vocabulary of ERC record types (not explained in this 1513 document) that permit knowledgeable recipients to apply defined sets 1514 of additional assumptions to the record. 1516 5.4. Advice to Web Clients 1518 ARKs are envisaged to appear wherever durable object references are 1519 planned. Library cataloging records, literature citations, and 1520 bibliographies are important examples. In many of these places URLs 1521 (Uniform Resource Locators) are currently used, and inside some of 1522 those URLs are embedded URNs, Handles, and DOIs. Unfortunately, 1523 there's no suggestion of a way to probe for extra services that would 1524 build confidence in those identifiers; in other words, there's no way 1525 to tell whether any of those identifiers is any better managed than 1526 the average URL. 1528 ARKs are also envisaged to appear in hypertext links (where they are 1529 not normally shown to users) and in rendered text (displayed or 1530 printed). A normal HTML link for which the URL is not displayed 1531 looks like this. 1533 Click Here 1535 A URL with an embedded ARK invites access (via `?info') to extra 1536 services: 1538 Click Here 1540 Using the [N2T] resolver to provide identifier-scheme-agnostic 1541 protection against hostname instability, this ARK could be published 1542 as: 1544 Click Here 1546 An NAA will typically make known the associations it creates by 1547 publishing them in catalogs, actively advertizing them, or simply 1548 leaving them on web sites for visitors (e.g., users, indexing 1549 spiders) to stumble across in browsing. 1551 5.5. Security Considerations 1553 The ARK naming scheme poses no direct risk to computers and networks. 1554 Implementors of ARK services need to be aware of security issues when 1555 querying networks and filesystems for Name Mapping Authority 1556 services, and the concomitant risks from spoofing and obtaining 1557 incorrect information. These risks are no greater for ARK mapping 1558 authority discovery than for other kinds of service discovery. For 1559 example, recipients of ARKs with a specified host (NMA) should treat 1560 it like a URL and be aware that the identified ARK service may no 1561 longer be operational. 1563 Apart from mapping authority discovery, ARK clients and servers 1564 subject themselves to all the risks that accompany normal operation 1565 of the protocols underlying mapping services (e.g., HTTP, Z39.50). 1566 As specializations of such protocols, an ARK service may limit 1567 exposure to the usual risks. Indeed, ARK services may enhance a kind 1568 of security by helping users identify long-term reliable references 1569 to information objects. 1571 6. References 1573 [ANVL] Kunze, J. and B. Kahle, "A Name-Value Language", 2014, 1574 . 1576 [ARK] Kunze, J., "Towards Electronic Persistence Using ARK 1577 Identifiers", IWAW/ECDL Annual Workshop Proceedings 3rd, 1578 August 2003, . 1580 [ARKagency] 1581 ARKs-in-the-Open, "ARK Maintenance Agency", 2019, 1582 . 1584 [DCKernel] 1585 Initiative, D. C. M., "Kernel Metadata Working Group", 1586 2001-2008, . 1588 [DOI] Foundation, I. D., "The Digital Object Identifier (DOI) 1589 System", February 2001, . 1591 [ERC] Kunze, J. and A. Turner, "Kernel Metadata and Electronic 1592 Resource Citations", October 2007, 1593 . 1595 [Handle] Lannom, L., "Handle System Overview", ICSTI Forum No. 30, 1596 April 1999, . 1598 [Kernel] Kunze, J., "A Metadata Kernel for Electronic Permanence", 1599 Journal of Digital Information Vol 2, Issue 2, 1600 ISSN 1368-7506, January 2002, 1601 . 1603 [N2T] Library, C. D., "Name-to-Thing Resolver", August 2006, 1604 . 1606 [NAANregistry] 1607 ARKs.org, "NAAN Registry", 2019, 1608 . 1610 [NAANrequest] 1611 ARKs.org, "NAAN Request Form", 2018, 1612 . 1614 [NLMPerm] Byrnes, M., "Defining NLM's Commitment to the Permanence 1615 of Electronic Information", ARL 212:8-9, October 2000, 1616 . 1618 [NOID] Kunze, J., "Nice Opaque Identifiers", April 2006, 1619 . 1621 [PStatements] 1622 Kunze, J., "Persistence statements: describing digital 1623 stickiness", October 2016, 1624 . 1626 [PURL] Shafer, K., "Introduction to Persistent Uniform Resource 1627 Locators", 1996, . 1629 [RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol 1630 Specification", STD 8, RFC 854, DOI 10.17487/RFC0854, May 1631 1983, . 1633 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1634 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1635 . 1637 [RFC2141] Moats, R., "URN Syntax", RFC 2141, DOI 10.17487/RFC2141, 1638 May 1997, . 1640 [RFC2288] Lynch, C., Preston, C., and R. Daniel, "Using Existing 1641 Bibliographic Identifiers as Uniform Resource Names", 1642 RFC 2288, DOI 10.17487/RFC2288, February 1998, 1643 . 1645 [RFC2611] Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom, 1646 "URN Namespace Definition Mechanisms", BCP 33, RFC 2611, 1647 DOI 10.17487/RFC2611, June 1999, 1648 . 1650 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1651 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1652 Transfer Protocol -- HTTP/1.1", RFC 2616, 1653 DOI 10.17487/RFC2616, June 1999, 1654 . 1656 [RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822, 1657 DOI 10.17487/RFC2822, April 2001, 1658 . 1660 [RFC2915] Mealling, M. and R. Daniel, "The Naming Authority Pointer 1661 (NAPTR) DNS Resource Record", RFC 2915, 1662 DOI 10.17487/RFC2915, September 2000, 1663 . 1665 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 1666 Resource Identifier (URI): Generic Syntax", STD 66, 1667 RFC 3986, DOI 10.17487/RFC3986, January 2005, 1668 . 1670 [RFC5013] Kunze, J. and T. Baker, "The Dublin Core Metadata Element 1671 Set", RFC 5013, DOI 10.17487/RFC5013, August 2007, 1672 . 1674 [THUMP] Gamiel, K. and J. Kunze, "The HTTP URL Mapping Protocol", 1675 August 2007, . 1678 Appendix A. ARK Maintenance Agency: arks.org 1680 The ARK Maintenance Agency [ARKagency] at arks.org has several 1681 functions. 1683 o To manage the registry of organizations that will be assigning 1684 ARKs. Organizations can request or update a NAAN by filling out a 1685 form [NAANrequest]. 1687 o To be a clearinghouse for information about ARKs, such as best 1688 practices, introductory documentation, tutorials, community 1689 forums, etc. These supplemental resources help ARK implementor in 1690 high-level applications across different sectors and disciplines, 1691 and with a variety of metadata standards. 1693 o To be a locus of discussion about future versions of the ARK 1694 specification. 1696 Appendix B. Looking up NMAs Distributed via DNS 1698 This subsection introduces an older method for looking up NMAs that 1699 is based on the method for discovering URN resolvers described in 1700 [RFC2915]. It relies on querying the DNS system already installed in 1701 the background infrastructure of most networked computers. A query 1702 is submitted to DNS asking for a list of resolvers that match a given 1703 NAAN. DNS distributes the query to the particular DNS servers that 1704 can best provide the answer, unless the answer can be found more 1705 quickly in a local DNS cache as a side-effect of a recent query. 1706 Responses come back inside Name Authority Pointer (NAPTR) records. 1707 The normal result is one or more candidate NMAs. 1709 In its full generality the [RFC2915] algorithm ambitiously 1710 accommodates a complex set of preferences, orderings, protocols, 1711 mapping services, regular expression rewriting rules, and DNS record 1712 types. This subsection proposes a drastic simplification of it for 1713 the special case of ARK mapping authority discovery. The simplified 1714 algorithm is called Maptr. It uses only one DNS record type (NAPTR) 1715 and restricts most of its field values to constants. The following 1716 hypothetical excerpt from a DNS data file for the NAAN known as 12026 1717 shows three example NAPTR records ready to use with the Maptr 1718 algorithm. 1720 12026.ark.arpa. 1721 ;; US Library of Congress 1722 ;; order pref flags service regexp replacement 1723 IN NAPTR 0 0 "h" "ark" "USLC" lhc.nlm.nih.gov:8080 1724 IN NAPTR 0 0 "h" "ark" "USLC" foobar.zaf.org 1725 IN NAPTR 0 0 "h" "ark" "USLC" sneezy.dopey.com 1727 All the fields are held constant for Maptr except for the "flags", 1728 "regexp", and "replacement" fields. The "service" field contains the 1729 constant value "ark" so that NAPTR records participating in the Maptr 1730 algorithm will not be confused with other NAPTR records. The "order" 1731 and "pref" fields are held to 0 (zero) and otherwise ignored for now; 1732 the algorithm may evolve to use these fields for ranking decisions 1733 when usage patterns and local administrative needs are better 1734 understood. 1736 When a Maptr query returns a record with a flags field of "h" (for 1737 host, a Maptr extension to the NAPTR flags), the replacement field 1738 contains the NMA (host) of an ARK service provider. When a query 1739 returns a record with a flags field of "" (the empty string), the 1740 client needs to submit a new query containing the domain name found 1741 in the replacement field. This second sort of record exploits the 1742 distributed nature of DNS by redirecting the query to another domain 1743 name. It looks like this. 1745 12345.ark.arpa. 1746 ;; Digital Library Consortium 1747 ;; order pref flags service regexp replacement 1748 IN NAPTR 0 0 "" "ark" "" dlc.spct.org. 1750 Here is the Maptr algorithm for ARK mapping authority discovery. In 1751 it replace with the NAAN from the ARK for which an NMA is 1752 sought. 1754 1. Initialize the DNS query: type=NAPTR, query=.ark.arpa. 1756 2. Submit the query to DNS and retrieve (NAPTR) records, discarding 1757 any record that does not have "ark" for the service field. 1759 3. All remaining records with a flags fields of "h" contain 1760 candidate NMAs in their replacement fields. Set them aside, if 1761 any. 1763 4. Any record with an empty flags field ("") has a replacement field 1764 containing a new domain name to which a subsequent query should 1765 be redirected. For each such record, set query= 1766 then go to step (2). When all such records have been recursively 1767 exhausted, go to step (5). 1769 5. All redirected queries have been resolved and a set of candidate 1770 NMAs has been accumulated from steps (3). If there are zero 1771 NMAs, exit -- no mapping authority was found. If there is one or 1772 more NMA, choose one using any criteria you wish, then exit. 1774 A Perl script that implements this algorithm is included here. 1776 #!/depot/bin/perl 1778 use Net::DNS; # include simple DNS package 1779 my $qtype = "NAPTR"; # initialize query type 1780 my $naa = shift; # get NAAN script argument 1781 my $mad = new Net::DNS::Resolver; # mapping authority discovery 1783 &maptr("$naa.ark.arpa"); # call maptr - that's it 1785 sub maptr { # recursive maptr algorithm 1786 my $dname = shift; # domain name as argument 1787 my ($rr, $order, $pref, $flags, $service, $regexp, 1788 $replacement); 1789 my $query = $mad->query($dname, $qtype); 1790 return # non-productive query 1791 if (! $query || ! $query->answer); 1792 foreach $rr ($query->answer) { 1793 next # skip records of wrong type 1794 if ($rr->type ne $qtype); 1795 ($order, $pref, $flags, $service, $regexp, 1796 $replacement) = split(/\s/, $rr->rdatastr); 1797 if ($flags eq "") { 1798 &maptr($replacement); # recurse 1799 } elsif ($flags eq "h") { 1800 print "$replacement\n"; # candidate NMA 1801 } 1802 } 1803 } 1805 The global database thus distributed via DNS and the Maptr algorithm 1806 can easily be seen to mirror the contents of the Name Authority 1807 Table file described in the previous section. 1809 Authors' Addresses 1811 John A. Kunze 1812 California Digital Library 1813 415 20th St, 4th Floor 1814 Oakland, CA 94612 1815 USA 1817 Email: jak@ucop.edu 1818 Emmanuelle Bermes 1819 Bibliotheque nationale de France 1820 Quai Francois Mauriac 1821 Paris, Cedex 13 75706 1822 France 1824 Email: emmanuelle.bermes@bnf.fr